Elite gearbox

[Mike-SEiW]

not sure I buy Nick's arguement. The ECU has to respond fast enough to time the spark. To do that it has to be able to accurately detect events of a degree or so at any revs.

The only way this could be the case is if the ECU deliberately waits a few cycles before cutting spark/fuel.

Looking at the code for megasquirt at least I don't think it does that. it measures revs each cycle and checks for the rev limit continuously in the main program loop.

Toby

I think the over rev due to ECU "lag" is in the magnitude of 50 or maybe 100 rpms at most. If not less.

However, I do agree it "feels" like it would over rev as the revs rise so "bl**** fast" when at WOT and no load.

I've been partaking in building an engine dyno for 3.5cc two-strokers that we use in 1/8 dragracers, and the engines rev at near 50.000 rpms. We had a guy design the electronics to measure revs, and we thought the ecu would never ever be able to detect the light beam shining through a hole in the flywheel at those speeds, but he just laughed at us and said when it's at about 10 times that, i'll have to start scratching my head. Modern electronics are      (btw he is a true rocket science engineer, which is kinda impressive    )

[SteveF]

I guess the potential for over-rev will be linked to the way the limiter logic works, as the ECU must be able to predict the 'new' revs between every spark/injector - as it needs to time these correctly based on things like the (crank) acceleration, and throttle position etc - so if the limiter works 'softly' by progressively cutting spark / fuel to the cylinders, then it may try cutting 1, then 2, then 3 etc. until the 'over-rev' is stopped - but it may take a full cycle to do this - hence you may get an 'over-rev'

[Nick M]

If I might point out, we have evidence in the form of Westfieldman's data logging from the ECU which shows that an over-rev situation did, in fact, arise.

[Toby Mack]

If I might point out, we have evidence in the form of Westfieldman's data logging from the ECU which shows that an over-rev situation did, in fact, arise.

a fair point, I missed that bit  

I still think there is no physical/electronic reason that this should happen but it does indicate the MBE didn't react fast enough. Perhaps it has some algorithms to prevent false triggering if it gets unexpected signals and this confused it?

[Nick M]

I'm guessing here, but I wonder if the ECU is purely passive (it receives data from sensors and reacts) or whether it's just a little bit active, i.e. if it's looking at the rate of change of, for example, fuelling under hard acceleration then it may "predict" what the requirements will be in future cycles rather than relying purely on data from the sensors to tell it what to do next and thus create a little bit of lag in the response of the ECU to the rev-limit being reached.

I know there's a setting in Easimap which looks at snap throttle fuelling which is supposed to deal with situations where the throttle is suddenly wide open and the engine needs to respond quickly.  If this is tied to the throttle position that's one thing, but if it also looks at the rate of change of engine revs then it might think the sudden rise in revs is just the car accelerating (especially if the throttle is wide open) and that may kick in some over-riding algorithm.

Like I say, this is now purely speculation and guesswork on my part so don't take it as anything more than that.

I too am curious as to why the ECU didn't catch it sooner and limit the revs.

[Bananaman]

If the rev's were @ say 5,500 then the engine with NO load on it would fly up to the Rev limiter say 7,800 and the inertia would have carried it on after the spark/fuel was cut. Unless of course it has some kind of predictive sensing built in as Nick M mentions. However with big power and sdden loss of load, i'd imagine that the rev's would get from 5,500 to 7,800 (and beyond) befor you could say 'doh S***'

[stu999]

If I might point out, we have evidence in the form of Westfieldman's data logging from the ECU which shows that an over-rev situation did, in fact, arise.

Apparently so. But it doesn't log the reason (which I suppose, is why we are having this discussion...).

For example, as you mentioned earlier in the thread, snagging the wrong gear would do the same...

And the shock would be enough to snap a weak input shaft?  

I do know that my old boss managed to select 2nd gear instead of 4th in his Mk4 Escort diesel, whilst ragging the   off it - and ripped the centre clean out of the clutch plate...  No ECU to detect over rev though...  

[Mike-SEiW]

If the rev's were @ say 5,500 then the engine with NO load on it would fly up to the Rev limiter say 7,800 and the inertia would have carried it on after the spark/fuel was cut.

No, that is physically impossible for it to "keep increasing" the revs after fuel/ignition is cut due to fast acceleration just previously. As soon as you stop feeding it energy, it will stop accelerating regardless of how fast it was accelerating before you removed the input of energy.

However, since the overrev did occur, perhaps the ECU didn't respond fast enough. That is the only explanation as I see it.

Can we see the datalog? Is it easy to export to a text format? I would really like to see what the ecu did to fuel and ignition and what happened afterwards. I'm only interested in rpms, fuel (ms) and igntion advance if those are exportable somehow?

