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Difficulty: MediumEngine Principles and Internal Components

In a four-stroke automotive engine, the camshaft is synchronized with the crankshaft via a timing chain or belt to actuate the intake and exhaust valves. If the engine's crankshaft is rotating at a speed of 2400 RPM2{}400\text{ RPM}, at what rotational speed does the camshaft turn?

  1. A
    4800 RPM4{}800\text{ RPM}
  2. 1200 RPM1{}200\text{ RPM}Answer
  3. C
    2400 RPM2{}400\text{ RPM}
  4. D
    600 RPM600\text{ RPM}

Answer

1200 RPM1{}200\text{ RPM}
In a four-stroke internal combustion engine, a full cycle (intake, compression, power, and exhaust) spans two complete revolutions (720∘720^\circ) of the crankshaft. Because the intake and exhaust valves must open only once per cycle, the camshaft completes one revolution for every two revolutions of the crankshaft (a 1:21:2 drive ratio). Therefore, when the crankshaft rotates at 2400 RPM2{}400\text{ RPM}, the camshaft rotates at half that speed, which is 1200 RPM1{}200\text{ RPM}.

Step-by-Step Solution

1
Determine the operating cycle relationship between the crankshaft and camshaft in a four-stroke engine.
A complete four-stroke cycle requires two full rotations (720∘720^\circ) of the crankshaft, during which each valve must open exactly once.
Because each valve opens once per complete cycle, the camshaft must complete one full rotation (360∘360^\circ) for every two rotations of the crankshaft (a 1:21:2 speed ratio).
2
Calculate the camshaft speed using the given crankshaft speed.
Camshaft RPM=Crankshaft RPM2=2400 RPM2=1200 RPM\text{Camshaft RPM} = \frac{\text{Crankshaft RPM}}{2} = \frac{2{}400\text{ RPM}}{2} = 1{}200\text{ RPM}
Dividing the crankshaft rotational speed by 22 yields the synchronized camshaft speed.

Key Concept

Four-stroke engine valve timing ratio between the camshaft and crankshaft
Estimated Time:1m 0s
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Topics

Engine Principles and Internal Components

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