20 14-4-5
Baidu Team: Life of People and Cars | CFA386
Analysis of engine cooling system
This paper summarizes the importance of cooling system to the normal and efficient operation of automobile engine, analyzes the structural characteristics of cooling system, and puts forward some suggestions for improvement and perfection.
1, summary
With the development of automobile industry, the engine adopts more compact design and larger specific power, and the waste heat density generated by the engine also increases obviously. Some key areas, such as the heat dissipation around the exhaust valve, need to be given priority, and even a small fault of the cooling system may have disastrous consequences in these areas. The cooling capacity of the engine cooling system should generally meet the cooling demand of the engine at full load, because the engine generates the most heat at this time. But at part load, the cooling system will lose power, that is, the coolant flow provided by the water pump exceeds the required flow. We hope that the shorter the cold start time of the engine, the better, because the more pollutants the engine emits at idle speed, the more fuel it consumes. The structure of the cooling system has great influence on the cold start time of the engine. Normal and efficient cooling system directly affects the fuel economy, acceleration, reliability and service life of the engine.
2. Characteristics of modern engine cooling system
The function of traditional cooling system is to protect the engine reliably, but it should also have the function of improving fuel economy and reducing emissions. Therefore, modern cooling system should comprehensively consider the following factors: (1) internal friction loss of engine; Power of cooling system water pump; Combustion boundary conditions, such as combustion chamber temperature, charging density and charging temperature. The advanced cooling system adopts a systematic and modular design method, and considers all the influencing factors as a whole, so that the cooling system can not only ensure the normal operation of the engine, but also improve the efficiency of the engine and reduce emissions.
2. 1 temperature set point
The limit value of engine operating temperature depends on the highest temperature in the area around the exhaust valve. Ideally, the cooling system should be controlled according to the metal temperature rather than the coolant temperature, so as to better protect the engine. Because the cooling temperature set by the cooling system is based on the maximum heat dissipation rate at full load, the engine and cooling system are in an unsatisfactory state at partial load, such as driving in urban areas and at low speed, which will produce higher fuel consumption and emissions. By changing the set point of coolant temperature, the performance of engine and cooling system under partial load can be improved. According to the temperature limit of the area around the exhaust valve, the temperature set point of coolant or metal can be raised or lowered. The point of increasing or decreasing the temperature has its own characteristics, which depends on the expected purpose.
2.2 Reducing the temperature set point and the working temperature of the cooling system can improve the charging efficiency of the engine and reduce the intake temperature. This is beneficial to the combustion process, fuel efficiency and emissions. Lowering the temperature set point can save the running cost of the engine and improve the service life of the components. The research shows that if the cylinder head temperature is reduced to 50℃, the ignition advance angle can be advanced by 3℃A without knocking, the charging efficiency can be improved by 2%, and the engine working characteristics can be improved, which is helpful to optimize the compression ratio and parameter selection and obtain better fuel efficiency and emission performance.
2.3 Split-flow cooling system
Split flow cooling system is another kind of cooling system. In this cooling system, the cylinder head and cylinder block are cooled by their own liquid flow circuits, and the cylinder head and cylinder block have different temperatures. Split-flow cooling system has unique advantages, which can make all parts of the engine work at the optimal temperature set point. The overall efficiency of the cooling system is maximized. Each cooling circuit will work at a different cooling temperature set point or flow rate to form an ideal engine temperature distribution. The ideal engine thermal working state is that the cylinder head temperature is low and the cylinder block temperature is relatively high. Lower cylinder head temperature can improve the charging efficiency and increase the intake air volume. Low temperature and large air intake can promote complete combustion, reduce the production of co, HC and nitrogen oxides, and improve the output power. Higher cylinder block temperature will reduce friction loss, directly improve fuel efficiency, and indirectly reduce the peak pressure and temperature in the cylinder. Split cooling system can make the temperature difference between cylinder head and cylinder block reach 100℃. The cylinder temperature can be as high as 150℃, while the cylinder head temperature can be reduced by 50℃, thus reducing the friction loss of the cylinder block and fuel consumption. Higher cylinder temperature reduces fuel consumption by 4%-6%, and HC decreases by 20%-35% under partial load. When the throttle valve is fully open, the set temperature of the cylinder head and cylinder block can be adjusted to 50℃ and 90℃, which improves the overall fuel consumption, power output and emission.
3 Conclusion
Several advanced cooling systems introduced before may improve the performance of the cooling system, thus improving fuel economy and emission performance. The controllability of the cooling system is the key to improve the cooling system. Controllability means that the key parameters of engine structural protection, such as metal temperature, coolant temperature and engine oil temperature, can be controlled to ensure that the engine works within the safety limit. The cooling system can quickly respond to different working conditions and save fuel and reduce emissions to the greatest extent without affecting the overall performance of the engine.