The outlet valve begins to open at the end of the expansion process ahead of LMW. at an angle φ o.v. \u003d 30h-75 ° (Fig. 20) and closes after a.m.t. with a delay by the angle φ z.v., when the piston moves in the filling stroke in the direction to the N.m.t. The beginning of the opening and closing of the intake valve are also shifted relative to the dead points: the opening begins before the TDC. leading by the angle φ 0. vp, and closing occurs after nm. with a delay by the angle φ c.v. at the beginning of the compression stroke. Most of the processes of release and filling are carried out separately, but near the center of oil production. the inlet and outlet valves are open for a while at the same time. The duration of valve overlap, equal to the sum of the angles φ З.в + φ о.вп, is small for piston engines (Fig. 20, a), and for combined ones it can be significant (Fig. 20, b). The total duration of gas exchange is φ o.v + 360 o + φ z.vp \u003d 400-520 o; it is higher for high-speed engines.

Gas exchange periods in two-stroke engines

In a two-stroke engine, gas exchange processes occur when the piston moves near the borehole. and occupy part of the piston stroke in the expansion and compression strokes.

In engines with a loop gas exchange scheme, both inlet and outlet windows are opened by a piston, therefore the valve timing and cross-sectional area diagrams of the windows are symmetrical relative to the l.w. (Fig. 24, a). In all engines with direct-flow gas exchange schemes (Fig. 24, b), the opening phases of the outlet ports (or valves) are performed asymmetric relative to the nominal pressure, thereby achieving a better cylinder filling. Typically, inlet ports and outlet ports (or valves) are closed at the same time or with little angle difference. It is also possible to carry out asymmetric phases in an engine with a loop gas exchange scheme,

if you install (at the inlet or outlet) additional devices - spools or valves. Due to the insufficient reliability of such devices, they are not currently used.

The total duration of gas exchange processes in two-stroke engines corresponds to 120-150 ° of the crankshaft rotation angle, which is 3-3.5 times less than in four-stroke engines. The opening angle of the outlet ports (or valves) φ r.v. \u003d 50-90 ° BC, and the pre-opening angle φ pr \u003d 10-15 0. In high-speed engines with valve exhaust, these angles are larger, and smaller in engines with window exhaust.

In two-stroke engines, the exhaust and filling processes take place for the most part jointly - with simultaneously open inlet (purge) and outlet ports (or exhaust valves). Therefore, air (or a combustible mixture) enters the cylinder, as a rule, provided that the pressure in front of the inlet ports is greater than the pressure behind the outlet ports (valves).

Literature:

Nalivaiko V.S., Stupachenko A.N. Sypko S.A. Methodical instructions for laboratory work on the course "Ship internal combustion engines", Nikolaev, NKI, 1987, 41p.

Internal combustion engines. Textbook / Yu.Ya. Fomin, A.I. Gorban, V.V. Dobrovolsky, A.I. Lukin et al. - L.: Shipbuilding, 1989 - 344 p.: Ill.

Internal combustion engines. Theory of piston and combined engines: Ed. A.S. Orlina, M.G. Kruglova –M .: Mechanical engineering, 1983yu - 372p.

Vansheidt V.A. Internal combustion engines. L. Shipbuilding, 1977.-392s.

Gas distribution phases

The location of the channels and the valve timing of the engine

The reciprocating motion (up and down) of the engine piston allows it to act as an air compressor. Initially, the air / fuel mixture moves into the crankcase under the piston, and then travels into the cylinder (above the piston) where it is compressed and ignited. As soon as the gases are burnt, the temperature and pressure rise rapidly. This pressure propels the piston to the underside of its stroke, where the exhaust is ultimately purged. Sounds simple, but very precise channel design - shape, size, position and timing - is essential if you want to achieve significant engine performance.

The wastegate pass the fresh air / fuel mixture into the cylinder before combustion while the exhaust gases are purged through the exhaust port.

