Engine at low temperature drop. Thermal engine on a new thermodynamic principle. Determination of the permissible amount of harmful substances

In the engine cylinder with some frequency, thermodynamic cycles are carried out, which are accompanied by a continuous change in the thermodynamic parameters of the working fluid - pressure, volume, temperature. The energy of fuel combustion when the volume changes turns into mechanical work. The condition for the transformation of heat into mechanical work is a clock sequence. These clocks in the internal combustion engine include the inlet (filling) of cylinders of a combustible mixture or air, compression, combustion, expansion and release. The changing volume is the volume of the cylinder, which increases (decreases) with the progressive movement of the piston. An increase in volume occurs due to the expansion of products when combustion of a combustible mixture, a decrease - when compressed by a new charge of a combustible mixture or air. Gas pressure for the cylinder walls and the piston with expansion tact turn into mechanical work.

The energy-accumulated energy turns into thermal energy when performing thermodynamic cycles, is transmitted by the walls of the cylinders by heat and light radiation, radiation and the walls of the cylinder - coolant and the mass of the engine through thermal conductivity and in the surrounding space from the surfaces of the free and forced engine surfaces

convection. In the engine there are all types of heat transfer, which indicates the complexity of the processes occurring.

The use of heat in the engine is characterized by an efficiency, the smaller the heat of the combustion of the fuel is given to the cooling system and into the mass of the engine, the more work is performed above the efficiency.

The operating cycle of the engine is carried out in two or four tact. The main processes of each working cycle are intake tacts, compression, working stroke and release. Introduction to the workflow of engine tact engines made it possible to reduce the cooling surface as much as possible and optimize the fuel combustion pressure. The combustion products are expanding according to the compression of a combustible mixture. Such a process reduces thermal losses in the cylinder walls and with exhaust gases, increase the pressure of gases to the piston, which significantly increases the power and economic indicators of the engine.

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

where?) R T is the amount of heat of the working fluid at the beginning of the intake clock; OS.TS - the amount of heat entered into the working fluid when contacting the heated surfaces of the inlet path and cylinder; Qo g - the amount of heat in the residual gases.

From the heat balance equation, you can determine the temperature at the end of the intake tact. We will take a massive value of the number of fresh charge t with s residual gases - t o g With a well-known heat capacity of fresh charge with p, residual gases c "R. and working mixture with R. Equation (2.34) is presented in the form of

where T S. h - the temperature of fresh charge before the inlet; BUT T NW - heated fresh charge when inlet of it in a cylinder; T. - The temperature of the residual gases at the end of the release. Perhaps with sufficient accuracy to assume that c "R. = with R. and with "r - s, with p, where with; - correction coefficient depending on T NW and composition of the mixture. At a \u003d 1.8 and diesel fuel

When solving equation (2.35) regarding T A. Denote by attitude

The formula for determining the temperature in the cylinder at the intake is

This formula is valid for both four-stroke and two-stroke engines, for turbocharging engines, the temperature at the end of the intake is calculated by formula (2.36), provided that q \u003d. 1. The assured condition does not contribute large errors. The parameter values \u200b\u200bat the end of the intake clock defined experimentally on the nominal mode are presented in Table. 2.2.

Table 2.2.

When the intake valve intake tact in the diesel engine is opened by 20 ... 30 ° to the arrival of the piston in the NMT and closes after the NMT passage by 40 ... 60 °. The duration of the opening of the ink valve is 240 ... 290 °. The temperature in the cylinder at the end of the previous tact - release is equal T. \u003d 600 ... 900 K. Air charge having a temperature is significantly lower, mixed with 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. Delta of temperatures in the cylinder between the output clocks and the inlet is equal ATA r \u003d t a - tInsofar as T A. ATA T \u003d 290 ... 550 °.

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

For diesel, the speed of temperature change when the intake tact when p. \u003d 2400 min -1 and Φ a \u003d 260 ° is CO d \u003d (2.9 ... 3.9) 10 4 hail / s. Thus, the temperature at the end of the intake tact in the cylinder is determined by the mass and temperature of the residual gases after the release tact and the heating of the fresh charge from the engine parts. Graphs of the function CO RT \u003d / (D E) intake tact for diesel engines and gasoline engines, presented to PA Fig. 2.13 and 2.14 indicate a significantly higher rate of temperature change in the gasoline engine cylinder in comparison with the diesel engine and, therefore, the greater the intensity of the heat flux from the working fluid and its growth with increasing the rotational speed of the crankshaft. The average estimated value of the temperature change rate when the diesel inlet tact within the speed of the crankshaft rotation of 1500 ... 2500 min -1 is equal to \u003d 2.3 10 4 ± 0.18 degrees / s, and in gasoline

the engine is within the frequency of 2,000 ... 6000 min -1 - with me \u003d 4.38 10 4 ± 0.16 deg / s. With the intake tact, the temperature of the working fluid is approximately equal to the operating temperature of the coolant,

Fig. 2.13.

