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Originally Posted by hat_man
So I guess I'm still a bit confused. It sounds like an engine running at peak efficiency(or as near as possible) could possibly use more fuel than one running at less than peak efficiency? I'm believing you all just not quite following you all.
I now have two more questions. If peak efficiency is the engine running at its smoothest, is less than peak efficiency harder on the engine from a wear and tear point of view?
Secondly, if it isn't harder on the engine, how do I find that point or range that is using less fuel?
And finally, (hopefully) if that point or range of "less fuel" is found would it be better to run somewhere between peak efficiency and less fuel for the sake of engine life or strictly run at the less fuel end?
You all have been great "teachers". Sometimes I wish I could be a better student.
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1) Sounds like you have it. Peak efficiency in an engine which is "too big" or geared too short will burn more fuel and accelerate the car. In order to get to peak efficiency without burning more fuel or accelerating, it's often necessary to have taller gears or a smaller engine. Even my Honda Insight's 1L engine with stupidly tall gearing is only running at around 50% load in 5th gear on the highway. Most cars are considerably less. CVT transmissions are nice in that they can lower RPMs down and keep load up at just about any speed.
2) I would think not. Wear is hard to define, but I figure higher RPM and expecting a higher specific power output (e.g. wide open throttle at high RPM) would be what ages an engine faster. Friction goes up exponentially with RPM, so anything you can do to slow the engine down will help it last longer - to a point.
3) It's generally best to run the engine as slowly as possible. The C7 Corvette has a 6.2L V8 an nearly 500HP and is relatively portly at ~3400lbs, but is capable of 35-40mpg at lower highway speeds because at ~50mph the huge engine is turning at only
900rpm. I expect more manufacturers don't give tall gears like this, which save both fuel AND engine wear, because people don't like to change gears in their car and have the perception the car is "slow" if it can't pass easily or climb mountains in top gear.