Timing Diagram of marine diesel engine is nothing but the angular representation of opening and closing of valves of engine. It is controlled by camshaft in 4-stroke and some 2-stroke engines. Intelligent Engines are camless engines.
Timing diagram is used to understand the operation of valves, piston and camshaft.
For the proper understanding of timing diagram, we have some basic assumption or considerations here:
- TDC (top dead centre) and BDC (bottom dead centre) are taken as reference for crank angles, 180 degree each other.
- Consider clockwise direction.
- Ideally, Inlet valve opens at TDC and closes at BDC (after 180 degree rotation).
- But due to Inertia and Momentum, practically it deviates.
4 Stroke Engine (Medium or High speed, Trunk type)
There are 4 steps or processes in combustion i.e. suction or intake, compression, expansion and exhaust.
Ideally inlet valve should open at TDC and should close at BDC. But,
Injection of fuel begins few degrees before TDC.
Inlet valve opens before TDC and closes after BDC.
Exhaust valve opens before BDC and closes after TDC.
Here, 1st stroke is intake, 2nd is compression, 3rd is power and 4th is exhaust stroke.
2 Stroke Engine (Slow speed, Crosshead type)
There are 2 steps or processes in combustion i.e. (intake+ compression) and (power + exhaust).
Fuel burns near or at TDC.
Fuel injection commences before TDC and closed after TDC.
Exhaust valve opens before Inlet valve. (after TDC)
Exhaust valve closes after Inlet valve.
Period between Inlet open and Inlet close is called Scavenging period.
Position of the scavenge ports is fixed, so opening and closing must be symmetrical about BDC.
Only exhaust valve can be varied symmetrically to alter the motion i.e (astern).