IF ENGINE OVERHEATED ...

Spring always brings problems for car owners. They arise not only among those who have kept their car in the garage or in the parking lot all winter, after which the car that has been inactive for a long time presents surprises in the form of system and component failures. But also for those who travel all year round. Some defects, "dormant" for the time being, make themselves felt as soon as the thermometer steadily passes into the region of positive temperatures. And one of these dangerous surprises is engine overheating.

Overheating is, in principle, possible at any time of the year - both in winter and in summer. But, as practice shows, the largest number of such cases occurs in the spring. The explanation is simple. In winter, all vehicle systems, including the engine cooling system, operate in a very difficult conditions... Large temperature differences - from "minus" at night to very high workers after a short movement - have a negative effect on many units and systems.

How to detect overheating?

The answer seems to be obvious - look at the coolant temperature gauge. In fact, everything is much more complicated. When there is heavy traffic on the road, the driver does not immediately notice that the pointer needle has moved far towards the red zone of the scale. However, there are a number of indirect signs, knowing which you can catch the moment of overheating without looking at the devices.

So, if overheating occurs due to a small amount of antifreeze in the cooling system, then the heater located at the high point of the system will be the first to respond to this - hot antifreeze stop going there. The same will happen when the antifreeze boils, because it starts in the hottest place - in the cylinder head near the walls of the combustion chamber - and the formed steam plugs block the passage of the coolant to the heater. As a result, the supply of hot air to the passenger compartment is interrupted.

The fact that the temperature in the system has reached a critical value is most accurately evidenced by a sudden detonation. Since the temperature of the walls of the combustion chamber during overheating is much higher than normal, this will certainly provoke the occurrence of abnormal combustion. As a result, an overheated engine, when you press the gas pedal, will remind you of a malfunction with a characteristic ringing knock.

Unfortunately, these signs can often go unnoticed: at elevated air temperatures, the heater is turned off, and detonation with good noise insulation of the cabin can simply not be heard. Then, with the further movement of the car with an overheated engine, power will begin to drop, and a knock will appear, stronger and more uniform than during detonation. Thermal expansion of the pistons in the cylinder will lead to an increase in their pressure on the walls and a significant increase in friction forces. If, however, this sign is not noticed by the driver, then during further operation the engine will receive serious damage, and, unfortunately, it is impossible to do without serious repairs.

Why overheating occurs

Take a close look at the cooling system diagram. Almost every element of it, in certain circumstances, can become a starting point for overheating. And its root causes in most cases are: poor cooling of antifreeze in the radiator; violation of the seal of the combustion chamber; insufficient amount of coolant, as well as leaks in the system and, as a result, a decrease in excess pressure in it.

The first group, in addition to the obvious external contamination of the radiator with dust, poplar fluff, foliage, also includes malfunctions of the thermostat, sensor, electric motor or fan clutch. There is also internal contamination of the radiator, but not due to scale, as happened many years ago after prolonged operation of the engine on water. The same effect, and sometimes much stronger, is given by the use of various radiator sealants. And if the latter is really clogged with such a tool, then cleaning its thin tubes is a rather serious problem. Usually, faults in this group are easily detected, and in order to get to a parking lot or service station, it is enough to replenish the liquid level in the system and turn on the heater.

Failure to seal the combustion chamber is also a fairly common cause of overheating. The products of fuel combustion, being under high pressure in the cylinder, penetrate through leaks into the cooling jacket and displace the coolant from the walls of the combustion chamber. A hot gas "cushion" is formed, which additionally heats the wall. A similar picture occurs due to burnout of the head gasket, cracks in the head and cylinder liner, deformation of the mating plane of the head or block, - most often due to the previous overheating. You can determine that such a leak is taking place by smell exhaust gases in expansion tank, the leakage of antifreeze from the tank when the engine is running, a rapid increase in pressure in the cooling system immediately after starting, as well as the characteristic water-oil emulsion in the crankcase. But to establish specifically what the leakage is connected with, it is possible, as a rule, only after partial disassembly of the engine.

Obvious leaks in the cooling system occur most often due to cracks in the hoses, loosening of the clamps, wear of the pump seal, malfunction of the heater valve, radiator and other reasons. Note that a radiator leak often appears after the tubes are "corroded" by the so-called "Antifreeze" of unknown origin, and a pump seal leak occurs after prolonged operation on water. Establishing that there is little coolant in the system is visually as simple as determining the location of the leak.

