Here’s a sketch of a four-stroke timing diagram showing valve operation:
Four stroke timing diagram
Labeling:
- TDC (Top Dead Center): Highest point of piston travel in the cylinder.
- BDC (Bottom Dead Center): Lowest point of piston travel in the cylinder.
- Intake Valve: Controls the flow of air-fuel mixture into the cylinder.
- Exhaust Valve: Controls the release of exhaust gases from the cylinder.
- Intake Stroke: Piston moves down, drawing in air-fuel mixture.
- Compression Stroke: Piston moves up, compressing the air-fuel mixture.
- Combustion Stroke (Power Stroke): Ignition occurs, burning the compressed mixture and pushing the piston down.
- Exhaust Stroke: Piston moves up, forcing exhaust gases out of the cylinder.
Valve Operation:
- Intake Stroke: Intake valve opens just before TDC (Top Dead Center) of the intake stroke and closes after BDC (Bottom Dead Center) of the intake stroke.
- Compression Stroke: Both valves are closed throughout the compression stroke.
- Combustion Stroke (Power Stroke): Both valves are closed throughout the combustion/power stroke.
- Exhaust Stroke: Exhaust valve opens just before TDC of the exhaust stroke and closes after BDC of the exhaust stroke.
Valve Overlap (b)
Valve Overlap refers to the period during the engine cycle where both the intake and exhaust valves are open slightly at the same time. This typically occurs near the end of the exhaust stroke and the beginning of the intake stroke, or near the end of the intake stroke and the beginning of the compression stroke, as shown in the diagram.
Purpose of Valve Overlap:
- Improved Scavenging: During the overlap at the end of the exhaust stroke and beginning of the intake stroke, some fresh air from the intake valve can help push out any remaining exhaust gases trapped in the cylinder. This improves scavenging and cylinder purging, leading to a more complete intake of fresh air-fuel mixture.
- Reduced Pumping Losses: The slight overlap at the end of the intake stroke and beginning of the compression stroke allows some of the already inducted air-fuel mixture to flow back out, reducing the pumping work required to fill the cylinder completely. This can improve engine efficiency at higher RPMs.
Note: The amount of valve overlap is carefully designed by the engine manufacturer to optimize performance and efficiency based on the engine’s intended application.