Volume 25 (2007) No. 6 Modeling and Numerical Simulation of Compressed Air/Fuel Hybrid Engine Working Process Fang Qinghua, Liu Wei, Chen Ying, Tao Guoliang (Zhejiang University National Key for Fluid Power Transmission and Control, Its Cylinder Parts and Tradition Similar to the internal combustion engine, the most significant difference is twofold: increased compressed air supply and injection devices; 2) compressed air and fuel injection using an electronically controlled management system. The compressed air supply and injection device is composed of a gas storage tank, a pressure reducing valve, a flow valve, an electromagnetic switching valve, and a compressed air nozzle. The injection timing of the compressed air is determined by the opening timing of the electromagnetic opening and closing valve, and the injection amount is determined by the opening position of the flow valve and the opening and closing time of the high-speed electromagnetic switching valve. The fuel injection timing and the injection amount are controlled by the electronically controlled fuel injection system to drive the opening and closing of the solenoid valve according to signals such as engine speed and load. The hybrid engine can operate in both compressed aerodynamic mode and internal combustion engine mode, with compressed air and fuel as the power source. The conversion of the two operating modes is managed by an engine control unit (not shown). When the engine is to be switched to the compressed air power mode, the engine control unit sends a signal to the electronically controlled fuel injection system to cut off the fuel supply, and then according to the rotational speed, Signals such as the crank angle indicate that the electromagnetic switch valve is opened and closed to operate the engine in the compressed air power mode; when the engine is to be switched to the internal combustion engine mode, the electromagnetic control valve closing signal is issued by the engine control unit to cut off the compressed air supply and then to the electric power. The control fuel injection system sends out a control signal, and the electronically controlled fuel injection system controls the injection of fuel to operate the engine in the internal combustion engine mode. 1 Compressed aerodynamic mode Similar to the 4-stroke reciprocating piston internal combustion engine 19, the compressed aerodynamic mode is also composed of 4 strokes: suction, compression, inflation expansion and exhaust stroke, 4 strokes complete one work cycle, the whole cycle consists of two The crankshaft rotation cycle is completed. The difference from the 4-stroke internal combustion engine is that the compressed air power mode is the work of injecting compressed air expansion near the top dead center of the compression stroke, and the internal combustion engine is driven by the high temperature and high pressure gas generated after the injection of the fuel into the fuel. In the start-up and low-speed phases, the engine is operated in a compressed aerodynamic mode, which can be characterized by low-speed, high-torque and zero-pollution emissions of the compressed air-powered engine. 2 The internal combustion engine mode is the same as the 4-stroke reciprocating piston internal combustion engine. The internal combustion engine mode is also composed of four strokes: suction, compression, fuel injection combustion expansion and exhaust stroke. One power cycle is completed by two crank cycle cycles. The electromagnetic switch valve single-cylinder hybrid engine structure shows that the hybrid engine engine model uses a single cylinder as a thermodynamic system. The system boundary consists of the piston crown, the cylinder head and the cylinder liner wall surface (such as the middle dotted line). In order to simplify the calculation process, the following assumptions are made: the gas in the cylinder is in a uniform state, and the temperature and pressure are the same at each point; the working fluid is an ideal gas, and the parameters such as specific heat, internal energy, and helium are only related to gas temperature and gas composition; gas inflow or The outflow cylinder is quasi-stable flow; (4 kinetic energy of the avoidance and outlet gases is negligible. 1Compressed aerodynamic mode mathematical model 21.1 Energy balance equation The in-cylinder process of compressed aerodynamic mode is the process of gas thermodynamic state change. The mathematical model of its working process can be established by thermodynamic analysis. The energy conservation equations that should be satisfied by changes in gas state are the intake, exhaust, and compressed air masses. In general, the internal energy U and mass m of the gas in the cylinder change simultaneously, so the specific energy of the actual gas is a function of pressure, temperature and gas composition. Since the gas in the cylinder is pure air with stable composition and the influence of pressure on the internal energy of the gas is very small, the change of the internal energy can be regarded as a function of the gas temperature T: 21.2 The mass change in the mass balance equation system should satisfy the mass conservation equation. , that is, the sum of the masses exchanged through the system boundary is equal to the change in the mass within the system: dmdm, ddA, do% degrees; for the flow function, when the value is calculated as P 2k(+1))1, it is a subcritical flow, because There is a combustion process. The heat source of the system mainly comes from the heat exchanged between the peripheral wall of the cylinder and the outside. According to the relevant relationship in heat transfer, the heat transfer amount per unit crank angle can be written as the critical flow, surface area; T is heat transfer. Average surface temperature. The gas heat transfer coefficient can be calculated by referring to the empirical formula of the internal combustion engine. 