Leakage of the cooling system in its upper part, including due to a malfunction of the radiator plug valve, leads to a drop in pressure in the system to atmospheric. As you know, the lower the pressure, the lower the boiling point of the liquid. If the operating temperature in the system is close to 100 degrees C, then the liquid can boil. Often, boiling in a leaky system occurs not even when the engine is running, but after it is turned off. You can determine that the system is really leaking by the lack of pressure in the upper radiator hose on a warm engine.

What happens when overheating

As noted above, when the engine overheats, the liquid begins to boil in the cooling jacket of the cylinder head. The resulting vapor lock (or cushion) prevents direct contact of the coolant with the metal walls. Because of this, the efficiency of their cooling decreases sharply, and the temperature rises significantly.

This phenomenon is usually local in nature - near the boiling region, the wall temperature can be noticeably higher than on the indicator (and this is all because the sensor is installed on the outer wall of the head). As a result, defects may appear in the block head, first of all, cracks. IN gasoline engines - usually between the valve seats, and in diesel engines - between the seat exhaust valve and a prechamber lid. In cast iron heads, cracks are sometimes found across the seat of the exhaust valve. Cracks also occur in the cooling jacket, for example on beds camshaft or along the holes in the bolts of the block head. It is better to eliminate such defects by replacing the head, and not by welding, which has not yet been possible to perform with high reliability.

When overheated, even if no cracks have occurred, the block head often receives significant deformation. Since at the edges the head is pressed against the block by bolts, and its middle part overheats, the following happens. Most modern engines have a head made of an aluminum alloy, which expands more when heated than the steel of the mounting bolts. With strong heating, the expansion of the head leads to a sharp increase in the compressive forces of the gasket at the edges where the bolts are located, while the expansion of the overheated middle part of the head is not restrained by the bolts. Because of this, on the one hand, deformation (failure from the plane) of the middle part of the head occurs, and on the other hand, additional compression and deformation of the gasket by forces significantly exceeding the operational ones.

Obviously, after the engine has cooled down in some places, especially at the edges of the cylinders, the gasket will no longer be clamped properly, which can cause leakage. With further operation of such an engine, the metal edging of the gasket, having lost thermal contact with the planes of the head and the block, overheats and then burns out. This is especially the case for engines with plug-in "wet" liners or if there are too narrow bridges between the cylinders.

To top it all off, the deformation of the head, as a rule, leads to a curvature of the axis of the camshaft beds located in its upper part. And without major repairs, these consequences of overheating cannot be eliminated.

Overheating is no less dangerous for the cylinder-piston group. Since the boiling of the coolant gradually spreads from the head to an increasing part of the cooling jacket, the cooling efficiency of the cylinders also sharply decreases. And this means that the heat removal from the piston heated by hot gases worsens (heat is removed from it mainly through the piston rings into the cylinder wall). The temperature of the piston rises, and its thermal expansion also occurs. Since the piston is aluminum and the cylinder is usually cast iron, the difference in thermal expansion of the materials leads to a decrease in the working clearance in the cylinder.

The further fate of such an engine is known - overhaul with block boring and replacement of pistons and rings with repair ones. The list of work on the block head is generally unpredictable. It's better not to drive the motor to this point. By periodically opening the hood and checking the fluid level, you can protect yourself to some extent. Can. But not 100 percent.

If the engine is still overheated

Obviously, you need to immediately stop on the side of the road or on the sidewalk, turn off the engine and open the hood - this will cool the engine faster. By the way, at this stage all drivers do this in such situations. But then they make serious mistakes, which we want to warn against.

Under no circumstances should the radiator cap be opened. It is not for nothing that they write on the traffic jams of foreign cars "Never open hot" - never open if the radiator is hot! After all, this is so understandable: with a working plug valve, the cooling system is under pressure. The boiling point is located in the engine, and the plug is located on the radiator or expansion tank. By opening the plug, we provoke the release of a significant amount of hot coolant - the steam will push it out, like from a cannon. At the same time, a burn to the hands and face is almost inevitable - a jet of boiling water hits the hood and ricochets into the driver!

Unfortunately, out of ignorance or despair, all (or almost all) drivers do this, apparently believing that by doing so they defuse the situation. In fact, they, having thrown out the remnants of antifreeze from the system, create for themselves additional problems... The fact is that the liquid boiling "inside" the engine, nevertheless, evens out the temperature of the parts, thereby reducing it in the most overheated places.

