Device in operation

Two-stroke engines with a crank-cam.blade purge do not have a special valve timing mechanism. Gas distribution is carried out using a cylinder, a piston and a crankcase, while the crankcase serves as a purge pump housing.

The cylinder has windows that are opened and closed by a moving piston. Through the windows, a combustible mixture from the crankcase enters the cylinder and exhaust gases exit the cylinder.

In two-stroke engines, loop and direct-flow blowdown schemes are used. Loop circuits are characterized by the rotation of the combustible mixture when it moves inside the cylinder in such a way that it forms a fly. Distinguish between return and transverse loop schemes.

In a direct-flow system, the combustible mixture usually enters from one end of the cylinder, and combustion products exit from the other end.

Engines with different types of timing systems are described below.

In fig. 54, a shows a cylinder with a purge port opposite the outlet port. When blowing, when the piston is near n. m. t., the combustible mixture, pre-compressed in the crankcase, enters the cylinder through the purge hole and is directed by the deflector on the piston up to the combustion chamber. Then the combustible mixture goes down, displacing the exhaust gases through the outlet port, which closes at the end of the purge. When the exhaust gas is displaced from the cylinder through the exhaust port, a slight leakage of the combustible mixture occurs.

The described cross-blow "is almost never used. A more perfect is the loop-back blowing carried out with a conventional piston with a flat or slightly convex head. Such pistons make it possible to use a combustion chamber close in shape to a hemispherical chamber.

During the reciprocating-loop scavenging, there are two scavenging ports in the engine cylinder (Fig. 54, b), directing two jets of the combustible mixture at an angle to one another to the cylinder wall located opposite the outlet port. The jets of the combustible mixture rise up to the combustion chamber and, making a loop, go down to the outlet window. Thus, the exhaust gases are displaced and the cylinder filled with a fresh mixture.

The most widespread is the return two-channel blowdown. It is used both in engines of domestic and foreign motorcycles (M-104, Kovrovets-175A, Kovrovets-175B and Kovrovets-175V, IZH Jupiter, Java, Panonia, etc. ).

Three-channel blowdown (Fig. 54, e) is used, for example, for Tsyundap engines, four-channel blowdown (Fig. 54, d) - for IZH-56 motorcycle engines, cross-shaped two-channel blowdown (Fig. 54, e) - for Ardi engines, four-channel (Fig. 54, e) -_. at Villiers engines.

With all the described methods of purging the single-piston engine has a symmetrical valve timing diagram (Fig. 55). This means that * if the intake phase begins before the arrival of the piston in the. m. t. (for example, for 67.5 °), then its end occurs through 67.5 ° of the angle of rotation of the crankshaft after v. m. t. Also begin and end relative to n. m.t. phases of release and purge. The exhaust phase is longer than the purge phase. Filling the cylinder with a combustible mixture occurs all the time with an open outlet window. This feature of symmetrical valve timing limits the ability to increase the engine's liter power. In addition, the compressed working mixture contains a relatively large amount of residual gases. To reduce the amount of residual gases and improve the filling of the cylinder with a combustible mixture, the blowing is improved. For this, the design of the engine is sometimes changed, although it is more expedient to achieve an increase in power in a conventional two-stroke engine without complicating its design. At the Dunelt engine (Fig. 56, a), a stepped piston is used to increase the amount of the incoming fuel mixture. The volume described by the bottom of the oversized piston is approximately 50% greater than the top of the cylinder.

The Bekamo engine (Fig. 56, b) has an additional large-diameter cylinder with a piston with a small stroke. The piston is driven by a connecting rod from an additional crank on the crankshaft. Such engines, in contrast to engines with superchargers, are called engines with "backwater" (engines of this type were installed, in particular, on some domestic sports motorcycles). These engines have symmetrical valve timing with a single piston. However, the outlet port closes later than the purge port. The piston delivers an additional amount of the mixture when the exhaust port is open, as a result of which the cylinder is not filled with a compressed fuel mixture, as is the case in a supercharged engine, in which the intake occurs partially with the exhaust port or valve closed.

To increase the filling of the engine with a combustible mixture, spool devices are also used, with the help of which the intake phase is increased. Possible options for the spool device are the installation of a spool on the cylinder instead of the nozzle for the carburetor (Fig. 57, a) or on the crankcase (Fig. 57, b), as well as the spool proposed by the author in the hollow main journal of the crankshaft. In the latter case, you can change the valve timing while the engine is running (Fig. 57, c) and use its vortex motion in the crankcase to form and stop the jets of the combustible mixture. This design, but without a device for changing the valve timing, is used, in particular, on the D-4 bicycle engine.

Record results are shown by engines manufactured in the GDR for the MZ motorcycle, in which the fuel mixture is fed into the central part of the crankcase through a device with a rotating spring-loaded valve (Fig. 57, d) made of sheet steel.

Engines with direct-flow scavenging, having two pistons in two cylinders with a common combustion chamber (so-called two-piston engines), are distinguished by high power.

The Junkers engine with direct-flow blowdown has the following device (Fig. 58, a). The cylinder contains two pistons moving towards each other. The middle part of the cylinder between the piston crowns when they are in position. m. t. serves as a combustion chamber. It contains a spark plug. The combustible mixture enters through the windows on the right side of the cylinder and displaces the exhaust gases into the exhaust ports located on the left side of the cylinder. In this case, the combustible mixture almost does not mix with the exhaust gases.

The cylinder can be fed in the usual way using a crank-chamber purge or a separate compressor feeding the mixture with a slide valve. Each piston is connected by a connecting rod to a separate crankshaft. The crankshafts are interconnected by gears so that when approaching n. m. t. the left piston opens the outlet ports approximately 19 ° earlier than the right piston opens the purge ports. The release of the exhaust gases starts earlier than in a single-piston engine, and accordingly the pressure in the cylinder is lower by the beginning of the purge. When the piston moves from n. m. sq. m. t., in contrast to single-piston engines, the exhaust ports are closed earlier than the purge ones and the cylinder is filled with closed exhaust ports for about a time corresponding to the rotation of the crankshaft by 29 *. The asymmetrical phase diagram of the purge and discharge in the direct-flow purge makes it possible to effectively use the supercharger to obtain high power.

The domestic engine of the GK-1 racing motorcycle has a similar structure.

Engines of this design are complex and expensive to manufacture, not. correspond to the layout adopted in the motorcycle industry and therefore have not received mass distribution.

There are ramjet engines that are more convenient for positioning on a motorcycle. In engines with direct-flow blowing according to the Zoller scheme, two pistons move in a U-shaped cylinder. The combustion chamber is located in the middle. The combustible mixture enters through the window on the right side of the cylinder, and the exhaust gases exit through the window on the left side of it. The piston movement, providing asymmetric purge and exhaust phases, is carried out using various crank mechanisms. In DKV engines (Fig. 58, b), one piston is mounted on the main connecting rod, and the other on a trailed one. The Pooh engine (Fig. 58, c) uses a forked connecting rod. In Triumph engines, which have a Zoller scheme, the crankshaft consists of two crankshaft displaced one relative to the other and two connecting rods (Fig. 58, d).

With direct-flow blowing, the cylinders can be placed at an acute angle, with the combustion chamber at the apex of the corner (Fig. 58, e). In this case, the combustion chamber is less stretched than with a U-shaped cylinder. Otherwise, such an engine is similar to that of the Juncker system.

Domestic engines with superchargers for racing motorcycles S-1B, S-2B and S-ZB, which are distinguished by high liter capacity, have direct-flow blowing and angled parts of the cylinder.

Service

Gas distribution in a two-stroke engine is disturbed most often when excess air enters it and when the resistance of the exhaust tract increases. It is necessary to monitor the tightness of the crankcase, timely tighten the connections, change damaged gaskets and oil seals, and also clean the cylinder outlet windows, pipe and muffler from carbon deposits.

