The working fluid entering the cylinder is pure air. Due to the low resistance of the intake system of the diesel engine, the intake end pressure pa = (0.85 ~ 0.95) p0, which is higher than that of the gasoline engine. The intake end temperature Ta=300～340K, which is lower than that of gasoline engine.
Since the working medium for compression is pure air, the compression ratio of the diesel engine is higher than that of the gasoline engine (generally ε=16-22). The pressure at the end of compression is 3 000 to 5 000 kPa, and the temperature at the end of compression is 750 to 1 000 K, which greatly exceeds the auto-ignition temperature of diesel (about 520 K).
When the compression stroke is nearing the end, under the action of a high-pressure oil pump, diesel is injected into the cylinder combustion chamber through a fuel injector at a high pressure of about 10 MPa, and it mixes with air in a short time and immediately ignites and burns. The pressure of the gas in the cylinder rises rapidly, up to 5 000 ~ 9 000 kPa, the highest temperature reaches 1 800 ~ 2 000K. Because the diesel engine burns on fire by compression, the diesel engine is called a compression ignition engine.
The exhaust of a diesel engine is basically the same as that of a gasoline engine, except that the exhaust temperature is lower than that of a gasoline engine. Generally Tr=700～900K. For a single-cylinder engine, the speed is uneven, the engine is not stable, and the vibration is large. This is because only one of the four strokes does work, and the other three strokes consume energy to prepare for work. In order to solve this problem, the flywheel must have a sufficiently large moment of inertia, which in turn will lead to an increase in the mass and size of the entire engine. The use of multi-cylinder engines can make up for the above shortcomings. Most modern vehicles use four-cylinder, six-cylinder and eight-cylinder engines.