Numerical simulation of compressed air engine system

Fund Project: Tsinghua University's national focus on vehicle safety and energy conservation. Where Vo represents the clearance volume; 1 represents the intake volume; V2 represents the cylinder volume.

After the intake valve is opened, the cylinder pressure is rapidly increased from the residual pressure of the previous cycle to the intake pressure (12). During the intake valve keep-on phase, the equal-pressure charge (23) is performed due to the replenishment of the high-pressure gas in the gas storage tank, and the intake valve is closed when the piston descends to a suitable position 3. The high pressure gas in the cylinder then begins to expand (34). This phase is generally a multi-variable process. Thereafter, the exhaust valve opens and the cylinder gas exits the cylinder (4-1).

The compressed air engine is powered by high-pressure gas expansion to achieve power output. The overall structure can be borrowed from the existing structural models of conventional engines, mainly in the form of reciprocating pistons and rotary pistons. The high-pressure compressed air is depressurized by a deceleration pressure valve, and is expanded by heat absorption of the heat exchanger, and further expanded after entering the cylinder to push the piston to output power. In order to improve the parameters of the compressed air engine and piping system in a targeted manner and improve the efficiency and energy utilization of the engine, it is necessary to carry out numerical simulation of the existing compressed air engine and piping system. Compressed air system dynamics, improve system efficiency, shorten work cycle, and save development costs.

2 System Modeling The overall composition of the compressed air engine system is shown.

The modeling objectives are divided into a gas distribution section, a compressed air engine section, and a connecting pipeline section. The gas distribution part provides high pressure gas for the compressed air engine, and is composed of a gas storage tank, an intake pressure reducing valve and a control valve. The compressed air engine section consists of an intake valve, an exhaust valve and a cylinder, which are the core of the system. The connecting pipe section runs through the entire system and connects the various components.

Compressed air is stored in high-pressure gas cylinders with a pressure of up to 15 MPa. Due to the limited pressure bearing capacity of the gas pipelines and valves and various components, the pressure relief valve is used to reduce the outlet pressure from a safety point of view. The general outlet pressure is adjusted to 2. A control valve is provided between the intake pressure reducing valve and the engine to regulate the output power of the compressed air engine, which is equivalent to the throttle of an ordinary engine. The pressure reducing valve, the control valve and the compressed air engine are connected by a steel wire braided hose with a high pressure bearing capacity, and the high pressure gas is expanded in the engine cylinder and discharged into the atmosphere through the exhaust pipe.

There are many popular fluid analysis softwares, such as FLOWMASTER for one-dimensional analysis, Fluent for three-dimensional analysis, and Ansys. FLOWMASTER is a professional analysis software for engineering fluid piping systems16 that specializes in the overall analysis of fluid piping systems. Its database contains a wealth of general engineering fluid components, such as valves, pipes, etc., and can use C or Fortran language to write their own component modules, building a system is convenient and fast. It can calculate the pressure, temperature, mass flow and Mach number of each node in the system, and the analysis results can be output in an intuitive graphic or in Excel file format. Other software, such as Fluent and Ansys, are more focused on locality than FLOWMASTER. Although it is capable of performing detailed 3D fluid calculations, modeling of valves and engines is difficult or computationally expensive and not suitable for system integration. analysis.

Considering that the main object of this paper modeling is the entire engine system, FLOWMASTER is used as the modeling tool software and the system calculation model is constructed.

Since there are no engine-characterized modules in the FLOWMASTER software, the engine intake, exhaust valve and cylinder components must be modeled as corresponding components such as valves, variable pressure sources, control modules and abrupt junctions. The resulting program is further combined with other components into an overall model of the system. The Engine simulation calculation model is shown.

No. Replicator; 0, 24 are valves; 21 is a pipe; 22, 23 are abrupt junctions. There is no combustion process in the compressed air engine, the change in the cylinder is relatively simple, and the FLOWMASTER software itself is only one-dimensional analysis. As a component in the system, it is meaningless to carry out multi-dimensional flow analysis, so the engine model adopts the engine quasi-dimensional model. Modeling.

Variable pressure sources with exactly the same pressure and temperature, they constitute the engine cylinder working model, and its output value is controlled by two controllers to control their output pressure and temperature. The controller can be programmed in VBScript to control the variable pressure source. The calculation model is to write code in this part. The controller can have up to 5 input variables, which can be transient state parameters of other components, such as mass flow, temperature and pressure. Three variables are used here, namely the intake air temperature, the intake mass flow rate and the exhaust mass flow rate, so that the differential equations are solved according to the engine quasi-dimensional model, and the pressure, temperature and working mass in the cylinder are calculated. Engine quasi-dimensional model differential equations ThcF / [Tv / - where is the heat transfer area; Twi is the average temperature of the heat transfer surface; c instantaneous surface heat transfer coefficient averaged by the heat transfer surface position.

