Slightly OT: Petrol or Diesel?

wel I have. Two :-)

Note where I said 'performance'

Power is the rate of doing work, strickly.

Hum, fairly sure thats the rev limit on my TD5 which is most definately not a racing engine (mind the 2+ tonnes of vehicle weight might have something to do with that).

I think I've only taken it that high a couple of times, once to see what happened (nothing much the revs just stop increasing, nothing dramatic as has been implied in this thread) and maybe a couple of times in second "to get out of trouble".

Maybe that's what you need to do in a little (

Tim Streater :

Quite right - thanks. It's fifty years since I was taught that stuff in school. It's a tribute to my teacher (Mr Ellis) that I still remember it (even imperfectly) after all that time.

Roots (blower)

zactly

energy is work done more or less. power is first differential of that wrt to time. torque is a measure of force applied around and axis. work done is torque times revolutions. Power istorque times number of revolutions in a given time. So (removing all the constants) power=torque times RPM.

Basically power as quoted for an engine, BHP, is simply a product of torque and revs.

The formula to calculate it is

-------------- 33,000

Where N is RPM and T torque in lb.ft

So there is always a direct relationship between the two.

"Dave Plowman (News)" :

Coming back to driving, the point is that maximum acceleration corresponds to maximum power, not maximum torque. Typically a petrol engine has much less than maximum power at cruising speed, so you need to change to a lower gear to accelerate briskly. A diesel engine gives close to maximum power at cruising revs, so you don't need to change down. (The difference is more obvious with a manual gearbox than an auto, because of the time and effort required to change down manually.)

WIWAL we compared the power of engines. Nowadays the pundits like to bang on about "torque", and diesel torque is often compared favourably with petrol torque. Torque is not a particularly useful indicator of available acceleration, *unless* you discount the possibility of changing gear (and why would you?).

MM. its is relevant because of the issues of number of gears and gear changes. The original F1 honda engine was massively powerful, but at such a narrow rev band that it didnt actually deliver best acceleration. They tamed it a bit and got better results.

If you are beyond 'wheelspin' power then tractability is more use.

What do you base that on? I think you've got it exactly wrong.

Nah, he's right. It might be maximum torque at the wheels, but the gearbox means it might well not be maximum torque at the engine.

give an infinitely adjustable gear ratio he is entirely correct.

Eh again? A gearbox multiplies engine torque.

The fasted rate of change of speed of the vehicle (acceleration) will be at maximum torque. In the lowest gear - assuming it retains traction. However, if you change up a gear at peak torque, you'll now likely be well below that in the higher gear so lower revs. A well designed gearbox would allow the engine speed to be kept roughly between peak torque and peak power for best acceleration.

at the wheels...

a big assumption at low speeds.

you dont. you change up when power delivered at lower rpm in the next gear is eual to the power delivered in the current gear.

You really haven't a clue have you?

In article , bert scribeth thus

Oh yes!, and the sound a couple of Napiers giving a bit of thrash and clag:))..

That would imply cruising speed ~= max speed.

ITYM max _torque_ at cruising revs. Max torque for my (petrol) car in top is well into the lost licence range.

That would fit with my limited experience of diesels - they feel fine, until you ask them for a bit more. And it isn't there.

Andy

What car do you have that can break traction without silly use of the clutch?

See the next sentence.

Eh? Did you actually read what I wrote?