Comments on: Kansas Wrap Up: What Caused all the Engine Failures?

By: diandra

diandra — Fri, 27 Apr 2012 22:46:23 +0000

The energy needed to rotate something with rotational speed omega is 1/2 * I (moment of inertia) times omega squared. If you think about the energy flow from the engine to the rear tires, losing energy means losing rotational speed because you can’t lose moment of inertia. You’re right that the gears tell you how they should rotate – but that’s not exact because the gear equations assume massless parts. DLP

By: Tom

Tom — Fri, 27 Apr 2012 22:01:56 +0000

I have to agree with jack that each revolution of the engine will complete a given revolution of the tire. friction can’t change that, except for the friction created because there is a difference of tire size between the left and right side tires. It still doesn’t alter the number of times the tire is rotated per engine RPM.

By: Jack

Jack — Thu, 26 Apr 2012 23:13:24 +0000

I will have to respectfully disagree with your statement about the engine having to run faster. Unless there is a fluid coupling in the drivetrain somewhere, the ratio between the engine and the wheels remains the same. Friction in the drivetrain, to me, means that the engine has to work harder to produce the same rotational speed at the wheels. It seems to me that if the engine was turning more rpms than were required to run(using your example) 180 mph, then the vehicle would be traveling more than 180. I guess I just don’t understand what you are trying to say. I will admit that I am old and don’t learn things as fast as I used to.

By: Diandra

Diandra — Thu, 26 Apr 2012 17:30:50 +0000

Jack: You are right that the gear ratios don’t vary; however, there is energy loss along the route. The rotational speed is proportional to the energy. If you lose energy, you can’t spin as fast — even though you are 100% right that the gear ratios do not change. Most of the energy is lost to friction – between the gear teeth, in bearings, etc. In order to run at, say 180 mph, the engine has to run even faster than the gear calculations would suggest because not all the energy produced at the engine makes it all the way back to the wheels.
Thank you for the compliment: I’m not sure my students find my classes ‘a hoot’ – especially during the final week of classes for the semester!

By: Jack

Jack — Thu, 26 Apr 2012 17:15:51 +0000

Diandra, I don’t understand how friction in gear trains would affect the ratio between engine rpm and tire rpm. Friction wouild make a difference in horsepower losses, but the gear ratios are fixed ratios and I don’t think friction would change that. Please correct me if I’m wrong. I wish I could attend some of your classes. I’ll bet they woul;d be a real hoot, and I’ll bet even I could learn something from them.