Subsection 6.1.3 Summary
A diesel engine converts the energy stored in the fuel’s chemical bonds into mechanical energy by burning the fuel. The chemical reaction of burning the fuel liberates heat, which causes the gases to expand, forcing the piston to rotate the crankshaft. Ignition occurs in a diesel by injecting fuel into the air charge which has been heated by compression to a temperature greater than the ignition point of the fuel.
A four-stroke engine requires two rotations of the crankshaft to complete one cycle. The event occur as follows:
- Intake — the piston passes TDC, the intake valve(s) open and the fresh air is admitted into the cylinder, the exhaust valve is still open for a few degrees to allow scavenging to occur.
- Compression — after the piston passes BDC the intake valve closes and the piston travels up to TDC (completion of the first crankshaft rotation).
- Fuel injection — As the piston nears TDC on the compression stroke, the fuel is injected by the injectors and the fuel starts to burn, further heating the gases in the cylinder.
- Power — the piston passes TDC and the expanding gases force the piston down, rotating the crankshaft.
- Exhaust — as the piston passes BDC the exhaust valves open and the exhaust gases start to flow out of the cylinder. This continues as the piston travels up to TDC, pumping the spent gases out of the cylinder. At TDC the second crankshaft rotation is complete.
A two-stroke engine requires one rotation of the crankshaft to complete one cycle. The events occur as follows:
- Intake — the piston is near BDC and exhaust is in progress. The intake valve or ports open and the fresh air is forced in. The exhaust valves or ports are closed and intake continues.
- Compression — after both the exhaust and intake valves or ports are closed, the piston travels up towards TDC. The fresh air is heated by the compression.
- Fuel injection — near TDC the fuel is injected by the injectors and the fuel starts to burn, further heating the gases in the cylinder.
- Power — the piston passes TDC and the expanding gases force the piston down, rotating the crankshaft.
- Exhaust — as the piston approaches BDC the exhaust valves or ports open and the exhaust gases start to flow out of the cylinder.
