11 replies to this topic

[Fausta]

What was Kimi referring to when he mentioned his rev limiter kicked in on one of the gears?

Is this good or bad for the engine?

Thanks,
Kelly

[Slumberer]

Actually the Macs have two different rev limiters:
One controlled by the engine's ECU stops the engine exceeding a certain rpm.
This can be varied by the driver so as to best trade off reliability against performance.

The other system used by the Macs uses a crude mechanical system which launches pieces of engine through other bits of the self same engine.
This is heralded by the emmision of plumes of smoke, and the revs are rapidly reduced.

To zero...

[LongJohnSilver]

He was saying that his 7th gear was a bit to short. So this meant that at the end of the straight and probably only in a draft, he would reach the very top end of his gearing and thus not be able to go any faster.

You could hear it on TV. I wasnt sure what it was at first and was expecting to see one of the previously mentioned coulds of smoke come from the rear of his car.

[Fausta]

Doed pushing the limits of these engines in this faashion cause undo stress on them?
Perhaps Kimi should trade out the engine for a new one in China.

Thanks for the clarification on the revs as I am not a motor specialist, just love the sport.
Kelly

[voice_of_reason]

It was windy!

The wind was variable in strength and direction.

In particular, there was a headwind on the straight Saturday (when the gear ratios were finalised) and a tail wind during the race.

Combination of tailwind and slipstream caused the McLarens (and others) to run out of revs in top gear. Dangerous for reliability, but nobody blew up (at least not in Japan!).

[zac510]

Or the rev limiter could have been set to the lower setting whereas the gearing was chosen for high rev limit setting?

[LongJohnSilver]

Originally posted by voice_of_reason
It was windy!

The wind was variable in strength and direction.

In particular, there was a headwind on the straight Saturday (when the gear ratios were finalised) and a tail wind during the race.

Combination of tailwind and slipstream caused the McLarens (and others) to run out of revs in top gear. Dangerous for reliability, but nobody blew up (at least not in Japan!).

Ahhh thats interesting. Was wondering how they would make such a mistake.

But, I dont think that hitting the rev limiter would do anything to reliabilty of the engine. That is just what it is there for, to keep you from going into the range that will hurt it. But I guess if the gearing is of a higher ratio than it should be, then over the course of a race (or any period of time) uou will have more rotations and faster rotations which will be more stressful.

The idea of changing Kimis engine is also intresting, but not because these circumstances have put undue stress on it. Instead they should think about changing it because others (Alonso and Fisi) will have engines that only have to last one race. They will have a definite advantage. However, I doubt it is an advantage worth taking a 10 spot penalty over.

Mercedes should bring a short life engine though so that if anything does happen which would warrant and engine change (probably a 50/50 shot for them ;) ) they can get that advantage back. I'm sure they will do this.

[voice_of_reason]

That it was windy is not speculation, it is fact.

Likewise that the direction changed Saturday to Sunday.

It was visible in real time on the weather page of FIA live timing feed.

It was not just McLaren that had trouble.

McLaren revs were normal from in-car camera during race (shifts about 18600 RPM), so no reason to believe settings were wrong. In any case, the driver has several engine settings to choose from on the steering wheel, he would be unlucky if they were all messed up.

With parc fermé rules, gear ratios cannot be changed after qualifying, so must be chosen on Saturday morning based on estimations of Sunday weather. Its not easy. To be a few hundred RPM off is normal if there is a big wind change.

[FordFan]

Since we're on the topic of rev limiters, there's something I always wondered.

Why is a tailwind a rev limit problem? I understand that the car goes faster, which means the wheels spin faster, and thus the crank, etc. But so what? Is it the mere moving of the bits that causes the problems for the engine, or the problem of controling the detonations at that speed? How fast could you run an engine without fuel, ignition sparks, valve action, etc (say, by just spinning the wheels)?

Seems like, instead of having a rev limit, what you should do is have an engine control system that staggers the ignition in the cylinders, etc., so that they aren't firing so regularly at the top speeds. That way you get the higher revs, and thus higher speeds, without having to overstress the engine.

That is, unless the mere speed of the moving parts is what causes the stress, in which case I'm just an idiot.

[Greg Locock]

It's the accelerations, which increase as the speed increases.

At high speed some engines see higher forces at full load, but when I used to work on dynos we used to think that the high speed no load case was more likely to break the engine.

So with a tailwind the engine can run that little bit faster, the accelerations are higher, so the forces are higher and suddenly there is a loud noise.

[McGuire]

Originally posted by FordFan
Why is a tailwind a rev limit problem? I understand that the car goes faster, which means the wheels spin faster, and thus the crank, etc. But so what? Is it the mere moving of the bits that causes the problems for the engine, or the problem of controling the detonations at that speed? How fast could you run an engine without fuel, ignition sparks, valve action, etc (say, by just spinning the wheels)?

It doesn't really matter a great deal if the wind or another condition -- say running downhill -- causes the engine to overspeed beyond its tested and rated limit. It's still bad. Some ways are more brutal in their methodology than others, for example downshifting. But that's just because it's more rpm, quickly.

How fast an engine could be rotated minus its torque or pressure loadings is sort of an interesting question. Bring your car around to the shop and let's find out.

Let's see, how we can perform this test...we take out your spark plugs and disable your fuel and ignition, put your car in first gear and tow it until the engine goes blammo. Then you tell us what the tach was reading when the big noise occurred.

But seriously... not much faster than running it normally, I expect. You could find some parts coming adrift sooner running under no or reduced load. Engines are fairly complicated in that regard. It is claimed that Mario Andretti once lost an Indy 500 by under-revving the engine. He had the race in the bag and was trying to baby it home and by keeping the revs down he put the engine into a bad vibration period and a part snapped in two -- a pump shaft or something. Or at least that's how the story goes at Ilmor.

[McGuire]

Originally posted by FordFan
Seems like, instead of having a rev limit, what you should do is have an engine control system that staggers the ignition in the cylinders, etc., so that they aren't firing so regularly at the top speeds. That way you get the higher revs, and thus higher speeds, without having to overstress the engine.

That is pretty much how it is done these days. Both the fuel and spark are manipulated to make the rev limit as gentle as possible...it's known as a "soft limiter." In many applications there is a soft limiter, then a hard limiter at some rpm above it as a failsafe.