Engine overheating is just the case when, not knowing what to do, it is better not to do anything. Ten to fifteen minutes, at least. During this time, boiling will stop, the pressure in the system will drop. And then you can start acting.

After making sure that the upper radiator hose has lost its former elasticity (which means there is no pressure in the system), carefully open the radiator cap. Now you can add the boiled liquid.

We do it carefully and slowly, because cold liquid, getting on the hot walls of the block head jacket, causes their rapid cooling, which can lead to the formation of cracks.

After closing the plug, we start the engine. Observing the temperature gauge, we check how the upper and lower radiator hoses heat up, whether the fan turns on after warming up and whether there are fluid leaks.

The most, perhaps, unpleasant thing is the failure of the thermostat. Moreover, if its valve "stuck" in the open position, there is no trouble. The engine will simply warm up more slowly, since the entire flow of coolant will be directed along a large circuit through the radiator.

If the thermostat remains closed (the pointer arrow, slowly reaching the middle of the scale, will quickly rush to the red zone, and the radiator hoses, especially the lower one, will remain cold), movement is impossible even in winter - the engine will immediately overheat again. In this case, you need to dismantle the thermostat or at least its valve.

If a coolant leak is found, it is advisable to eliminate it or at least reduce it to reasonable limits. Usually the radiator "leaks" due to corrosion of the tubes on the fins or at the soldering points. Sometimes such tubes can be muffled by biting them and bending the edges with pliers.

In cases where it is not possible to completely eliminate a serious malfunction in the cooling system on the spot, you need to at least drive to the nearest service station or village.

If the fan is faulty, you can continue driving with the heater turned on at "maximum", which takes on a significant part of the heat load. It will be "a little" hot in the cabin - it doesn't matter. As you know, "couples of bones do not ache."

Worse if the thermostat has failed. We have already considered one option above. But if you cannot cope with this device (do not want, do not have tools, etc.), you can try another method. Start moving - but as soon as the pointer arrow approaches the red zone, turn off the engine and coast. When the speed drops, turn on the ignition (it is easy to make sure that after only 10-15 seconds the temperature will be lower), start the engine again and repeat all over again, continuously following the arrow of the temperature indicator.

With a certain degree of care and appropriate road conditions (there are no steep climbs) in this way, you can drive tens of kilometers, even when there is very little coolant left in the system. At one time, the author managed to overcome about 30 km in this way without causing any significant damage to the engine.

During the operation of the electric motor, part of the electrical energy is converted into heat. This is due to energy losses due to friction in bearings, and to magnetization reversal in the stator and rotor steel, as well as in the stator and rotor windings. Energy losses in the stator and rotor windings are proportional to the square of their currents. Stator and rotor current is proportional
load on the shaft. The rest of the motor losses are almost independent of the load.

With a constant load on the shaft, a certain amount of heat is generated in the engine per unit of time.

The engine temperature rises unevenly. At first, it increases rapidly: almost all the heat goes to increase the temperature, and only a small amount of it goes into the environment. The temperature difference (the difference between the engine temperature and the ambient temperature) is still small. However, as the engine temperature rises, the differential increases and the heat transfer to the environment increases. The engine temperature rise slows down.

Electric motor temperature measurement circuit: a - according to the circuit with a switch; b - according to the scheme with a plug.

The temperature of the engine stops rising when all of the newly generated heat has been completely dissipated into the environment. This engine temperature is called steady-state. The steady-state temperature of the motor depends on the load on its shaft. Under heavy load, the a large number of heat per unit of time, which means that the steady-state temperature of the engine is higher.

After shutdown, the engine is cooled down. Its temperature first decreases rapidly, since its drop is large, and then, as the drop decreases, slowly.

The value of the permissible steady-state temperature of the motor is determined by the insulation properties of the windings.

Most general-purpose motors use enamels, synthetic films, impregnated cardboard, cotton yarn to insulate the windings. The maximum permissible heating temperature for these materials is 105 ° C. The temperature of the motor winding at rated load must be 20 ... 25 ° C below the maximum permissible value.

The significantly lower engine temperature corresponds to its operation with a light load on the shaft. In this case, the efficiency of the engine and its power factor are low.

Operating modes of electric motors

There are three main modes of operation of engines: continuous, intermittent and short-term.

