The physics of cars - a question sequence.

[The Natural Philosopher]

Even then what Plowperson said is plain wrong.

You want to arrange gear shifts symmetrically around peak *power*.

[Roger Mills]

Maybe you'd better remind us what your original statement was, since it got lost in the mists of time!

Seems to me that we are arguing over semantics and may even be in agreement over the engineering principles (though I'm not 100% convinced!). So let's try again.

Q1: How do you maximise the acceleration of a car in a given gear?

A1: By using full throttle and running the engine at its maximum torque speed.

Q2: How do you maximise the acceleration of a car at a given speed?

A2: By using full throttle and by choosing a gear which enables the engine to develop its maximum power (or as near to it as the choice of gearing permits).

Unless I misunderstand you, you are saying that the answer to Q2 is A1. If so, you are simply WRONG!

[Dave Plowman (News)]

And you think I meant anything else?

Given the gearbox multiplies the torque from the engine, what else would you expect?

Have we now got to the point where everyone agrees that *in any one gear* the best acceleration is at peak torque, not peak BHP?

That is and was the crux of the matter. Before all the red herrings and moving of goal posts started.

[Dave Plowman (News)]

Can't be bothered. Many seem unable to read or understand a simple statement.

I'm not arguing over semantics.

If you ran the car at maximum torque there'd be no acceleration, since that occurs at specific revs.

And that's where you're wrong.

As I said before, some seem totally incapable of understanding a very simple and basic condition.

[Roger Mills]

Of course there would. It would only be at its max value at the max torque point itself - which may actually cover a range of speeds since some engine torque curves are flatish rather than peaky at max torque. We're talking about a dynamic rather than static situation anyway. Acceleration is the RATE OF CHANGE of speed and is a continuously variable function. If you're defining acceleration as a step change in speed it's little wonder that you're living on a different cloud from everyone else.

Maybe I need to change A1 for your benefit, thus:

A1: By using full throttle and allowing the car to accelerate over a speed range which encompasses the max torque speed of the engine. Maximum acceleration will occur as the engine passes through its max torque point.

[In actual fact, it may not! As the car speed increases, the aerodynamic drag increases as the square of the speed. The effect of this - which we have so far ignored in this discussion - is that maximum acceleration MAY occur slightly below the max torque speed because the increase in drag may more than offset the increase in torque.]

I think we agree on THAT! What we DON'T agree on is who those people ARE!!!!

[The Natural Philosopher]

And that's where the staggerring incompetence of Plowperson emerges.

You and Dave, basically.

[bert]

Yes and No - Windows smart phone I'll have a look for an app.

[Roger Mills]

Pray what simple and basic condition have *I* failed to understand?

[The Natural Philosopher]

Gearboxes. The relationship between power torque acceleration and speed. Basic mechanical engineering. Take your pick.

[Roger Mills]

Rubbish! I challenge to you quote anything I wrote which backs up your statement.

[Vir Campestris]

I've given up on Plowman, he must be trolling. Or too embarrassed to admit he's wrong.

Your A1 as it is written is perfectly true, but irrelevant for performance.

To get the greatest acceleration for a car you need to maximise the amount of power that the engine generates.

If you think of acceleration as increasing your kinetic energy this should be obvious.

If you follow through some of the examples I gave earlier it should also help. To remind you:

400nM at 2000 RPM geared down to 1000RPM -> 800nM at the wheel. 200nM at 5000 RPM geared down to 1000RPM -> 1000nM at the wheel.

Andy

[Roger Mills]

Yes, I'm aware of that - and was simply trying to explain to Mr Plowman that his assertion only applies if you restrict yourself to one gear.

Yes, indeed. That's what I've been saying all along. That's why I gave the example of using a continuously variable transmission to hold the engine at its max power (*NOT* max torque) speed as the car accelerates.

It's obvious when you consider that power = thrust x speed. At any given speed, the higher the power the higher the thrust.

[Dave Plowman (News)]

That's fair enough, but how is that any different from saying 'in any one gear' as I've constantly stipulated in this discussion?

Adding in drag etc is simply attempting to move the goalposts yet again. Drag depends on speed. And that is irrelevant to the discussion.

You've been wrong all the time and have attempted every trick in the book to try and prove yourself right.

Totally relevant to the original point. And perfectly accurate.

Right. Once again it seems you don't understand what power is. You need to maximise the torque at the driven wheels under all conditions. If talking about through the gears, the same applies.

Why would you think of it as that?

Congratulations. You've discovered that a gearbox multiplies the torque.

[Dave Plowman (News)]

If talking about the maximum accleration point in an engine's output over its speed range, why would you not restrict it to one gear? Surely you realise that what applies in one gear will also apply to another?

No it's bollocks. You need to maximise the torque. Not power.

Care to explain why a CVT holds the engine at maximum torque, then?

I've no idea what you mean by 'thrust' That's something usually associated with a jet engine. Power is a function of torque and RPM. And the higher the power does *not* mean the higher the torque.

[Roger Mills]

Your silence is deafening, Mr "Philosopher". I'm still waiting for your abject apology!

[The Natural Philosopher]

Sorry. I didn't see the previous post.

The mere fact that you agree with D plowman is sufficient evidence. I rest my case.

[Roger Mills]

You need to maximise the torque at the *wheels*. Don't forget, there's a gearbox between the engine and the wheels. As you keep reminding us, a gearbox magnifies the torque. But that comes at the expense of speed. So

- as should be obvious - you need a higher engine speed to maintain a given road speed in a low gear than you would in a higher gear. And when the engine is at its max power point you get the highest available product of output speed and torque - which is what you want.

I'm not aware that it does. If I were to design one, it certainly wouldn't!

Newton's first law! A body remains at rest or moves at a constant speed in a straight line unless acted upon by "forces".

In order to accelerate a car, you need an external force. That force is imparted to the car through the contact patch. The torque imparted on the wheels through the transmission manifests itself as a force at the contact patch which tries to push the road backwards. But the road usually wins, and pushes the car forwards instead. *That* is the thrust to which I was referring.

[The Natural Philosopher]

And the point you and plowperson refuse to accept is that you will get more thrust at the contact patch by running the engine at peak power and using a deeper reduction ratio, than at peak (engine) torque with less reduction ratio.

[Roger Mills]

If you are trying to maximise acceleration, drag is highly relevant.

If you simply want to maximise thrust at the wheels, it isn't.

But the force which actually accelerates the car is the *difference* between the thrust at the wheels and the aerodynamic drag.

That's why every car has a maximum speed - when it reaches the point where the drag equals the maximum thrust available so there's nothing left to accelerate it any more.

And you may care to reflect on why it is that the maximum speed invariably coincides with the engine's maximum *power* output in an appropriate gear - not at its max torque point.

[Dave Plowman (News)]

Yes it does. But throughout the range of engine revs in that gear.

You seem to be wanting to pick the torque produced by the engine at peak BHP in one gear and compare it to the torque produced in another gear.

Are you trying to say acceleration is uniform at all points in every gear?

But then you're obviously not into car design. ;-)

And the 'force' in this case is torque.