Ideal Thermal Cycle Analysis of Engine Compressed Air Regenerative Braking

Analysis of ideal thermal cycle of engine compressed air regenerative braking Wang Lei, Li Daofei, Xu Huanxiang, Fan Zhipeng, Yu Xiaoli (Institute of Power Machinery and Vehicle Engineering, Zhejiang University, Hangzhou 310027, China) High-efficiency, low-cost recycling of braking energy. The vehicle is operated in urban road conditions as the application object. Based on the traditional four-stroke engine, three kinds of engine compressed air regenerative braking energy recovery schemes are proposed. By establishing the ideal thermodynamic cycle common to the three schemes, the brake cycle was analyzed with the maximum pressure of the recoverable gas, the mass per unit of recycled gas per cycle, the coefficient of cyclic performance (COP) and the cyclic mean pressure as the evaluation index. . The results show that increasing the compression ratio, reducing the volume and displacement ratio of the exhaust pipe buffer chamber or reducing the opening angle of the exhaust valve can increase the maximum pressure of the recovered gas, and should be reduced as much as possible under the conditions of the mechanical structure. The volume of the small exhaust pipe buffer chamber; during the braking process, reducing the exhaust valve opening advance angle can obtain a higher cycle average indicating pressure and gas recovery quality; as the back pressure of the gas tank increases, the exhaust valve is controlled The opening advance angle is reduced from large to small, and the optimal braking cycle performance can be obtained. Theoretically, the two-stroke braking cycle COP is the same as the four-stroke braking cycle, but the two-stroke braking cycle gas recovery mass flow rate and braking power are 2 times the four-stroke brake.

During the driving process of the urban area, the vehicle frequently consumes and stops the calculation of the cloth, and a large part of the energy in the FTP75 (federaltestprocedure 75) urban condition. According to the energy consumption of a typical car, the energy consumed by braking accounts for the traction energy fund project: the National Key Basic Research and Development Program (973 Program) funded project (2011CB707205); the National Natural Science Foundation of China (50976104) .

The pressure of the impulse is reduced to 55% of the four systems. Therefore, the implementation of braking energy recovery is an important technical way to improve fuel economy.

At present, the commonly used engine braking method is to install an actuator on the exhaust valve, so that the exhaust valve is slightly opened (lifting about 12mm) at the end of the compression stroke of the engine to release the compressed gas in the cylinder and reduce the expansion of the gas in the cylinder. The amount of work done on the piston by the stroke produces engine braking force. However, this kind of engine braking only has the effect of slowing down the vehicle, and there is no energy recovery function.

One of the main advantages of a pneumatic-internal combustion hybrid vehicle is the ability to achieve brake energy recovery. Through proper modification, when the vehicle brakes, the engine works in the brake cycle mode to convert the kinetic energy of the vehicle into compressed air energy storage; when the vehicle is driven, the recovered compressed air can be used for vehicle starting, low speed driving and engine intake. Pressure, at the same time can achieve energy saving and emission reduction, so it has a good application prospect and practical value.

At present, most researchers use the camless full variable valve mechanism to modify the engine to achieve the switching of different working modes of hybrid power. However, the camless fully variable valve mechanism is complicated and costly, and it takes a certain time from the actual application. In addition, Zhao et al. proposed a low-cost brake energy recovery technology with engine brake actuators, which has a large change to the engine intake system and may affect the normal intake of the engine.

Based on the traditional engine exhaust auxiliary brake, the author proposes a method to recover compressed air from the exhaust pipe, discusses the technical scheme of the engine two-stroke brake and four-stroke brake, and carries out the ideal heat for the braking process. Cycle analysis.

1 Compressed air regenerative braking principle The engine compressed air regenerative brake researched by the author is based on the traditional four-stroke engine and is suitable for vehicles operating in urban road conditions, such as buses and taxis. As shown, there are four-stroke cycle brakes and two-stroke cycle brakes according to the number of strokes of the work cycle. Among them, the four-stroke brake mode can be divided into compression stroke recovery and exhaust due to different compression air recovery strokes. Trip recovery. The opening and closing rules of the intake and exhaust valves and the inflation valve for different braking cycles are as shown.

The four-stroke braking scheme based on exhaust stroke recovery does not require modification of the engine cylinder head. It is only necessary to install a butterfly valve and a gas recovery pipeline in the exhaust pipe, and the recovery pipeline is connected to the gas storage tank through the inflation valve and the pipeline. . When the engine is working normally, the butterfly valve in the exhaust pipe is opened and the inflation valve is closed; when the engine brakes, the controller cuts off the fuel supply, and at the same time closes the butterfly valve, and opens the inflation valve at a certain moment of the exhaust stroke to allow the gas in the cylinder to enter the gas storage tank. .

The four-stroke braking scheme based on compression stroke recovery requires the use of an exhaust valve actuator in addition to the installation of the exhaust butterfly valve and the gas recovery line. When the engine is working, the controller cuts off the fuel supply, closes the exhaust valve butterfly valve, and activates the exhaust valve actuator, so that the exhaust valve opens slightly at the end of the engine compression stroke, and remains open, but by the variable mass system The thermodynamic theory is activated to the end of the exhaust stroke, and the inflation valve is opened at a certain moment of the compression stroke to recover the compressed air.

