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Re: Shifting Efficiently
Well any internal combustion engine has more complicated & interdependent processes going on than a DC electric motor, so simple absolute linear rules of behavior are not there (peak torque at half max rpm etc).
The processes such as combustion, gas thermodynamics, compressible fluid flow of the air in/exhaust gasses out are all interrelated and vary with engine load, rpm, air temperature, resonant frequencies of the intake/exhaust plumbing etc.
However, that being said keep in mind that all ICE are heat engines which function by converting the heat energy of a fuel into expanding a gas that pushes on and moves the pistons/crankshaft.
So work is done - force x distance.
Here are rules of thumb I keep in mind:
1. I recall from somewhere that the maximum combustion efficiency of a motor is at it's peak torque (maximum area under the torque/speed curve).
For most car motors that's around 3000 to 4000 rpm.
2. On the other hand, internal sliding friction of the pistons/bearings is essentially linear for each revolution of the motor. So, all else being equal, if you can get your desired output hp (say to maintain 30 mph) at half the motor rpm, you will eliminate half the engine revolutions and half the sliding friction in the motor to go a given distance. Less wasted energy in the motor = higher efficiency & mpg.
3. The optimum efficiency for a given motor is at lowest rpm that still gives good combustion efficiency. Crossover of the friction loss & combustion efficiency curves. Motor's with good torque at low speed, good combustion effieciency and low thermal losses win.
...Any FIRST engineers out there from the motor city with some more detail? Hey speak out FORD, GM, Daimler guys!
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2004 UTC New England Champions with 716 & 230
Engineering Lead
Team Paragon #571
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