Diesel Engines - Mode Of Operation Of Diesel Engines

In Diesel Engines (2- stroke or 4-stroke), combustion takes place in a combustion chamber or a cylinder like other reciprocating engines. It, like spark internal combustion engine, also takes five events processes to operate. However, It employs what is called direct injection methods thus making it different. Let’s see how that happen in diesel engines by considering the mode of operation of both the 2 and 4 stroke diesel engines.

The diesel internal combustion engine are compression ignition engines. They differs from their gasoline powered or gas powered counterparts which depend on spark plugs for the ignition of their fuel-air charge. Diesel engines ignites by using highly compressed hot air to ignite the fuel rather than using a spark plug.Lets take a look at their circle.

 4-Stroke Diesel Engine

• Unlike the gasoline engine, air alone enters the cylinder on the intake stroke.

• Compression, forcing the air into a small volume. This compression heats the air. At the end of this stroke, diesel engine fuel is injected into the combustion chamber.

• Ignition, burning the mixture. It is just the heat of the compressed air that ignites the fuel. That’s why diesels are called compression ignition engines.

• Power, where energy released from combustion generates the force to turn the crankshaft.

• And Exhaust, removing leftover gases.

This brings the system back to where it began, ready for another cycle.In a 4-stroke diesel engine, these fine steps takes place in four different strokes. Hence we have the Intake stroke, Compression stroke, Power stroke and exhaust stroke all as different strokes.

 2-Stroke Diesel Engine

• First is intake, when a blower forces air alone into the cylinder.

• Compression, squeezing the air into a small volume. This is when fuel is sprayed into the chamber.

• Ignition., the heat of the compressed air ignites the mixture of air and fuel. There is no spark plug.

• Power, forcing the piston down.

• Exhaust, removing burned gases.

But what makes it a 2 stroke engine is that it does all that in just 2 strokes.And the whole process is continues again in a cycle. The five key events occur, but in a different way from the case of a 4 stroke engine.

• At the start of air intake, the piston is moving toward bottom dead center at the end of a power impulse. The piston uncovers the inlet ports and the blower delivers fresh air into the cylinder. The exhaust valve opens. The piston reaches bottom dead center and starts its move upwards. Incoming air scavenges exhaust gases out of the cylinder.So both the intake and exhaust takes place in the same stroke. It occurs with the power stroke

• The piston continues up, covers the air inlet ports and cuts off the air supply from the blower. The exhaust valve closes. The piston approaches top dead center, compressing the air and raising its temperature. Just before the piston reaches top dead center, the injector sprays in fuel. The heat of the compressed air ignites the fuel. Pressure from the expanding gases forces the piston down the cylinder in a power impulse.

Before the piston reaches bottom dead center, the exhaust valve opens and burned gases escape. The inlet port is uncovered and it admits fresh air from the blower. The piston reaches bottom dead center.
The cycle is ready to repeat itself. All five events occur in just two strokes. Thus the crank makes only one revolution per cycle.