The two-stroke braking scheme has made a big change to the original engine air distribution mechanism. The author proposes a second cam to achieve the braking work. When the engine brakes, the controller cuts off the fuel supply, and the engine mode cam switches to the brake mode cam through the cam switching mechanism, and at the same time cooperates with the butterfly valve and the inflation valve in the exhaust pipe to recover the gas in the exhaust stroke.

The above transformation is only for the braking cycle, and the original working cycle remains unchanged. When switching from the duty cycle to the brake cycle, the controller first cuts off the fuel supply, causes the engine to be in a flameout state, then switches the gas distribution scheme, and finally closes the exhaust butterfly valve. The entire operation undergoes an intermediate transition state of about 2 or 3 cycles. The high temperature exhaust gas in the original working cycle is discharged.

2 Thermal cycle modeling In the analysis of the engine compressed air regenerative braking cycle, in order to understand the gas recovery characteristics and cycle indication characteristics, the actual cycle needs to be simplified, and the typical thermal process is used to represent the brake work process, making it easy to establish. Mathematical model, therefore made the following assumptions: 1 working fluid is ideal gas; 2 cylinder gas and exhaust pipe buffer chamber gas state parameters are evenly distributed; 3 intake and exhaust valve opening and closing instantaneous completion, gas inlet and outlet valves are not Throttle loss; 4 cylinder gas exists in the form of pressure energy and heat energy, regardless of friction loss; 5 cylinder wall pistons are all adiabatic wall surface, while not considering leakage loss. The engine compression ratio is Vd, the exhaust pipe buffer volume is Fexh, the ratio of Fexh to Vd is r, the ratio of intake pressure to tank pressure is p, p, r, m and V are respectively The pressure, temperature, mass and volume, W is the output work, and cv, cp, and k are the ideal gas ratio constant volume heat capacity, specific pressure heat capacity and specific heat ratio, respectively. Under the above assumptions, the three braking schemes mentioned above are different in specific implementation methods, but in the ideal case, the braking process contains the same thermodynamic cycle. For the ideal thermodynamic cycle, V map.

In the middle of the intake process, it can be regarded as the ideal constant pressure adiabatic process. The initial pressure and temperature are the same as the intake state, respectively. The compression process is 5: the gas quality in the 2-3 process cylinder is constant, which is adiabatic. During the compression process, 4 is the process of mixing the gas in the cylinder with the gas in the buffer chamber of the exhaust pipe when the exhaust valve is opened. It is assumed that the gas pressure and temperature in the exhaust buffer chamber before mixing are the same as 1 point, which is obtained by m3U3+mexhUexh=m4U4. If 4 is less than still, then 4-5 is the common compression process between the gas in the cylinder and the gas in the buffer chamber of the exhaust pipe. The gas pressure in the cylinder at 5 o'clock is equal to the pressure of the gas cylinder, and 5-6 is obtained as the inflation process, which is regarded as constant pressure. In the adiabatic process, an equal volume process is obtained, and the remaining gas in the cylinder is discharged to the atmosphere at the moment when the intake valve is opened.

Based on the above formula, the cycle indication function can be expressed as the ratio of the amount vd, and the relationship between it and the exhaust valve opening angle is the crank angle of the furnace when the exhaust valve is opened (0180 before top dead center); t is the crank connecting rod of the engine ratio.

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3 cycle performance calculation bookmark11 engine compressed air regenerative braking ideal thermal cycle similar to the maximum pressure of the recovered gas (exhaust valve open 90 ° before top dead center) Fig.5Maximumpressureof under fixed compression ratio (19) conditions, as shown As the exhaust valve opening advance angle decreases, the maximum pressure of the recovered gas gradually rises; and there is an angle (about 70 in the figure, when the exhaust valve opening advance angle is larger than this angle (70), the opening angle is recovered The maximum pressure of the gas has a weak influence; when less than 70, the opening angle has a gradual strong influence on the maximum pressure of the recovered gas (expressed as the slope of the curve). Thus, for the compressed air regenerative braking of the engine, the compression ratio and the exhaust pipe The volume of the buffer chamber and the advance angle of the exhaust valve jointly affect the maximum pressure of the recovered gas. In the actual design process, the engine displacement, compression ratio and other parameters are fixed, and it is necessary to combine the specific structure and select the exhaust pipe buffer according to the above relationship. The cavity volume to displacement ratio and the exhaust valve opening advance angle. For the sum, the r value is preferably 0.2.