The hc expressions are also different based on different engine models.

And Ts is calculated by FOWMATER analysis and passed into the calculation model through the controller variable input interface. According to the simultaneous equations (1) and (7), numerical calculations can be performed to obtain numerical solutions of pT and m. The calculation process of FLOWMASTER is a special iterative calculation process, with the set analysis time and step size as the process parameters of the iterative calculation. The analysis starts from zero seconds, and a time step is added after each analysis time is calculated, until the set analysis time value is reached.

The heat transferred into or out of the boundary; W is the mechanical work acting on the piston; the ratio of the working medium; 9 is the crank angle.

The calculation formula of mechanical work is where V is the cylinder volume; it is the in-cylinder pressure.

For a certain engine, V is a function of the crank angle, and its expression is the starting point of the program to calculate the start of the gas expansion work, that is, the moment when the intake valves are closed. At this time, the mass flow rate of intake and exhaust is 0, the mass of the working fluid in the cylinder is constant during the whole expansion process, and the mass of the cylinder in other processes changes. This point is used as the starting point of calculation, and the initial value calculation is easier to determine than other processes. , so take it as the starting point for the calculation. The system model calculation process is as shown.

N is the number of calculation steps; TotalN is the total number of calculation steps; s, Th are the intake and exhaust temperatures; qms, qml, respectively, the intake and exhaust mass flow. The calculation is based on the following conditions and assumptions: Since the pressure of the cylinder after decompression through the pressure reducing valve fluctuates little with the engine operating cycle, it can be simplified as a constant pressure source, the output pressure is constant, and the temperature is constant at room temperature, assuming the exhaust pipe The outlet pressure and temperature are the same as atmospheric pressure and room temperature, respectively.

Braided hose parts are replaced by general steel pipes of the same diameter.

For the running operation, the control valve opening is set to a maximum of 1. Including 3 working cycles, the time step is 10-5s, which is about 0.0022 crank angle. A slightly larger time step may cause the calculation to fail to converge.

3 Calculation results and comparative analysis The 10CA intake valve is opened before the top dead center. At this time, the internal and external pressures of the cylinder are relatively small, the cylinder volume is small, and the piston is still rising. Although the valve opening is small, the intake air volume is small, and the pressure is rapidly increased. . As the piston descends, the rate of increase of the cylinder volume is gradually increased. The internal and external pressure ratio of the cylinder is increased, the intake rate is decreased, and the pressure rises slowly, and a platform area appears. At the beginning of 10CA before the intake valve closes, the intake air flow rate decreases due to the decrease of the intake valve opening degree, the cylinder volume increase rate increases continuously, and the pressure begins to decrease. After the top dead center, the 90CA intake valve is closed, the gas continues to expand, and the pressure continues to decrease. After the top dead center, the 155CA exhaust valve opens and begins to exhaust. The mass of the working fluid in the cylinder is reduced, and the pressure drop rate is slightly accelerated. As the pressure is continuously reduced, the pressure drop tends to be gentle when approaching one atmosphere, and the minimum pressure is basically equal to one atmosphere. equal. In the initial stage of intake air volume change is small, it is considered that this process is a constant volume inflation process. The rate of pressure rise is greater than the rate of mass increase. It is known from equation (6) that the temperature rise is advantageous for the expansion process, and the gas temperature of the expansion process is raised as a whole.

As the piston descends, the gas expands and the temperature decreases due to a decrease in the rate of pressure increase. Before the end of the exhaust gas, the gas pressure is close to one atmosphere, and the piston works upwards, and the gas temperature rises slightly.

The test was carried out on a modified R175 single-cylinder diesel engine, Chen Hong, Xu Hong. Research and development of compressed air powered vehicles. Mechanical Xu Hong. Compressed air powered cars. Automotive technology, 2 (1) 2 Xu Hong. Feasibility study of compressed air powered vehicles. Chinese mechanics Yu Xiaoli, Yuan Guangjie, Shen Yuming, and so on. Theoretical analysis of the working cycle of a pneumatic engine. Journal of Mechanical Engineering, 2002 Anda, Tan Jian, Zuo Chengji. Design and preliminary experiments of compressed air engines. Journal of Hefei University of Technology (Natural Science Edition) 2 (1) 5, Zuo Chengji Li Ning. The domestic 4125 diesel engine was converted into a natural gas engine.

Jiang Deming. The principle of advanced internal combustion engines. Xi'an: Xi'an Jiaotong University Press, Shen Hengrong, Liu Ji, Gao Zhengguan, and so on. Research on the performance of internal combustion engines. Beijing: Mechanical Industry Press, 1981.1370 (Editor-in-Chief Lu Jie)

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