Theoretical and Actual valve timing diagram For Four Theoretical and Actual valve timing diagram For Four Stroke SI Engine Theoretical valve timing diagram: The exact moment at which each of the valves opens and closes with reference to the position of piston and crank can be shown graphically in a diagram. This diagram is known as “valve timing diagram”. In theoretical valve timing diagram, inlet and exhaust valves open and close at both dead Two stroke engine - Energy Education The pressure volume diagram (PV diagram) that models the changes the fuel-air mixture undergoes in pressure and volume in any gasoline engine is called the Otto cycle. The changes in these will create heat, and use this heat to move the vehicle or machine (hence why it's a type of heat engine). The Otto cycle can be seen in Figure 2 (real Otto Cycle) and Figure 3 (ideal Otto Cycle). Stages of combustion in SI engine - ExtruDesign Jun 16, 2019These are the different Stages of combustion in SI engine. Conclusion. We have discussed the pressure vs crank angle diagram [p-θ Diagram] for the actual cycle and the theoretical cycle. And also the different stages of combustion in SI engine, those are ignition lag, flame propagation and the afterburning.