3.2 The mass of gas recovered per cycle During the regenerative braking of the compressed air of the engine, the pressure inside the cylinder is continuously accumulated. If the isothermal change of the gas in the tank is considered, the energy recovered depends on the mass of the recovered gas. The mass of gas recovered per cycle can be determined from the results of Section 2 derivation. Assuming an atmospheric temperature of 300K and an ideal gas constant of 287V (kgK), in order to make the gas recovery quality per cycle more general, divide it by the engine displacement to obtain the gas recovery quality per unit displacement. Shown is the change in the mass of gas recovered per cycle per unit of displacement when the engine compression ratio 19 and the exhaust pipe buffer volume and displacement ratio are 0.2. As the back pressure of the gas tank is increased, the mass of the gas recovered by the cycle is reduced, and reducing the opening angle of the exhaust valve is advantageous for gas recovery. At the same engine speed, since the two-stroke brake is 1/2 of the four-stroke brake per cycle time, the mass flow rate of the two-stroke brake gas recovery is theoretically twice that of the four-stroke brake.

According to the definition of COP according to equation (20), the cyclic COP calculates the cycle performance coefficient under different tank back pressure and exhaust valve opening advance angle. As shown, under the condition that the engine compression ratio is 19, the exhaust pipe buffer volume and the displacement ratio are 0.2, the COP gradually decreases as the back pressure of the gas tank increases. Under low tank back pressure, the large exhaust valve opening advance angle is conducive to energy recovery, while in the higher tank back pressure condition, reducing the exhaust valve opening advance angle is conducive to energy recovery. Therefore, in the process of engine compressed air braking, it is necessary to control the exhaust valve opening advance angle from large to small to obtain the best COP. It can be seen that the engine compression ratio is selected as 19, the exhaust pipe buffer chamber volume and displacement When the ratio is selected to be 0.2, the average indicated pressure of the brake cycle is increased as the back pressure of the cylinder is increased, and is larger as the opening angle of the exhaust valve is decreased. If it is desired to obtain a higher braking power under low tank back pressure conditions, the exhaust valve opening advance angle is controlled to be close to the compression top dead center. Under the same cyclic mean indicating pressure condition, since the two-stroke braking cycle time is 1/2 of the four-stroke braking, the two-stroke engine braking power is twice that of the four-stroke braking.

Parameter value Engine 290F compression ratio 19 single cylinder displacement / cm3477 cylinder diameter / mm90 stroke / mm75 crank connecting rod ratio 0.333 exhaust pipe buffer chamber volume / cm3110 (large), 80 (small) intake air continuous range / (CA) 0 180 Exhaust gas continuous range / (CA) 180360 Inflator valve check valve gas tank volume / L18 4 Two-stroke brake preliminary test For the two-stroke brake, the brake mode cam was designed, and one of the engine was modified, and the single Preliminary test of compressed air brake for cylinder engine. The test sample parameters are shown in Table 1.

Table 1 Test sample parameters 0 is the engine compressed air brake gas mass recovery characteristics of two groups of different sizes of exhaust pipe buffer chamber volume, 720. The crank angle is recorded as one cycle. The engine speed is 1 200r/min, which can be seen from 0. The gas recovery quality per cycle per unit volume decreases with the increase of the back pressure of the gas tank. It is confirmed that the recovery condition of the exhaust pipe buffer cavity volume is smaller than the larger one. Case. It can be seen from 1 that the maximum recovery gas pressures of the large and small exhaust pipe buffer chamber volumes are about 0.6 MPa and 0.7 MPa. 2 respectively, when the exhaust pipe buffer chamber volume is 80 cm 3 and the exhaust valve opening advance angle is 180. The engine compressed air brake is measured and the ideal in-cylinder dynamometer diagram. The measured compression process curve in the figure is consistent with the ideal cycle trend. After fitting, the measured variable index of the compression process line is about 1.39, which is close to the adiabatic index of 1.4, indicating that the actual compression process is close to adiabatic compression.

5 Conclusion The compression ratio, the exhaust pipe buffer cavity volume and displacement ratio and the exhaust valve opening advance angle together affect the maximum pressure of the recovered gas. Increasing the compression ratio, reducing the volume of the exhaust pipe buffer chamber to the displacement ratio, or reducing the exhaust valve opening advance angle can increase the maximum pressure of the recovered gas. In the actual design, the volume of the exhaust pipe buffer chamber should be reduced as much as possible under the conditions allowed by the mechanical structure.

After determining the structural parameters (compression ratio, exhaust pipe buffer volume), the exhaust valve opening advance angle is reduced to obtain a higher cycle average indicating pressure and gas recovery quality.

Under the condition of low gas tank back pressure, the smaller the exhaust valve opening advance angle is, the smaller the cycle performance coefficient COP is. Under the high gas tank back pressure condition, the smaller the exhaust valve opening advance angle is, the larger the cycle performance coefficient COP is. Therefore, as the back pressure of the gas tank rises, the control exhaust valve opening advance angle is reduced from large to small, and the optimum brake cycle performance (COP) can be obtained.

The two-stroke brake cycle performance coefficient (COP) is the same as the four-stroke brake cycle, but the two-stroke brake cycle gas recovery mass flow and brake power are theoretically twice that of the four-stroke brake cycle.

Among the three braking schemes, the two-stroke braking scheme has the highest transformation cost, and the four-stroke braking scheme based on the exhaust stroke recovery has the lowest transformation cost. In practical applications, it is necessary to consider the braking demand and the transformation cost to select a specific braking scheme.

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