THE BASES

If you're curious enough to take your engine apart, you've probably seen holes in the liner and crankshaft. These holes are known as ducts or holes, and in a two-stroke engine they have 3 functions:

1. Intake - Allows fresh air / fuel mixture to enter the crankcase below the piston.

2. Bypass - movement of the air / fuel mixture from the crankcase to the cylinder above the piston.

3. Exhaust - This is where the exhaust gases leave the engine after combustion.

The holes are opened and closed by the movement of the piston and crankshaft, and unlike engines with mechanical valves, they do not require additional energy from the engine to function.

The holes you see are necessary for the two-stroke engine to function properly.

CHANNEL TYPES

INLET. Car engines use an intake system based on a crankshaft rotary valve. How it works: A bore made in the shaft journal aligns with the air intake hole in the engine housing (under the carburetor) at every revolution of the shaft. The air / fuel mixture passes through an open hole in the surface of the crankshaft journal and then through a channel in the center of the crankshaft and finally into the crankcase.

The intake port in the crankshaft "metes out" how much air and fuel is entering the engine. The air / fuel mixture then enters the crankcase through a channel in the center of the crankshaft.

BYPASS HOLES. These holes are made in the cylinder wall and are alternately closed and opened by a piston. The air / fuel mixture from the crankcase (below the piston) moves through bypass channels outside the cylinder to the bypass ports.

Two-stroke car engines use a variety of bypass combinations. There can be anywhere from two to 10-11 bypass holes of various shapes and sizes - plus an exhaust hole or holes (yes, there may even be multiple exhaust holes).

LOCATION OF SHNURLE CHANNELS: Two-stroke engines use a variety of bypass and exhaust port configurations, but self-similar engines use a basic configuration known as Schnurle duct placement, so we will only discuss that option.

In the Schnurle system, the two by-pass ports are directed upward and away from the single exhaust port that lies between them. Fresh fuel mixture is deliberately directed to the point farthest from the exhaust port. At this point, the fresh mixture loops towards the cylinder head and pushes the exhaust gases out through the exhaust port.

Schnurle holes direct the air / fuel mixture away from the exhaust port.

BOOST HOLE: The boost hole is an important improvement in the basic arrangement of the Schnurle channels. It is located opposite the exhaust port and is easily distinguishable from the rest of the cylinder bores by its sharp upward angle. The boost hole not only creates another path through which the air / fuel mixture can enter the cylinder, but also does so at an angle that directs the mixture towards the glow plug at the top of the cylinder. This promotes better cylinder filling and improved exhaust gas purging.

The boost port is opposite to the exhaust port. Its sharp upward angle helps direct the fresh air / fuel mixture towards the glow plug at the top of the cylinder.

A LOT - NOT ALWAYS GOOD: More important than the number of ports are valve timing (i.e. when the ports open and close), duration (how long they stay open) and area (port size), so don't be impressed by the number of ports announced for a given engine. A properly designed 3-channel motor can be more powerful than a poorly designed 7-channel motor.

Properly designed passages help direct the flow of the air / fuel mixture and exhaust gases. More channels sometimes equals more power, but not always.

GAS DISTRIBUTION PHASES

The valve timing indicates the points in the engine cycle at which the holes open and close. These points are usually measured from TDC (top dead center) or BDC (bottom dead center), from the one to which the piston is closer.

In addition to opening and closing the holes, the valve timing tells us how long the hole remains open (duration). This is important in determining the operating speed of an engine, high speed motors move gases longer than low speed motors.

Most experts measure the opening and closing of holes in degrees of crankshaft rotation. Some designers and engineers use a system that measures bore opening and closing as a percentage of TDC (TDC). While there are technical advantages to using the latter system, the former is the most used.

To measure the events of the valve timing, a goniometer wheel is attached to the crankshaft. The stationary gauge aligns with the gage wheel and precisely matches the piston position at TDC, providing intake, bypass and exhaust phase measurements.

All you need to start measuring your engine's camshaft timing is a protractor wheel, a pointer and a sturdy engine mount. This method is used by all engine designers to map valve timing and identify potential improvements.

DUCTS AND PURGE

In engine terminology, "purge" means volume scrubbing — in other words, scrubbing the exhaust gas from the cylinder and moving the fresh air / fuel mixture from the crankcase to the cylinder. For an engine designer, cleaning the cylinder from exhaust gases is only half of the problem, while replacing these gases with a fresh air / fuel mixture is another problem.

