Why do rev limits go beyond peak power

[cps13]

kambites said:

Fatman2 said:

Maybe this only applies to NA engines but I thought power = revs x torque / 5252

Yes it does (although that 5252 is only down to the weird units we use), what has that got to do with the proportionality of torque and revs?

The level or torque you get changes through the gears. Its the purpose of a gearbox.

If you have a drive gear with 10 teeth driving a gear with 20 teeth you will get a different level of torque if the same drive gear is driving a gear with 30 teeth. It's similar to levers, a gear with more teeth acts like a lever with a longer handle. Which is why the yanks measure it in lb/ft. How many lb of torque you get per ft of bar. The lb/ft of torque you get is also proportional to the amount of movement required by the bar, with a large gear you will get more torque but it will have to turn further to achieve the same distance moved as a small gear.

Therefore you have larger gears to get moving as you get more lb/ft of torque but they need to work harder to achieve the same distance as a smaller gear.

Which is why if you have a gear ratio allowing a greater top speed you will lose acceleration - because the torque is less. and vice versa. When you are already moving you need les torque to keep going.

[kambites]

cps13 said:

Which is why if you have a gear ratio allowing a greater top speed you will lose acceleration - because the torque is less. and vice versa. When you are already moving you need les torque to keep going.

Yes, the torque is less for a given engine torque output. It is not less for a given engine power output. The acceleration is only slower because the engine is generating less power at lower revs than at higher revs.

If you had an engine with a flat power curve (although I'm not quite sure how you'd make one), it wouldn't ever matter what gear it was in.

Edited by kambites on Friday 31st December 13:59

[Talksteer]

otolith said:

What you need to illustrate how it works is a cascade chart to show how much torque you have at the wheels against road speed for each gear. Then you can see why you might want to hold onto a lower gear beyond peak power.

Like this one:

There are some pretty big gaps between the ratios on that plot, it does also show the potential benefits of a 7 speed close ratio gearbox..

[cps13]

kambites said:

Yes, the torque is less for a given engine torque output. It is not less for a given engine power output.

But are these two not directly related? You will get greater torque (generally speaking before the curve drops) at a higher engine speed. It is impossible to have an engine that will generate less power and torque at 3000 rpm than 4000 rpm if the peak output was 6000rpm. You will always get the same amount of power from an engine if it had no gearbox.

The purpose of a gearbox is therefore surely to deliver the correct amount of power/torque for the given situation. Be it a tractor requiring high torque or a ford fiesta requiring less torque. Which is why on the diagram you get different levels of torque in each gear.

[cps13]

kambites said:

No. Power is an absolute figure throughout the drive train. Torque is not, it's multiplied up and down by gearing.

So are you saying that an engine running at 2000 rpm would have the same amount of power as it would at 4000 rpm?

Or is power at 4000rpm in 1st is the same as 4000rpm in 4th?

[kambites]

Wheel torque on that graph varies depending on gear because the car is travelling at a different speed (so the wheel speed is different). The comments above were in relation to a car travelling at the same speed in two different gears with the engine generating the same power in each case.

[kambites]

cps13 said:

kambites said:

No. Power is an absolute figure throughout the drive train. Torque is not, it's multiplied up and down by gearing.

So are you saying that an engine running at 2000 rpm would have the same amount of power as it would at 4000 rpm?

Or is power at 4000rpm in 1st is the same as 4000rpm in 4th?

The latter.

What I was actually saying is that wheel power is the same at 4000rpm in first as 4000rpm in 4th. Wheel torque is not the same in the two cases.

Edited by kambites on Friday 31st December 14:20

[busta]

kambites said:

busta said:

Really? Explain how. I thought the effect of a gearbox was to increase/decrease the torque at the wheels depending on the gear. Hence why you can wheel spin easily in 1st gear and not at all in 5th. Surely with the same torque at the wheels in any gear it'd be just as easy at 70mph in 5th?

The effect of the gearbox is to change the ratio of engine revolutions to wheel revolutions. That changes the ratio of engine torque to wheel torque, but not the ratio of engine power to wheel torque. That's why flywheel torque figures are utterly meaningless.

