If lift and duration remain constant and the lobe center angle decreases, then __.
a. The valve overlap decreases
b. The effective lift increases
c. The effective duration increases
d. The valve overlap increases
The correct answer and explanation is :
The correct answer is d. The valve overlap increases.
Explanation:
The camshaft in an internal combustion engine plays a critical role in controlling the timing and movement of the engine’s valves. It is responsible for regulating when the intake and exhaust valves open and close in relation to the piston’s position in the engine cycle. The lobe center angle (LCA) is the angle between the intake and exhaust cam lobes at the point where the camshaft’s lobes are at their peak lift (typically measured in degrees of crankshaft rotation). The LCA directly affects valve timing and can influence engine performance characteristics, including valve overlap.
Valve overlap refers to the period when both the intake and exhaust valves are open simultaneously. It is crucial because it allows the exhaust gases to be expelled and the intake charge to be drawn in at the same time, which can improve engine efficiency and performance at certain engine speeds.
Here’s how the LCA affects valve overlap:
- Decreasing the lobe center angle essentially brings the intake and exhaust lobes closer together. As a result, the exhaust valve remains open longer into the intake stroke, and the intake valve opens earlier as the exhaust valve closes. This increases the valve overlap, as the intake and exhaust valves are open for a longer period during the engine cycle.
- Increased valve overlap can lead to better performance at higher engine speeds due to the improved scavenging effect (more exhaust gases are expelled, and fresh intake air is drawn in), but it can hurt low-end torque because it may lead to a loss of compression.
In conclusion, when the lobe center angle decreases, the intake and exhaust valves open and close closer together in time, leading to increased valve overlap.