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Difficulty: MediumEngine Principles and Internal Components

In a four-stroke internal combustion engine, completing all four strokes requires the crankshaft to complete two full rotations (720∘720^\circ). How many full rotations does the camshaft make during one complete four-stroke cycle?

  1. A
    Two full rotations, turning at the same speed as the crankshaft
  2. One full rotation, turning at half the speed of the crankshaftAnswer
  3. C
    Four full rotations, turning at twice the speed of the crankshaft
  4. D
    One-half rotation, turning at one-fourth the speed of the crankshaft

Answer

The camshaft completes one full rotation, operating at half the rotational speed of the crankshaft.
In a four-stroke internal combustion engine, one full thermodynamic cycle spans four piston strokes, which equals two complete rotations (720∘720^\circ) of the crankshaft. Because each valve needs to open only once during this entire sequence, the camshaft is geared to rotate at exactly half the speed of the crankshaft, making exactly one full rotation per cycle.

Step-by-Step Solution

1
Determine the crankshaft rotation required for one four-stroke cycle.
Two complete revolutions (720∘720^\circ) of the crankshaft occur per cycle.
Each of the four strokes (intake, compression, power, exhaust) corresponds to 180∘180^\circ of crankshaft rotation (4×180∘=720∘4 \times 180^\circ = 720^\circ).
2
Determine the required frequency of valve operation.
Each cylinder's intake and exhaust valves must open and close exactly once per four-stroke cycle.
A single cam lobe actuates its designated valve once per complete rotation of the camshaft.
3
Calculate the drive gear ratio between the crankshaft and camshaft.
The gear ratio is 2:1, meaning 2 crankshaft turns produce 1 camshaft turn.
To actuate valves once every two crankshaft revolutions, the camshaft must turn at half the engine speed (1 camshaft rotation/2 crankshaft rotations1 \text{ camshaft rotation} / 2 \text{ crankshaft rotations}).

Key Concept

Crankshaft to camshaft 2:1 speed and timing relationship in four-stroke engines
Estimated Time:1m 0s
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Topics

Engine Principles and Internal Components

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