Cruise control, over-rev - how bad?
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Originally Posted by Geoffrey
Noone is arguing that the DME is not fast enough. An ECU works at speeds much higher than the engine operates, nanoseconds vs milliseconds. Simply stopping spark does not stop an engine. You are forgetting about the physics involved.
Nordschleife Master
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From: Kingston, NY
The engine is accelerating at a given rate, say 1000rpm/sec. When the fuel and spark is cut off, it is still accelerating at 1000rpm/sec up to that point, then the ACCELERATION RATE has to DECELERATE to 0rpm/sec before the engine can begin a deceleration rate. The ENTIRE time the engine is accelerating above 0, the engine RPM will INCREASE! That is why! And, we are talking more like 3000-4000 rpm/sec in neutral WOT, not 1000rpm/sec. There is no more additional energy being added to the engine to maintain or increase the acceleration rate, however, cutting fuel and spark will not instantly overcome the momentum of the engine and stop the acceleration rate. If it did, you'd be picking the rods out of the head as the rod bolts broke. This is basic high school physics we are talking about.
Three Wheelin'
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From: Halifax, Nova Scotia , Canada
" I think the test Geoff was mentioning is probably different than the way a 964 ECU is setup and programmed. "
Fortunately I know a lot more about the 964 DME operation than most ! as writing motor management software is what I do !
Motronic systems ,including the current 997 ,all have the same thought processes in operation software as far as the rev limit is concerned.
The rev limit is a very clear name . The system is set to inhibit further rpm once the rev limit rpm is reached.
That little story was to show that a simple ignition cut will limit the maximum rpm even with the throttle at WOT . All simple stuff !!
Geoff
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KS400200,the oldest 964 on Rennlist,unless you know differently !
Fortunately I know a lot more about the 964 DME operation than most ! as writing motor management software is what I do !
Motronic systems ,including the current 997 ,all have the same thought processes in operation software as far as the rev limit is concerned.
The rev limit is a very clear name . The system is set to inhibit further rpm once the rev limit rpm is reached.
That little story was to show that a simple ignition cut will limit the maximum rpm even with the throttle at WOT . All simple stuff !!
Geoff
----------------------------------------------------------------
KS400200,the oldest 964 on Rennlist,unless you know differently !
Drifting
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From: behind the Corn Curtain
"There is no more additional energy being added to the engine to maintain or increase the acceleration rate, however, cutting fuel and spark will not instantly overcome the momentum of the engine and stop the acceleration rate."
Geoffrey,
Okay, that was worth the wait. :-) Maybe the key word here is 'momentum'. 'Acceleration' and 'momentum' are not interchangeable, but perhaps they've been incorrectly used or thought as the same by myself. Acceleration rate is the principle idea that helps me understand.
If one considers a rocket accelerating through the atmosphere, the thrust being suddenly cut, the rocket continues upward for a short time while decelerating. Did I get that right, or is the light at the top of my hole getting smaller?
Noah
Geoffrey,
Okay, that was worth the wait. :-) Maybe the key word here is 'momentum'. 'Acceleration' and 'momentum' are not interchangeable, but perhaps they've been incorrectly used or thought as the same by myself. Acceleration rate is the principle idea that helps me understand.
If one considers a rocket accelerating through the atmosphere, the thrust being suddenly cut, the rocket continues upward for a short time while decelerating. Did I get that right, or is the light at the top of my hole getting smaller?
Noah
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Noah,
Your analogy is spot on. A rev limiter doesn't in any way "brake" or arrest the engine. In many ways the name "rev limiter" is a mis-nomer. It should be called a fuel and spark cutoff. It really doesn't "limit" the revs.
Hitting WOT in neutral will most certainly damage or destroy an engine regardless of the rev limiter.
Your analogy is spot on. A rev limiter doesn't in any way "brake" or arrest the engine. In many ways the name "rev limiter" is a mis-nomer. It should be called a fuel and spark cutoff. It really doesn't "limit" the revs.
