Does the piston travel farther in the first 90 degrees or the second 90 degrees?

• A. First 90 degrees
• B. Second 90 degrees
• C. Both 90 degrees
• D. Neither 90 degrees

Answer: (A)

In a slider-crank arrangement, the piston’s movement is the projection along the cylinder axis of the crank pin as the crank rotates. Because the connecting rod has finite length, the motion isn’t a perfect sine wave, so the distance the piston covers in each quarter of crank rotation isn’t exactly the same.

From the top-dead-center position to 90°, the geometry produces a larger axial advance per degree than from 90° to 180°. As the crank continues past 90°, the connecting rod becomes more inclined, which reduces the rate of axial displacement and makes that second 90-degree segment cover slightly less total travel than the first. If the connecting rod were infinitely long, the motion would be nearly perfectly symmetric and the two 90-degree segments would be about equal.

So the piston travels farther in the first 90 degrees.