Compression-ignition engines:
Basic 4-stroke diesel principles
4-stroke diesel engine cycle
Basic 2-stroke diesel principles
2-stroke diesel engine cycle
Three phases of combustion
Basic 4-stroke diesel principles
This is one cylinder of a 4-stroke diesel engine. This model uses what is called direct injection. It is an internal combustion engine, with the 5 events common to all internal combustion engines. Let’s see how they happen in diesel engines.
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
4-stroke diesel engine cycle
A 4-stroke diesel engine has a cycle of 4 strokes. A stroke is the distance from top dead centre to bottom dead centre. The piston travels down for 1 stroke on intake, up for compression, down for power, and back up for exhaust.
In intake, or induction, the inlet valve opens and the piston starts to move down from top dead centre. Air enters the cylinder through the inlet port. When the piston reaches bottom dead centre, the cylinder is full of air. The inlet valve closes.
The piston starts up from bottom dead centre. The exhaust valve is closed so the cylinder is sealed. The piston’s upward motion compresses the air. When the piston reaches top dead centre, the air is compressed to about one-sixteenth of its original volume. This is higher compression than in a similar petrol engine. Compressing the air also heats it.
Both valves stay closed as the piston rises. Just before it reaches top dead centre, an injector sprays fuel into the chamber. It mixes with the very hot compressed air and ignites. Combustion occurs, the temperature rises much higher and the gases expand and force the piston down in a power stroke. The piston reaches bottom dead centre, the exhaust valve opens.
With the exhaust valve open and inlet valve closed, the piston moves up, forcing exhaust gases out of the exhaust port. The piston reaches top dead centre, the exhaust valve closes, the inlet valve opens and the cycle starts again.
Basic 2-stroke diesel principles
The 2-stroke diesel engine is an internal combustion engine with the 5 events common to all such engines.
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. Just as with a 4-stroke diesel engine, 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.
All in just 2 strokes. The whole process is a cycle. Everything is back where it began, ready to run again.
2-stroke diesel engine cycle
Once again, the 5 key events occur, but in a different way.
At the start of air intake, the piston is moving toward bottom dead centre 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 centre and starts its move upwards.
Incoming air scavenges exhaust gases out of the cylinder.
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 centre, compressing the air and raising its temperature.
Just before the piston reaches top dead centre, 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 centre, 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 centre.
The cycle is ready to repeat itself. All 5 events occur in just 2 strokes.
How many revolutions does the crankshaft do in one engine cycle? 1 revolution per cycle.
Three phases of combustion
The graph shows the variations in cylinder pressure plotted on a continuous crank angle base.
Phases of combustion This is the time taken (or angle turned by the crank) between the start of injection to the commencement of the pressure rise. During this important period, the injected fuel particles are being heated by the hot air to the temperature required for the fuel to self-ignite.
Phase 2: Flame spread causes a sharp pressure rise due to the sudden combustion of the fuel that was injected during the first phase. The rate of pressure rise governs the extent of the combustion knock. This is commonly called 'diesel knock' and is considered to be the main disadvantage of the CI engine.
Phase 3: Direct burning of the fuel as it enters the chamber gives a more gradual pressure rise. When the engine is operating at less than full load, this phase does not exist.
