Lightweight flywheel casing overrevs?
07-05-2018, 09:31 PM
#17
Frank, i see the graph, but how is this even remotely possible? if the throttle closes, there is no more increase in power, how does the engine increase rpm with no added energy? It reminds me of the graphs of dyno runs. the graph shows jumps in RPM at the end of a high power pull, but it is just electrical noise.
The lightweight flywheel can indeed cause overrev counts. Some aftermarket light flywheels have the tooth form for the crank position sensor cut incorrectly. This causes the crank position signal to the engine ECU to be incorrect under some conditions. Normally the engine will run perfectly, but the ECU will think there are random misfires and overrevs.
Mark, do you think it is impossible for the last combustion cycle prior to the fuel cut to push the rpm of an unloaded engine above the fuel cut rpm? I am not sure, but I think it might be possible, especially with a turbo engine.
RPM data can lie. The sampling rate of the data can show things that are not really happening. You have the ECU getting a possibly noisy signal from the crankshaft position sensor, the ECU then reporting that signal to a CAN bus at an unknown sampling rate, and with an unknown amount of filtering, where it is picked up by the data acquisition system at a known sampling rate with possibly unknown filtering. Lots of opportunity for error there. Realistically it is +/- 50 rpm.
Chris Cervelli
Cervelli Technical Service
07-05-2018, 09:41 PM
#18
[QUOTE=mark kibort;15123051]... when fuel cut off is at 6600rpm for example, there is NO more fuel at 6600rpm.
There is some fuel for the next cycle at least, and maybe more. The fuel is pulled off of the port walls and valves by the high port vacuum that occurs once the throttle closes. It is arguable whether those cycles can produce any power.
Chris Cervelli
Cervelli Technical Service
There is some fuel for the next cycle at least, and maybe more. The fuel is pulled off of the port walls and valves by the high port vacuum that occurs once the throttle closes. It is arguable whether those cycles can produce any power.
Chris Cervelli
Cervelli Technical Service
07-06-2018, 08:46 AM
#19
Three Wheelin'
Mark - if the car is not in gear and throttle is depressed (e.g. errant blip when shifting or accidental neutral shift), the engine can go past fuel/ignition cutoff RPM due to momentum from the reciprocating mass. I'd guess it's only possible in neutral.
07-06-2018, 09:32 AM
#20
Rennlist Member
Chris hit it on the head, I have lab scoped duel mass and aftermarket flywheels, At redline during the transition from load to unloaded there is a lot of hash in the pattern with lightweight flywheels that is not there with the duel mass flywheels. How the ecm is interpreting this I cannot tell you but I would think that the ecm is not programmed to filter out the hash that the duel mass flywheel is buffering. I can say that my 996 frequently overreves on downshifts by 200-300 rpm and onetime when I hit 1st rather than 3rd on a downshift and stopped the motor it registered 12500 rpm that is impossible without causing damage.
07-06-2018, 11:10 AM
#21
Rennlist Member
momentum doesn't work that way........it may seem intuitively that it can continue to rev higher, without additioal fuel, but it cant. there was a discussion like that about guys accelerating through the gears and there was some resistance to the concept that between gear shifts you not only stop acceleration as soon as the clutch is pushed in, but actual deceleration. as long as the fuel stops at a certain RPM , there is no more force on the piston to accelerate it to a higher RPM than the redline limit.
