Subtopic: Selection, operation, maintenance and repair of I.C. engines
Question Statement:
Which of the following dynamic phenomena in an IC engine valve train can lead to catastrophic valve-to-piston collision at high engine rotational speeds?
Options:
(A) Valve float (valve lifter loses contact with the cam profile due to insufficient spring force)
(B) Valve spring surge (standing resonant waves excited in the helical coil spring)
(C) Excessive tappet clearance causing reduced duration of valve opening
(D) Weakened or fatigued valve return springs
Answer Key:
A, B, D
Step-by-Step Solution & Explanation
- **A is correct:** Valve float occurs when the inertia force of the moving valve assembly exceeds the spring return force, preventing full closure.
- **B is correct:** Spring surge creates high-amplitude resonant oscillations, reducing effective seating force and causing bounce.
- **C is incorrect:** Excessive clearance causes noisy tapping and delayed opening, but does not prevent the spring from closing the valve.
- **D is correct:** Fatigued springs have reduced spring stiffness, lowering the float threshold speed.