Technical analysis of Audi TDI engine
When it comes to diesel engines, people’s first reaction is often the heavy trucks on the road and the walking tractors in the fields. The rough torque, huge noise and rolling black smoke have become the consistent impression of Chinese people on diesel engines. As time goes by, engineers have never stopped their research on diesel engines, just like gasoline engines. At present, the world’s leading passenger car diesel engine technology has completely overturned our inherent concept. In this regard, the passenger car diesel engines launched in China include ouyide, Huatai and other brands, and the ones that have done better abroad include Audi TDI, Land Rover TD series and so on.
Why is the “extensive” diesel engine used in luxury models? Why do we seldom see high-end diesel engine models in China? The purpose of this paper is to introduce the current leading diesel engine technology in detail by using Audi TDI series diesel engine, and briefly analyze the obstacles encountered in the promotion of advanced diesel engine in China.
Before technical analysis, it is necessary for the editor to briefly introduce the working principle of diesel engine. At present, most cars use four stroke diesel engines, while two-stroke diesel engines are more used in large trucks and tractors. If you understand the working process of the gasoline engine of your own car, then the four stroke diesel engine is not difficult to understand.
The working principle of four stroke diesel engine is similar to that of gasoline engine. Each working cycle consists of four strokes: suction, compression, work and exhaust. The difference is that due to the low ignition point of diesel (220 ℃), the fuel mixture in the cylinder can be ignited by compression ignition without ignition device. Therefore, the compression ratio of diesel engine is generally high, between 16-22. High compression ratio brings more noise. The most intuitive feeling is that the tractor (two-stroke) is “bumpy”, and even the horn is saved. On the other hand, the compression torque of diesel engine is obviously higher than that of diesel engine. However, after the speed increases (generally 3000rpm), the torque of the diesel engine decreases significantly, and the power also reaches the peak value, which can not be improved any more.
The content of carbon atoms in diesel is high and the composition is complex. Combined with the process of compression ignition, the fuel utilization of traditional diesel engine is not high. Due to incomplete combustion, the content of nitrogen oxides, sulfur-containing compounds and carbon containing particles in emissions is also very high, “black smoke and strong taste” is precisely this reason.
To sum up, traditional diesel engines have the following characteristics:
Compression ignition mode → high compression ratio → low to high torque, high noise and vibration
High carbon content of diesel → high energy density → low fuel consumption
Complex composition + compression ignition mode → low fuel utilization and emission problems
In this way, it seems that the diesel engine is only suitable for simple and rough large trucks. But why does Audi insist on diesel engine for 25 years and expand the displacement to 1.4l-6.0l? Because engineers firmly believe that diesel engines can become more refined and cleaner with the advantage of low fuel consumption and high torque, and they have done it. Let’s take a look at what technologies are used in advanced diesel engines, from “silly big black thick” to “small jasper”. It should be noted that many of the technologies described below are not developed by Audi, but are equipped with their own TDI series engines after adjustment and improvement.