07-06-2018, 11:50 AM
#22
Rennlist Member
Yes, we know there is some built in de-coupling of the physcial pedal for the the reasons you describe. I think we are also talking of two different topics here. one , from the OP, being worried about erroneous RPM hits and worried about if they are real. the other is whether or not ,a lightweight flywheel because of its weight , can can cause overrev. i was addressing the later. Yes, when these aftermarket flywheels are made. and they do not have the same sensing configurations, issues that have been described can happen with registering accurate RPM data.
as far as the last cycle of a combustion cycle..
as far as the last combustion cycle goes, no there is not enough force or time for this to likely increase engine speed to register 400rpm over redline. the reason being, is that at 6000rpm, thats 100 revolutions in a second. if you were to over rev your engine by 400rpm, that would be an additional 6 revolutions in a second, or 12 revs in 1/2 sec, or 24 revs in 1/4 sec or near 50 revs in a 1/10 of a sec. so, one combustion cycle is not likely capable of increasing the speed by 400rpm after a throttle lift
as i mentioned above, i dont think there is enough time, and force to accelerate the engine with one or two engines cycles. yes, fuel is pulled off the walls of the port , back of valve, but there is not enough volume in the intake plennum to produce any combustible mixture, this is why a lot of race cars are flame throwers when they are off throttle....... the fuel that is remaining, and being allowed in , is carried out the hot exhaust, and doesnt combust, util it reaches open air at the tail pipe. so no, a "cycle" with the throttle closed will not produce any power.
[QUOTE=CTS;15123866]
i get the noise and difference in the sensor patterns cut in the flywheel. makes sense it could generate noise.in registers. im trying to understand what you are saying about overevs on downshifts, yes, a downshift is over and above fuel and spark control. .......these are mechanical forces that can over rev the engine by near 35% in almost an instant. again, we are also talking of two things here. the lightweight flywheel being a problem for overrev out of gear (neutral) , the lightweight flywheel cuts that the ECU gets readings from, vs a stock flyweel, and misshift forces to spin up the engine from the the rear wheel forces.
as far as the last cycle of a combustion cycle..
as far as the last combustion cycle goes, no there is not enough force or time for this to likely increase engine speed to register 400rpm over redline. the reason being, is that at 6000rpm, thats 100 revolutions in a second. if you were to over rev your engine by 400rpm, that would be an additional 6 revolutions in a second, or 12 revs in 1/2 sec, or 24 revs in 1/4 sec or near 50 revs in a 1/10 of a sec. so, one combustion cycle is not likely capable of increasing the speed by 400rpm after a throttle lift
It is possible because Bosch_PPS is not throttle position. It is pedal position. Throttle position does not faithfully follow pedal position in many cases. The throttle closes slowly in order to reduce the emissions of the engine. This pattern is very common on drive by wire engines.
The lightweight flywheel can indeed cause overrev counts. Some aftermarket light flywheels have the tooth form for the crank position sensor cut incorrectly. This causes the crank position signal to the engine ECU to be incorrect under some conditions. Normally the engine will run perfectly, but the ECU will think there are random misfires and overrevs.. ......
Mark, do you think it is impossible for the last combustion cycle prior to the fuel cut to push the rpm of an unloaded engine above the fuel cut rpm? I am not sure, but I think it might be possible, especially with a turbo engine.
RPM data can lie. The sampling rate of the data can show things that are not really happening. You have the ECU getting a possibly noisy signal from the crankshaft position sensor, the ECU then reporting that signal to a CAN bus at an unknown sampling rate, and with an unknown amount of filtering, where it is picked up by the data acquisition system at a known sampling rate with possibly unknown filtering. Lots of opportunity for error there. Realistically it is +/- 50 rpm.
Chris Cervelli
Cervelli Technical Service
The lightweight flywheel can indeed cause overrev counts. Some aftermarket light flywheels have the tooth form for the crank position sensor cut incorrectly. This causes the crank position signal to the engine ECU to be incorrect under some conditions. Normally the engine will run perfectly, but the ECU will think there are random misfires and overrevs.. ......
Mark, do you think it is impossible for the last combustion cycle prior to the fuel cut to push the rpm of an unloaded engine above the fuel cut rpm? I am not sure, but I think it might be possible, especially with a turbo engine.
RPM data can lie. The sampling rate of the data can show things that are not really happening. You have the ECU getting a possibly noisy signal from the crankshaft position sensor, the ECU then reporting that signal to a CAN bus at an unknown sampling rate, and with an unknown amount of filtering, where it is picked up by the data acquisition system at a known sampling rate with possibly unknown filtering. Lots of opportunity for error there. Realistically it is +/- 50 rpm.
