TomF

What about the inertia of the flywheel? It has significant mass. Cutting the spark and fuel won't cause the engine to immediately stop. I would suspect it will go through several strokes before friction and compression stop it. It is probably imperceptible to us, since 7000rpms equates out to ~117 revolutions per sec...

N51

Tom,

From my first post, I could not accept that the engine could accelerate beyond the point of no energy being put to it. But I have come to accept(at least for now ) that the stored energy in the flywheel could contribute to an increase in rpm beyond the point of fuel/ignition shutoff.

Geoff and Geoffrey have conducted similiar tests and have made different observations. The great likelyhood of dissimilar control parameters(weight of flywheel, for one) could resolve this.

To carry the thought to the extreme, imagine an engine, with no flywheel, under the same test. Catastrophic.

Noah

Felix

But a water wheel tends to reach equilibrium (steady speed) quite quickly. When the water supply is stopped some of the "buckets" still contain water so the wheel will still turn but slow down fairly rapidly once the "buckets" are empty.

What we're trying to understand here is the situation where an accelerating system appears to continue to accelerate when force is removed. That seems to me to be a contradiction to the laws of motion. There might be a latent ignition stroke that hasn't finished but other than that there's no more energy being added.

If this were true you could accelerate to 60 mph and at exactly 60 stomp on the clutch and the car would continue to accelerate. Or a skier accelerating down a ski slope would continue to accelrate once hitting a flat section. Force = mass times acceleration or accelearation = force dividied by mass. When Force = 0 then accelleration must =0 or perpetual motion is just around the corner.

(I spent a year as a Physics major before I changed to applied Math because the girls were slightly better looking.)

From Charles Probst's book "Bosch Fule Injection and Engine Management" there's a paragraph titled "RPM Limitation" under the Motronic section. It reads "If the RPM signal is greater than max-allowable rom stored in the computer's memory the control-unit signals a cutback of the fuel injection waunity. A scope set to read four pulses for simulaneous injection for four cylinders shows the limitation cutback of every other pulse, one pulse every other crankshaft revolution."

pcar964

Completely wrong - I thought everyone understood this....

once energy stops being added, the engine will CEASE TO ACCELERATE. The inertia of the engine will keep it spinning, but NOT ACCELERATING.

Felix

I'm in vehement agreement.

N51

And I'm still listening. :-)

The energy stored in the flywheel does not and cannot exceed the energy required to increase the engine rpm's beyond the point of fuel/ignition cutoff.

Have I got it right, yet?

Noah

Terry Adams

I was wrong. Just tested mine. With CC engaged, depressed clutch. Revs dropped to idle and stayed at idle as long as clutch was depressed.

Cincy_Ron

Lol! That why I took so many Communication electives in college!

Felix

Yes, sort of. The flywheel is bolted to the crankshaft so it's rotating at the same speed. Without an outside influence the engine's rpm can't rise. The greater the mass of the flywheel the longer it will take for the engine to slow down when spark/fuel is removed.

jimbo3

Correct. Actually, at 7000 RPM with no load and no spark, I'd expect that the engine would take 3 to 4 seconds to coast to a stop. Without doing the math, that would be around 150 revolutions or so. However, it still won't (can't) accelerate at any point after cutting the ignition, which is the point.

-Jim

Red rooster

Just to bring some heavy duty stuff in , how do you think an F1 motor is inhibited at 19,000 rpm +/- 50 rpm ? with the accelerator on the floor .

Hate to disapoint but its done by rev limit + fuel cut !! Same old stuff that has always worked .

The mistake creeping in is using a moving coil tachometer ( Rev counter ) which has the dynamics of a toffee apple ! The normal car rev counter is to provide the driver with information from a low cost system .It is not precision .

To really understand this a proper data accquisition is needed.

Does that help ??

Geoff
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KS400200,the oldest 964 on Rennlist,unless you know differently !

Felix

But the F1 car also has an automatic transmission to shift when the revs hit the limit...

And they don't have cruise!

Red rooster

Felix,

When an F1 is in any gear, including top, there is a mandatory rev limit .

It is part of the FIA regulations .

The automatic gearbox is a sophisticated , electronic/hydraulic manual type gear selection system with driver paddle control and not to be confused with the 4 gear Auto shifter / torque converter as in a nice saloon !!!


Does that help ??

Geoff
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KS400200,the oldest 964 on Rennlist,unless you know differently !

N51

For now, I'm on the sidelines. You're all so much fun!

Thanks,
Noah

Performance music major. They excell beyond the math majors.:-)

Matt Lane

Yeah, but for the Doofus that started this (me) not so much fun!

My conclusion:

1. I probably have not done any damage in neutral on account of the rev limiter.

2. I will be really careful from now on wrt disengaging cruise.

3. If my "just turned 20K mile" beauty has a fatal malady in the next 100K miles, I'll have to assume point one above was incorrect!

Cheers~!