[Nick M]

If the rev's were @ say 5,500 then the engine with NO load on it would fly up to the Rev limiter say 7,800 and the inertia would have carried it on after the spark/fuel was cut. Unless of course it has some kind of predictive sensing built in as Nick M mentions. However with big power and sdden loss of load, i'd imagine that the rev's would get from 5,500 to 7,800 (and beyond) befor you could say 'doh S***'

For the revs to keep on rising you need to keep adding energy - if you cut the fuel and / or spark then the crank will not continue to rotate ever faster.

This is something Mike pulled me up on in relation to my use of the word inertia.  I was talking about a situation where the energy had been removed (spark / fuel had been cut) but the inertia in the bottom end meant it continued to rotate above the rev limit because the ECU cannot *extract* energy to slow the crank down, it can only stop adding energy.

Edited to say - Mike beat me to it !!

[Bananaman]

If the rev's were @ say 5,500 then the engine with NO load on it would fly up to the Rev limiter say 7,800 and the inertia would have carried it on after the spark/fuel was cut. Unless of course it has some kind of predictive sensing built in as Nick M mentions. However with big power and sdden loss of load, i'd imagine that the rev's would get from 5,500 to 7,800 (and beyond) befor you could say 'doh S***'

For the revs to keep on rising you need to keep adding energy - if you cut the fuel and / or spark then the crank will not continue to rotate ever faster.

This is something Mike pulled me up on in relation to my use of the word inertia.  I was talking about a situation where the energy had been removed (spark / fuel had been cut) but the inertia in the bottom end meant it continued to rotate above the rev limit because the ECU cannot *extract* energy to slow the crank down, it can only stop adding energy.

Edited to say - Mike beat me to it !!

Are you sure that the acceleration would not still be in the components

It would be interesting to find out the recation times of the ECU/Sensor's.

Any idea what speed an engine would increase it's rev's with no load

[Nick M]

The "inertia" explanation is possibly explained by looking at a pendulum.

Imagine yourself stood in a big circle with a pendulum (say a 100kg weight) dangling from the ceiling.  Now, grab hold of the weight and walk outwards toward the edge of the circle.  If you *push* the pendulum away from you (thus adding energy) it will come back and smack you in the face.  

If, however, you simply let go (thus not adding any more energy) it will only swing back to about the point at which you held it (it will use up some energy - over-coming air resistance, etc.- and thus not swing back quite as far) so you won't get smacked in the face.

Same with an engine - keep adding energy and it will accelerate.  Stop adding energy and it cannot keep accelerating.

[Bananaman]

The "inertia" explanation is possibly explained by looking at a pendulum.

Imagine yourself stood in a big circle with a pendulum (say a 100kg weight) dangling from the ceiling.  Now, grab hold of the weight and walk outwards toward the edge of the circle.  If you *push* the pendulum away from you (thus adding energy) it will come back and smack you in the face.  

If, however, you simply let go (thus not adding any more energy) it will only swing back to about the point at which you held it (it will use up some energy - over-coming air resistance, etc.- and thus not swing back quite as far) so you won't get smacked in the face.

Same with an engine - keep adding energy and it will accelerate.  Stop adding energy and it cannot keep accelerating.

So were looking at ECU reaction times then

[Nick M]

Yes, and how they cope with a sudden mechanical failure which causes the revs to increase very rapidly, because all of the energy from the engine that was being put through the transmission suddenly isn't - it has nowhere to go but the engine.  

Can the ECU stop supplying energy (fuel / spark) quickly enough to prevent the over-rev when there is already an unexpected amount, maybe even an excess (more energy than is needed to operate at a particular level of engine revs), of energy in the engine.

[Bananaman]

Wonder if there is any data on the reaction time of the MBE and data on how fast an engine under the loads mentioned would accelerate...........

[ACW]

Ive got a Quaife 6 speeder on my sprint car with a vauxhall, which as it is ready for a rebuild, and due to me mincing with the shift, occationally misses a gear like 3rd. When this happens I am going WOT with nothing by the first input shaft rotating. The engine has a 5" flywheel and clutch and steel bits and will sit there bouncing off the limiter which is set at 9K5 but never exceeds 9K6 in these situations. Due to the rotating intertia it gets there bloddy quickly.

I would guess that if there was a continuation of engine rpms, that this could be somthing like

1. timer based event in the ECU processor rather than immediate interupt driven, or interupt driven but at too lower priority. thus giving a dely for the processor to service the spark cut.

2. Continuation to burn fuel without sparks (time to decoke the engine)  the ECU only cuts sparks generally to prevent leaning at rev limit.

3. Conservation of inertia ? ie if you suddenly loose some of the rotating mass ?