Chris Cervelli
Cervelli Technical Service
[QUOTE=CTS;15123866]
... when fuel cut off is at 6600rpm for example, there is NO more fuel at 6600rpm.
There is some fuel for the next cycle at least, and maybe more. The fuel is pulled off of the port walls and valves by the high port vacuum that occurs once the throttle closes. It is arguable whether those cycles can produce any power.
Chris Cervelli
Cervelli Technical Service
There is some fuel for the next cycle at least, and maybe more. The fuel is pulled off of the port walls and valves by the high port vacuum that occurs once the throttle closes. It is arguable whether those cycles can produce any power.
Chris Cervelli
Cervelli Technical Service
Chris hit it on the head, I have lab scoped duel mass and aftermarket flywheels, At redline during the transition from load to unloaded there is a lot of hash in the pattern with lightweight flywheels that is not there with the duel mass flywheels. How the ecm is interpreting this I cannot tell you but I would think that the ecm is not programmed to filter out the hash that the duel mass flywheel is buffering. I can say that my 996 frequently overreves on downshifts by 200-300 rpm and onetime when I hit 1st rather than 3rd on a downshift and stopped the motor it registered 12500 rpm that is impossible without causing damage.
Last edited by mark kibort; 07-06-2018 at 02:31 PM.
07-06-2018, 02:39 PM
#23
Rennlist Member
07-06-2018, 04:52 PM
#24
Eluding to my first response, if RPM is actually sensed by the flywheel and the replacement flywheel causes trash on the RPM sensor that the DME does not know about, then bogus overrev's are possible. It would take someone very familiar with the motor in question to figure it out. With the number of cars on the market, I doubt it is worth the hassle to figure it out. Even if true, imagine trying to unload this car when the OP wants to sell it in the future.
-Mike
07-06-2018, 06:18 PM
#25
Rennlist Member
Per the bolded, nothing happens instantly. The factory fuel cutoffs here are intended to be last line of defense and are certainly not intended for everyday use. A such, they are not that sophisticated.
Eluding to my first response, if RPM is actually sensed by the flywheel and the replacement flywheel causes trash on the RPM sensor that the DME does not know about, then bogus overrev's are possible. It would take someone very familiar with the motor in question to figure it out. With the number of cars on the market, I doubt it is worth the hassle to figure it out. Even if true, imagine trying to unload this car when the OP wants to sell it in the future.
-Mike
Eluding to my first response, if RPM is actually sensed by the flywheel and the replacement flywheel causes trash on the RPM sensor that the DME does not know about, then bogus overrev's are possible. It would take someone very familiar with the motor in question to figure it out. With the number of cars on the market, I doubt it is worth the hassle to figure it out. Even if true, imagine trying to unload this car when the OP wants to sell it in the future.
-Mike
07-06-2018, 06:59 PM
#26
I don't know about flywheels continuing to accelerate after fuel cutoff but I do know that a car will accelerate through wet grass. Einstein studied this and found that the acceleration was so great that in twins where one stayed on the track and the other went in the grass, the off track twin would be nearly 80 years older when he finally came to rest.
07-06-2018, 08:40 PM
#27
I don't know about flywheels continuing to accelerate after fuel cutoff but I do know that a car will accelerate through wet grass. Einstein studied this and found that the acceleration was so great that in twins where one stayed on the track and the other went in the grass, the off track twin would be nearly 80 years older when he finally came to rest.
-Mike
07-06-2018, 08:57 PM
#28
Rennlist Member
I don't know about flywheels continuing to accelerate after fuel cutoff but I do know that a car will accelerate through wet grass. Einstein studied this and found that the acceleration was so great that in twins where one stayed on the track and the other went in the grass, the off track twin would be nearly 80 years older when he finally came to rest.