Those who are connected with racing automobile or motorcycle technology or are simply interested in the design of sports cars are familiar with the name of engineer Wilhelm Wilhelmovich Beckman - the author of the books "Racing Cars" and "Racing Motorcycles". More than once he spoke on the pages of "Behind the Wheel".

Recently, the third edition of the book "Racing Motorcycles" was published (the second was released in 1969), revised and supplemented with information about new design solutions and an analysis of the trend in the further development of two-wheeled vehicles. The reader will find in the book an essay on the history of the birth of motorcycle sport and its influence on the development of the motorcycle industry, receive information about the classification of cars and competitions, get acquainted with the design features of engines, transmissions, chassis and ignition systems of racing motorcycles, and learn about ways to improve them.

Much of what is first used in sports cars is then introduced to production road bikes. Therefore, acquaintance with them allows you to look into the future and imagine the motorcycle of tomorrow.

The overwhelming majority of motorcycle engines currently under construction in the world operate on a two-stroke cycle, so motorists are most interested in them. We bring to the attention of our readers an excerpt from the book by V.V. Beckman, dedicated to one of the most important issues in the development of two-stroke engines. We made only minor abbreviations, changed the numbering of figures and brought some names in line with those used in the magazine.

Currently, two-stroke racing engines outperform their four-stroke rivals in the 50 to 250 cc classes: in the larger displacement classes, four-stroke engines are still competitive. since the high boost of two-stroke engines of these classes is more difficult, and the known disadvantage of the two-stroke process becomes more noticeable - increased fuel consumption, which requires an increase in the volume of fuel tanks and more frequent stops for refueling.

The prototype of most modern two-stroke engines of the racing type is the design developed by the MC (GDR) company. The work on the improvement of two-stroke engines carried out by this company provided the MC racing motorcycles of 125 and 250 cm3 classes with high dynamic qualities, and their design was copied to one degree or another by many companies in other countries of the world.

MTs racing engines (Fig. 1) have a simple design and are similar both in structure and appearance to conventional two-stroke engines.

A - general view; b - the location of the gas distribution channels

For 13 years, the power of the MC 125 cm3 racing engine has grown from 8 to 30 hp. from.; already in 1962 a liter capacity of 200 liters was reached. s. / l. One of the essential elements of the engine is the rotary disc valve proposed by D. Zimmerman. It allows asymmetric intake phases and an advantageous shape of the intake tract to be achieved: this increases the crankcase filling ratio. The disc spool is made of thin (about 0.5 mm) sheet spring steel. The optimal disc thickness has been found empirically. The disc spool acts as a septum valve, pressing against the intake port as the combustion mixture is compressed in the crankcase. With increased or decreased spool thickness, accelerated disc wear is observed. A disc that is too thin bends towards the intake channel, which entails an increase in the frictional force between the disc and the crankcase cover; increased disc thickness also leads to increased frictional losses. As a result of the refinement of the design, the service life of the disc spool was increased from 3 to 2000 hours.

The disc spool does not add much complexity to the engine design. The spool is mounted on the shaft by means of a sliding keyed or spline connection so that the disc can take a free position and not be pinched in the narrow space between the crankcase wall and the cover.

Compared to a conventional intake port control system, the lower edge of the piston spool allows the intake port to be opened earlier and kept open for a long time, which contributes to increased power at both high and medium speeds. With a conventional gas distribution device, early opening of the intake port is inevitably associated with a large delay in its closing: this is useful for obtaining maximum power, but is associated with a reverse emission of the combustible mixture at medium conditions and a corresponding deterioration in the torque characteristics and starting qualities of the engine.

On two-cylinder engines with parallel cylinders, disc spools are installed at the ends of the crankshaft, which, with carburetors protruding to the right and left, gives large dimensions in the width of the engine, increases the frontal area of \u200b\u200bthe motorcycle and worsens its external shape. To eliminate this drawback, a design was sometimes used in the form of two twin-angle single-cylinder engines with a common crankcase and air cooling (Derby, Java).

Unlike the Java engine, the cylinders of paired engines can be vertical: this requires water cooling, since the rear cylinder is obscured by the front one. One of the MC 125 cm3 racing engines was manufactured according to this scheme.

Suzuki's three-cylinder engine (50 cc, about 400 hp / l) with disc spools essentially consisted of three single-cylinder engines with independent crankshafts combined in one block: two cylinders were horizontal. one vertical.

Engines with intake valves were also designed in four-cylinder versions. Typical examples are Yamaha engines manufactured as two gear-coupled parallel-cylinder twin-cylinder engines; one pair of cylinders is horizontal, the other is at an upward angle. The 250 cm3 engine developed up to 75 hp. with., and the power of the 125 cm3 variant reached 44 liters. from. at 17,800 rpm.

The four-cylinder Java engine (350 cm3, 48x47) with spools at the inlet, which is two twin water-cooled two-cylinder engines, is designed in a similar way. It develops a capacity of 72 liters. from. at 1300 rpm. The power of the four-cylinder Morbidelli engine of the 350 cm3 class of the same type is even greater - 85 hp. from.

Due to the fact that disc spools are mounted at the ends of the crankshaft, power take-off in multi-cylinder designs with this intake system is usually done through a gear on the middle journal of the shaft between the crankcase compartments. With disc spools of the type under consideration, an increase in the number of engine cylinders over four is impractical, since further pairing of two-cylinder engines would lead to a very cumbersome design; even in the four-cylinder version, the engine is obtained at the limit of permissible dimensions.

Recently, some Yamaha racing engines have used automatic diaphragm valves in the intake port between the carburetor and the cylinder (Fig. 2, a). The valve is a thin elastic plate that bends under the action of the vacuum in the crankcase and frees the passage for the combustible mixture. To avoid valve breakage, their travel stops are provided. In medium operating conditions, the valves close quickly enough to prevent the backflow of the combustible mixture, which improves the engine torque characteristic. These valves, based on practical observation, can function normally at speeds up to 10,000 rpm. At higher speeds, their performance is problematic.

: a - device diagram; b - the beginning of filling the crankcase; c - suction of the mixture through the valves into the cylinder; 1 - limiter; 2 - membrane; 3 - a window in the piston

In engines with diaphragm valves, to improve filling, it is advisable to maintain communication between the intake port and the sub-piston space or purge port when the piston is near N.M.T. For this purpose, appropriate windows 3 are provided in the piston wall from the intake side (Fig. 2, b). Diaphragm valves provide additional suction of the combustible mixture when a vacuum is formed in the cylinders and crankcase during purging (Fig. 2, c).

High power is also developed by two-stroke engines, in which the piston controls the intake of the fuel mixture into the crankcase, as in the vast majority of conventional mass-produced engines. This mainly applies to engines with a displacement of 250 cm3 and more. Examples are Yamaha and Harley-Davidson motorcycles (250cc - 60hp;

350 cm3 - 70 HP pp.), as well as a Suzuki motorcycle with a two-cylinder engine of the 500 cm3 class with a capacity of 75 hp. with., took first place in the race T.T. (Tourist Trophy) 1973. Forcing these engines is carried out in the same way as in the case of using disc spools, by careful design study of the gas distribution bodies and on the basis of studying the mutual influence of the intake and exhaust tracts.

Two-stroke engines, regardless of the intake control system, have a straightened intake tract, which is directed into the sub-piston space, where the combustible mixture enters; in relation to the axis of the cylinder, the intake tract can be perpendicular or inclined from bottom to top or from top to bottom. This shape of the intake tract is favorable for exploiting the effect of resonant boost. The flow of the combustible mixture in the intake tract continuously pulsates, and waves of rarefaction and high pressure arise in it. Adjustment of the intake tract by selecting its dimensions (length and flow sections) allows for the closing of the intake port at a certain speed interval at the moment of the high pressure wave entering the crankcase, which increases the filling factor and increases engine power.