Hitting WOT in neutral will most certainly damage or destroy an engine regardless of the rev limiter.
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Originally Posted by Geoffrey
The engine is accelerating at a given rate, say 1000rpm/sec. When the fuel and spark is cut off, it is still accelerating at 1000rpm/sec up to that point, then the ACCELERATION RATE has to DECELERATE to 0rpm/sec before the engine can begin a deceleration rate. The ENTIRE time the engine is accelerating above 0, the engine RPM will INCREASE! That is why! And, we are talking more like 3000-4000 rpm/sec in neutral WOT, not 1000rpm/sec. There is no more additional energy being added to the engine to maintain or increase the acceleration rate, however, cutting fuel and spark will not instantly overcome the momentum of the engine and stop the acceleration rate. If it did, you'd be picking the rods out of the head as the rod bolts broke. This is basic high school physics we are talking about.
Nordschleife Master
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From: Kingston, NY
FWIW, I just checked my car and in neutral at 2000rpm steady then WOT, the RPM needle will go to the 7200mark and then drop back to the 6800 mark the three times I did it.
You are correct, I am mistakenly using "acceleration rate" instead of "velocity", and I should know better. However, the principal remains the same.
You are correct, I am mistakenly using "acceleration rate" instead of "velocity", and I should know better. However, the principal remains the same.
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I think whats happening here is that the power has been applied, then cut when the gas/spark is cut. But power is still stored temporarily in the inertia of all the individual parts.
If you look at the engine as one piece then yes, cut the gas and spark and it will immediately stop accelerating, but if you think of all the individual little pieces, non powered pistons, the crank, flywheel etc. They all have stored energy that they are releasing.
Think of it like a water wheel, when you shut off the water, the potential energy that applies force to the wheel is still there for a bit afterwards applying force to the wheel.
So Pcar964, yes, when you stop applying force something will decelerate, but there is still stored energy in the moving parts that has to dissipate before the apparatus as a whole stops getting force applied to it. The motor, under gas and spark, is presumed to be driven at anyone time by only one piston. This actually is not true if you think about it. Each piston contributes to a myriad of moving metal masses that store energy and dissipate it over a period, albeit short, of time. So the power of the engine is actually coming from several pistons at a time. Even the ones not firing, because the force of their last explosion is still contributing to the overall.
An electrical analogy would be that of a capacitor. The individual moving parts in the engine act as capacitors and discharge for a moment after the current is applied.
Anyway, thats my shot at it. . .
If you look at the engine as one piece then yes, cut the gas and spark and it will immediately stop accelerating, but if you think of all the individual little pieces, non powered pistons, the crank, flywheel etc. They all have stored energy that they are releasing.
Think of it like a water wheel, when you shut off the water, the potential energy that applies force to the wheel is still there for a bit afterwards applying force to the wheel.
So Pcar964, yes, when you stop applying force something will decelerate, but there is still stored energy in the moving parts that has to dissipate before the apparatus as a whole stops getting force applied to it. The motor, under gas and spark, is presumed to be driven at anyone time by only one piston. This actually is not true if you think about it. Each piston contributes to a myriad of moving metal masses that store energy and dissipate it over a period, albeit short, of time. So the power of the engine is actually coming from several pistons at a time. Even the ones not firing, because the force of their last explosion is still contributing to the overall.
An electrical analogy would be that of a capacitor. The individual moving parts in the engine act as capacitors and discharge for a moment after the current is applied.
Anyway, thats my shot at it. . .
Drifting
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From: behind the Corn Curtain
Geoffrey,
If you would allow me, here is your amended statement:
"There is no more additional energy being added to the engine to maintain or increase the acceleration rate, however, cutting fuel and spark will not instantly overcome the [upward] momentum of the engine..."
Better, I think. What it all boils down to is...it's all your fault!
Noah
If you would allow me, here is your amended statement:
"There is no more additional energy being added to the engine to maintain or increase the acceleration rate, however, cutting fuel and spark will not instantly overcome the [upward] momentum of the engine..."