Continuous operation is called a mode of operation of the engine at constant load with a duration not less than that necessary to achieve a steady-state temperature at a constant ambient temperature.

Intermittent operation is a mode of operation in which a short-term constant load alternates with engine shutdowns, and during a load, the engine temperature does not reach a steady-state value, and during a pause, the engine does not have time to cool down to the ambient temperature.

A short-term mode is called a mode in which during the engine load time its temperature does not reach the steady-state value, and during the pause time it has time to cool down to the ambient temperature.

Figure 1. Scheme of heating and cooling of motors: a - continuous operation, b - intermittent, c - short-term

In fig. 1 shows the heating and cooling curves of the engine and the input power P for three operating modes. For continuous operation, three heating and cooling curves 1, 2, 3 (Fig. 1, a) are shown, corresponding to three different loads on its shaft. Curve 3 corresponds to the highest load on the shaft; in this case, the input power is P3\u003e P2\u003e Pi. In the intermittent operation of the engine (Fig. 1, b), its temperature during the load does not reach the steady state. The engine temperature would rise along a dashed curve if the load times were longer. Motor ON times are limited to 15, 25, 40 and 60% of the cycle time. The duration of one cycle tts is taken equal to 10 minutes and is determined by the sum of the load time N and the pause time R, i.e.

For intermittent operation, motors are produced with a duty cycle of 15, 25, 40 and 60%: duty cycle \u003d N: (N + R) * 100%

In fig. 1c shows the heating and cooling curves of the engine during short-term operation. For this mode, motors are made with a period duration of a constant rated load of 15, 30, 60, 90 minutes.

The heat capacity of the engine is a significant value, so it can be heated to a steady temperature for several hours. The intermittent mode motor does not have time to heat up to the established temperature during the load period, therefore it operates with a greater load on the shaft and more power input than the same motor for continuous operation. The intermittent duty motor also operates with a higher shaft load than the same continuous duty motor. The shorter the duration of the motor activation, the greater the permissible load on its shaft.

For most machines (compressors, fans, potato peelers, etc.), asynchronous motors for general use with continuous operation are used. For lifts, cranes, cash registers, intermittent operation engines are used. Intermittent duty motors are used for machines used during repairs such as electric hoists and cranes.

some liquid will work in the cylinder. And from the movement of the piston, as in a steam engine, with the help crankshaft both the flywheel and the pulley will begin to rotate. Thus, a mechanical

This means that you only need to alternately heat and cool some working fluid. For this, arctic contrasts were used: water from underneath alternately happens to the cylinder. sea \u200b\u200bicethen cold air; the temperature of the liquid in the cylinder changes rapidly, and such an engine starts to work. It doesn't matter if the temperatures are above or below zero, you just need to have a difference between them. In this case, of course, the working fluid for the engine must be taken such that it would not freeze at the lowest temperature.

Already in 1937 a temperature difference engine was designed. The design of this engine was somewhat different from the described scheme. Two pipe systems were designed, one of which must be in the air and the other in the water. The working fluid in the cylinder is automatically brought into contact with one or the other pipe system. The liquid inside the pipes and the cylinder does not stand still: it is driven all the time by pumps. The engine has several cylinders, and they alternately come to the pipes. All these devices make it possible to accelerate the process of heating and cooling the liquid, and, therefore, the rotation of the shaft to which the piston rods are attached. As a result, such speeds are obtained that they can be transmitted through a gearbox to the shaft of an electric generator and, thus, the thermal energy received from the temperature difference can be converted into electrical energy.

The first temperature difference engine was only designed for relatively large temperature differences, of the order of 50 °. It was a small 100-kilowatt power plant operating

on the temperature difference between air and water from hot springs, which are available here and there in the North.

On this installation, it was possible to check the design of the multi-temperature engine and, most importantly, to accumulate experimental material. Then an engine was built that uses smaller temperature differences - between sea water and cold arctic air. The construction of differential temperature stations became possible everywhere.

Somewhat later, another multi-temperature source of electrical energy was constructed. But it was no longer mechanical engine, but an installation that acts like a huge galvanic cell.

As you know, a chemical reaction occurs in galvanic cells, as a result of which electrical energy is obtained. Many chemical reactions associated with either the release or the absorption of heat. You can choose such electrodes and electrolyte that there will be no reaction as long as the temperature of the cells remains unchanged. But as soon as they are warmed up, they will begin to give current. And here the absolute temperature does not matter; it is only important that the temperature of the electrolyte begins to rise relative to the temperature of the air surrounding the installation.

