Why do rev limits go beyond peak power

[christofmccracke]

Say peak power is at 7k why does the rev limit stop at 8500rpm
surely it would be more sensible to limit the revs at 7500rpm

[busta]

Because the rate of acceleration beyond peak power in a lower gear can still be higher than acceleration at or just below peak power in the next gear.

eg. Same speed, 3rd gear at 7000rpm vs 4th at 5000pm, peak power at 6000rpm. At 7000rpm you might have 10hp less than peak power but in 3rd the overall gear ratio is, say, 1:0.75. At the same speed in 4th you might be at 5000rpm, still 10hp below peak power but with a higher gear ratio of 1:1, so the torque at the wheels is less.

Edited by busta on Friday 31st December 13:08

[kambites]

It needs to be, if you change up a gear at exactly peak power you will typically drop quite a long way down the power curve. You want the rev limiter at (or beyond) the point where power before and after the gear change is roughly equal.

[h0b0]

christofmccracke said:

Why does the rev limit go beyond peak power? Why not change it so it still goes beyond peak power?

What a strange question

[kambites]

busta said:

Because the rate of acceleration beyond peak power in a lower gear can still be higher than acceleration at or just below peak power in the next gear.

eg. Same speed, 3rd gear at 7000rpm vs 4th at 5000pm, peak power at 6000rpm. At 7000rpm you might have 10hp less than peak power but in 3rd the overall gear ratio is, say, 1:0.75. At the same speed in 4th you might be at 5000rpm, still 10hp below peak power but with a higher gear ratio of 1:1, so the torque at the wheels is less.

Torque at the wheels is directly proportional to power at the flywheel (assuming constant losses in the gearbox), regardless of gear ratio.

[busta]

kambites said:

busta said:

Because the rate of acceleration beyond peak power in a lower gear can still be higher than acceleration at or just below peak power in the next gear.

eg. Same speed, 3rd gear at 7000rpm vs 4th at 5000pm, peak power at 6000rpm. At 7000rpm you might have 10hp less than peak power but in 3rd the overall gear ratio is, say, 1:0.75. At the same speed in 4th you might be at 5000rpm, still 10hp below peak power but with a higher gear ratio of 1:1, so the torque at the wheels is less.

Torque at the wheels is directly proportional to power at the flywheel (assuming constant losses in the gearbox), regardless of gear ratio.

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?

[CraigyMc]

kambites said:

busta said:

Because the rate of acceleration beyond peak power in a lower gear can still be higher than acceleration at or just below peak power in the next gear.

eg. Same speed, 3rd gear at 7000rpm vs 4th at 5000pm, peak power at 6000rpm. At 7000rpm you might have 10hp less than peak power but in 3rd the overall gear ratio is, say, 1:0.75. At the same speed in 4th you might be at 5000rpm, still 10hp below peak power but with a higher gear ratio of 1:1, so the torque at the wheels is less.

Torque at the wheels is directly proportional to power at the flywheel (assuming constant losses in the gearbox), regardless of gear ratio.

If you really think that, then what do you think a gearbox is for?

[kambites]

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.

[kambites]

CraigyMc said:

If you really think that, then what do you think a gearbox is for?

Allowing you to keep the engine in part of the rev range that generates enough power to propel the car. What do you think it's for?

A particular car generating 100bhp at a particular speed will instantaneously accelerate at a particular rate; it doesn't matter what gear it's in. E=1/2MV^2. If you're putting a given amount of energy per unit time (power) in above what is being lost to drag, the car will gain kinetic energy at a given rate. The energy can't just vanish.

Edited by kambites on Friday 31st December 13:25

[Fatman2]

Isn't it the torque before the gearbox that's proportional to the revs ? The gearbox is then the torque multiplier.

But OP, the rev range passes peak power because it's still within the useful range of the engine. Just because you pass the peak power output it doesn't mean the engine is past being useful.

You could, in principle, level the same arguement for peak torque as the engine works most efficiently (and I don't mean efficiency in terms of mpg) at a lower rev range than the peak power value.

Edited by Fatman2 on Friday 31st December 13:32

[kambites]

Fatman2 said:

Isn't it the torque before the gearbox that's proportional to the revs ? The gearbox is then the torque multiplier.

Neither torque now power is usually proportional to revs, although power comes closer in a typical engine. Torque curves tend to be fairly flat.

[Fatman2]

kambites said:

Fatman2 said:

Isn't it the torque before the gearbox that's proportional to the revs ? The gearbox is then the torque multiplier.

Neither torque now power is usually proportional to revs, although power comes closer in a typical engine. Torque curves tend to be fairly flat.

Maybe this only applies to NA engines but I thought power (bhp) = revs (rpm) x torque (lb ft) / 5252

Edited by Fatman2 on Friday 31st December 13:36

[WeirdNeville]

This can be easiliy explained by reference to the "area under a power curve".

If you rev the engine PAST peak power, then shift up to the next gear, the engine drops fewer revs and is therefore making more power from the outset as you accelerate through the new gear.
Vs Revving to precisely peak power, then changing, you extract more power from the engine overall throught the acceleration phase.

It also works on pedal bikes - you pedal past your peak efficiency in "leg" rpm in order to be closer to that peak efficiency in the next gear.

[otolith]

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:

[kambites]

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?

[Fatman2]

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?

Ok my bad, but they are directly related to each other.

At the end of the day though I guess it's immaterial as the OP's question was why revs go past peak power, which is ultimately down to the useful range of an engine.

[kambites]

Fatman2 said:

At the end of the day though I guess it's immaterial as the OP's question was why revs go past peak power, which is ultimately down to the useful range of an engine.

Yes. As someone said above, that's best described by the area under the used part of the power curve (I think it's the power curve, not the torque curve, as some people believe).

[Scuffers]

kambites said:

Fatman2 said:

At the end of the day though I guess it's immaterial as the OP's question was why revs go past peak power, which is ultimately down to the useful range of an engine.

Yes. As someone said above, that's best described by the area under the used part of the power curve (I think it's the power curve, not the torque curve, as some people believe).

exactly...

if you drop 2,000Rpm between one gear and the next, then optimum would be to change gear when the power curve (past peak) = the power curve -2,000Rpm.

[WeirdNeville]

Scuffers said:

kambites said:

Fatman2 said:

At the end of the day though I guess it's immaterial as the OP's question was why revs go past peak power, which is ultimately down to the useful range of an engine.

Yes. As someone said above, that's best described by the area under the used part of the power curve (I think it's the power curve, not the torque curve, as some people believe).

exactly...

if you drop 2,000Rpm between one gear and the next, then optimum would be to change gear when the power curve (past peak) = the power curve -2,000Rpm.

Yep, power curve as that integrates out to total power produced over time, which will be higher when revved past the peak power figure than it will if changed at the peak power figure.

[Engineer1]

Because power is a curve not a sawtooth or square wave, past peak it declines but it doesn't fall off a cliff. Also if going just over peak gets you to 60 with one less gear change the figures look better, fuel economy considerations come in as well.