If the crankcase filling ratio is greater than unity, the two-stroke engine would have to develop twice the power compared to the four-stroke. In reality, this does not happen due to significant losses of the fresh mixture in the exhaust and mixing of the charge that entered the cylinder with the residual gases from the previous working cycle. The imperfection of the working cycle of a two-stroke engine is due to the simultaneous occurrence of the processes of filling the cylinder and cleaning it from combustion products, while in a four-stroke engine these processes are separated in time.

Gas exchange processes in a two-stroke engine are very complex and still difficult to calculate. Therefore, the forcing of engines is carried out mainly through the experimental selection of the ratios and sizes of the structural elements of the gas distribution elements from the carburetor inlet pipe to the exhaust pipe end pipe. Over time, a lot of experience has been accumulated in forcing two-stroke engines, described in various studies.

In the first designs of MC racing engines, a return-loop blowing of the "Shnurle" type with two blowing channels was used. A significant improvement in performance was obtained by the addition of a third purge duct (see Figure 1) located in front of the exhaust ports. A special window is provided on the piston for bypassing through this channel. An additional purge passage eliminated the formation of a cushion of hot gases under the bottom of the piston. Thanks to this channel, it was possible to increase the filling of the cylinder, improve cooling and lubrication with a fresh mixture of the needle bearing of the upper connecting rod head, and also facilitate the temperature regime of the piston bottom. As a result, engine power increased by 10 percent, and piston burnouts and upper connecting rod bearing failures were eliminated.

The quality of the blowdown depends on the degree of compression of the combustible mixture in the crankcase; on racing engines, this parameter is maintained in the range of 1.45 - 1.65, which requires a very compact design of the crank mechanism.

Achieving high liter capacities is possible due to the wide distribution phases and the large width of the gas distribution windows.

The width of the windows of racing engines, measured by the center angle in the cross section of the cylinder, reaches 80 - 90 degrees, which creates a difficult working condition for piston rings. But with such a width of windows in modern engines, they do without jumpers prone to overheating. Increasing the height of the purge ports shifts the maximum torque to a lower rpm region, while increasing the height of the exhaust ports has the opposite effect.

Figure: 3. Purge systems:a - with the third blowing window, b - with two additional blowing channels; c - with branching blowing channels.

The purge system with a third additional purge port (see Fig. 1) is convenient for engines with a spool, in which the inlet port is located on the side, and the cylinder area opposite the outlet port is free to accommodate the purge port; the latter can be bridged as shown in fig. 3, a. An additional scavenging window facilitates the formation of a flow of a combustible mixture that envelopes the cylinder cavity (loop scavenging). The entry angles of the purge channels are very important for the efficiency of the gas exchange process; the shape and direction of the mixture flow in the cylinder depend on them. The horizontal angle a, ranges from 50 to 60 degrees, with a larger value corresponding to a higher engine boost. The vertical angle a2 is 45 - 50 degrees. the ratio of the cross-sections of the additional and main blow-out ports is about 0.4.

On engines without a spool, the carburetors and intake ports are usually located on the rear of the cylinders. In this case, a different blowing system is usually used - with two additional side blowing channels (Fig. 3, b). The horizontal angle of entry a, (see Fig. 3, a) of the additional channels is about 90 degrees. The vertical angle of entry of the purge nanals varies for different models within a fairly wide range: on the Yamaha TD2 model of 250 cm3 class, it is 15 degrees for the main purge channels, and 0 degrees for additional ones; on the Yamaha TD2 model of 350 cm3 class, 0 and 45 degrees, respectively.

Sometimes a variant of this blowdown system with branching blowdown channels is used (Fig. 3, c). Additional purge ports are located opposite the outlet port, and, therefore, such a device approaches the first of the systems considered, having three ports. The vertical angle of entry of additional purging channels is 45 - 50 degrees. The cross-section ratio of the additional and main blow-out ports is also about 0.4.

Figure: 4. Schemes of gas movement in the cylinder: a - with branching channels; b - with parallel ones.

In fig. 4 shows diagrams of gas movement in the cylinder during the purging process. With an acute angle of entry of the additional purging channels, the fresh mixture flow coming from them removes a ball of exhaust gases in the middle of the cylinder, which is not captured by the mixture flow from the main purging channels. Other variants of blowdown systems are possible according to the number of blowdown ports.

It should be noted that on many engines the duration of opening additional purge ports is 2 - 3 degrees less than that of the main ones.

On some Yamaha engines, additional purge channels were made in the form of grooves on the inner surface of the cylinder; the inner wall of the channel is here the wall of the piston at its positions near N.M.T.

The purge process is also affected by the profile of the purge channels. Smooth shape without sharp bends results in lower pressure drops and improves engine performance, especially at intermediate speeds.

The information in this section shows that two-stroke engines stand out for their simplicity in design.

The increase in power density of engines of this type during the last decade has not been accompanied by any significant changes in the basic design; it was the result of careful experimental selection of ratios and sizes of previously known structural elements.

The time intervals from the beginning of the moment of opening the engine valves to their complete closure relative to the dead points of the piston movement were called the valve timing. Their influence on engine performance is very great. So, the efficiency of filling and cleaning cylinders during engine operation depends on the duration of the phases. This directly determines the fuel economy, power and torque.

The essence and role of valve timing

At the moment, there are motors in which the phases cannot be changed forcibly, and motors equipped with mechanisms (for example, CVVT). For the first type of engines, the phases are selected experimentally when designing and calculating the power unit.

Fixed and variable valve timing

Visually, they are all displayed on special valve timing diagrams. Upper and lower dead points (TDC and BDC, respectively) represent the extreme positions of the piston moving in the cylinder, which correspond to the largest and smallest distance between an arbitrary point of the piston and the axis of rotation of the engine crankshaft. The starting points for opening and closing valves (phase length) are shown in degrees and are considered relative to the rotation of the crankshaft.

Phase control is carried out using (timing), which consists of the following elements:

• cam camshaft (one or two);
• chain or belt drive from the crankshaft to the camshaft.

Gas distribution mechanism

It always consists of strokes, each of which corresponds to a certain position of the valves at the inlet and outlet. Thus, the beginning and the end of the phase depend on the angle of the crankshaft position, which is associated with the camshaft that controls the valve position.

In one revolution of the camshaft, the crankshaft makes two revolutions and its total angle of rotation per working cycle is 720 °.

Camshaft timing diagram

We will consider the operation of the valve timing for a four-stroke engine using the following example (see picture):

1. Inlet... At this stage, the piston moves from TDC to BDC and the crankshaft rotates 180º. The outlet valve is closed and the inlet valve is opened. The latter occurs with an advance of 12º.
2. Compression... The piston moves from BDC to TDC and the crankshaft rotates 180º (360º from start). The exhaust valve remains closed and the intake valve remains open until the crankshaft rotates 40º.
3. Working stroke... The piston goes from TDC to BDC under the action of the ignition force of the air-fuel mixture. The intake valve is in the closed position and the exhaust valve opens ahead of time when the crankshaft has not yet reached 42º to BDC. At this stroke, the full turn of the crankshaft is also 180º (540º from the initial position).
4. Release... The piston goes from BDC to TDC and at the same time pushes the exhaust gases. At this moment, the intake valve is closed (opens 12º before TDC), and the exhaust valve remains in the open position even after the crankshaft reaches TDC by another 10º. The total amount of crankshaft rotation at this stroke is also 180º (720º from the starting point).

Timing timing also depends on the profile and position of the camshaft cams. So, if they are the same at the inlet and outlet, then the valve opening time will also be the same.