Thus, in this case too, if such an installation is placed in cold, Arctic air and “warm” sea water is supplied to it, electrical energy will be obtained.

Differential temperature installations were already quite common in the Arctic in the 50s. They were quite powerful stations.

These stations were installed on a T-shaped pier deeply protruding into the sea bay. Such an arrangement of the station reduces the pipelines connecting the working fluid of the differential-dark installation with the sea water. The installation requires a considerable depth of the bay for a good pabota. There must be large masses of water near the station so that when it is cooled due to the transfer of heat to the engine, it does not freeze.

Differential temperature power plant

The power plant, using the temperature difference between water and air, is installed on an iola, which cuts deep into the bay. Cylindrical air radiators are visible on the "roof of the power plant building. From the air radiators there are pipes through which working fluid is supplied to each engine. Pipes also go down from the engine to a water radiator immersed in the sea (not shown in the figure). The motors are connected to electric ones. "generators through gearboxes (in the figure they are visible on the opened part of the building, in the middle between the engine ^ a generator), in which, with the" worm gear the number of revolutions increases. From the generator, electrical energy goes to the transformers, which increase the voltage (the transformer / pores are on the left side

building, not opened in the figure), and from the transformers to the distribution boards (upper floor in the foreground) and then to the transmission line. Some of the electricity goes to huge heating elements immersed in the sea (they are not visible in the figure). These l create a frost-free port.

Sent:

Considering the topic of obtaining electricity in the field, we somehow completely lost sight of such a converter of thermal energy into mechanical energy (and further into electricity), like external combustion engines. In this review, we will consider some of them that are available even for self-made amateurs.

Actually, the choice of designs for such engines is small - steam engines and turbines, a Stirling engine in various modifications and exotic engines, such as vacuum ones. Steam machines we will discard for now, because so far nothing small and easily repeatable has been done on them, but we will pay attention to Stirling engines and vacuum ones.
Provide classification, types, operating principle, etc. I will not be here - whoever needs it will easily find it all on the Internet.

In the most general terms, almost any heat engine can be thought of as a generator of mechanical vibrations that uses a constant potential difference (in this case, thermal) for its operation. The self-excitation conditions of such an engine, as in any generator, are provided by delayed feedback.

Such a delay is created either by a rigid mechanical connection through the crank, or by means of an elastic connection, or, as in a "slow heating" engine, by means of the thermal inertia of the regenerator.

Optimally, from the point of view of obtaining the maximum amplitude of oscillations, the removal of maximum power from the engine, when the phase shift in the movement of the pistons is 90 degrees. In engines with a crank mechanism, this shift is set by the shape of the crank. In motors where such a delay is performed by means of elastic coupling or thermal inertia, this phase shift is performed only at a certain resonant frequency at which the motor power is maximum. However, engines without a crank mechanism are very simple and therefore very attractive to manufacture.

After this short theoretical introduction, I think it will be more interesting to look at those models that were actually built and that can be suitable for use in mobile conditions.

YouTube features the following:

Low temperature Stirling engine for low temperature differences,

Stirling engine for large temperature gradients,

"Slow heating" engine, other names are Lamina Flow Engine, thermoacoustic Stirling engine (although the latter name is incorrect, since there is a separate class of thermoacoustic engines),

Stirling engine with free piston (free piston Stirling engine),

Vacuum motor (FlameSucker).

The appearance of the most typical representatives is shown below.

Low temperature Stirling engine.

High temperature Stirling engine.
(By the way, the photo shows a burning incandescent light bulb powered by a ganerator connected to this engine)

Lamina Flow Engine

Free piston engine.

Vacuum engine (flame pump).

Let's consider each of the types in more detail.

Let's start with a low-temperature Stirling engine. Such an engine can operate from a temperature difference of literally several degrees. But the power removed from it will be small - fractions and units of Watt.
It is better to watch the work of such engines on video, in particular, on sites like YouTube great amount working instances. For instance:

Low temperature Stirling engine

In this engine design, the upper and lower plates must be at different temperatures because one of them is a heat source, the other is a cooler.

The second type of Stirling engines can already be used to obtain power in units or even tens of watts, which allows powering most electronic devices in field conditions. An example of such engines is shown below.