How do you explain that with reference to the graph otolith posted up? Engine power is clearly the same in all gears, and yet wheel torque is dramatically less in higher gears than lower gears. Therefore the ratio of wheel torque to engine power does change with the gears, surely?

[kambites]

busta said:

How do you explain that with reference to the graph otolith posted up? Engine power is clearly the same in all gears, and yet wheel torque is dramatically less in higher gears than lower gears. Therefore the ratio of wheel torque to engine power does change with the gears, surely?

The wheel speed is different, we were discussing the case where the car was travelling at the same speed in different gears and hence had the same wheel rotation speed.

[carl_w]

otolith said:

What you need to illustrate how it works is a cascade chart to show how much torque you have at the wheels against road speed for each gear. Then you can see why you might want to hold onto a lower gear beyond peak power.

Like this one:

Interesting, I would have thought there would be more overlap than that. What that graph tells me is that for max acceleration you should always change gear at the limiter (well, just before of course).

[kambites]

carl_w said:

Interesting, I would have thought there would be more overlap than that. What that graph tells me is that for max acceleration you should always change gear at the limiter (well, just before of course).

I would have thought that would be true of every car? Otherwise, why not lower the rev limiter?

[alock]

Talksteer said:

otolith said:

What you need to illustrate how it works is a cascade chart to show how much torque you have at the wheels against road speed for each gear. Then you can see why you might want to hold onto a lower gear beyond peak power.

Like this one:

There are some pretty big gaps between the ratios on that plot, it does also show the potential benefits of a 7 speed close ratio gearbox..

What gaps do you think cause a problem? To me the gear ratios look pretty good and put you at about peak torque after each gear change.

[Magic919]

Here's another chart. It shows why it's good for me to get up to the rev limiter in the lower gears. This chart has me changing just below the limiter and shows steps.

[kambites]

alock said:

What gaps do you think cause a problem? To me the gear ratios look pretty good and put you at about peak torque after each gear change.

Looks about perfect to me.

[carl_w]

kambites said:

carl_w said:

Interesting, I would have thought there would be more overlap than that. What that graph tells me is that for max acceleration you should always change gear at the limiter (well, just before of course).

I would have thought that would be true of every car? Otherwise, why not lower the rev limiter?

They tend to be pretty simple affairs, rather than adaptive according to gear. In the example that magic919 gives, for the last gearchange he could change up a bit earlier.

[kambites]

Why? the 4th gear line is still at or above the 5th gear one at the point at which he changes.

[Jim Campbell]

kambites said:

cps13 said:

Which is why if you have a gear ratio allowing a greater top speed you will lose acceleration - because the torque is less. and vice versa. When you are already moving you need les torque to keep going.

Yes, the torque is less for a given engine torque output. It is not less for a given engine power output. The acceleration is only slower because the engine is generating less power at lower revs than at higher revs.

If you had an engine with a flat power curve (although I'm not quite sure how you'd make one), it wouldn't ever matter what gear it was in.

Edited by kambites on Friday 31st December 13:59

The higher the gear the less torque is put to the ground. ie in 6th you will have far less torque transfered to the wheels than in first. The reason we have gear boxes in cars is because piston engines don't make nearly enough torque and spin far to fast. As and example a long wheelbase defender on 30 inch tires on a 30 degree hill needs 32 000NM (23598.82 lb ft) of torque just to overcome the effect of gravity and move up the hill from a stop. The enginge makes about 350NM so some serious torque multiplication is needed. Thats why first gear is so far below 1:1 and the diff ratio is often about 4:1 because in the case off the diff ratio you make the tires spin 4 times slower but you get 4 times the torque.

Another example of how piston engines spin to fast is if you spun a 31inch(not unusual for offroaders) tyre at 3000rpm your speed would be about 180mph.

Ta

[kambites]

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.

[busta]

kambites said:

busta said:

How do you explain that with reference to the graph otolith posted up? Engine power is clearly the same in all gears, and yet wheel torque is dramatically less in higher gears than lower gears. Therefore the ratio of wheel torque to engine power does change with the gears, surely?

The wheel speed is different, we were discussing the case where the car was travelling at the same speed in different gears and hence had the same wheel rotation speed.

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.

[kambites]

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.

Indeed.