OT: Driving electric cars in winter

[charles]

If I keep my foot on the brake, the engine is automatically stopped, until I lift it off again. So, do I dazzle the car behind, or asphixiate its driver?

[charles]

I was told by my main dealer that it takes half an hour of clear motoring to recharge the battery after a cold start.

[The Natural Philosopher]

I think that has to be a VERY cold start.

the ratio of discharge to charge with an alternator is about 300A to 15A.

i.e. 20:1

Do you really need to crank your engine for a minute and a half to start it?

Mine usually fires in less than 3 seconds..

[Dave Plowman (News)]

And you've been consistently wrong.

[Dave Plowman (News)]

Are you still driving an Austin 7? ;-)

[Dave Plowman (News)]

Many modern cars have the alternator controlled by the engine ECU and use pretty high charge rates into a low battery.

I live in Central London and would be hard pressed to find 1/2 hour of clear driving.

If I go to the local supermarket, I'll take the long way round to make sure the engine is fully hot. Takes about 1/4 hr. Come back the direct route - about 5 minutes. Never had a problem with a flat battery with that sort of use.

Of course to get the battery absolutely fully charged may well take longer. But why would you want to?

[charles]

I never tried one of those

[charles]

It's a lot colder in the country out of London, so battery capacity will be reduced on freezing mornings. Is your car kept in a garage?

[Mike Barnes]

I haven't driven one of those, but I somehow doubt that they were designed for maximum acceleration regardless of economy etc. So that doesn't answer the question.

You seem disinclined to give any consideration to the CVT case (why, I wonder), so I'll say one more thing then leave you to it.

You're maximising acceleration in a certain gear. Peak torque is the answer.

Others, including me, are maximising acceleration given the freedom to vary the gearing. Peak BHP is the answer.

That's it, in a nutshell. I can see both sides of the "argument", but you apparently can't.

[Clive George]

I really don't understand how you're failing to get this. It obviously needs a gearbox, ideally CVT, to keep the engine at maximum power regardless of road speed. You seem to be fixated on the idea that one is in a single fixed gear, because that's the only case where maximum acceleration is achieved at maximum engine torque.

Does kick-down enable you to accelerate faster? Does kick-down move the engine towards maximum power or maximum torgue?

The answer to the first question is very obviously true, otherwise there's no point in having it (ok, unless making it sound noisier is better...)

I'm pretty sure the answer to the second question is that it moves it towards maximum power. IME when you floor it in an auto it goes the gear where the engine is closest to the rev limit but not above it.

[Dave Plowman (News)]

You jest, I assume? ;-)

[Dave Plowman (News)]

It would be a very poor auto which didn't give its best acceleration when you floor the throttle.

You apparently want to use the figure from the engine in one gear where it suits you and compare it to a different figure in a different gear.

It is very simple. Ignore the gearbox. The engine gives maximum acceleration at maximum torque.

If you use a lower gear to increase the torque at high revs, it will also increase the torque at other engine speeds.

In other words, the gearbox of any type is a red herring.

[Dave Plowman (News)]

No gearbox ever made can do that.

And you seem to be unable to understand that any torque multiplication from a gearbox applies across the rev range of the engine. So if you use a lower gear to increase the torque at peak BHP it will also increase the torque at the peak torque part of the engine output. So that peak torque will always be higher than any figure at peak RPM.

Not at 130 mph on one of my cars.

Most autos won't kick down if that lower gear brings the engine close to its maximum RPM. It would be pointless. And on most engines, peak BHP isn't that far away from maximum revs. Peak torque, on the other hand, is always at lower RPM than peak BHP.

[Dave Plowman (News)]

I take it you were a civil servant?

You've produced a set of meaningless figures to prove a lie. Exactly as you claim is done with renewable energy.

[Clive George]

You can't ignore the gearbox, it's what gives you the maximum acceleration.

For a given gear, yes. But we are not stuck in one gear.

Yes, it will - but you can't use those other engine speeds because the wheels would be going at the wrong speed for the road.

It is true that if you're in say 3rd, that maximum acceleration in 3rd is achieved at maximum engine torque. But for the same road speed you could change down to 2nd where there will be more power, and you will accelerate faster.

It is then true that you would accelerate faster in 2nd at maximum torque, but that's at a lower road speed so isn't at all helpful.

The fastest momentary acceleration will always be in first gear (or whatever the lowest ratio the gearbox can provide is) at maximum torque (assuming you've got enough grip, which often isn't true), but that is only useful for a brief moment - to get maximum acceleration overall you need to change gear to keep the engine closest to maximum power. So with idealised CVT I mentioned earlier, you'd be in lowest ratio up to maximum power then start changing ratio. With a more conventional gearbox, choose gear to keep closest to maximum power, which is what an auto with kickdown will do.

(you did mention DAFs - I'm sure I've heard owners say they screamed when you asked them to go as fast as possible, rather than chugging at maximum torque. There are probably plenty of people who don't ask their cars to go as fast as possible though).

No, the gearbox is the critical thing.

[Dave Plowman (News)]

Forget it. Even I'm bored with this. Believe what you want. You're in good company with all the other idiots.

[Mike Barnes]

There it is in a nutshell again. In the real world, cars have gearboxes, but you argue as if they don't. Obviously you're not going to agree with people who acknowledge the purpose and value of gearboxes.

[Clive George]

I've got a good idea who the idiots are on this NG. Obviously I'm going to claim I'm not one, and you're not normally one either. But for a change, TNP is right and you're not.

We could prove who's right given a set of gear ratios and a torque curve for an engine - would that help?

is a fairly generic torque curve pic - would that do for a sample? We could work with both the blue and the orange lines.

Gear ratios ought to be easy enough to find - any preferences? Obviously I'd like to use a theoretical CVT to prove my point, and in that case the orange engine would be run at about 7Krpm, the blue at 5750rpm for my run, and orange would run at any speed between 4000-5500rpm for your run and blue between 2500-4000rpm.

(using the theoretical CVT does have the advantage of making the sums much more easy)

[The Natural Philosopher]

I am nearly always right.

[Dave Plowman (News)]

The discussion was about where in an engine's rec range the best acceleration occured. Bringing gearboxes into it simply clouds the issue.

But I'm so used to that on here with some that I shouldn't be surprised.