OT: Driving electric cars in winter

[Dave Plowman (News)]

But in any gear, the torque will always be at maximum at the engine's peak torque figure. Not at peak bhp. Makes absolutely no different what type of gearbox is used.

[Dave Plowman (News)]

And like I keep on saying is wrong. In any gear there will be less torque applied to the wheels at peak bhp than at peak torque. Why is this so hard to understand?

You apparently don't.

[Clive George]

While what you've written is true, that doesn't give you maximum acceleration. At a given road speed, the acceleration in a gear chosen for peak engine torque will be lower than the acceleration in a gear chosen for peak engine power. The engine torque at peak power may be lower, but it is more than made up for by the change in gear ratio.

In real life you know this - you change down to accelerate hard, or kick down in the auto to achieve the same thing.

Many people are thinking this. You seem to be assuming that the gear ratio is fixed. This isn't true, we have the option of picking different gear ratios, and that option means that the maximum acceleration is achieved when you run the engine closer to peak engine power, not peak engine torque. I did try and hint that with my theoretical CVT example, but you didn't seem to understand it.

[Mike Barnes]

That "in any gear" is at odds with "given suitable gearing", and is the crux of the misunderstanding. I'm trying to reveal where the misunderstanding lies, not to point out faulty logic or arithmetic.

So, here's an exercise that might help. Consider a CVT (continuously variable transmission) where the engine speed is always the same, as specified by the designer, regardless of road speed. You're that designer, and you want to maximise acceleration.

What engine speed do you choose? Peak torque RPM, peak power RPM, some other RPM? And why?

[dennis]

To accelerate an object you have to input energy. The kinetic energy is mv2 so the faster you can input the energy the faster it accelerates. On a car you choose the gear that can put the energy into the object the quickest.

I will leave the audience to think about it.

[NY]

And by extension, you choose the engine revs in Mike Barnes' CVT example which puts energy into the object the quickest. Oh look, it's the max-power RPM :-)

As a matter of interest, how do the power-versus-rpm and torque-versus-RPM curves look for an electric motor as used in an electric car? Can an electric car achieve constant acceleration over a range of road speeds (for example 0-70 mph), assuming a fixed gearing (which may be direct drive), and no CVT, fixed-selection-of-ratios or torque-converter gearbox is interposed.

[Clive George]

I tried that one :-)

[bert]

Yes the forward movement of the vehicle depends on the fact that the tyre grips the ground. The wheel at that point of contact is stationary otherwise you have wheel spin. Totally irrelevant to the discussion.

As has been stated without dispute many times power and torque are related by speed. Acceleration is determined by torque which is in turn related to power.

You will always have better acceleration by dropping a gear to keep the engine on peak torque than by changing up to peak power.

Hurrah.

This discussion is becoming rather repetitive. I'm out.

[Vir Campestris]

Once you've passed that point in first gear you either change up to 2nd or let the revs go up towards peak power.

You'll lose less by going up the revs to lower torque than you would by going to a higher gear.

Acceleration is controlled by producing a force over a distance in a time - newtons/metre/second. That's the same units as power.

Andy

[Dave Plowman (News)]

Decided to move the goalposts? Of course you can get near any torque you want by the use of a gearbox. But what that gearbox can't do is change the relationship between the two while in the same gear.

And in any one gear, the best acceleration will be at maximum torque...

[Dave Plowman (News)]

If you've ever driven a CVT - even the original DAF - you'd know on full throttle the engine stays at around maximum torque, not BHP. Hence the normal complaint of them droning. If it went to peak BHP, the complaint would be of screaming.

[Dave Plowman (News)]

No it's not. To determine BHP, you also need to know the engine speed. Without that, it's torque. ;-)

[Clive George]

Peak power is at higher revs - you drop a gear to go to peak power rather than peak torque.

There's certainly a lot of confusion in some minds.

[Clive George]

No, I've consistently said the same thing : that maximum acceleration is achieved by keeping the engine at maximum power rather than maximum torque.

Yes, that's what many people have been saying.

I don't think anybody has said otherwise.

But nobody is saying you should stay in one gear for best acceleration.

[Steve Walker]

I think that the currently taught recommendation is to put your handbrake on, but keep you footbrake depressed until a vehicle approaches and stops behind you. Thus making sure that they see you and stop and that you don't keep dazzling them.

SteveW

[The Natural Philosopher]

Completely and utterly WRONG.

I am beginning to realise why people 'believe in renewable energy' and 'climate change'

You remind me of those people who said that a bee inside a train couldn't fly, because the train was already travelling faster than the bee could fly..

[The Natural Philosopher]

Or that you will stay ant best torque as the car goes faster.

[NY]

Yes, that makes sense. I will sometimes do that if I'm stopped in a particularly vulnerable spot such as a turning-right lane in the middle of a fast road.

Likewise for putting rear fog lights on in fog until someone comes up behind.

The operative thing is to turn brake/fog lights off as soon as someone

*does* come up behind. Many people, especially in automatics, sit with their foot on the brake permanently when they are stopped at a junction or in a traffic jam, instead of putting the handbrake on.

[Tim Streater]

Or turning the engine off.

[NY]

Ultimately, if the traffic jam is along one with no movement at all. If the traffic keeps moving forwards a few feet every minute or so, I tend to leave the engine running, especially if it's cold weather so the heater remains on. I've forgotten what our new car, which has auto-stop on the engine, does about leaving the fan and the heater going. Obviously if the engine isn't run for a while the cooling water will cool down so the heater will not be as hot, but I'm talking about brief outages at traffic lights.