A technician connects a diagnostic tachometer to a running four-stroke automotive engine a… — Armed Services Vocational Aptitude Battery (ASVAB)

A technician connects a diagnostic tachometer to a running four-stroke automotive engine and measures a crankshaft rotational speed of $2{}400\text{ RPM}$ . Under these conditions, at what rotational speed does the engine's camshaft spin?

A
$2{}400\text{ RPM}$ , because the camshaft and crankshaft maintain a direct one-to-one rotational speed ratio
$1{}200\text{ RPM}$ , because the camshaft rotates at one-half the speed of the crankshaftAnswer
C
$4{}800\text{ RPM}$ , because the camshaft must rotate at twice the speed of the crankshaft to operate each valve
D
$600\text{ RPM}$ , because the camshaft completes only one rotation for every four complete crankshaft cycles

Answer

The camshaft rotates at

1{}200\text{ RPM}

because it turns at half the speed of the crankshaft in a four-stroke engine.

In a four-stroke engine, each cylinder requires two complete crankshaft rotations (

720^\circ

) to complete all four strokes. Because each intake and exhaust valve opens only once per four-stroke cycle, the camshaft must turn at exactly half the speed of the crankshaft. Dividing the crankshaft speed of

2{}400\text{ RPM}

2

gives a camshaft speed of

1{}200\text{ RPM}

Step-by-Step Solution

Determine the rotational relationship between the crankshaft and camshaft in a four-stroke cycle.

A complete four-stroke cycle (intake, compression, power, and exhaust) requires two full rotations of the crankshaft (

720^\circ

). During this complete cycle, each valve must open exactly once, meaning the camshaft completes one full rotation for every two crankshaft rotations (a

1:2

speed ratio).

Establishing the mechanical ratio is required to calculate the valvetrain operating speed.

Calculate the camshaft rotational speed using the established

1:2

ratio.

\text{Camshaft RPM} = \frac{\text{Crankshaft RPM}}{2} = \frac{2{}400\text{ RPM}}{2} = 1{}200\text{ RPM}

Dividing the crankshaft RPM by two provides the exact rotational speed of the camshaft.

Key Concept

In a four-stroke engine, a complete operational cycle requires two full crankshaft revolutions (

720^\circ

) for every one camshaft revolution (

360^\circ

), establishing a fixed

1:2

speed ratio.

Estimated Time:45s

Question

Answer

Step-by-Step Solution

Key Concept

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