Why are the valves delayed and advanced?

In order to improve the filling of the cylinders, as well as to provide a more intensive cleaning of the exhaust gases, the actuation of the valves does not occur at the moment the piston reaches the dead points, but with a slight advance or lag. So, the opening of the intake valve is performed until the piston passes the TDC (from 5 ° to 30 °). This allows for a more intensive injection of a fresh charge into the combustion chamber. In turn, the closing of the intake valve occurs with a delay (after the piston has reached bottom dead center), which allows the cylinder to continue filling with fuel due to inertial forces, the so-called inertial boost.

The exhaust valve also opens ahead of time (from 40 ° to 80 °) until the piston reaches BDC, which allows most of the exhaust gases to escape under its own pressure. The closing of the exhaust valve, on the other hand, occurs with a delay (after the piston passes the top dead center), which allows the inertial forces to continue removing the exhaust gases from the cylinder cavity and makes it more effective for cleaning.

Lead and lag angles are not common to all engines. More powerful and faster ones have larger values \u200b\u200bof these intervals. Thus, their valve timing will be wider.

The stage of engine operation in which both valves are open at the same time is called valve overlap. Typically, the amount of overlap is about 10 °. At the same time, since the overlap time is very short and the valve opening is negligible, no leakage occurs. This is a fairly favorable stage for filling and cleaning the cylinders, which is especially important at high rpm.

At the beginning of the opening of the intake valve, the current pressure level in the combustion chamber is higher than atmospheric pressure. As a result, the exhaust gases move very quickly to the exhaust valve. When the engine switches to the intake stroke, a high vacuum will be established in the chamber, the exhaust valve will fully close, and the intake valve will open to a cross section sufficient for intensive filling of the cylinder.

Features of variable valve timing

At high speeds, the car engine needs more air volume. And since in unregulated timing, the valves can close before enough of it enters the combustion chamber, the engine is ineffective. To solve this problem, various methods have been developed for adjusting the valve timing.

Camshaft timing valve

The first motors with a similar function made it possible to perform step adjustment, which made it possible to change the phase length depending on the achievement of certain values \u200b\u200bby the engine. Over time, stepless designs have appeared, allowing for smoother and more optimal tuning.

The simplest solution is the phase shift system (CVVT), which is realized by turning the camshaft relative to the crankshaft by a certain angle. This allows you to change the moment of opening and closing the valves, but the actual duration of the phase remains unchanged.

In order to directly change the duration of the phase, a number of cars use several cam mechanisms, as well as oscillating cams. For the precise operation of the regulators, complexes of sensors, controller and actuators are used. The control of such devices can be electrical or hydraulic.

One of the main reasons for the introduction of timing systems is the tightening of environmental standards for the level of toxicity of exhaust gases. This means that for most manufacturers, the issue of valve timing optimization remains one of the most important.

Go Kart Design - Forcing Engines

There will be no ready-made recipes for forcing specific types of engines. All engines are different, on different chassis the dimensions of individual elements (for example, the exhaust system) will change, and the characteristics will also change. Therefore, some specific recipes, in which, nevertheless, there will be a lot of white spots, can only lead to useless work.

In particular, the fundamentals of the theory of the processes occurring in the engine will be considered, with special emphasis on those issues that are fundamental when forcing the engine. Of course, in the proposed chapter, only those sections of the theory are considered, the knowledge of which is necessary so that a novice karting fan does not spoil the engine in an effort to squeeze maximum power out of it. General recommendations are also given on the directions in which engine modifications should be carried out in order to achieve positive results. General instructions are illustrated with examples from practical work on boosting kart engines. In addition, a number of comments and practical recommendations are given regarding seemingly small changes, the introduction of which will improve the operation of the engine, increase its reliability, and save us from sometimes costly learning from our own mistakes.

Gas distribution phases

The valve timing is expressed by the angles of rotation of the crankshaft at which the corresponding cylinder windows open and close. In a two-stroke engine, consider three phases: opening the inlet port, opening the outlet port, and opening the bypass ports (Figure 9.3).

The phase of opening a window, for example, an exhaust one, is the angle of rotation of the crankshaft, measured from the moment when the upper edge of the piston opens the outlet window, until the moment when the piston, moving back, closes the window. Similarly, you can define the phases of the opening of other windows.

Figure: 9.3. Valve timing diagrams:

a -symmetrical; b - asymmetrical; OD and ZD - inlet opening and closing. OP and ZP - bypass opening and closing; OW and ZW - issue opening and closing; a, y- opening angles of the inlet and outlet windows, respectively; B - opening angle of bypass windows

Figure: 9.4. Comparison of time-sections (area under curves) for windows of different shapes

In a conventional piston engine, all windows are opened and closed by a piston, so the valve timing diagram is symmetrical (or almost symmetrical) about the vertical axis (Fig.9.3, and).In kart engines, in which the crank chamber is filled with a combustible mixture using a rotating valve, the intake phase may not depend on the movement of the piston, so the valve timing diagram is usually asymmetrical (Fig.9.3, b).

Valve timing are comparable values \u200b\u200bfor engines with different piston strokes, that is, they serve as universal characteristics. When comparing engines with the same piston stroke, the valve timing can be replaced by the distances from the windows, for example, to the top plane of the cylinder.

In addition to the valve timing, an important parameter is the so-called time-section. When the window is gradually opened by the piston, the shape of the channel depends on how the open surface of the window increases, depending on the angle of rotation of the crankshaft (or time). The wider the window, the more surface will open when the piston is moved downward. For the same time, a greater amount of combustible mixture will pass through the window. It is advisable that when the window is opened by the piston, its area is immediately as large as possible. In many engines, for this, the window is extended upward. This achieves the effect of quickly opening the window without increasing its surface.

The diagram of the growth of the open surface of windows of different shapes depending on time at a constant FW of the engine is shown in Fig. 9.4. The total area of \u200b\u200bthe windows is the same in both cases. The area under the curves of the diagram characterizes the time-section value. For an irregularly shaped window, the time-section is larger.

Cylinder Purge Systems

Figure: 9.10. Diagram of cylinder blowdown systems and corresponding cylinder mirror sweeps:

a - two-channel system; b - three-channel system; c - four-channel system; d - five-channel system

Cylinder purge systems used in kart engines are schematically shown in Fig. 9.10. The location of the bypass windows on the scan of the cylinder mirror for each of the systems: two-, three-, four- and five-channel is shown next. In engines where the crankcase filling is controlled by a piston, covers and does not close the intake port. In this case, the inlet is not made in the cylinder, and it becomes possible to place an additional bypass channel.

The role of the exhaust system

In a two-stroke engine, the exhaust system plays a huge role, which consists of an exhaust pipe (in the cylinder and behind the cylinder), an expansion chamber and a muffler. At the moment the outlet port is opened, there is some pressure in the cylinder, which is reduced in the exhaust system. The gas expands, shock waves appear, which are reflected from the walls of the expansion chamber. Reflected shock waves cause a new increase in pressure near the exhaust port, as a result of which some of the exhaust gases again enters the cylinder (Fig.9.11).

Figure: 9.11. Schematic representation of sequential exhaust phases:

a - opening the outlet window; b - full opening of the window; c - closing the window

It seems that it would be more advantageous to obtain a vacuum at the outlet when it is fully open. This will cause the gases to be pumped out of the cylinder and thereby fill the cylinder with a fresh mixture. However, in this case, part of this mixture, together with the exhaust gases, will enter the outlet pipe. Therefore, it is necessary to achieve increased pressure at the outlet port when it is closed. In this case, the combustible mixture that has entered the exhaust pipe along with the exhaust gases will be returned to the cylinder, significantly improving its filling. This happens after the piston closes the bypass ports. As in the intake system, the wave phenomena in the exhaust system have a positive effect only near the resonant CV. By changing the dimensions, and especially the length of the exhaust system, it is also possible to shape the speed characteristics of the engine. The effect of changes in the size of the exhaust system on engine performance is more significant than the change in the size of the intake system.