When the engine is running, some of the fresh mixture transferred to the cylinder mixes with the blown exhaust gases and reduces the efficiency and power of the engine. Many duct systems have been tried over the years to minimize this mixing and fouling, the design has been improved, but this phenomenon continues to affect the performance of two-stroke engines. The size, position, and direction of these holes determines how successful the purge will be and how well the engine will perform.

The air / fuel mixture flows out of the bypass port on the left, fills the cylinder for the next combustion cycle, and helps "blow" exhaust gases through the exhaust port on the right.

GAS DISTRIBUTION PHASES

In a two-stroke engine, several events occur simultaneously. They overlap and affect each other, and their effect is difficult to track simply by looking at the valve timing. The valve timing diagram makes these numbers easier to understand.

In the example of the diagram, the exhaust port opens at 80 degrees before BDC (BBDC). It is also 100 degrees After TDC (ATDC). As the exhaust port opens closer to BDC, the phase is measured from this position. The total opening time (duration) of any channel is determined by adding individual rotations.

PRACTICAL USE

The Mungen MT12 engine used to drive the Yokomo GT-4R showed flat power despite having a very significant increase in peak power. This was achieved by optimizing the valve timing for racing.

I recently spoke with renowned engine modification expert Dennis Ritchie from Texas. Dennis modified hundreds of engines for his buyers' boats and cars every year, in fact, he modified Steve Pond's Mugen MT12 engine for the Yokomo GT-4R and it worked very well. He kindly set aside his time for a discussion about ducts, valve timing and duct modifications.

Dennis Ritchie sees a significant difference in valve timing philosophy between expensive 12 and 15 displacement engines and 21 displacement engines. According to Denis, small engines have much more conservative valve timing.

Here's a typical example:

• INLET - opens at 40 degrees After BDC, closes at 48 degrees After TDC, duration 188 degrees.
• EXHAUST - opens at 78 degrees Before BDC, closes at 78 degrees After BDC, duration 156 degrees.
• BYPASS - opens at 60 degrees Before BDC, closes at 60 degrees After BDC, duration 120 degrees.

He said, "Although the exhaust and bypass durations are somewhat low, the greatest increase in high rpm performance comes from the longer intake times." According to my calculations, if the inlet opening remains unchanged and the closing advances to about 65 degrees After TDC (ATDC), then the inlet duration expands to 205 degrees - a 9% increase. The best displacement engines. 21 (3.44 cc) always have advanced valve timing.

Here are some typical times for an advanced 21cc engine. inch (3.44 cc):
- intake 210 degrees;
- exhaust 180 degrees;
- bypass 126 degrees.

Dennis said these engines "safely" use fuel with 30% nitromethane and, after modifications, their peak power is between 33,000 and 34,000 rpm.

The bypass and exhaust ports allow compressed gas to escape from the top and bottom of the piston during engine cycles. Having enough time (phase duration) for this is only half the story. Having a sufficiently large hole (hole area) is the other half. To put it another way: the time it takes to move some gas through the hole depends on the area of \u200b\u200bthe hole.

An analogy might be helpful: 50 people have 30 seconds to leave the premises after a fire alarm sounds. If the door is fully open, they will easily leave the room within the allotted time. If the door is faulty and only partially open, people can still exit, but there is a crush at the door that will allow a maximum of 35 people to leave the premises at the appointed time. Arithmetic shows that a partially open door will allow only 70% of people to leave at the appointed time. A similar situation exists for gases trying to pass through the bypass and exhaust ports. If the flow is too limited, the hole can be widened to increase its area, or it can be made higher to increase both its area and phase duration. Each solution has a different effect. Deciding which one is best is a matter of long study and experience.

Most engine mods aim to increase power. The easiest way to do this is to get the engine running faster. When the maximum RPM is increased, the channels remain open for a shorter time. Based on experience with a particular engine, the modifier expands the hole or increases its height - or a combination of both. This practice is known as "porting" (modifying channels or holes).

