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Difficulty: HardEngine Principles and Internal Components

An automobile's four-stroke gasoline engine operates at a steady speed of 2,400 RPM. For any single cylinder in this engine, how many power strokes occur each minute, and at what rotational speed does the camshaft turn?

  1. A
    2,400 power strokes per minute, with the camshaft turning at 2,400 RPM
  2. 1,200 power strokes per minute, with the camshaft turning at 1,200 RPMAnswer
  3. C
    1,200 power strokes per minute, with the camshaft turning at 4,800 RPM
  4. D
    2,400 power strokes per minute, with the camshaft turning at 1,200 RPM

Answer

1,200 power strokes per minute, with the camshaft turning at 1,200 RPM
In a four-stroke cycle, completing all four strokes (intake, compression, power, exhaust) requires two complete rotations (720∘720^\circ) of the crankshaft. Consequently, each cylinder produces one power stroke every two revolutions, yielding 2,400/2=1,2002,400 / 2 = 1,200 power strokes per minute. Because the intake and exhaust valves must each actuate only once per complete four-stroke cycle, the camshaft is geared to rotate at half crankshaft speed (2,400/2=1,200 RPM2,400 / 2 = 1,200\text{ RPM}).

Step-by-Step Solution

1
Determine the relationship between crankshaft revolutions and power strokes in a four-stroke cycle.
A single cylinder completes one full four-stroke cycle (intake, compression, power, exhaust) every 2 crankshaft revolutions (720∘720^\circ), producing exactly 1 power stroke per cycle.
Four distinct piston strokes require two complete rotations of the crankshaft.
2
Calculate the number of power strokes per minute for a single cylinder at 2,400 RPM.
2,400 RPM2 revolutions/cycle=1,200 power strokes per minute\frac{2,400\text{ RPM}}{2\text{ revolutions/cycle}} = 1,200\text{ power strokes per minute}.
The cylinder fires once every two crankshaft revolutions.
3
Determine camshaft speed relative to crankshaft speed.
The camshaft rotates at a 1:2 ratio relative to the crankshaft: 2,400 RPM2=1,200 RPM\frac{2,400\text{ RPM}}{2} = 1,200\text{ RPM}.
Each intake and exhaust valve opens only once per four-stroke cycle, requiring the camshaft to complete one full revolution for every two revolutions of the crankshaft.

Key Concept

Four-stroke engine cycle kinematics and camshaft-to-crankshaft 1:2 timing relationship
Estimated Time:1m 15s
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Topics

Engine Principles and Internal Components

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