Subtopic: Selection, operation, maintenance and repair of I.C. engines
Question Statement:
Which of the following statements regarding the valve timing diagram of a high-speed four-stroke tractor diesel engine is/are CORRECT?
Options:
(A) The inlet valve opens before Top Dead Center (TDC) to utilize incoming air inertia and ensure maximum cylinder filling
(B) The exhaust valve opens before Bottom Dead Center (BDC) during the power stroke to allow high-pressure blow-down of exhaust gas
(C) Valve overlap is the brief period around TDC when both the inlet and exhaust valves are simultaneously open
(D) Valve overlap must be strictly zero degrees in diesel engines to avoid intake manifold backfire
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
- **A is correct:** Inlet valve lead (typically $10^\circ\text{--}25^\circ$ before TDC) allows the valve to reach significant opening area when the piston starts descending.
- **B is correct:** Exhaust valve lead ($40^\circ\text{--}50^\circ$ before BDC) reduces backpressure during the exhaust stroke without substantial loss of expansion work.
- **C is correct:** Overlap occurs at the end of the exhaust stroke / beginning of the intake stroke, promoting cylinder scavenging through exhaust momentum.
- **D is incorrect:** Diesel engines benefit from significant valve overlap ($20^\circ\text{--}40^\circ$) because only fresh air is inducted, so there is no risk of backfire.