21.3 Gas state equation The gas state change in the system should satisfy the state equation as the heat transfer surface area of ​​the P/=mRT cylinder changes with the crank angle. The average friction pressure of the friction model of 21.4 can be determined by the empirical formula 111 proposed by WneboneDE et al. The formula calculates the fuel injection amount; 丨 is the combustion quality index; Acp is the combustion continuous angle; 3 is the combustion starting angle. The calculation of cylinder wall heat transfer, mechanical work, intake and exhaust flow, and friction loss are the same as the compressed aerodynamic mode. As the compressed air is injected into the cylinder, the mass of the gas in the cylinder rises sharply, and the injected cold air mixes with the hot air compressed in the cylinder, causing the temperature in the cylinder to rapidly decrease, and the pressure continues to rise after the compression stroke is over. High, the cylinder gas pushes the piston to output torque externally, and the output torque rises rapidly; after the compressed air intake ends, the gas quality in the cylinder remains unchanged, the gas in the cylinder continues to expand, and the temperature, pressure and output torque drop sharply; After the expansion work is completed, the exhaust stroke and the intake stroke are entered, and the process is similar to that of the 4-stroke internal combustion engine (see). Compressed aerodynamic mode output torque change average indication pressure is an extremely important indicator to measure the actual working cycle dynamic performance of the engine. According to the average indicated pressure, other important performance indicators such as indicated power and torque can be calculated. Keeping the position of the flow valve opening unchanged, the average indicated pressure can be seen by changing the continuous angle of the compressed air intake. It can be seen that the average indicated pressure and the average effective torque are increased with the increase of the compressed air intake angle. When the air intake angle is 50°CA, the compressed air intake is about 087R/cyce, the maximum output torque is 139N.m, the average effective torque is 9. 1N.m, the effective power is 0, 95kW, the average indicating pressure is also reached. The average indicated pressure of the 615kP and 4-stroke non-supercharged diesel engine under normal conditions is generally 0 95MPa. The average indicated pressure of the compressed aerodynamic mode is not significantly lower than that of the internal combustion engine, indicating that the compressed air power mode is used in the performance of the internal combustion engine at low speed. There is no big reduction compared to it. 322 compressed aerodynamic mode simulation results test evaluation Due to test conditions, we converted a 462 4-stroke petrol engine to a 2-stroke air motor operating with compressed air. The compressed air-power mode was evaluated based on a 2-stroke: pneumatic engine test. Simulation results. The modified engine is a 4-cylinder, 2-stroke, 8-valve overhead camshaft with a cylinder diameter of 62m. The piston stroke is 66mm. The total displacement is 0 8L. The compression ratio is 8. 7. As long as the 2-stroke air motor is compressed during the test. The gas available energy is equal to the available energy of the compressed air power mode compressed air intake. According to the engine related theory, the power and torque of the 2-stroke air motor at the same speed should be twice that of the 4-stroke air motor (compressed aerodynamic mode). The available energy E of the compressed air is the gas pressure. The test measures the compressed air inlet pressure to 1.2MPa and the speed is 1rmn. The single-cylinder intake mass per cycle is 1.2g. The available air energy is 251. The effective power is 1.75kW. The average effective torque is 167Nm. The simulated compressed aerodynamic mode is compressed. The air inlet pressure is 3MPa, the speed is 1000r/mn, the intake angle is 50°A, and the intake air mass per cycle is 087g. The available air energy is 249 effective power and twice the average effective torque, respectively, 2Nm. Under the condition that the compressed air intake energy can be equivalent, the double value of the compressed aerodynamic mode effective power and the average effective torque are slightly higher than the 2-stroke pneumatic engine test results, and the simulation results are sufficiently accurate. 33 Internal combustion engine mode engine numerical simulation and test 331 internal combustion engine mode performance numerical simulation can be obtained engine speed is 1 500rmn injection advance angle is 10 °CA combustion continuous angle is 70 °CA excess air coefficient is 1.1, cylinder temperature, pressure And the output torque varies with the crank angle. The maximum temperature in the internal combustion engine mode cylinder is much higher than the compressed air power mode. The cylinder pressure in the 2350K internal combustion engine mode is much higher than the compressed air power mode. The highest pressure reaches 52 MPa. The higher cylinder pressure also makes the maximum output torque as high as 210N°. m crank angle 0 / ° CA internal combustion engine mode in-cylinder pressure and temperature change aerodynamic mode, its main performance indicators compared with non-pressurized 4-stroke internal combustion engine, there is no significant reduction; because it does not consume fuel and emissions of pure air, No harmful emissions, so it can also achieve the purpose of reducing fuel consumption and emissions. (2) The main performance indexes of the internal combustion engine mode are similar to those of the non-supercharged 4-stroke internal combustion engine, and the internal combustion engine mode is usually operated at a higher rotational speed or load, thus avoiding the low economy or low load, the economy is poor, and the harmful emissions are high. Shortcomings. The performance indicators of the two working modes can meet the needs of small car engines, and the increasingly mature engine electronic control technology also provides technical support for the switching of the two working modes, which indicates that the compressed air/fuel hybrid engine is feasible.
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