Combustion Basics

For a better understanding of the engine's operation, it is necessary to say a few words about the processes occurring in the engine combustion chamber. The growth of pressure in the cylinder depends on the course of the combustion process, which determines the engine power.

The results of fuel combustion, perceived as the operation of the crank mechanism, primarily depend on the composition of the combustible mixture. The theoretically ideal composition of the combustible mixture is the so-called stoichiometric composition, that is, one in which the mixture contains so much fuel and oxygen that after combustion there is no fuel or oxygen in the exhaust gases. In other words, all the fuel in the combustion chamber will burn, and all the oxygen contained in the combustible mixture will be consumed for its combustion.

If there was an excess of air in the combustion chamber (lack of fuel), then this excess would not be able to help the combustion process. However, it would become an additional mass of gas that must be "pumped" through the engine and heated using heat, which, without this additional mass, would increase the temperature and, therefore, the pressure in the cylinder. A combustible mixture with excess air is called lean.

Lack of air (or excess fuel) is equally unfavorable. This would lead to incomplete combustion of the fuel and, as a result, to less energy. The excess fuel will then be passed through the engine and evaporate. A combustible mixture with a lack of air is called rich.

In practice, to obtain the highest power, it is advisable to use a slightly rich mixture. This is due to the fact that local inhomogeneities in the composition of the combustible mixture are always formed in the combustion chamber, arising from the fact that it is impossible to achieve ideal mixing of fuel with air. The optimal composition of the mixture can only be determined empirically.

The volume of the combustible mixture sucked into the cylinder each time is determined by the working volume of this cylinder. But the mass of air in this volume depends on the air temperature: the higher the temperature, the lower the density of the air. Thus, the composition of the combustible mixture depends on the air temperature. Because of this, it is necessary to "tune" the engine depending on the weather. On a hot day, warm air enters the engine, therefore, to maintain the correct composition of the combustible mixture, it is necessary to reduce the fuel supply. On a cold day, the mass of the incoming air increases, so more fuel must be supplied. It should be noted that air humidity also affects the composition of the combustible mixture.

As a result of all this, the temperature of even an ideal mixture composition under these conditions significantly affects the degree of filling of the crank chamber. In a constant crankcase volume at a higher temperature, the mass of the combustible mixture will be less and, thus, after its combustion, there will be a lower pressure in the cylinder. Because of this phenomenon, they try to give the engine elements such a shape, especially the crankcase (ribbing), in order to achieve their maximum cooling.

The combustion of the mixture in the combustion chamber occurs at a certain speed; during the combustion, the crankshaft rotates at a certain angle. The pressure in the cylinder increases as the mixture burns. It is advisable to obtain the highest pressure at the moment when the working stroke of the piston has already begun. To achieve this, the mixture must be ignited a little earlier, with a certain advance. This advance, measured by the crankshaft angle, is called the ignition timing. It is often more convenient to measure the ignition timing by the distance that remains for the piston to travel to top dead center.

Range of modifications

Before we start working on the engine, we need to decide what figure we want to achieve. In five-, six-speed motors of the racing category, we can strive to increase the CV, although it is known that as a result of this CV of the maximum torque it approaches the CV of maximum power; we reduce the range of working revolutions, seeking more power in return.

In engines of the popular category, and these are the Damba engines with a volume of 125 cm 3 with a three-speed gearbox, one should not strive to achieve too high CV, it is necessary to achieve the greatest range of operating CV. In such engines (using its own components and assemblies), it is possible to achieve a power of more than 10 kW at a rotational speed of the order of 7000-8000 rpm.

It is also necessary to determine the range of improvements that we are going to perform. You need to know in advance whether this will be the introduction of improvements to the engine under development, or the range of improvements will be so wide that in the end we will get a practically new engine while maintaining several original (but modified) units, as required by the rules.

Assuming the revision of the engine, preference should be given to those operations that will significantly increase the performance of the engine. However, it is not worth (at least at this stage of work) to provide for the implementation of such operations that require significant labor and which are known in advance that they will give insignificant results. Such operations include polishing of all bores of the engine cylinder, despite the fact that there is a general belief in the effectiveness of this operation. Bench tests of many engines have shown that polishing the cylinder bores increases the engine power by 0.15-0.5 kW. As you can see, the effort spent on doing this work is completely incommensurate with the results.

Here are the operations that will undoubtedly affect the increase in engine performance: increasing the compression ratio; change in valve timing; changing the shape and size of channels and cylinder windows; correct selection of the parameters of the intake and exhaust systems; optimization of ignition timing.

Changing the compression ratio

An increase in the compression ratio obtained by reducing the volume of the combustion chamber leads to an increase in engine power. An increase in the compression ratio leads to an increase in the combustion pressure in the cylinder by increasing the compression pressure, improving the circulation of the mixture in the combustion chamber and increasing the combustion rate.

The compression ratio cannot be increased to any arbitrary value. It is limited by the quality of the fuel used and the thermal and mechanical strength of the engine components. Suffice it to say that when the effective compression ratio increases from 6 to 10, the forces acting on the piston almost double; that is, the load, for example, on the crank mechanism doubles.

Taking into account the strength of engine parts and the detonation properties of available fuels, it is not recommended to use a geometric compression ratio greater than 14. Increasing the compression ratio to this value requires not only removing the gasket (if any), but also shaping the cylinder head and sometimes the cylinder. To facilitate the calculation of the volume of the combustion chamber for different degrees, you can use the diagram shown in Fig. 9.17. Each of the curves refers to a specific cylinder displacement.

Figure: 9.17. Diagram of the dependence of the compression ratio a on the volume of the combustion chamber V 1 \u003d 125 cm 3 and V 2 -50 cm 3

In some engines with a relatively low compression ratio, its significant increase is possible only by machining. In this case, the combustion chamber is melted and processed again. It also allows you to change the shape of the camera. Most modern engines used in karting have a hat-shaped combustion chamber. This shape should not be changed when modifying the engine.

The only way to accurately determine the volume of the combustion chamber is to fill it with engine oil through the spark plug hole (Fig. 9.18) with the piston at top dead center. With this method of measurement, the volume of the plug hole must be subtracted from the volume of the filled oil. The volume of the candle hole for a candle with a short thread is 1-1.1 cm ’1, for a candle with a long thread - 1.7-1.8 cm 3.

Cylinder head gaskets are either not used at all in racing engines, or they are replaced by thin copper rings. In both cases, the joint surfaces of the cylinder and the head must be ground in. The use of gaskets made of a material with a low thermal conductivity coefficient is contraindicated, because it will impede the outflow of heat from the upper part of the cylinder liner, which carries a significant thermal load, to the head and its cooling fins. The cylinder head gasket must under no circumstances protrude into the combustion chamber. The protruding edge of the gasket will heat up and become a source of glow ignition.

Figure: 9.18. Determination of the volume of the combustion chamber

The octane rating of the gasoline used must match the compression ratio. However, it must be borne in mind that the compression ratio is not the only factor that determines the possible detonation of the fuel.

Detonation depends on the course of the combustion process, on the movement of the mixture in the combustion chamber, on the ignition method, etc. The type of fuel for a particular engine is selected empirically. However, it does not make sense to use high-octane fuel for a low compression ratio engine because engine performance is not improved.

Purge the cylinder

The selection of the appropriate valve timing in a two-stroke engine is of great importance for removing exhaust gases from the cylinder and filling it with fresh mixture. In addition, it is necessary to direct the jets of the mixture coming from the bypass windows so that they pass through all the nooks of the cylinder and combustion chambers, blowing out the remaining exhaust gases from them and directing them to the exhaust window.

