Question

In four stroke cycle engines

• A. The cam-shaft rotates at one-half the crankshaft speed
• B. The crankshaft rotates at one-half the cam-shaft speed
• C. The crankshaft rotates at same speed as that of cam-shaft
• D. The crankshaft and cam-shaft speeds are independent of each other

Hint:

Remember the speed ratio of 4-stroke engines:
\[ \text{Crankshaft Speed} : \text{Camshaft Speed} = 2 : 1 \] This is a fixed gear or chain-drive ratio.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
A four-stroke engine cycle requires four piston strokes (two full revolutions of the crankshaft, or \(720^\circ\)) to complete one thermodynamic power cycle.

Step 3: Detailed Explanation:
During one complete four-stroke cycle:
1. The intake valve opens and closes once.
2. The exhaust valve opens and closes once.
The opening and closing of these valves are controlled by cams mounted on the camshaft.
Since each valve must operate exactly once per engine cycle (every two crankshaft revolutions), the camshaft must complete exactly one full revolution (\(360^\circ\)) for every two revolutions of the crankshaft.
Thus, the camshaft rotational speed \(N_c\) must be exactly half of the crankshaft rotational speed \(N_r\):
\[ N_c = \frac{1}{2} N_r \]

Step 4: Final Answer:
The correct option is 1, which corresponds to "The cam-shaft rotates at one-half the crankshaft speed".