The physics of cars - a question sequence.

[Roger Mills]

If they meant maximum *engine* acceleration, I'd have to agree with them. But if they meant maximum *vehicle* acceleration, they clearly haven't understood the question!

[Roger Mills]

The very first post which started this thread *actually* said the following:

"Q1: If an engine is capable of a peak torque of 400nM, what is the force available at the wheels at a speed of 10m/S?"

. . . which is impossible to answer without knowing anything about the gearing - and which makes no mention whatsoever about acceleration.

It has morphed - as many threads do - into a discussion about something which is not directly related to the original subject.

And the fact remains that - at any given speed - the highest instantaneous rate of change in speed (acceleration) is obtained by maximising the amount of power transmitted to the wheels. Simple laws of Physics!

[The Natural Philosopher]

It is all very reminiscent of the 'fly inside the cabin of a 707 and how fast is it flying' malarkey.

[Dave Plowman (News)]

Good grief. ;-)

[Dave Plowman (News)]

No, they understood it perfectly. Plenty on here don't, though. Those who don't understand BHP and torque. Must admit to being amazed how many don't.

[Dave Plowman (News)]

I take it you don't remember the original thread some weeks ago? it was obvious that Vir Campestris was attempting to prove he was now right about that.

Sigh. Not thrust now? Power is measured in BHP. So you are perfectly sure of this? Because you're wrong. It happens at maximum torque.

[michael adams]

Surely maximising the amount of power transmitted to the wheels to change the speed from say 1 mph to say 5mph would simply result in wheelspin. As many boy racers at traffic lights know to their cost, never mind many stars in reasonably prices cars*. And the same would presumably also apply in wet or icy conditions* at much higher speeds

Not that the same doesn't apply equally well to torque.

michael adams

*Top Gear *Ice Road Truckers

...

[Tim Streater]

Doesn't necessarily follow. Studded tires come to mind.

[michael adams]

But depending on weight distribultion, and assuming the studded tires dont simply carve a big hole in the road surface, all that that might achieve might be to send the front of the car into the air, if not flip it over completely.

And overal studded tires would be a specal case dependent on an optimum road surface, and optimum weigh distribution and would only be applicable up to a certain speed.

michael adams

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[Roger Mills]

Get yourself a large mirror!

[Roger Mills]

So are you telling me that everything you've written relates to

*another* thread rather than this one? I can only cope with one thread at a time!

What I said is correct. If you maximise power at a given speed you also maximise thrust at that speed since thrust = power / speed. You are also maximising the drive-shaft torque at that speed. Where we differ is that you say this coincides with engine max torque and I - and everyone else as far as I can ascertain - says that this coincides with max engine

*power*. Do the sums with some actual numbers!

[Roger Mills]

Well yes, of course. But I think we're assuming we have a dry road with plenty of adhesion.

[I did in fact cite icy conditions at one point when trying to explain to My Plowman what I meant by "thrust"].

[Tim Streater]

Front wheel drive.

All cases are special cases and depend on all the factors you mention.

[michael adams]

What was being proposed, was a general principle.

If all cases were special cases, then there would be little point in formulating general principles at all, would there ?

michael adams

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[Vir Campestris]

The whole point was that it's impossible to say. Torque on its own is useless - you need revs too, and that means it's power. Force times speed.

Andy

[Tim Streater]

It might spin or it might not. Depending on all the special case variables. So a statement of the bleeding obvious, really.

[Dave Plowman (News)]

No car engine produces torque without revs. Any more red herrings you'd like to introduce?

[Johnny B Good]

But at a lower road speed.

I did have that in mind but I thought I'd leave that little factoid for someone else to consider. It doesn't matter who else was going make that observation but it does help that you're well aware of this effect. :-)

Choosing to use the engine's peak torque point does optimise something. It's just not maximum possible acceleration. What it's most likely to optimise is most *fuel economic* maximum acceleration to any given target cruising speed since max torque revs approximates very closely to maximum engine efficiency revs.