The shapes, sizes and positions of the holes are very critical to engine performance and you cannot make one change without affecting engine performance elsewhere. It's always a compromise.

The performance of an internal combustion engine in a car depends on many factors, such as power, efficiency, cylinder volume.

The valve timing is of great importance in the engine, and the efficiency of the internal combustion engine, its throttle response, and the stability of idling depend on how the valves overlap.
In standard simple engines, timing changes are not provided, and such motors are not highly efficient. But recently, more and more often on cars of leading companies such as Honda, Mercedes, Toyota, Audi, power units with the ability to change the displacement of the camshafts as the number of revolutions in the internal combustion engine change.

Camshaft timing diagram for two-stroke engine

A two-stroke engine differs from a four-stroke engine in that its operating cycle takes one revolution of the crankshaft, while on a 4-stroke ICE it takes two revolutions. The gas distribution phases in the internal combustion engine are determined by the duration of the opening of the valves - exhaust and intake, the angle of valve overlap is indicated in degrees of position to / in.

In 4-stroke engines, the cycle of filling the working mixture occurs 10-20 degrees before the piston reaches top dead center, and ends after 45-65 degrees, and in some internal combustion engines even later (up to one hundred degrees), after the piston has passed bottom point. The total admission time in 4-stroke engines can last 240-300 degrees, which ensures good filling of the cylinders with the working mixture.

In 2-stroke engines, the duration of the intake of the air-fuel mixture lasts approximately 120-150º at the turn of the crankshaft, and the purging also lasts less, therefore filling with the working mixture and cleaning the exhaust gases in two-stroke ICEs is always worse than in 4-stroke power units. The figure below shows the valve timing diagram of a two-stroke motorcycle engine of the K-175 engine.

Two-stroke engines are rarely used on cars, as they have lower efficiency, worse efficiency and poor cleaning of exhaust gases from harmful impurities. The last factor is especially relevant - due to the tightening of environmental standards, it is important that the engine exhaust contains a minimum amount of CO.

Still, 2-stroke ICEs have their own advantages, especially for diesel models:

• power units are more compact and lighter;
• they are cheaper;
• a two-stroke motor accelerates faster.

On many cars in the 70s and 80s of the last century, carburetor engines with a "trambler" ignition system were mainly installed, but many advanced car manufacturing companies already then began to equip motors with an electronic engine control system, in which all the main processes were controlled by a single block (ECU). Now almost all modern cars have ECMs - the electronic system is used not only in gasoline, but also in diesel internal combustion engines.

In modern electronics, there are various sensors that monitor the operation of the engine, sending signals to the unit about the state of the power unit. Based on all the data from the sensors, the ECU decides how much fuel should be supplied to the cylinders at certain loads (revolutions), what to set the ignition timing.

The valve timing sensor has another name - the camshaft position sensor (DPRV), it determines the position of the timing relative to the crankshaft. It depends on its readings in what proportion the fuel will be supplied to the cylinders, depending on the number of revolutions and the ignition timing. If the DPRV does not work, it means that the timing phases are not controlled, and the ECU does not "know" in what sequence it is necessary to supply fuel to the cylinders. As a result, fuel consumption increases, since gasoline (diesel fuel) is simultaneously supplied to all cylinders, the engine runs erratically, on some car models the internal combustion engine does not start at all.

Camshaft adjuster

In the early 90s of the 20th century, the first engines with automatic timing change were produced, but here it was no longer the sensor that controlled the position of the crankshaft, but the phases themselves were shifted directly. The principle of operation of such a system is as follows:

• the camshaft is connected to a hydraulic clutch;
• also with this clutch has a connection and a camshaft;
• at idle and low speeds, the camshaft gear with a camshaft is fixed in the standard position, as it was installed according to the marks;
• with an increase in speed under the influence of hydraulics, the clutch turns the camshaft relative to the sprocket (camshaft), and the timing phases shift - the camshaft cams open the valves earlier.