Fig. 2.14.

the heat of the cylinder walls is spent on the heating of the working fluid and does not have a significant effect on the cooling fluid temperature of the cooling system.

For tact of compression There are quite complex heat exchange processes inside the cylinder. At the beginning of the compression tact, the charge temperature of the combustible mixture is less than the temperature of the surfaces of the walls of the cylinder and the charge is heated, continuing to take the 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), the temperature range of the surface of the cylinder and the charge of the mixture is leveled, as a result of which heat exchange between them is terminated. With further compression, the temperature of the combustible mixture exceeds the temperature of the surfaces of the cylinder walls and the heat flux changes the direction, i.e. The heat enters the cylinder walls. The overall return of heat from the charge of a combustible mixture is insignificant, it is about 1.0 ... 1.5% of the amount of heat entering with the fuel.

The temperature of the working fluid at the end of the intake and its same temperature at the end of compression are related to the compression polytropic equation:

where 8 is a compression ratio; p l - Indicator polytropags.

The temperature at the end of the compression tact of the general rule is calculated by the average constant for the entire process of the polytropic indicator. sh. In a particular case, the polytropic indicator is calculated on the balance of heat during the compression process in the form of

where and S. and and "- Internal energy of 1 km of fresh charge; and A. and and "-internal energy of 1 km of residual gases.

Joint solution of equations (2.37) and (2.39) with a known temperature value T A. Allows you to determine the indicator of polytropags sh. The polytropope indicator affects the intensity of the cooling of the cylinder. At low cooling fluid temperatures, the surface temperature of the cylinder is below, therefore, and p L. will be less.

The values \u200b\u200bof the terminal parameters of the compression tact are shown in Table. 2.3.

Table23

With the intake and exhaust valve compression tact, the piston moves to the VTC. Take time of compression tact in diesel engines at a speed of 1500 ... 2400 min -1 is 1.49 1 SG 2 ... 9.31 kg 3 C, which corresponds to the rotation of the crankshaft at an angle f (. \u003d 134 °, in 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 in the cylinder between compression and intake clocks At from _ a = T C - T A Diesels are within 390 ... 550 ° C, in gasoline engines - 280 ... 370 ° C.

The rate of temperature change in the cylinder for compression tact 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 degrees / s, gasoline engines at a rotational speed of 2000 ... 6000 min -1 - ( 3.2 ... 9.5) x x 10 4 hail / s. The heat flux with compression tact is directed from the working fluid in the cylinder to the walls and into the coolant. Graphics function CO \u003d f (N. e) for diesel engines and gasoline engines are presented in Fig. 2.13 and 2.14. It follows that the rate of changes in the temperature of the working fluid in diesel engines compared with gasoline engines at one rotational speed above.

The heat exchange processes with compression tact are caused by the temperature drop between the surface of the cylinder and the charge of a combustible mixture, relatively small surface of the cylinder at the end of the tact, mass of the combustible mixture and limitedly a short period of time at which heat transfer occurs from a combustible mixture to the surface of the cylinder. It is assumed that compression tact does not have a significant effect on the cooling system temperature.

Expansion tact It is the only tact of the engine working cycle, in which useful mechanical work is performed. This clock is preceded by the combustion process of a combustible mixture. The result of combustion is to increase the internal energy of the working fluid transformed into the work of the expansion.

The combustion process is a complex of physical and chemical phenomena of fuel oxidation with intensive selection

warm. For liquid hydrocarbon fuels (gasoline, diesel fuel), the combustion process is chemical reactions of carbon and hydrogen compound with air oxygen. The heat of combustion of the charge of a combustible mixture is spent on the heating of 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 block cartridge and other parts of the engine, as well as the coolant. The thermodynamic process of the real workflow, taking into account the loss of heat of the combustion of fuel, taking into account the incompleteness of combustion, heat transfer in the walls of the cylinders and so on is extremely complex. In diesel engines and gasoline engines, the combustion process varies and has its own characteristics. In diesel engines, combustion occurs with different intensity depending on the stroke of the piston: first intensively and then slowed down. In gasoline engines, combustion occurs instantly, it is believed that it is performed at a constant volume.

To account for heat in the component of losses, including heat transfer in the walls of cylinders, the coefficient of use of heat combustion The coefficient of use of heat is determined experimentally for diesel engines \u003d 0.70 ... 0.85 and gasoline engines?, \u003d 0.85 ... 0.90 of the equation of state states at the beginning and end of the expansion:

where is the degree of preliminary expansion.

For diesel engines

then

For gasoline engines then

Values \u200b\u200bof parameters in the combustion process and at the end of the engine expansion clock)