Question

In four stroke internal combustion engine, the period during which both inlet and exhaust valves remain open, is called:

• A. Overlap period
• B. Blowdown period
• C. Supercharging period
• D. Scavenging period

Hint:

Think of the word "Overlap" literally. If two things are happening at the same time (both valves being open), they are overlapping. This happens at the end of the exhaust stroke to give the fresh air a "head start."

Answer 1

The Correct Option is A

Valve timing is the regulation of the opening and closing of the inlet and exhaust valves. In a theoretical cycle, valves open and close at the exact dead centers, but in real engines, they are timed to take advantage of gas inertia. 1. Defining Valve Overlap:

Valve Overlap occurs at the end of the exhaust stroke and the beginning of the intake stroke (near Top Dead Center). During this brief interval, both the inlet valve has already started to open (Inlet Valve Opening - IVO) and the exhaust valve has not yet fully closed (Exhaust Valve Closing - EVC).

2. The Purpose of Overlap: Overlap is used to improve the

volumetric efficiency of the engine:

• Scavenging: The momentum of the escaping exhaust gases helps pull the fresh air-fuel mixture into the cylinder.

• Cooling: The fresh incoming charge can help cool the exhaust valve and the combustion chamber.

3. Impact on Engine Performance:

• Low Speeds: Too much overlap can cause "exhaust gas recirculation" at idle, leading to a rough idle or poor emissions.

• High Speeds: High overlap is beneficial as it allows more time for the gases to move, which is why high-performance racing engines have very large overlap periods.