One of the first such developments (VANOS) was applied on BMW M50 engines, the first engines with variable valve timing appeared in 1992. It should be noted that at first VANOS was installed only on the intake camshaft (M50 engines have a two-shaft timing system), and since 1996, the Double VANOS system was used, with which the position of the exhaust and intake p / shafts was already adjusted.

What is the advantage of the timing controller? At idle, valve timing is practically not required, and in this case it even harms the engine, since when the camshafts shift, exhaust gases can enter the intake manifold, and some of the fuel will enter the exhaust system without completely burning out. But when the engine is operating at maximum power, the phases should be as wide as possible, and the higher the rpm, the more valve overlap is necessary. The timing clutch makes it possible to efficiently fill the cylinders with a working mixture, which means to increase the efficiency of the motor and increase its power. At the same time, at idle speed, the r / shafts with the coupling are in their original state, and the combustion of the mixture is in full. It turns out that the phase regulator increases the dynamics and power of the internal combustion engine, while fuel is consumed quite economically.

The variable valve timing system (CIFG) provides lower fuel consumption, reduces the level of CO in the exhaust gases, and allows more efficient use of the engine power. Different world car manufacturers have developed their own CIFG, they apply not only the change in the position of the camshafts, but also the level of valve lift in the cylinder head. For example, Nissan uses a CVTCS system that is controlled by a variable valve timing valve (solenoid valve). At idle, this valve is open and does not generate pressure, so the camshafts are in their original state. An opening valve increases the pressure in the system, and the higher it is, the greater the angle the camshafts move.

It should be noted that CIFGs are mainly used on engines with two camshafts, where 4 valves are installed in the cylinders - 2 inlet and 2 outlet.

Camshaft timing accessories

In order for the engine to work without interruption, it is important to correctly set the timing phases, to set the camshafts in the desired position relative to the crankshaft. On all engines, the shafts are set according to marks, and a lot depends on the accuracy of the installation. If the shafts are not aligned correctly, various problems arise:

• the motor runs unstably at idle;
• ICE does not develop power;
• there are shots at the muffler and pops in the intake manifold.

If you mistake a few teeth in the marks, it is possible that the valve may bend and the engine will not start.

On some models of power units, special devices have been developed for setting the valve timing. In particular, for engines of the ZMZ-406/406/409 family, there is a special template with which the angles of the camshafts are measured. The template can check the existing corners and if they are not aligned correctly the shafts must be reinstalled. The attachment for 406 motors is a set consisting of three elements:

• two protractors (for the right and left shaft, they are different);
• protractor.

When the crankshaft is set to TDC of the 1st cylinder, the camshaft cams should protrude above the top plane of the cylinder head at an angle of 19-20 ° with an error of ± 2.4 °, and the intake shaft cam should be slightly higher than the exhaust camshaft cam.

There are also special devices for installing camshafts on BMW M56 / M54 / M52 engines. The kit for installing the valve timing of the internal combustion engine BVM includes:

Malfunctions of the variable valve timing system

It is possible to change the valve timing in various ways, and recently the most common rotation of the p / shafts, although the method of changing the amount of valve lift is often used, the use of camshafts with modified cams. Periodically, various malfunctions occur in the gas distribution mechanism, due to which the engine starts to work intermittently, "dulls", in some cases it does not start at all. The causes of problems can be different:

• defective solenoid valve;
• the phase change coupling is clogged with dirt;
• the timing chain is stretched;
• chain tensioner defective.

Often when malfunctions occur in this system:

• idle speed decreases, in some cases the internal combustion engine stalls;
• fuel consumption increases significantly;
• the engine does not develop speed, the car sometimes does not even accelerate to 100 km / h;
• the motor does not start well, it has to be driven by the starter several times;
• a chirp is heard coming from the SIFG coupling.

By all indications, the main cause of problems with the engine is the failure of the SIFG valve, usually with computer diagnostics revealing an error of this device. It should be noted that the Check Engine diagnostic lamp does not always light up in this case, so it is difficult to understand that failures occur precisely in the electronics.

Often timing problems arise due to clogged hydraulics - poor oil with abrasive particles clogs the channels in the clutch, and the mechanism gets jammed in one of the positions. If the clutch “wedges” in the initial position, the internal combustion engine works quietly at XX, but does not develop revs at all. If the mechanism remains in the position of maximum valve overlap, the engine may not start well.

