The physics of cars - a question sequence.

[Dave Plowman (News)]

Ok. You've now confirmed you are an idiot.

[Thomas Prufer]

Marketing-speak would say it's got non-discrete ratios, hence they are continuous, hence infinitely many ratios, hence infinitely variable.:-)

But, yeah, it's bounded.

(And the Wiki entry on the Variomatic efficiency may be a poor translation from another language? It's oddly phrased: "Due the fact the engine runs most of the time in its best fuel economic speed, the fuel consumption of this car was at accepted level...)

Plus the two extra-special modes, outside this range! Only available for a short period!

(These are "goes nowhere with the engine running" and "goes somewhere even with the engine off" -- if pushed, or rolling downhill.)

I'll concede that the extra-special modes are a bit spurious. (But one is very fuel-efficient!)

Thomas Prufer

[Dave Plowman (News)]

But it doesn't have an infinite number of ratios. That's my point. It's just the usual adspeak to take in those who don't understand mechanics. Which seems to include quite a few on here. ;-)

[Roger Mills]

That is true of *that* gear at *that* road speed.

HOWEVER, the engine won't be developing its maximum power when running at its max torque speed. So - at that roadspeed - you could actually get

*more* thrust and *more* acceleration by choosing a different gear which allowed the engine to run at at its max power speed. The engine would be producing less torque than in the first case, but this would be more than offset by extra torque multiplication from the lower gearing.

Tell me, in your boy racer days, how did you decide when to change gear in order to achieve the best possible 0-100 time?

[Roger Mills]

See my previous post. At any given road speed the max available thrust at the wheels occurs when the engine is developing its max *power* not its max *torque*. You choose the gearing to suit.

[Clive George]

Are we still doing this? What you've said there is true, however it is not the way to accelerate the car the fastest.

[Roger Mills]

It has an upper and lower limit of course. But it is continuously variable between those limits - in an infinite number of infinitesimal steps!

The concept first appeared in the Austin Hayes transmission

in the 1930's - long before Daf's belt drive system.

[Roger Mills]

No-one would argue with that.

But that only occurs at one road speed which corresponds to the engine's peak torque speed in that gear.

I'm sure you would agree that you could more acceleration at that road speed by choosing a different gear which allowed the engine to operate at its max power speed. If you *don't*, please explain what's wrong with that statement.

[Dave Plowman (News)]

There can only be one road speed in one gear and at one engine speed.

Of course the rate of change of speed (acceleration) depends on the gear in use and where the engine is in its torque curve.

But the maximum rate of change happens at peak torque. I really can't see how so many fail to understand this very simple thing.

[Dave Plowman (News)]

The discussion was about accleration - rate of change of speed - at any one point in an engine's range.

[Dave Plowman (News)]

I'd long since given up. But someone decided to revive it.

We're not talking 0-60 times here. Just the best rate of change of speed. No vehicle with an IC engine gives a linear rate of increase in speed from

0 to top speed. It will obviously produce the best acceleration in the lowest gear, slightly less in the next one up, and so on. But that is a red herring as regards when the engine produces the best acceleration in any one gear.

[bert]

If you chose the correct gearing would the car achieve maximum seed at maximum power output? In which instant the acceleration would be zero.

[bert]

Mine (DAF44 could easily out accelerate my Renault 18 (1.8) over the first few hundred yards.

As fast as it could go forwards :-)

[The Natural Philosopher]

Peak power is always the place of maximum speed and maximum acceleration, given suitable choices of gears

[newshound]

Not convinced. Peak power is certainly the place of max speed, I would have said that peak torque is the place of max acceleration.

[Roger Mills]

OK, can we agree on this?

In a given gear, the maximum rate of change of road speed will be achieved at the engine's maximum torque speed. The road speed at which that occurs will be determined by the engine speed and the gearing.

At a given road speed, the maximum rate of change of road speed will be achieved at the engine's maximum power point. The necessary gear ratio to achieve this will be determined by the chosen road speed and the engine's maximum power speed.

Yes or no please - no equivocation!

[Clive George]

Ok. Which is achieved with the gear chosen so the engine at its maximum power, not maximum torque.

I think people aren't concerned about when the engine produces the best acceleration in any one gear, they're concerned about what is necessary to accelerate the car the fastest, which means choosing gears which will put the engine closest to maximum power, not maximum torque.

[Dave Plowman (News)]

At least now you've come out with a definitive statement. No more crap about gears. Definitive, but wrong. No surprise there.

[Dave Plowman (News)]

Yes. Assuming you have a gear which allows that road speed at maximum BHP. That would by nature be a lower gear than one which place the engine at peak torque at that road speed. And a lower gear multiples the torque.

Doesn't need any equivocation. Of course a lower gear multiplies the engine's torque.

[Dave Plowman (News)]

You should join Turnip in some basic mechanical theory lessons.