In the engine cylinder, thermodynamic cycles are carried out with some periodicity, 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 is converted into mechanical work. The condition for the transformation of heat into mechanical work is the sequence of cycles. These cycles in an internal combustion engine include the intake (filling) of the cylinders with a combustible mixture or air, compression, combustion, expansion and exhaust. Variable volume is the volume of a cylinder that increases (decreases) as the piston moves forward. An increase in volume occurs due to the expansion of products during the combustion of a combustible mixture, a decrease - due to the compression of a new charge of a combustible mixture or air. 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 accumulated in the fuel is converted into thermal energy when performing thermodynamic cycles, is transferred to the cylinder walls by thermal and light radiation, radiation and from the cylinder walls - to the coolant and engine mass by thermal conduction and to 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 ongoing processes.
The use of heat in the engine is characterized by efficiency, the less heat of combustion of the fuel is given to the cooling system and to the mass of the engine, the more work is done and the higher the efficiency.
The working cycle of the engine is carried out in two or four cycles. The main processes of each working cycle are the 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 simultaneously increase the combustion pressure of the fuel. Combustion products expand according to the compression of the combustible mixture. This process allows to reduce heat losses in the cylinder walls and with 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 the engine differ significantly from theoretical ones based on the laws of thermodynamics. The theoretical thermodynamic cycle is closed, and a mandatory condition 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 compression ratios, the temperature of the combustible mixture at the end of the intake stroke is 310... .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 in 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 o g With a known heat capacity of a fresh charge with R, residual gases s"r and working mixture with p equation (2.34) is represented as
Where T s h - fresh charge temperature before intake; A T sz- heating of a fresh charge when it enters the cylinder; T g is the temperature of the residual gases at the end of the outlet. It is possible to assume with sufficient accuracy that s"r = with p And s "r - s, s p, where s; - correction factor depending on T sz and composition of the mixture. With a = 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 is
This formula is valid for both four-stroke and two-stroke engines; for turbocharged engines, the temperature at the end of the intake is calculated using formula (2.36), provided that q = 1. The accepted condition does not introduce large errors into the calculation. The values of the parameters at the end of the intake stroke, determined experimentally in the nominal mode, are presented in Table. 2.2.
Table 2.2
|
Four-stroke internal combustion engines |
Two-stroke internal combustion engines |
||
|
Index |
with spark ignition |
with direct-flow gas exchange scheme |
|
|
Residual gas coefficient y |
|||
|
Exhaust gas temperature at the end of exhaust G p K |
|||
|
Fresh charge heating, K |
|||
|
The temperature of the working fluid at the end of the inlet T a, TO |
|||
During the intake stroke, the inlet valve in the diesel engine opens by 20...30° before the piston reaches TDC and closes after passing BDC by 40...60°. The duration of the intake valve opening 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 much 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 = 310 ... 350 K. The temperature difference in the cylinder between the exhaust and intake strokes is AT a. g \u003d T a - T g. Because the T a AT a. t = 290...550°.
The rate of temperature change in the cylinder per unit time per cycle is:
For a diesel engine, the rate of temperature change during the intake stroke at p e\u003d 2400 min -1 and f a \u003d 260 ° is so 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. Graphs of the function co rt = / (D e) intake stroke for diesel and gasoline engines, presented in par fig. 2.13 and 2.14 indicate a significantly higher rate of temperature change in the cylinder of a gasoline engine compared to a diesel engine and, consequently, a greater intensity of the heat flux from the working fluid and its growth with increasing crankshaft speed. The average calculated value of the temperature change rate at the diesel intake stroke within the crankshaft speed of 1500 ... 2500 min -1 is = 2.3 10 4 ± 0.18 deg / s, and for gasoline
engine within the speed range of 2000...6000 min -1 - co i = = 4.38 10 4 ± 0.16 deg/s. During the intake stroke, the temperature of the working fluid is approximately equal to the operating temperature of the coolant,
Rice. 2.13.
Rice. 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 of the cooling system.
At compression stroke quite complex heat transfer processes occur inside the cylinder. At the beginning of the compression stroke, the charge temperature of the combustible mixture is less than the temperature of the surfaces of the cylinder walls and the charge heats up, continuing to take heat from the cylinder walls. 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 are equalized, as a result of which the heat exchange between them stops. With further compression, the charge temperature of the combustible mixture exceeds the temperature of the surfaces of the cylinder walls and the heat flow changes direction, i.e. heat is transferred to the walls of the cylinder. 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 intake and its temperature at the end of the compression are interconnected by the equation of the compression polytrope:
where 8 - compression ratio; p l - polytropic index.
The temperature at the end of the compression stroke, as a general rule, is calculated from the average constant value of the polytropic index for the entire process sch. In a particular case, the polytropic index is calculated from the heat balance in the compression process in the form
Where and with And And" - internal energy of 1 kmole of fresh charge; and a And And" - internal energy of 1 kmole of residual gases.
Joint solution of equations (2.37) and (2.39) for a known temperature T a allows you to determine the polytropic index sch. The polytropic index is affected by the intensity of cooling of the cylinder. At low coolant temperatures, the surface temperature of the cylinder is lower, and therefore p l will be less.
The values of 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 and 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 1SG 2 ... 9.31 KG 3 s, which corresponds to the rotation of the crankshaft at an angle φ (. = 134 °, for gasoline engines at a speed of 2400 ... 5600 min -1 and cp g \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 with _ a = T s - T a for diesel engines it is in the range of 390 ... 550 ° C, for gasoline engines - 280 ... 370 ° C.
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. Graphs of the function co = f(n e) for diesel and gasoline engines are presented 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.
The 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 surface of the cylinder at the end of the stroke, the mass of the combustible mixture, and the limitedly short period of time during which heat is transferred from the combustible mixture to the cylinder surface. It is assumed that the compression stroke does not significantly affect the temperature regime of the cooling system.
Extension stroke is the only stroke of the engine cycle during which useful mechanical work is performed. This step is preceded by the process of combustion of the combustible mixture. The result of combustion is an increase in the internal energy of the working fluid, which is converted into the work of expansion.
The combustion process is a complex of physical and chemical phenomena of fuel oxidation with intensive 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, performing mechanical work. Part of the heat from the working fluid through the walls of the cylinders and the head heats the crankcase and other parts of the engine, as well as the coolant. The thermodynamic process of a real working process, taking into account the loss of heat of combustion of the fuel, taking into account the incompleteness of 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 intensity 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 heat by loss components, including heat transfer to the cylinder walls, the coefficient of utilization of the heat of combustion is introduced. The coefficient of utilization of heat is determined experimentally, for diesel engines = 0.70 ... 0.85 and gasoline engines?, = 0.85 ... 0.90 from the equation of state of gases at the beginning and end of expansion:
where is the degree of pre-expansion.
for diesels
Then 
For petrol engines 
Then
Parameter values during combustion and at the end of the expansion stroke for engines )