Unfortunately, SIFG is not installed on Russian-made engines, but many motorists are tuning the internal combustion engine, trying to improve the characteristics of the power unit. The classic version of the engine modernization is the installation of a "sports" camshaft, which has shifted cams, changed their profile.

This p / shaft has its advantages:

• the engine becomes throttle, responds clearly to pressing the gas pedal;
• the dynamic characteristics of the car are improved, the car literally tears from under itself.

But this tuning has its drawbacks:

• idle speed becomes unstable, they have to be set within 1100-1200 rpm;
• fuel consumption increases;
• it is quite difficult to adjust the valves, the internal combustion engine requires careful adjustment.

Quite often, VAZ engines of models 21213, 21214, 2106 undergo tuning. The problem of VAZ engines with a chain drive is the appearance of "diesel" noise, and often it arises from a failed tensioner. Modernization of the VAZ ICE consists in installing an automatic tensioner instead of the standard factory one.

Often, a single-row chain is installed on the VAZ-2101-07 and 21213-21214 engine models: the engine runs quieter with it, moreover, the chain wears out less - its resource is on average 150 thousand km.

Those who are associated 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 analysis of the trend of 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 competitors 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 racing-type two-stroke engines 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 design 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 inlet port when the combustion mixture is compressed in the crankcase. With increased or decreased spool thickness, accelerated disc wear is observed. Too thin a disc bends towards the intake channel, which entails an increase in the friction 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 the classic intake port control system, the lower edge of the piston allows the valve to open the intake port earlier and keep it open for a long time, which contributes to increased power at both high and medium speeds. With a conventional gas distribution device, the 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 located horizontally, the other 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, was 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 more powerful - 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 bulky 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 return of the fuel mixture, which improves the torque characteristic of the engine. 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 blowing (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 motorcycles "Yamaha" and "Harley-Davidson" (250 cm3 - 60 HP;

350 cm3 - 70 HP from.), as well as a motorcycle "Suzuki" with a two-cylinder engine of class 500 cm3 with a capacity of 75 liters. 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 to 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 appear 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 fill factor is greater than unity, a two-stroke engine would have to develop twice the power compared to a four-stroke. In fact, 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 Fig. 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 promotes the formation of a flow of a combustible mixture around the cylinder cavity (loop scavenging). The entry angles of the purge channels are of great importance 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 carburettors 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 considered systems 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 purge 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 purge 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 of design.

The increase in the power density of engines of this type over the past 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.

Types of purging the combustible mixture of an internal combustion engine.

There are two main types of blowdown: deflector (transverse) and deflectorless (return or loop).

A deflector is a special protrusion - a visor - on the piston bottom, which serves to ensure the correct direction of the flow of the combustible mixture entering the cylinder through the blowout window. In fig. 44 shows a schematic of the deflector blowing.

The mixture compressed in the crankcase through the purge channel and the window enters the cylinder, meeting a deflector on its way. The flow of the mixture deviates upward into the combustion chamber, and from there goes down to the exhaust port, forcing the exhaust gases out of the cylinder through it. With such a scavenging system, the exhaust port is positioned opposite the scavenging port, which to some extent contributes to an increase in the loss of the working mixture through the exhaust port during cylinder purging. Deflector-blown engines have higher fuel consumption. The presence of a deflector on the piston crown increases its weight and impairs the shape of the combustion chamber. Nevertheless, for a number of design considerations, deflector blowing is widely used for outboard motors: this is how, for example, the "Moscow" motor with a capacity of 10 liters is arranged. from.

Somewhat higher efficiency is achieved by using a deflectorless blowdown. The scheme of a return, two-channel blowdown is shown in Fig. 45.

In this case, the piston is made with a flat or slightly convex bottom. The blowdown streams collide and rise upward along the cylinder wall, forcing exhaust gases into the exhaust port. By the number of blowing channels and the nature of the mixture movement, this type of blowing is called two-channel, loop.