Better, I think. What it all boils down to is...it's all your fault!
Noah
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Originally Posted by N51
Geoffrey,
If you would allow me, here is your amended statement:
"There is no more additional energy being added to the engine to maintain or increase the acceleration rate, however, cutting fuel and spark will not instantly overcome the [upward] momentum of the engine..."
Better, I think. What it all boils down to is...it's all your fault!
Noah
If you would allow me, here is your amended statement:
"There is no more additional energy being added to the engine to maintain or increase the acceleration rate, however, cutting fuel and spark will not instantly overcome the [upward] momentum of the engine..."
Better, I think. What it all boils down to is...it's all your fault!
Noah
Can't establish or gain momentum with no energy as momentum is simply energy that's already been stored.
To the earlier comment about the tach reading higher after blipping the throttle- a tach is relatively slow to respond, so you're seeing the delayed reading.
I could buy into the ignition taking a split second to respond to a WOT over-rev and therefore gain a couple hundred revs beyond where it's supposed to cut out, but us mortals can't get something from nothing.
-Jim
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Originally Posted by Amfab
I think whats happening here is that the power has been applied, then cut when the gas/spark is cut. But power is still stored temporarily in the inertia of all the individual parts.
If you look at the engine as one piece then yes, cut the gas and spark and it will immediately stop accelerating, but if you think of all the individual little pieces, non powered pistons, the crank, flywheel etc. They all have stored energy that they are releasing.
Think of it like a water wheel, when you shut off the water, the potential energy that applies force to the wheel is still there for a bit afterwards applying force to the wheel.
So Pcar964, yes, when you stop applying force something will decelerate, but there is still stored energy in the moving parts that has to dissipate before the apparatus as a whole stops getting force applied to it. The motor, under gas and spark, is presumed to be driven at anyone time by only one piston. This actually is not true if you think about it. Each piston contributes to a myriad of moving metal masses that store energy and dissipate it over a period, albeit short, of time. So the power of the engine is actually coming from several pistons at a time. Even the ones not firing, because the force of their last explosion is still contributing to the overall.
An electrical analogy would be that of a capacitor. The individual moving parts in the engine act as capacitors and discharge for a moment after the current is applied.
Anyway, thats my shot at it. . .
If you look at the engine as one piece then yes, cut the gas and spark and it will immediately stop accelerating, but if you think of all the individual little pieces, non powered pistons, the crank, flywheel etc. They all have stored energy that they are releasing.
Think of it like a water wheel, when you shut off the water, the potential energy that applies force to the wheel is still there for a bit afterwards applying force to the wheel.
So Pcar964, yes, when you stop applying force something will decelerate, but there is still stored energy in the moving parts that has to dissipate before the apparatus as a whole stops getting force applied to it. The motor, under gas and spark, is presumed to be driven at anyone time by only one piston. This actually is not true if you think about it. Each piston contributes to a myriad of moving metal masses that store energy and dissipate it over a period, albeit short, of time. So the power of the engine is actually coming from several pistons at a time. Even the ones not firing, because the force of their last explosion is still contributing to the overall.
An electrical analogy would be that of a capacitor. The individual moving parts in the engine act as capacitors and discharge for a moment after the current is applied.
Anyway, thats my shot at it. . .
Theory is good for a theoretical maximum of 1/2 of a revolution, assuming that the ignition was cut immediatly after the last cylinder fired. The energy released in the firing of that last cylinder will accelerate that one piston to the bottom of the stroke and the rest of them are along for the ride. No more accelleration after that unless more energy is applied.
Drifting
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From: behind the Corn Curtain
Originally Posted by jimbo3
Can't establish or gain momentum with no energy as momentum is simply energy that's already been stored.
Noah
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What it boils down to is... the engine does not continue to accelerate once the spark is cut, except perhaps the theoretical 1/2 revolution of the last piston on the power stroke after the last ignition.