Stirling's engine

There are many such engines on YouTube, and some are made of this stuff ... but they work.

Captivates with its simplicity. Its diagram is shown in the figure below.

Slow heating motor

As already mentioned, the presence of a crank here is also optional, it is only needed to convert the oscillations of the piston into rotation. If the removal of mechanical energy and its further transformation are performed using the schemes already described, then the design of such a generator may turn out to be very, very simple.

Free piston Stirling engine.
In this engine, the displacing piston is connected to the force piston through an elastic connection. In this case, at the resonant frequency of the system, its movement lags behind the oscillations of the power piston, which is about 90 degrees, which is required for normal excitation of such an engine. In fact, a generator of mechanical vibrations is obtained.

Vacuum motor, unlike others, it uses the effect in its work compression gas when it cools. It works as follows: first, the piston sucks the burner flame into the chamber, then the movable valve closes the suction hole and the gas, cooling and contracting, forces the piston to move in the opposite direction.
The operation of the engine is perfectly illustrated by the following video:

Vacuum engine operation diagram

And below is just an example of a manufactured engine.

Vacuum motor

Finally, we note that although the efficiency of such homemade motors is, at best, a few percent, but even in this case, similar mobile generators can generate an amount of energy sufficient to power mobile devices... Thermoelectric generators can serve as a real alternative to them, but their efficiency is also 2 ... 6% with comparable weight and size parameters.

In the end, the thermal power of even simple alcohol lamps is tens of watts (and by the fire - kilo watts) and the conversion of at least a few percent of this heat flux into mechanical, and then electrical energy, already allows you to get quite acceptable powers suitable for charging real devices ...

Let's remember that, for example, the power of a solar battery recommended for charging a PDA or a communicator is about 5 ... 7W, but even these watts the solar battery will give only under ideal lighting conditions, actually less. Therefore, even when generating several watts, but independent of the weather, these engines will already be quite competitive, even with the same solar panels and thermal generators.

Few links.

A large number of drawings for the manufacture of models of Stirling engines can be found on this site.

The www.keveney.com page contains animated models of various engines, including Stirlings.

I would also recommend to look at the page http://ecovillage.narod.ru/, especially since the book "Walker G. Machines working on the Stirling cycle. 1978" is posted there. It can be downloaded as a single file in djvu format (about 2MB).

In the engine cylinder, thermodynamic cycles are carried out with some frequency, which are accompanied by a continuous change in the thermodynamic parameters of the working fluid - pressure, volume, temperature. The energy of fuel combustion with a change in volume turns into mechanical work. The condition for the transformation of heat into mechanical work is the sequence of strokes. To these strokes in the engine internal combustion include intake (filling) of cylinders with a combustible mixture or air, compression, combustion, expansion and release. The variable volume is the volume of the cylinder, which increases (decreases) with the translational movement of the piston. An increase in volume occurs due to the expansion of products during the combustion of a combustible mixture, a decrease - when a new charge of a combustible mixture or air is compressed. The forces of gas pressure on the walls of the cylinder and on the piston during the expansion stroke are converted into mechanical work.

The energy stored in the fuel is converted into thermal energy during thermodynamic cycles, is transferred to the cylinder walls by thermal and light radiation, radiation and from the cylinder walls - the coolant and the engine mass by thermal conduction and into the surrounding space from the surfaces of the engine free and forced

convection. All types of heat transfer are present in the engine, which indicates the complexity of the processes taking place.

The use of heat in the engine is characterized by efficiency, the less the heat of combustion of the fuel is given to the cooling system and to the engine mass, the more work is done and the higher the efficiency.

The engine runs in two or four strokes. The main processes of each working cycle are intake, compression, stroke and exhaust strokes. The introduction of a compression stroke into the working process of engines made it possible to minimize the cooling surface and at the same time increase the fuel combustion pressure. Combustion products expand according to the compression of the combustible mixture. This process allows to reduce heat losses in the cylinder walls and exhaust gases, to increase the gas pressure on the piston, which significantly increases the power and economic performance of the engine.

