The physics of cars - a question sequence.

[Roger Mills]

Because the torque which the engine produces at wide open throttle varies with speed. If you plot torque against engine speed, you get a curve which is convex upwards. Peak torque typically occurs at 3000 -

3500 rpm.

If you then calculate power at each speed (speed x torque) and plot that, you get a different curve. That rises steadily with speed until it reaches its peak - maybe at 5000 rpm, and then starts to drop off again.

So, at peak torque, the engine is producing more torque than it is at peak power. Because power is the product of speed and torque, it continues to rise even after the torque has peaked.

So, to return to your question, you can get more torque - but less power

- out of an engine by running it a speed lower than its max power speed.

Or, to put it another way, peak torque and peak power don't occur at the same speed as each other.

Hope that helps!

[Roger Mills]

Absolutely. What puzzles me is why that isn't patently obvious to anyone who claims to know anything about cars and engineering!

[michael adams]

Thanks. I found a website with the curves etc.

" Typically, the torque peak will occur at a substantially lower RPM than the power peak."

My problem really is that the above statement isn't intuitively obvious. To me, at least.

I suspect understanding these things requires the ability to visualise the interplay of at least two different cocepts at the same time; that once that's been achieved then it all fits into place; but for people who've never bothered with this, beyond a certain age, without 3 D models and intensive tuition it may be a bit of a lost cause.

Especially when there's so much else out there to get confused about.

Which people who have been familiar with these concepts since their youth may find a bit difficult to understand.

michael adams

...

[Vir Campestris]

No, let's not.

You can get better acceleration off the line by slipping the clutch. But you'll waste fuel and wreck your clutch.

Andy.

[stvlcnc43]

It seems that you're saying that becase Power is obtained at the TOP of the power curve and Peak Torque much lower on the power curve, that a car will accelerate more rapidly between 90 and 100 mph than it will between 40 and 50 MPH.

[Vir Campestris]

Torque is force, not power. I suspect he doesn't know the difference.

Andy

[Vir Campestris]

Power can be defined as force times speed.

For a rotating engine that's torque times revs.

If the torque is the same you will have more power at higher revs - in fact, if you doubled the revs you'd get double the power.

Highest power comes when the engine is struggling to get enough air through, and the torque has fallen off so much that the increase in revs doesn't give you more power.

Andy

[Roger Mills]

No-one is saying anything remotely like that!

For a start, you're not going to be using the same gear at 40 mph as at

90 mph - you select the best gear for the speed you're at, to enable the engine to develop something approaching max power at all road speeds.

Since power = force x speed, the same amount of power at a high road speed gives you less force to accelerate the car than is available at low speed. *And* the aerodynamic drag is 4 times as high at 80 as it is at 40. Putting that all together, a car accelerates much *less* rapidly as high speed.

When it reaches it's maximum speed - the point at which drag equals the available tractive effort - it doesn't accelerate at all!

[Dave Plowman (News)]

Hasn't stopped many trying. And failing.

Would that be 'thrust'? ;-)

[Dave Plowman (News)]

How many many times do I have to say I'm only commenting on the best acceleration *in any one gear* - so entirely due to engine output.

Perhaps there's something missing from the way I've made that point. Anyone who's ever driven a car knows that you'll get better acceleration by using a lower gear. It's so obvious it didn't ever need stating.

However, if making the point I am, saying to use a different gear is simply nonsense.

And I'm utterly amazed so few understand the relationship between torque and power.

[Dave Plowman (News)]

I'm beginning to lose the will to live.

Power is a function of torque and engine speed. It therefore goes without saying that maximum power will always be delivered at higher revs than maximum torque in practice. An *ideal* engine would produce constant torque throughout its rev range, so BHP would increase linearly with engine speed. But no engine is ideal, therefore it will have both a torque peak and BHP peak.

[Dave Plowman (News)]

Have said this countless times. But didn't stop others saying this was wrong - or introducing countless red herrings and moving of goal posts.

It's what started the original thread and caused Vir to start this one.

But it seems he's not alone in having little understanding of basic mechanics.

[Roger Mills]

And, to a man, everyone agrees with you on that narrow point.

But that doesn't mean that any discussion about to get the best acceleration when you're not constrained to use just one gear is invalid.

That's the problem - we *do*! We know that power = torque x speed (or torque = power/speed) - with the result that, in order to get the maximum possible output torque from a gearbox at any given speed (which is what accelerates a car) you need to put the maximum possible power in.

Since there are very cars in existence with only one gear, it is perfectly legitimate to take the gearbox into account when discussing acceleration - even though you regard it as a "red herring".

[Tim Lamb]

In message , "Dave Plowman (News)" writes

To try and introduce an element of humour.... I was intrigued reading an Autosport? magazine report on 0-60mph trials for a series 2 Morgan around 1960 to learn that *all upward gear changes were carried out at full throttle*:-)

>

[The Natural Philosopher]

It seems you are Dave Plowman, and I claim my £5

Engine RPM is not directly and irrevocably poroprtional to road speed

WE have a gearbox.

[michael adams]

That may well go without saying for you, but it isn't intuitively obvious to me.

It may have said something similar on some website, complete with diagrams and coloured lines, so I'm not going to tear my hair out over it, either way.

michael adams

...

[Dave Plowman (News)]

If only. Lots and lots say it's at maximum power, not maximum torque.

That's what started this entire 'discussion'.

[Dave Plowman (News)]

Easy to tell the difference as I only talk sense.

And you win the cigar for stating the bleeding obvious.

Now go and find out about torque and BHP. Even an idiot can use Google. Get one to show you how.

[Dave Plowman (News)]

It is simple maths. If BHP is a function of RPM and torque, and the torque remains constant, then higher RPM means higher BHP.

If you looked at graphs showing an engine's output relative to RPM, you'd see two plots, one for BHP and the other for torque. Often shown in the same graph. Both will rise and then fall as RPM increases. The torque one will have its peak at lower RPM than the BHP.

It's something I've known since a child. ;-)

[michael adams]

But why should the torque remain constant ?

But doesn't that contradict what you were saying earlier about torque ? Now you seem to be saying peak power is reached at peak RPM and not peak torque.

michael adams

...