Quote:
Originally Posted by toast
If a machine can generate 100 lbs of force it does so regardless of how fast it is applied. You have a tougher time pushing a faster moving wheel because of timing issues and the fact that your are not a machine; if you were mechanically constrained to the rotating system and your 100 pounds of force did not require a ramp-up time like muscles do your force would always be applied at the correct time and you would generate the same amount of rotational acceleration.
|
The combustion process has time dependence that is somewhat comparable to a muscle ramping up in the merry-go-round example. As the combustion gas expansion drives the piston down the cylinder, the cylinder volume is also increasing via piston travel. As the rpms are decreased, the gas expansion rate might want to stay about the same, but it's contending with a slower cylinder volume expansion rate, so the result is a harder push against the piston over less degrees of crank rotation.
From that it follows that the resultant torque can be the same if combustion expansion is working over a more efficient range of leverage on the crank. For example, 1000 lb average force acting on an average crank throw of 2" would generate 2000 lb-in of torque. But if the combustion process is completed in a shorter amount of crank rotation (aka lower rpm's), you might have 1200 lb of force acting on an average throw of 1.67". Same torque in both situations, but more force at low rpm.