What is the working principle of a diesel engine?

A diesel engine is an engine that burns diesel fuel to release energy. Today, let's take a look at the working principle of a diesel engine.

1. Working Principle of a Two-Stroke Diesel Engine

(1) Intake and compression stroke

The piston moves upward from bottom dead center, and the exhaust turbocharger delivers fresh air at a certain pressure into the cylinder. When the piston reaches a certain position, it closes the scavenging port and the cylinder becomes a sealed chamber; as the piston continues upward, the pressure inside the cylinder gradually increases.

(2) Expansion and exhaust stroke

At a certain moment before the piston reaches top dead center, fuel is injected by the injector into the combustion chamber in a finely atomized state, where it mixes with the air and auto-ignites.

After the piston passes top dead center, the gas pressure and temperature inside the cylinder rise sharply. Driven by the high-temperature, high-pressure gas, the piston moves from top dead center to bottom dead center, transmitting power to the crankshaft through the connecting rod, and the crankshaft outputs power in rotary form.

As the piston moves downward, the cylinder volume gradually increases, and the pressure and temperature of the combustion gas gradually decrease until the exhaust valve opens.

2. Working Principle of a Four-Stroke Diesel Engine

The working process of a diesel engine is actually similar to that of a gasoline engine. Each operating cycle also goes through four strokes: intake, compression, power and exhaust.

(1) Intake stroke

The working medium entering the cylinder is pure air. Because the intake system resistance of a diesel engine is relatively low, the pressure at the end of the intake stroke pa= (0.85~0.95)p0, which is higher than that of a gasoline engine. The temperature at the end of intake Ta=300~340K, which is lower than that of a gasoline engine.

The purpose of the intake process is to fill the cylinder with fresh air. When the intake stroke begins, the piston is at top dead center, and some exhaust gas remains in the combustion chamber inside the cylinder.

As the crankshaft rotates, the connecting rod drives the piston from top dead center to bottom dead center. At the same time, the mechanism linked to the crankshaft opens the intake valve.　

As the piston moves downward, the volume above the piston inside the cylinder gradually increases, causing the air pressure in the cylinder to fall below the pressure in the intake manifold, so outside air continuously flows into the cylinder.

(2) Compression stroke

Because the compressed working medium is pure air, the compression ratio of a diesel engine is higher than that of a gasoline engine (generally ε=16~22). The pressure at the end of compression is 3 000~5 000kPa, and the temperature at the end of compression is 750~1 000K, far exceeding the auto-ignition temperature of diesel fuel (about 520K).

3. Power stroke

As the compression stroke approaches its end, diesel fuel is injected by the injector into the cylinder combustion chamber at a high pressure of about 10MPa by the action of the high-pressure fuel pump, where it mixes with air and immediately auto-ignites within a very short time.

The pressure of the gas inside the cylinder rises rapidly, reaching up to 5 000~9 000kPa, and the maximum temperature reaches 1 800~2 000K. Because diesel engines rely on compression ignition for combustion, they are called compression-ignition engines.

4、 Exhaust stroke

Diesel engine exhaust is basically the same as gasoline engine exhaust, except that the exhaust temperature is lower than that of a gasoline engine. Generally, Tr=700~900K.

For a single-cylinder engine, speed fluctuation is uneven, operation is unstable, and vibration is large. This is because only one of the four strokes is a power stroke, while the other three consume power in preparation for producing work.

To solve this problem, the flywheel must have sufficient rotational inertia, but this also increases the overall engine weight and size. Using multi-cylinder engines can compensate for the above shortcomings. Modern vehicles generally use four-cylinder, six-cylinder and eight-cylinder engines.

Because the working principle of a diesel engine is to compress the combustible mixture itself and rely on combustion to generate power, while also driving the flywheel to store energy for the next power stroke, it produces more exhaust pollution; therefore, more and more vehicles are gradually replacing diesel with gasoline or even new energy sources.

• PREV: Challenges and Opportunities C···
• NEXT: Characteristics of Diesel Engi···