To increase the CW of the engine and, as a consequence, its power, it is necessary to significantly expand the exhaust phase, or rather, to increase the difference between the exhaust and purge phases. As a result of this, the time during which the exhaust gases expand and leave the cylinder increases. In this case, at the moment of opening the bypass ports, the cylinder is already empty, the fresh charge entering it only slightly mixes with the residual exhaust gases.

The release phase is increased by shifting (cutting) the upper edge of the window. The release phase in racing engines reaches 190 ° compared to 130-140 ° in production engines. This means that the top edge can be cut down a few millimeters. However, it must be taken into account that as a result of an increase in the height of the outlet port, the stroke of the piston on which the work is performed decreases. Therefore, an increase in the height of the outlet port pays off only if the losses in the operation of the piston are compensated for by the improvement of cylinder blowing.

Due to the expediency of achieving the maximum difference between the exhaust and purge phases, the opening angle of the purge ports usually remains unchanged.

The size and shape of the bypass channels and windows have a significant influence on the quality of the blowdown. The direction of the mixture inlet into the cylinder from the bypass channel must correspond to the adopted blowdown system (see paragraph 9.2.4, Fig. 9.10). In two- and four-channel blowing systems, the jets of the combustible mixture entering the cylinder are directed above the piston to the cylinder wall opposite to the exhaust port, and in the four-channel system, the jets emanating from the windows located closer to the exhaust port are usually directed toward the cylinder axis. In systems with three or five bypass ports, one port must be located opposite the outlet port, the channel of this port must direct the stream of the combustible mixture upward at a minimum angle to the cylinder wall (Figure 9.19). This is a necessary condition for the effective action of this additional jet, usually obtained by reducing its cross section, as well as the later opening of this window.

The manufacture of an additional (third or fifth) port is the rule for motors with a rotating spool or diaphragm valve. In engines in which the piston controls the filling of the crank chamber, an intake port is located in place of the classic third (or fifth) bypass. In such engines, there may be additional bypass channels, and the inlet port must be of the appropriate shape; a similar solution is shown in Fig. 9.20. This engine has three additional small bypass ports connected by a common bypass, the entrance to which is located above the inlet port. The required intake phase is ensured here by the corresponding shape of the intake port.

Figure: 9.19. Influence of the shape of the third bypass channel on the movement of the charge in the cylinder:

a - irregular shape; b- correct form

When a rotary spool is installed on a conventional engine, it becomes possible to make a bypass in the cylinder opposite the outlet port. Here it is convenient to make a strongly curved short canal (Fig.9.21, and),the flow of the mixture into which for some time is closed by the piston skirt.

The disadvantage of this solution is that the movement of the piston disrupts the normal flow of the combustible mixture, but it has two important advantages: the small volume of the channel only slightly increases the volume of the crank chamber, and the combustible mixture, passing through the piston, cools it perfectly. In practice, such a channel is easy to do as follows. Two holes are made in the cylinder (the bypass window and the entrance to the channel), in this place the ribs are cut out and the lining with the channel cut through it is screwed on (Fig. 9.21.6). You can also try to cut a vertical groove in the cylinder mirror between the channel entrance and the window, the width of the groove is equal to the width of the channel. However, in this case, the downward movement of the piston will cause some turbulization of the combustible mixture in the channel (Fig. 9.21, c).

The bypass channels should taper towards the ports in the cylinder.

Figure: 9.21. Additional bypass channel with mixture flow through the piston:

a - principle of action; b - part of the channel passes through the outer pad; c - channel cut in the cylinder mirror

The inlet to the bypass must have an area 50% larger than the bypass. Obviously, the channel cross-section must be changed along its entire length. The corners of the windows and channel sections should be rounded with a radius of 5 mm to increase the laminar flow.

Any errors when joining parts of channels located in different engine parts are unacceptable. This remark primarily concerns the junction of the cylinder with the crankcase, where the gasket can become a source of additional turbulence of the mixture, and the joints of the inlet and outlet pipes with the cylinder. Vortices in the flow of the mixture can also occur at the junction of the cast jacket of the cylinder with the cast or pressed sleeve (Fig. 9.22). Discrepancies in sizes in these places must be corrected unconditionally.

In some engines, the cylinder windows are split by a rib. This applies primarily to intake and exhaust ports. It is not recommended to reduce the thickness of these ribs and, even more so, to remove them when the window area increases. These ribs prevent the piston rings from getting into the wide windows and therefore from breaking. It is only permissible to streamline the rib of the intake port, but only on the outside of the cylinder.

Figure: 9.22. Charge movement disturbances caused by incorrect

the relative position of the cylinder liner and the cast cylinder jacket

It is impossible to give an unambiguous recipe for obtaining certain effects of modifications. In general, it can be said that an increase in the opening of the exhaust window increases the engine power, simultaneously increasing the CW of maximum power and maximum torque, but narrowing the range of operating CW. An increase in the size of the windows and cross-sections of the channels in the cylinder has a similar effect.

These tendencies are well illustrated by changes in the speed characteristics of an engine (Fig. 9.23) with a volume of 100 cm (cylinder diameter 51 mm, piston stroke 48.5 mm), obtained as a result of changes in the dimensions and valve timing (Fig. 9.24). In fig. 9.24, andthe dimensions of the windows are given at which the engine develops the maximum power (curves N Aand M din fig. 9.23). The exhaust phase is 160 °, the purge phase is 122 °, and the intake phase is 200 °. The inlet window opened at 48 ° from TDC and closed at 68 ° from TDC. The diameter of the carburetor diffuser is 24 cm.

In fig. 9.24, bthe dimensions of the windows are shown at which the largest operating range of NW is achieved (see Fig. 9.23, curves N Band M c).The exhaust phase is 155 °, the purge is 118 ° and the intake phase is 188 °, the inlet is opened at an angle of 48 ° after BDC and closes at an angle of 56 ° after TDC. The diameter of the carburetor diffuser is 22 mm.

It should be noted that relatively small changes in the dimensions and valve timing have significantly changed the characteristics of the engine. At the engine ANDmore power, but it is practically useless at speeds below 6,000 rpm. Option INapplicable in a much wider range of CW, and this is the main advantage of an engine without a gearbox.

Although the example considered concerns an engine of a class not used in Poland, it illustrates well the relationship between the shape of the windows and cylinder bores and the parameters of its operation. However, we must remember that whether our modifications have led to the desired results, we will know only after they have been completed and the engine has been checked at the stand (or subjectively during running in). The preparation of a racing engine is an endless cycle of modifications and checks of the results of this work, new modifications and checks, and in fact other engine units (carburetor, exhaust system, etc.) also have a huge influence on engine performance, the optimal parameters of which can only be determined empirically ...

It is also necessary to emphasize the great importance of the geometric symmetry of all windows and channels in the cylinder. Even a slight deviation from symmetry will have a negative effect on the movement of gases in the cylinder. A slight difference in the height of the bypass ports on both sides of the cylinder (Fig. 9.25) will cause an asymmetric movement of the mixture and disrupt the operation of the entire purge system. An excellent indicator that allows you to directly assess the correctness of the direction of the mixture flows coming from the bypass ports are traces on the piston bottom. After some time of engine operation, part of the piston crown becomes covered with a layer of soot. The same part of the bottom, which is washed by jets of fresh combustible mixture entering the cylinder, remains shiny, as if it had been washed.