Real thermal processes in an engine differ significantly from theoretical ones based on the laws of thermodynamics. The theoretical thermodynamic cycle is closed, a prerequisite for its implementation is the transfer of heat to a cold body. In accordance with the second law of thermodynamics and in a theoretical heat engine, it is impossible to completely convert thermal energy into mechanical energy. In diesel engines, the cylinders of which are filled with a fresh charge of air and have high degrees compression, the temperature of the combustible mixture at the end of the intake stroke is 310 ... 350 K, which is explained by the relatively small amount of residual gases, in gasoline engines the intake temperature at the end of the stroke is 340 ... 400 K. The heat balance of the combustible mixture during the intake stroke can be represented as

where?) p t - the amount of heat of the working fluid at the beginning of the intake stroke; OS.ts - the amount of heat that entered the working fluid upon contact with the heated surfaces of the intake tract and cylinder; Qo g - the amount of heat in the residual gases.

From the heat balance equation, the temperature at the end of the intake stroke can be determined. We take the mass value of the amount of fresh charge t with z, residual gases - t about g With a known heat capacity of the fresh charge with P, residual gases with "p and working mixture with p equation (2.34) is represented as

where T with h - temperature of the fresh charge before inlet; AND T sz - heating of a fresh charge when it is injected into the cylinder; T g - temperature of residual gases at the end of the discharge. It is possible to assume with sufficient accuracy that with "p = with p and s "p - s, s p, where s; - correction factor depending on T sz and the composition of the mixture. With a \u003d 1.8 and diesel fuel

When solving equation (2.35) with respect to T a denote the relation

The formula for determining the temperature in the cylinder at the inlet has the form

This formula is valid for both four-stroke and two-stroke engines, for turbocharged engines, the end-of-intake temperature is calculated using formula (2.36), provided that q \u003d 1. The accepted condition does not introduce large errors into the calculation. The values \u200b\u200bof the parameters at the end of the intake stroke, determined experimentally at the nominal mode, are presented in table. 2.2.

Table 2.2

During the intake stroke, the intake valve in the diesel engine opens by 20 ... 30 ° before the piston reaches TDC and closes after passing the BDC by 40 ... 60 °. The opening time of the intake valve is 240 ... 290 °. The temperature in the cylinder at the end of the previous stroke - exhaust is equal to T g \u003d 600 ... 900 K. The air charge, which has a temperature significantly lower, is mixed with the residual gases in the cylinder, which reduces the temperature in the cylinder at the end of the intake to T a \u003d 310 ... 350 K. The temperature difference in the cylinder between the exhaust and intake strokes is AT a. r \u003d T a - T g.Insofar as T a AT a. t \u003d 290 ... 550 °.

The rate of temperature change in the cylinder per unit of time per cycle is equal to:

For a diesel engine, the rate of temperature change during the intake stroke at n e \u003d 2400 min -1 and φ a \u003d 260 ° is with d \u003d (2.9 ... 3.9) 10 4 deg / s. Thus, the temperature at the end of the intake stroke in the cylinder is determined by the mass and temperature of the residual gases after the exhaust stroke and the heating of the fresh charge from the engine parts. The graphs of the function co rt \u003d / (D e) of the intake stroke for diesel and gasoline engines, presented in Fig. 2.13 and 2.14, indicate a significantly higher rate of temperature change in the cylinder of a gasoline engine in comparison with a diesel engine and, consequently, a higher intensity of the heat flow from the working fluid and its growth with an increase in the crankshaft speed. The average calculated value of the rate of temperature change during the diesel intake stroke within the crankshaft speed of 1500 ... 2500 min -1 is \u003d 2.3 10 4 ± 0.18 deg / s, and for the gasoline

engine within the speed of 2000 ... 6000 min -1 - with i \u003d 4.38 10 4 ± 0.16 deg / s. At the intake stroke, the temperature of the working fluid is approximately equal to operating temperature coolant,

Figure: 2.13.

Figure: 2.14.

the heat of the cylinder walls is spent on heating the working fluid and does not significantly affect the temperature of the coolant in the cooling system.

When compression stroke rather complex processes of heat exchange occur inside the cylinder. At the beginning of the compression stroke, the temperature of the charge of the combustible mixture is less than the temperature of the surfaces of the walls of the cylinder and the charge heats up, continuing to take away heat from the walls of the cylinder. The mechanical work of compression is accompanied by the absorption of heat from the external environment. In a certain (infinitely small) period of time, the temperatures of the surface of the cylinder and the charge of the mixture equalize, as a result of which the heat transfer between them stops. With further compression, the temperature of the charge of the combustible mixture exceeds the temperature of the surfaces of the cylinder walls and the heat flux changes direction, i.e. heat goes to the cylinder walls. The total heat transfer from the charge of the combustible mixture is insignificant, it is about 1.0 ... 1.5% of the amount of heat supplied with the fuel.

