Idle thoughts on idle speed...

One factor might be that a V6 is not inherently well-balanced. All other things being equal, an in-line 6 should be able to idle smoothly at a lower speed than a V6 for example.

Chris

Indeed. Anyone with a straight six care to share their idle speed? Our cars both idle at 750rpm.

Tim

Bit of a simplification.

A nice heavy flywheel and a very gentle camshaft is a way to get a slow idle regardless of number of cylinders.

However, lots of other factors. Things like keeping the alternator producing some charge. Emissions. Air-con. Perhaps even chatter from the gears on a manual gearbox.

Here's a few figures plucked at random from Autodata:

BMW 530d 700 BMW 530i 730 BMW M5 780 (10 cylinders)

Audi A6 3l 720

Jaguar XJ6 3l 730

The range 700 to 800 seems to be the norm for pretty much all engines, regardless of fuel type, capacity, or number of cylinders.

At a given rotational speed, the more cylinders, then the more 'bangs per minute', so that may make the engine with more cylinders sound 'busier'.

Chris

pre emissions regs the idle would be 500 rpm , with tighter emission control the same engine was to be set to 800 rpm, (Land rover 6 pot)

Why? The power needed at idle is almost proportionate to the number of cylinders, and the conditions for smooth ignition independent.

Thanks. I thought idle speeds used to be lower in the old days. Maybe it's all down to emission control these days.

Tim

Its ironic that while the emissions as a % of total exhaust volume might be lower, the absolute quantity may well be higher than for a lower tick-over.

I'd guess - given the peak RPM will be in the same ballpark - it's the speed the alternator gives a useful charge.

Emission testing generally isn't interested in CO2. It's the toxic waste they're minimising.

Which is less than that needed with a generator. (Do I hear people asking what that is?).

And when one has a failed coil-cap, it's REALLY bad. Mine gets fixed on Monday.

IIRC, a dynamo produces more output at lower dynamo revs. Hence an alternator spinning to much higher RPM itself than a dynamo. Although of course it is also happy at much higher RPM due to not having high current brushes. I've a feeling the output curve from a dynamo is actually better suited to what's needed than an alternator. But in practice the alternator being so much more reliable wins out.

Remember a Rolls Royce specialist saying why they didn't recommend fitting an alternator to the '50s RR six cylinder. The dynamo actually gave more output at its slow idle speed. And with the restricted engine rev range, altering the gearing for an alternator wouldn't improve matters overall. Of course that was many years ago. Things may have changed now, with a much wider range of alternators available.

The problem is more that the dynamo can't handle high revs. The commutators tend to self-destruct! At idle, most dynamos are actually not charging at all; that's why the charge light often flickers. If they were driven at a speed that gave a decent output at idle they would destroy themselves at peak revs.

The other issue is efficiency. A typical dynamo was rated at 17A peak output, although even with some unofficial tweaking of the dire regulator,

Alternators give a much higher output at speeds from idle all the way to peak revs, with full output achieved at around 2,500 (engine) rpm, and as you say are infinitely more reliable.

Chris

An alternator requires rectifiers, usually 6 in a full-wave configuration, for example see:

I doubt very much whether there were suitable rectifiers available in the 1950's - suitable for a car alternator, that is.

Indeed. But then an alternator has a rev limit too. So it depends on how it is geared relative to engine speed.

Again, depends on the rev range of the engine. Which is why I specifically mentioned that old RR 6. Very slow idle due to a low CR and modest valve timing. And a low peak RPM too.

There is no intrinsic reason why a dynamo has a low maximum output. Apart from cost. But then the same applies to an alternator. Modern cars have to have a higher output due to much higher loads.

I think you'll find a dynamo can give a more linear output not so related to speed. Not that it matters, as an alternator is the obvious choice these days.

You could considerably improve the performance of a dynamo in practice by changing to a decent electronic regulator.

I was rather obviously referring to fitting one later on - when they were available. But it is a fairly limited example as in 99% of cases an alternator will be the best bet. Assuming, of course it is an adequate size - not all were, even as a factory fit.

I very much doubt that it would be possible to produce a dynamo capable of, say, 90A output in a package anywhere near the size of an alternator.

In some implementations the alternator id driven via a two-speed gearbox, to give maximum output at idle!

Indeed. In the past I've spent many an (un)happy hour poking about in those old electro-mechanical regulator boxes. Even by old Jo's low standards, they were crap.

Chris

Probably not. Have you seen those alternators made to look like the old Lucas dynamos?

Yes - I did wonder about some form of fluid coupling to restrict the maximum speed too. So you could run it faster at low engine revs. A bit like some fans.

They dated back to before transistors - so really the only practical way. An engineer pal of mine made an electronic regulator which fitted in the old Lucas casing. He said the performance was way better - and easy to limit the maximum output to something safe.