Figure: 9.25. Influence of differences in the height of bypass windows

on both sides of the cylinder on the symmetry of the charge movement

Piston and piston rings

Figure: 9.28. The dependence of the throughput of the inlet channel of the carburetor on the forums of its section

Modern engines use pistons made of a material with a low coefficient of linear expansion, so the clearance between the piston and the cylinder liner can be small. If we assume that the clearance along the circumference and the length of the piston skirt in a heated engine will be the same everywhere, then after cooling the piston will be deformed. Therefore, the piston must obtain the appropriate shape even during machining, which is done in practice. Unfortunately, this form is too complicated and can only be obtained on special machines. From this it follows that the shape of the piston cannot be changed by locksmith operations, and all kinds of turning of the piston skirt with a file or a sharpener, used everywhere after the piston is jammed, will lead to the piston losing its correct shape. In case of urgent need, such a piston can be used, but there is no doubt that its interaction with the cylinder mirror will be much worse.

One must be warned against using sandpaper for emergency cleaning of the piston skirt. Grains of abrasive material dig into the soft material of the piston, after which they rinse the entire cylinder mirror. This will require the cylinder to be bored to the next oversize.

An approximate temperature distribution on the piston is shown in Fig. 9.29. The highest heat load falls on the bottom and top, especially on the side of the outlet window. The lower skirt temperature is lower and depends primarily on the piston shape. The shape of the inner surface of the piston should be such that there are no narrowings in the cross section of the piston that impede heat transfer (Fig. 9.30). Heat from the piston to the cylinder is transferred through the piston rings and the contact points of the piston skirt with the cylinder.

To reduce the mass of the piston and, thus, to reduce the forces that noticeably increase at a high engine speed, it is possible to remove some of the material inside the piston, but only in its lower part. Usually, the lower edge of the piston ends with a shoulder inside, which is the technological base for processing the piston. This bead can be removed, leaving the skirt thickness at this point about 1 mm. The wall thickness of the piston should increase smoothly towards the bottom. You can slightly increase the notches in the piston skirt under the bosses. The shape and dimensions of these cutouts must match the cutouts in the bottom of the cylinder liner (Fig. 9.31). To change the time-section, it is easiest to undercut the lower edge of the piston from the side of the intake port, although the selection of the undercut value is more difficult.

To reduce the heat load on the upper piston ring, it is recommended to make a bypass groove above it 0.8-1 mm wide and 1-2 mm deep. Sometimes a similar groove (or even two) is made between the rings. These notches direct heat flow to the bottom of the piston, reducing the temperature of the piston rings.

In general, we cannot change the appearance and arrangement of the rings. We can only control the gap in the lock (cut) of the ring, which should not exceed 0.5% of the cylinder diameter. It is also necessary to carefully determine the angular position of the locks so that they never fall on the windows when the piston moves (Fig. 9.32). When working on the cylinder, it is also necessary to take into account the position of the piston ring locks.

Sometimes a simple method is used to reduce the elasticity of the piston ring by chamfering from its inner edges. This ensures a better fit of the rings to the cylinder bore. This method is especially useful when changing rings without grinding the cylinder.

Crank mechanism

As already mentioned, in the 501 engine -Z3Ait is advisable to rearrange the crankshaft cheeks. After disassembling with a press, the following operations must be performed over the shaft.

1. Deepen in the cheeks of the shaft of the socket for the lower head of the connecting rod by the thickness of the additional discs attached to the outer surface of the cheeks (Fig. 9.35, size e).

2. Squeeze out the axle shafts from the cheeks to the thickness of additional
disks.

3. Reduce the thickness of the connecting rod (Fig. 9.36) on the grinding machine. Manual processing is used only for finishing.
The thickness can be reduced even to 3.5 mm, provided that the connecting rod is polished. Each scratch on the connecting rod is a stress concentrator from which crack propagation can begin. Also, all fillets must be done very carefully. When modifying the connecting rod, it is advisable to make slots in the upper and lower heads to improve the access-mixture to the bearings.

4. Shorten the crank pin to size from(Fig. 9.36), equal to the width of the shaft after rearranging the cheeks, but before attaching additional discs. The pin should be shortened on both sides, this will allow the bearing roller raceways to remain in their old place.

5. Weigh the upper and lower connecting rod heads as shown in fig. 9.37.

6. Assemble the crankshaft. Pressing in the crank pin can be done using a press or a large vise.

Of course, after such an assembly, it is difficult to achieve the alignment of the axle shafts. An error can be detected by applying a steel plate to one of the cheeks (Figure 9.38), which will lag behind the other cheek. This can be corrected by striking one of the cheeks with a mallet (Figure 9.39). More precisely, we will check the runout of the shaft when it rotates in bearings. On the semiaxis covered with chalk, the starter will indicate the places in which the runout must be reduced (Fig. 9.40). When assembling the shaft, remember to maintain a gap between the lower connecting rod head and the shaft cheeks. This gap must be at least 0.3 mm. Too small a clearance in many cases will cause the connecting rod bearing to jam.

7. Balance the crankshaft. This is done using a static method. We will rest the shaft on prisms and, having hung the weight in the upper head of the connecting rod, we will select the balanced mass (not to be confused with the weight of the weight) so that the shaft remains at rest in any position. The mass of the sinker is the fraction of the masses involved in the reciprocating motion that must be balanced. Suppose that the mass of the upper connecting rod head is 170 g, and the mass of the piston with rings and piston pin is 425 g. The reciprocating mass is 595 g. Assuming that the balance coefficient is 0.66, we obtain that the mass, which must be balanced, is equal to 595X0.66 \u003d 392.7 g. Subtracting from this value the mass of the upper connecting rod head, we obtain the mass of the weight G suspended on the head.

The state of static equilibrium of the crankshaft is achieved by drilling holes in the shaft cheeks on the side that is pulling.

8. Make additional steel discs and attach them to the shaft with three MB screws with countersunk tapered heads. Before mounting the discs, it is advisable to lubricate the plane of the joint with the shaft with sealant. Counter screws by punching.

We add that additional disks can be attached not to the shaft, but motionlessly to the inner walls of the crankcase. However, due to the loose fit of the disc to the wall, heat transfer may deteriorate. It should be noted that the displacement of the crankshaft cheeks does not exclude the use of a thin "horseshoe".

Before starting the modifications to the cylinder, you need to make a tool for measuring the valve timing, using a circular protractor with a 360 ° scale for this purpose (Fig. 9.42). Install the protractor on the engine crankshaft, and attach the wire arrow to the engine.

To unambiguously determine the opening and closing times of the windows, you can use a thin wire inserted through the window into the cylinder and pressed by the piston in the upper edge of the window. The thickness of the wire will hardly affect the measurement accuracy, but this method will facilitate the work. It is especially useful in determining the opening angle of the intake port.

Taking impressions from the cylinder mirror will greatly facilitate the work of changing the valve timing and the size of the channels and windows. Such an impression can be obtained as follows:

put a piece of cardboard inside the cylinder and adjust it so that it lies exactly along the cylinder mirror; its upper edge should coincide with the upper plane of the cylinder;

with the blunt end of a pencil, squeeze out the contours of all windows;

on the cardboard removed from the cylinder, we get an imprint of the cylinder mirror; cut out the displayed windows in cardboard along the lines of the prints.

On the resulting scan of the cylinder mirror, you can measure the distance from the edges of the windows to the upper plane of the cylinder and calculate the valve timing corresponding to them (using the formulas available in each book about engines).

Now let's look at how to fix the new valve timing in a modified engine. To do this, set the required angles on the goniometer in turn, measuring each time the distance from the upper edge of the piston to the upper plane of the cylinder. The measured distances are applied to the previously made pattern.

Now we can outline the new shape of the windows, and then cut them out on the pattern. All that remains is to insert the pattern into the cylinder and enlarge the windows so that their shape coincides with the designed ones. Using a pattern will save us from having to repeatedly check the corners when enlarging the windows.

