Theoretical valve timing diagram :
Theoretical valve timing diagram is of two types-
- For Four stroke cycle engine
- For Two stroke cycle engine
Theoretical valve timing diagram for four-stroke cycle engine:
- In The theoretical valve timing diagram for a four-stroke cycle engine, a Piston moves from TDC (Top dead center) to BDC (Bottom dead center). the inlet valve opens and closed during movement of crankshaft.
- In this diagram,the piston moves from TDC to BDC, the inlet valve opens at IVO and the suction process takes place from IVO(inlet valve open) to IVC(inlet valve closed).
- The crankshaft revolves through 180º and the piston moves from BDC to TDC. the inlet valve and exhaust valve both are close and the compression takes place from IVC (inlet valve close) to IS (Ignition start).
- The crankshaft revolves through 180º and the piston moves from TDC to BDC At IS, the fuel is fired and the expansion takes place from IS to EVO (Exhaust valve open).
- The crankshaft revolves through 180º and the piston again moves from TDC to BDC. At EVO, the exhaust valve opens and the exhaust takes place from EVO (Exhaust valve open) to EVC (Exhaust valve close) The crankshaft again revolves through 180º and the piston moves back to TDC.

Theoretical valve timing diagram for the Two-stroke cycle engine:
- In The theoretical valve timing diagram for a Two-stroke cycle engine, a Piston moves from TDC (Top dead center) to BDC (Bottom dead center). the inlet valve opens and closed during movement of crankshaft.
- In this diagram, the fuel is fired at A and the expansion of gases takes place from A to B.
- The crankshaft revolves through approximately 120º and the piston moves from TDC to BDC. At B, the valves open, and suction, as well as exhaust, take place from B to C.
- The crankshaft revolves through approximately 120º and the piston moves first to BDC and then little upwards. At C. both the valves close and compression takes place from C to A. The crankshaft revolves through approximately 120º and the piston moves to TDC.

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