The temperature of the working fluid at the end of the inlet and its temperature at the end of compression are related by the equation of the compression polytrope:

where 8 is the compression ratio; n l - polytropic indicator.

Temperature at the end of the compression stroke general rule calculated by the average constant for the entire process value of the polytropic index n. In a particular case, the polytropic exponent is calculated from the balance of heat during compression in the form

where and with and and "- internal energy of 1 kmole of fresh charge; and a and and "-internal energy of 1 kmol of residual gases.

Joint solution of equations (2.37) and (2.39) at a known temperature T a allows you to determine the polytropic indicator n. The polytropic index is influenced by the intensity of cylinder cooling. At low coolant temperatures, the cylinder surface temperature is lower, therefore, n l will be less.

The values \u200b\u200bof the parameters of the end of the compression stroke are given in table. 2.3.

Table23

On the compression stroke, the intake and exhaust valves are closed, the piston moves to TDC. The time of the compression stroke for diesel engines at a speed of 1500 ... 2400 min -1 is 1.49 1СГ 2 ... 9.31 KG 3 s, which corresponds to the rotation of the crankshaft at an angle φ (. \u003d 134 °, for gasoline engines at a rotational speed of 2400 ... 5600 min -1 and cf r \u003d 116 ° - (3.45 ... 8.06) 1 (G 4 s. The temperature difference of the working fluid in the cylinder between the compression and intake strokes AT s _ a = T s - T a for diesel engines it is within 390 ... 550 ° С, for gasoline engines - 280 ... 370 ° С.

The rate of temperature change in the cylinder per compression stroke is:

and for diesel engines at a speed of 1500 ... 2500 min -1 the rate of temperature change is (3.3 ... 5.5) 10 4 deg / s, for gasoline engines at a speed of 2000 ... 6000 min -1 - ( 3.2 ... 9.5) x x 10 4 deg / s. The heat flow during the compression stroke is directed from the working fluid in the cylinder to the walls and into the coolant. Function graphs with \u003d f (n e) for diesel and gasoline engines are shown in Fig. 2.13 and 2.14. It follows from them that the rate of change in the temperature of the working fluid in diesel engines is higher than in gasoline engines at one speed.

Heat transfer processes during the compression stroke are determined by the temperature difference between the cylinder surface and the charge of the combustible mixture, the relatively small cylinder surface at the end of the stroke, the mass of the combustible mixture and a limitedly short time interval during which heat transfer from the combustible mixture to the cylinder surface occurs. It is assumed that the compression stroke has no significant effect on temperature regime cooling systems.

Expansion cycle is the only stroke in the engine's operating cycle during which useful mechanical work is performed. This cycle is preceded by the combustion process of the combustible mixture. The result of combustion is an increase in the internal energy of the working fluid, which is converted into work of expansion.

The combustion process is a complex of physical and chemical phenomena of fuel oxidation with intense release

warmth. For liquid hydrocarbon fuels (gasoline, diesel fuel) the combustion process is a chemical reaction of the combination of carbon and hydrogen with atmospheric oxygen. The heat of combustion of the charge of the combustible mixture is spent on heating the working fluid, making mechanical work... Part of the heat from the working fluid through the cylinder walls and the head heats the crankcase and other engine parts, as well as the coolant. The thermodynamic process of a real working process, taking into account the loss of the heat of combustion of the fuel, taking into account incomplete combustion, heat transfer to the cylinder walls, etc., is extremely complex. In diesel and gasoline engines, the combustion process is different and has its own characteristics. In diesel engines, combustion occurs with different intensities depending on the piston stroke: at first intensively, and then slowly. In gasoline engines, combustion occurs instantaneously; it is generally accepted that it occurs at a constant volume.

To take into account the heat by the components of losses, including heat transfer to the cylinder walls, the coefficient of utilization of the combustion heat is introduced.The coefficient of utilization of heat is determined experimentally, for diesel engines \u003d 0.70 ... 0.85 and gasoline engines ?, \u003d 0.85 ... 0.90 from the equation of state of gases at the beginning and end of expansion:

where is the degree of preliminary expansion.

For diesel engines

then

For gasoline engines then

Values \u200b\u200bof parameters during combustion and at the end of the expansion stroke for engines)