How much does the analytical compression ratio affect the engine?

In the last article, it was mentioned that the air or mixture in the cylinder should be compressed before ignition, and the compression ratio of diesel engine should be larger. The definition and function of engine compression ratio were also briefly described in the last article. And how much does the compression ratio affect the engine? Today, Bian Xiao made an in-depth analysis of it.

Let's review the concept of compression ratio: the ratio of the maximum volume of the bottom dead center cylinder to the minimum volume of the top dead center cylinder is the compression ratio. We assume that a cylinder has a cylinder diameter of 84mm, a stroke of 90mm, a piston area of about 55.5cm2 (the actual value should be smaller considering the clearance) and a displacement of 499.5ml. If the top combustion chamber volume is known to be about 55.5ml, the compression ratio can be obtained as 10: 1. Similarly, if the compression ratio is 65438+, the math problems in middle school need not be explained.

Next, we analyze the significance of compression ratio through several questions. Why do you want to compress gas? We mentioned in the chapter "Interpretation of Automobile Engines (I) Birth of Internal Combustion Engine" that 1858, lenoir invented a two-stroke gas engine, which ignited the mixture without compression, with low efficiency. Why is it inefficient not to compress? This is because: the increase of pressure can make the density of gas increase and the distance between molecules decrease, so that the distance between fuel molecules and oxygen molecules is closer and the combustion speed is faster; Temperature can make gas molecules move faster, fuel molecules and oxygen molecules interact more easily, and the mixed gas can be ignited more easily. Moreover, the smaller combustion space can complete combustion faster, the combustion process is accelerated, and the performance is also improved. Why do you want to increase the compression ratio? We also established a very simple mathematical model for gasoline engine cylinder. The cylinder diameter is 84mm, the stroke is 90mm, the piston area is about 55.5cm2, and the displacement is 499.5ml:

When the compression ratio is 2: 1, it is assumed that the intake pressure at this time is normal 1 atmospheric pressure, that is, 0. 1Mpa. After the 90mm intake stroke, 499.5ml of mixed gas enters the cylinder, and the pressure after the compression stroke is 0.2Mpa. After the fuel in the cylinder is completely burned, the temperature and pressure rise ratio is set to be 5 times of the initial value under adiabatic conditions, that is. Multiplied by 55.5cm2, the downward pressure on the piston at this time is 4995N, divided by 9.8, which is 5 10Kg equivalent. (Although this value looks great, it is an instantaneous maximum, which is far from the continuous torque of the whole cycle. Moreover, after the crankshaft is converted, the converted torque is much smaller. Let's look at the compression ratio 10: 1: the intake pressure is 0. 1Mpa, and the in-cylinder pressure becomes 1Mpa after compression. If adiabatic combustion continues, the pressure will increase by 5 times, that is, 5Mpa, 4.9Mpa after deducting 1 atmosphere, and 490N/ unit after conversion. Multiplied by 55.5cm2, it is about 27200N, which is 2775Kg! According to the numerical value, it is roughly inferred that the torque is more than five times higher than before, and the power will be greatly improved. Through the above experiments, it is enough to show that the improvement of compression ratio corresponds to the improvement of engine performance and efficiency. Note: The above algorithm is not completely accurate, and it does not take into account the pumping loss caused by the throttle valve, the scavenging situation of the combustion chamber, the conversion of non-adiabatic pressure, the initial Kelvin temperature of the mixture, and the specific heat capacity of air, fuel vapor and combustion tail gas. Moreover, the degree of fuel combustion is different under different working conditions, so the in-cylinder pressure cannot be analyzed by proportional increase, but this trend exists and can be used to prove the power increase brought by compression ratio. Why can't the compression ratio be raised too high? According to the analysis in the last paragraph, we know that since a higher compression ratio can bring huge power benefits, how about increasing the compression ratio to 20, 30 or even higher? Of course not. Our previous article specifically talked about knocking, and high compression ratio is one of the factors leading to knocking. Although the compression ratio of modern gasoline engine is getting higher and higher, it is improved on the premise of the continuous development of science and technology and the higher and higher gasoline label. After all, this is a process, not overnight. Early engine technology was not advanced, and the cylinder could not bear too much pressure. And even if the engine allows, there is no high-grade gasoline or it is not popular in the market. For example, in the 1980s and 1990s, 2 12, and in 2020, the compression ratio is only 7: 00, which can burn 75 oil or even lower. The imported Cherokee gasoline below 85% doesn't work normally. Moreover, for diesel engines, a larger compression ratio will inevitably produce higher pressure, which is also a test of the robustness of parts. Therefore, in general, the higher the compression ratio of an engine with the same displacement, the better its power and economy. However, there is a limit to everything. In order to pursue high performance, it is geometrically difficult to simply increase the compression ratio and make it work normally. Moreover, the domestic oil products are really not so good at present, which also limits the promotion of some excellent engines in China. At this time, some people will ask why the compression ratio of turbocharged engines is not particularly high.