Difference Between Gasoline Engine and Diesel Engine (2)

The molecular weight of gasoline is small, so the boiling point is only 20-220 °C, but the flash point is above 400 °C. The molecular weight of diesel is large, the carbon chain is longer, and the boiling point is higher, but the flash point is only 220 °C. This difference in fuel properties also results in a big difference between gasoline and diesel engines.

Due to the high ignition point of gasoline, after the piston of the engine is compressed, it must be ignited by a spark plug, while a diesel engine does not need a spark plug. The temperature rise after gas compression can directly cause diesel to spontaneously ignite. This compression ignition effect It also allows diesel engines to have many advantages, such as no ignition system required, lower failure rate; compression ignition method allows the compression ratio to be very high, the compression ratio of ordinary gasoline engines is 13:1, which is already very high, but diesel engines can easily do it. To 17:1, through the longer piston connecting rod, the movement stroke of the piston is increased, the torque of the engine is greatly improved, and the fuel economy of the diesel engine is better under the same displacement.

Gasoline engines can prefabricate a mixture, which is to mix gasoline and air first, and then inject it into the cylinder, or direct injection in the cylinder, use high-pressure fuel injectors to atomize the gasoline, mix and compress in the cylinder, and then compress it in the cylinde liner). When the engine piston is ignited by the spark plug at the top dead center, in a word, the gasoline and air are mixed first and then burned, but the diesel oil has high viscosity and cannot be effectively mixed with the air, so the diesel engine will first inhale the air, and then compress the air to heat up. The diesel is injected only when the piston is close to the top dead center, and the diesel is ignited by using high-temperature air. The whole combustion process is also divided into pre-combustion period, fast-burning period, slow-burning period and post-combustion period. This method has an obvious The disadvantage is that it requires a longer combustion time. The above-mentioned increase in the piston stroke is also the purpose. Therefore, the speed of the diesel engine cannot be made too high. Too fast speed will not only make the diesel unable to burn completely and emit black smoke madly, but also the phenomenon of torque attenuation. For example, the largest engine in the world is installed on a ship. The diesel engine has a rated speed of only 102r/min. The revs of the diesel versions of many cars are also far lower than the gasoline versions.

If diesel is added to the gasoline engine by mistake, before the spark plug is ignited, the diesel has already achieved compression ignition, and the engine will have obvious knocking. Not enough to ignite the oil-air mixture.

Diesel engines are fuel-efficient, durable, and have powerful torque, so they are very popular in Europe. The proportion of diesel cars in Europe exceeds 50%, and 80% of taxis are diesel cars. However, it is difficult to popularize diesel cars in our country. The main reason is nothing more than that The quality of diesel does not match, the vibration and noise are relatively loud, and many people have some resistance from the bottom of their hearts. Even though diesel engine technology is becoming more and more advanced, it is very close to gasoline engine, but it is still difficult to achieve large-scale application in China.

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