The return loop blowdown can be three- and four-channel; in the latter case, the blowing channels are located side by side, in pairs or crosswise.

Figure: 45. Scheme of return (loop) deflectorless blowdown

Reverse, two-channel blowing is more common. Outboard motors ZIF-5M and Strela have such a blow-off.

The use of deflectorless blowing allows to obtain high compression ratios with the most advantageous shape of the combustion chamber, which makes it possible to remove a large liter power from the engine. Crank-blown racing two-stroke motors generally have a two- or three-way return loop blowing.

The flow of the process of purging and filling the crankcase of a two-stroke engine with a fresh working mixture depends to a large extent on the size of the windows and the duration of their opening by the piston. The beginning of the opening and closing of the intake, purge and exhaust ports of the cylinder, as well as the duration of intake, purge and exhaust, expressed in degrees of crankshaft angle, can be seen on the engine timing diagram (Fig. 46).

The period corresponding to the angle of rotation of the crankshaft when the crankcase is filled with fresh working mixture through the open intake port is called the intake phase. The periods corresponding to the angles of rotation of the crankshaft at the opening of the purge and exhaust ports are called the purge and exhaust phases.

In fig. 46 shows a diagram of the gas distribution of the Strela engine. In this engine, the valve timing, expressed in degrees of the crankshaft angle, is: intake phase to the crankcase - 120 °, purge - 110 ° and exhaust - 140 °.

It can be seen from the diagram that the right and left parts of the diagram are symmetrical about the axis passing through the dead points. This means that if the inlet port starts to open by the piston 60 ° before TDC, it will close 60 ° after TDC. The opening and closing of the inlet and venting windows is carried out in the same way. The duration of the exhaust phase is usually 30-35 ° longer than the duration of the purge phase. The described engine is called three-window.

Symmetrical valve timing of a two-stroke engine with crank-chamber blowing negatively affects its liter power and efficiency.

Figure: 46. \u200b\u200bDiagram of gas distribution of engines of outboard motors ZIF-5M and "Strela"

The short duration of the intake phase reduces the filling of the crankcase and therefore the engine power. An increase in the height of the inlet window has its limit: it increases the amount of mixture sucked into the crankcase during the ascending stroke of the piston, but it leads to its losses due to the ejection of the mixture back into the carburetor through the open window when the piston moves down. The length of the intake phase depends on the engine speed. If the engine makes no more than 3000-4000 rpm, the intake phase does not usually exceed 110-120 ° of the crank angle. In racing engines developing 6000 rpm and more, it reaches 130-140 °, but when operating at low speeds, such an engine throws the mixture back into the carburetor.

The exhaust phase for high-speed engines is also increased and amounts to 150-160 °. At the same time, the exhaust window is 7- "8 mm higher in height than the blowdown window. The need to expand the phases for racing multi-turn engines is explained by the fact that at high speeds the time (duration) of opening the windows decreases, as a result of which the filling of the cylinders with the working mixture and the engine power drop.

Figure: 47. Diagram of two-stroke engines with spool valve timing: a- with disc spool on crankshaft; b- with a drive cylindrical spool, (valve)

It is possible to increase the crankcase filling of a two-stroke engine by using an intake system through a rotary valve or plate valves.

In the first case, on the crankshaft neck, inside the crankcase, a disc with a hole is installed to let the working mixture sucked into the crankcase pass. The second hole is located in the upper wall of the crankcase, against which the spool is pressed by a spring. During the rotation of the crankshaft, the spool rotates with it; when the hole in the spool coincides with the inlet window in the crankcase wall, the mixture fills the inner volume of the crankcase. The diagrams of the motor with suction through a rotary valve are shown in fig. 47.

The advantage of such a device is the ability to fully use the upward stroke of the piston and bring the intake phase to 180-200 ° of the crankshaft rotation angle. The intake of the mixture into the crankcase begins as soon as the upper edge of the piston closes the purge port. The inlet ends at 40-50 °, passing TDC (Fig. 48).

The intake phase diagram of such an engine is asymmetrical.

Figure: 48. Diagram of the gas distribution of a two-stroke engine with spool control by the release of the combustible mixture into the crankcase