Why do rev limits go beyond peak power
Discussion
busta said:
Yes I see what you mean now. So if you are doing 40mph and the engine is making 100hp, the torque at the wheels is the same regardless of the gear or engine rpm.
Yes, though it's unlikely that you'd need 100bhp to maintain 40mph unless towing a small bungalow up a steep hill.kambites said:
Torque at the wheels is directly proportional to power at the flywheel (assuming constant losses in the gearbox), regardless of gear ratio.
Torque at the wheels is proportional to torque at the flywheel. Since flywheel power continues to rise for quite some time after the flywheel torque has dropped off, you can't say the two are proportional.Mr2Mike said:
Torque at the wheels is proportional to torque at the flywheel. Since flywheel power continues to rise for quite some time after the flywheel torque has dropped off, you can't say the two are proportional.
Not if you change gear it isn't, which is what this whole thread is about, really. The OP was essentially asking why, at a given speed, you'd rather be in one gear than another. Under those constraints, wheel torque is proportional to power NOT engine torque. Edited by kambites on Friday 31st December 15:27
kambites said:
Mr2Mike said:
The OP was essentially asking why, at a given speed, you'd rather be in one gear than another. Under those constraints, wheel torque is proportional to power NOT engine torque.
Right, I see what you are getting at. Ignoring losses the flywheel power is always proportional to wheel power, but if the road speed is fixed the wheel rotates at a constant rate so wheel torque becomes proportional to engine power.
Mr2Mike said:
busta said:
Yes I see what you mean now. So if you are doing 40mph and the engine is making 100hp, the torque at the wheels is the same regardless of the gear or engine rpm.
Yes, though it's unlikely that you'd need 100bhp to maintain 40mph unless towing a small bungalow up a steep hill.kambites said:
You only have less torque transferred to the ground in 6th than in 1st because the engine generates less power in 6th than in 1st and the speed at which it would be rotating in those gears at that speed.
If 30mph was 7000rpm in 1st and 1000rpm in 6th, and your engine developed 100bhp and both 7000rpm and 1000rpm (however unlikely that may be), the wheel torque would be the same.
Incorrect, the purpose of gearing in a piston engined car is to multiply torque. Why do you say the engine makes less power in 6th? it doesn't. Gearing multiples torque and leaves power alone. In the previous case of a dif gear of 4:1 for every 4 revolutions of the driveshaft the axle will only spin once, you get 4 times the torque but a quarter of the speed. You see how on the power equation Hp = torque x rpm/5252 gearing has no bearing on power as you change both vareables by the same amount so the end result is constant.If 30mph was 7000rpm in 1st and 1000rpm in 6th, and your engine developed 100bhp and both 7000rpm and 1000rpm (however unlikely that may be), the wheel torque would be the same.
Remember cars make very little torque. As i said before a long wheelbase defender on a 30 degree incline need 32000 NM just to overcome gravity and set off up the hill and the engine make about 350 NM so its all torque multiplication via gears that gets it moving.
To show how little torque car engines make and example. Say i weight 100kg and lean with all my mass on a 0.5 meter breaker bar, that will be 500nm of torque being applied to stubborn nut or bolt. If i increase the bar length to 1m thats a 1000NM of torque. Thats more torque than all most any production car except the AMG V12's and the Veryon. The reason i can't make anywhere near the power they can is me leaning on a breaker bar is very slow and those engines can do it at 3000 to 6000 revolutions a minute.
Gearing has no effect on power only torque.
Ta
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