Importance of Engine's Valve Timing Diagram
The valve timing of an engine is set to give the best possible performance. This means that the valves must be opened and closed at very precise times.
The valve timing diagram is referred to when designing a 2-stroke or 4-stroke engine to ensure that the movement of the piston from the top dead center (TDC) to the bottom dead center (BDC) aligns with the optimal timing for the opening and closing of the intake and exhaust valves. By carefully coordinating these valve actions with the piston's position, the engine can achieve efficient and effective air intake, combustion, and exhaust processes. The valve timing diagram serves as a blueprint to ensure that the valve events occur at the right moments during the piston's stroke, ultimately contributing to the engine's overall performance, power output, and fuel efficiency.
Valve Timing Diagram for 4-Stroke Engine
The valve timing diagram of 4 stroke engine indicates that the valve timing is completed through four distinct strokes: Suction, Compression, Expansion, and Exhaust. By analyzing the valve timing diagram, engineers can ensure that the intake and exhaust valves open and close at optimal moments, allowing for efficient air-fuel mixture intake, compression, combustion, and proper exhaust expulsion. The valve timing diagram provides a valuable tool for understanding the intricate relationship between the valves and piston movement in a 4-stroke engine. The valve timing diagram of 4 stroke engine in the theoretical and the actual processes is given below.
Theoretical Process
Suction Stroke:
The engine cycle commences with the suction stroke, where the piston, located at the top dead center (TDC), moves towards the bottom dead center (BDC), causing the inlet valve to open. This motion allows the air-fuel mixture (for petrol engines) or fresh air (for diesel engines) to enter the cylinder until the piston reaches BDC.
Compression Stroke:
Next is the compression stroke, where the piston moves from BDC to TDC, compressing the air-fuel mixture (petrol engine) or fresh air (diesel engine). This compression raises the pressure inside the cylinder, preparing it for efficient fuel combustion. During this process, the inlet valve closes to seal the chamber for effective compression.
Expansion (Power) Stroke:
The expansion stroke follows, ignited by the combustion of the fuel. The combustion pushes the piston, located at TDC, towards BDC, releasing the pressure generated by the combustion and generating output power.
It's important to note that in petrol engines, combustion occurs due to the spark produced by the spark plug, while in diesel engines, combustion happens due to the high compression provided during the compression stroke, raising the temperature inside the cylinder to the auto-ignition temperature of the diesel and air charge. In petrol engines, the air-fuel mixture enters the cylinder during the suction stroke, whereas in diesel engines, fresh air enters during this stroke, with fuel being sprayed by fuel injectors into the air.
Exhaust Stroke:
Finally, the exhaust stroke takes place. After the expansion stroke, the piston, located at BDC, moves towards TDC, and the exhaust valve opens to remove the residual combustion gases from the cylinder. The exhaust valve closes once the piston reaches TDC, completing the four-stroke engine cycle.
Actual Process
Valve Timing Diagram for a Four Stroke Cycle Petrol Engine - The petrol engines are also known as spark ignition engines. The valve timing diagram for a four stroke cycle petrol engine is shown in Figure below:
- During the suction stroke of a 4-stroke engine, the inlet valve opens approximately 10-20 degrees in advance of the top dead center (TDC). This timing allows for proper intake of air-fuel mixture (petrol) or air (diesel), while also facilitating the cleansing of any remaining combustion residuals within the combustion chamber.
- The compression stroke begins as the piston reaches the bottom dead center (BDC). The piston then starts moving towards TDC, and during this compression stroke, the inlet valve closes approximately 25-30 degrees past BDC. This closure ensures complete sealing of the combustion chamber, enabling effective compression of the air-fuel mixture (petrol engine) or air (diesel engine).
- Within the compression stroke, as the piston progresses towards TDC, fuel combustion occurs roughly 20-35 degrees before reaching TDC. This timing ensures proper fuel combustion and the optimal propagation of the flame.
- Following combustion, the expansion stroke commences. This stroke is initiated by the release of pressure inside the combustion chamber due to the fuel combustion, resulting in the rotation of the crankshaft. The piston moves from TDC to BDC during the expansion stroke, spanning approximately 30-50 degrees before BDC.
- To begin the exhaust stroke, the exhaust valve opens approximately 30-50 degrees before BDC. This enables the expulsion of combustion residuals as the piston moves from BDC to TDC. The exhaust stroke continues until roughly 10-20 degrees after the piston reaches TDC.