Figure: 9.42. Simple goniometer for measuring valve timing

So, what is it and what is it for. I will not describe the basics of the operation of 2T engines, since everyone knows them, but not everyone understands what the valve timing is and why they are exactly the same and not others.
The valve timing is the period of time during which the windows in the cylinder open and close when the piston moves up and down. They are counted in degrees of rotation of the knees of the engine shaft. For example, an exhaust phase of 180 degrees means that the exhaust port will start to open, open and then close at half a revolution (180 out of 360) of the engine crankshaft. It must also be said that the windows open when the piston moves down. And they open to the maximum at the bottom dead center (BDC). Then, when the piston moves up, they close. Due to this design feature of 2T engines, the valve timing is symmetrical about the dead points.

To complete the picture of the gas distribution process, it is also necessary to say about the area of \u200b\u200bthe windows. The phase, as I already wrote, is the time during which the windows are opened and closed, but the area of \u200b\u200bthe window plays an equally important role. Indeed, at the same window opening time, the mixture (blowdown) will pass more through the window that is larger in area and vice versa. The same is for the exhaust, more exhaust gases will leave the cylinder if the window area is larger.
The general term characterizing the entire process of gas flow through the windows is called time-section.
And the larger it is, the higher the engine power and vice versa. That is why we see such huge cross-sectional channels for purging, intake and exhaust, as well as high valve timing on modern highly accelerated 2T engines.

So, we see that the functions of gas distribution are performed by the cylinder windows and the piston, which opens and closes them. However, because of this, time is lost during which the piston would do useful work. In fact, engine power is generated only before the exhaust port is opened, and with further downward movement of the piston, no or very little torque is generated. In general, the volume of the 2T engine, in contrast to the 4T, is not fully used. Therefore, the primary task of the designers is to increase the time - the section at minimum phases. This gives better indicators of the curves of the moment and economy than, at the same time, the section, but in higher phases.
But since the cylinder diameter is limited, and the width of the windows is also limited, in order to achieve a high level of engine boost, it is necessary to increase the valve timing.
Many people, wanting to achieve more power, begin to enlarge the windows in the cylinder either at random, or on someone's advice or after reading the advice somewhere, but they do not really understand what they will get in the end, and whether they are doing it right. Or maybe they want something completely different?
Let's say we have some kind of engine and we want to get more out of it. What should we do with phases? The first thing that comes to mind for many is to cut the exhaust ports up, or raise the cylinder with the gasket, and also cut the intake down or cut the piston from the intake side. Yes, in this way we will achieve an increase in phases and, as a consequence of time, a cross section, but at what cost. We have reduced the time during which the piston will do useful work. Why does the power increase at all with increasing phases, and not decrease? The time increases - the section, you say, is it so. But do not forget that this is a 2T engine and in it the whole principle of operation is based on resonant pressure and vacuum waves. And for the most part, the exhaust system plays a key role here. It is she who creates a vacuum in the cylinder at the beginning of the release, pulling out the exhaust gases, and also then pulls out the mixture from the purge channels, increasing the purge time-section. It also refills the escaped mixture from the cylinder back into the cylinder. As a result, we have an increase in power with increasing phases. But we must also not forget that the exhaust system is tuned to a certain speed, beyond which the mixture escaping from the cylinder does not return back, and the useful piston stroke is reduced due to high phases. So there is a drop in power and excessive fuel consumption at non-resonant engine frequencies.
So can you get the same power and reduce dip and fuel consumption? Yes, if you achieve the same time - sections without increasing valve timing!
But what does this mean in practice? The increase in the width of the windows and the cross-section of the channels is limited by the thickness of the walls of the channels and the limiting values \u200b\u200bof the width of the windows due to the operation of the rings. But while there is a reserve, it must be used, and only then the phases should be increased.
So, if you yourself do not really know what you want and, as many say, I want power, but so that the lows do not disappear, then increase the bandwidth of channels and windows without increasing the phases. If this is not enough for you, increase the phases gradually. For example, the optimal outlet will be 10 degrees, purge by 5 degrees.
I would like to step back a little and say separately about the intake phase. Here we were very lucky when people came up with a plate check valve, in the common people a petal valve (LK). Its plus is that it automatically changes the intake phase and intake area. Thus, it changes the intake time-section according to the needs of the engine at a given moment. The main thing is to select and install it correctly from the beginning. The valve area should be 1.3 times larger than the carburetor cross-sectional area, so as not to create unnecessary resistance to the mixture flow.

The intake ports themselves should be even larger, and the intake phase should be as large as possible so that the LC starts working as early as possible. Ideally, from the very beginning of the upward movement of the piston.
An example of how you can achieve the maximum intake phase can be the following photos of intake modifications (not Java, but the essence remains the same):

This is one of the best intake options available. In fact, the intake here is a combined version of the intake to the cylinder and the intake to the crankcase (the intake port is permanently connected to the crank chamber, KShK). This also increases the resource of the NGSH due to better airflow with the fresh mixture.

To form this channel connecting the intake channel with the KShK, the maximum possible amount of metal is selected in the crankcase, which is located on the intake side near the liner.

In the sleeve itself, additional windows are made below the main ones.

In the cylinder jacket, the metal near the liner is also selected.
Correctly installed LK allows once and for all to solve the problem with the selection of the intake phase.
Whoever decided to achieve more power and knows what he is aiming at, is ready to sacrifice the lower classes for the sake of explosive pickup at the top, he can safely increase the valve timing. The best solution would be to use someone else's experience in this matter.
For example, the following recommendations are given in foreign literature:

I would exclude the Road race option, because the phases are very extreme, designed for road-ring races and when driving on ordinary roads are not practical. And most likely they are designed for a power valve, which reduces the exhaust phase at low and medium speeds to an acceptable level. In any case, it is not worth making the release phase more than 190 degrees. The best option, as for me, is 175-185 degrees.

As for the purge ... everything is more or less indicated optimally. However, how do you know how much your engine will turn? You can look for the improvements of people and find out from them, or you can just take the average numbers. This is in the region of 120-130 degrees. Optimally 125 degrees. Higher numbers refer to smaller engine volumes.
And yet, with an increase in the purging phases, it is also necessary to raise its pressure, i.e. crankcase compression. To do this, you need to reduce the volume of the crank chamber as much as possible, removing unnecessary voids. For example, first by plugging the balancing holes in the crankshaft. The plugs must be made of the lightest material possible so that they do not affect the balancing of the HF. They are usually cut out of wine corks (cork wood) and driven into balancing holes, after which they are coated with epoxy on both sides.

As for the intake, I wrote above that it is better to put the LC and not rack your brains with the selection of the phase.

So, let's say you have decided how you will modify your engine, what valve timing it will have. Now, what is the easiest way to calculate how much it is in mm.? Very simple. There are mathematical formulas for determining the stroke of the piston that can be adapted to our purposes, which I did. Once I entered the formulas into the Excel program and received a program for calculating the valve timing of purge and exhaust ( link to download the program at the end of the article).
You just need to know the length of the connecting rod (Java 140mm, IZH Jupiter, sunrise, Minsk 125mm, IZH ps 150mm. If you wish, you can find the length of almost any connecting rod on the Internet) and the piston stroke.
The program is made in such a way that it determines the distance from the upper edge of the window to the edge of the sleeve. Why so, and not just say the height of the window? Because this is the most accurate phase definition. Top dead center piston crown MUST to be on the same level with the edge of the liner due to squish (features of the shape of the combustion chamber for detonation-free operation), and if it is suddenly not on the same level, then you will have to adjust the cylinder in height (for example, by selecting the thickness of the gasket under the cylinder). But at bottom dead center, the piston crown is usually not at the same level with the edges of the windows, but slightly higher, i.e. the piston does not fully open the windows! Such design features, there is nothing to be done. But this means that the windows do not work at their full height, and therefore the phases can not be determined by them!