Normally, assuming maximum fuel efficient acceleration is not your prime concern (and this is generally true for almost every fuel consumption conscious driver on the planet), minimising journey fuel consumption is achieved by 'short changing' so as to achieve the most economic gear ratio (top gear) for the cruise part of the journey as soon as possible without labouring the engine and without using higher revs than necessary.

Obviously, this depends on road conditions (starting off in an uphill direction may require more engine revs and in a 30mph limited section of road may force you to stay in a lower gear until you hit the level or a downhill section) but despite this, the aim remains the same, don't dawdle in an unnecessarily low ratio unless road conditions leave you no choice.

Since typical urban driving conditions require far less acceleration demands than the capabilities of even modestly specced motor cars, the change up points are often well below the peak engine torque revs anyway. It matters not that the engine is reving comfortably below its maximum torque revs on part throttle when changing up, the importance point is that you still get the best possible fuel economy in spite of the less than ideal driving conditions when choosing the tallest gear ratio the engine can comfortably handle.

However, when attempting to outdrag another vehicle on a clear motorway (there are very few of those about - the only one that comes to mind is the M58 into Liverpool early to late evening on a Sunday), you change up just at or after the maximum BHP revs which, as you observed, will put you close to the maximum torque point, probably providing close to the torque you had at the road wheels just prior to 'running out of steam' in the previous gear.

Further acceleration will see the accelerating force from the driven wheels continue to fall off as you increase road speed until you finally reach a limiting speed in top gear where the thrust finally balances the steady counterforce of drag and friction. Strangely, top gear isn't designed to limit the engine to maximum torque revs at the maximum road speed rating of the car if the manufacturer wants to offer the highest maximum road speed possible from the engine.

However, the manufacturer may well offer an 'overdrive' ratio to maximise motorway cruising economy at speeds between 70 and 80mph against modest headwinds and gentle uphill slopes where the engine runs close to maximum torque revs at 80mph but, aside from such considerations of fuel economy, gearing is generally configured for drive-ability at a small sacrifice in best possible fuel economy.

Since the product of thrust from the road wheels and road speed is an expression of power (energy delivery rate) translated from the engine's output shaft via the transmission system, you need to ideally arrange for the engine to be delivering its maximum power output throughout the whole range of road speeds if you wish to maximise the acceleration performance of the vehicle either by using a perfect continuously variable ratio automatic gearbox that holds the engine close to its maximum power output revs or else by approximating this ideal with skilfully chosen gear changes with a manual box.

If we observe the performance of such a vehicle with a perfect continuous ratio box automatically holding the engine to its peak power output at maximum throttle opening on a dynamometer rolling road you will observe an inverse relationship between thrust generated at the wheels and the speed of the rollers.

If we observe, for example 300 Lbs thrust at 40mph, it will drop to

150Lbs at 80 mph representing the constant steady state maximum engine power translated via our intelligent automatic gearbox into speed/thrust combinations with equal products of power delivered to the rolling road.

With a perfect gearbox, we can plot a continuous curve of falling thrust with increase of road speed up to the car's transmission system / max engine power output limit which we might well expect to correspond to the car's maximum rated road speed due to drag and rolling friction forces anticipated by the designers of the car.

If we reconfigure the gearbox to hold the engine at its peak torque rpm during such a test run we will see a lower product of road speed and thrust which corresponds to less acceleration and a slower top speed.

At the end of the day, it's all a matter of imparting kinetic energy into the moving mass of the vehicle at the highest possible rate (maximum engine power) to maximise the vehicle's acceleration courtesy of a variable gear ratio box (manual or perfect continuously variable automatic box) to provide as much effective thrust which declines with the increasing speed being imparted to the vehicle. Since it's engine power and not torque alone that's required to accelerate the vehicle, shifting up at peak torque output of any piston ICE powered car does not offer the best acceleration performance.

[Thomas Prufer]

A standing car would never accelerate...

Thomas Prufer

[Roger Mills]

Indeed.

Yes. Maximum speed occurs when the thrust vs speed curve intersects the drag vs speed curve. This is not necessarily in the highest available gear if that gear is an "overdrive".

Indeed. But my head is black and blue from being hit against a brick wall trying to get Mr Plowman to understand that!