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Post  Post subject: The Text was Wrong!  |  Posted: Thu Aug 01, 2013 7:16 am

Joined: Thu Aug 01, 2013 5:50 am
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I sat in Calculus class at DeVry Technical Institute, long ago, hearing the instructor reiterate the statement in our text that "pistons in an internal combustion engine move in simple harmonic motion." (we were studying SHM). My friend, Allan Youngs, recently out of the Army, immediately raised his hand, denouncing that statement.

Taken aback, the instructor wisely promised to cover the question the next day. Upon arriving, he stated that Mr. Youngs was right, the text was wrong. The Army did a good job of explaining engines to him!

The whole thing revolves around the incorrect belief that as a piston moves in a cylinder, connected to a crankshaft by a connecting rod, if the piston location at top of the cylinder is called zero degrees, that when rotation of the crank drags the piston downward, that at 90 degrees of rotation, it has travelled 1/2 of the way down. NOT SO! At 90` it has travelled somewhat LESS than half way.

This physical fact explains why piston pins (which attach the piston to the rod) are bored off center, to bring the motion more closely to true SHM.

Book was wrong, teacher (initially) was wrong, Allan was right, and I will never forget the incident.

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