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Old 10th March 2011, 11:44   #46
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Default re: RPM shoots up when changing gears

Ambiguous question, Sutripta, the ambiguity being whether you are talking of a 'not normal' carb (FEM on your question) or one is talking of an electronically-regulated fuel 'supply' (as in 'one supplies').

The second interpretation is answerable: With electronic regulation, one does not 'supply' any fuel, so there is no koshchen of rich/just right/lean/too lean, or - expressed differently - 'too lean where fuel = 0'. On a normal carb engine (one doesn't supply but) the air stream pulls the equivalent of idling fuel, which is for the over-run circumstances (fuel not needed to produce energy) 'too rich'.

*running away before the Prof blows his top*
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Old 10th March 2011, 15:20   #47
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Default re: RPM shoots up when changing gears

^^^^
Hi,
[CLARIFICATION]
leans out. CR LF

In 'if one supplies', replace 'one' with 'God'.
[/CLARIFICATION]

'too rich'. As in more fuel than necessary, or >>15:1?

Actually, wanted to ask the cause of the pops and crackles one heard on the exhaust note of older sports cars on the overrun. But decided against it. In modern racing cars too many variables complicate the matter.

BTW, for this NON-professor, rubbing hands in glee far more satisfying than blowing ones top.

Hasta La Vista
Sutripta

Last edited by Sutripta : 10th March 2011 at 15:30.
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Old 12th March 2011, 12:29   #48
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Originally Posted by Sutripta View Post
... In 'if one supplies', replace 'one' with 'God'.
[/CLARIFICATION] ...
Cease and desist, lest you be subjected to blasphemy laws. It is all by man's design, do not malign the Almighty.

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Originally Posted by Sutripta View Post
... 'too rich'. As in more fuel than necessary, or >>15:1?
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Originally Posted by DerAlte View Post
... over-run circumstances (fuel not needed to produce energy) 'too rich'. ...
The same (the ratio doesn't matter if fuel is not needed anyway).

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Originally Posted by Sutripta View Post
... Actually, wanted to ask the cause of the pops and crackles one heard on the exhaust note of older sports cars on the overrun. But decided against it. In modern racing cars too many variables complicate the matter. ...
Really weird coincidence! While writing the previous post I had actually written the same about what happens to the resultant unburnt fuel in over-run. And then, thinking that the passage would be OT, I deleted it before submitting. Don't know about the crackles, but the pop is due to the unburnt fuel ignited by the hot spots after the exhaust manifold. The pop is succeeded by the rather phlegmatic harrumph of the flame wave propagating down the exhaust pipe.

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... this NON-professor ...
Please don't expect others to believe that.
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Old 12th March 2011, 19:36   #49
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Cease and desist, lest you be subjected to blasphemy laws. It is all by man's design, do not malign the Almighty.
When we are no longer in Dharmaniropekho Bharat, I'll change it omni(potent, present, scient).

Really weird coincidence!
......
The pop is succeeded by the rather phlegmatic harrumph of the flame wave propagating down the exhaust pipe.
I agree, though I'd prefer exhaust system to manifold. But where does the unburnt fuel come from?

Actually, not a coincidence. Because when talking of overrun conditions, that is the first thing that comes to mind. And I thought of using the example of the overrun condition because it is so easily observable.

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Old 15th March 2011, 13:04   #50
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Default re: RPM shoots up when changing gears

Er... why did this interesting discussion take a holiday?

@Sutripta, nitrogary, DerAlte, etc.
What, if any, was the role of the little bit of fuel being 'dragged' out of the idle jets in an old carb engine on overrun, in keeping the valves and tops of pistons cool? Also applicable to 2-strokes, what was the role of this little bit of fuel in lubricating the engine when a petrol + 2-stroke oil pre-mix (not a lube injection pump) was used?

And if there was a role, how is that being compensated for when the ECU gives zero supply on overrun in throttle closed position?
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Old 15th March 2011, 13:37   #51
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Default re: RPM shoots up when changing gears

^^^ The micrograms of idle fuel (vapor form) pulled from the carb's idle jet does not do any significant cooling of the inlet valve(s) or the top of the piston. One gets app. 80ms or less per stroke for the heat transfer at idling, 30ms or less at 2000rpm. The mass of air pulled during the suction stroke, which is in grams, would do most of it. The temperature rise in the charge after firing will be far more more than what the charge brings down pre-firing.

In the cooling-heating duty cycle, time taken for each cooling part-cycle (induction stroke) is far less that of the heating part-cycle (firing through exhaust) - so expecting significant cooling from the incoming charge is wrong expectancy IMHO. By design we are relying on the thermodynamic equilibrium (including external forced cooling) for things not burning out, the way Hr. Dr. Ferdinand Porsche exploited it. With 2-strokes this would be slightly better, not very significantly. Equations, anyone?

With ECUs, this doesn't change - the injectors are not told to open.

Last edited by DerAlte : 15th March 2011 at 13:39.
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Old 15th March 2011, 20:24   #52
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Er... why did this interesting discussion take a holiday?
We were waiting for you! Welcome. (Given your relative absence, I take it that your 'Mad March' starts before ours.)

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^^^ The micrograms of idle fuel (vapor form) pulled from the carb's idle jet does not do any significant cooling of the inlet valve(s) or the top of the piston.
....
With ECUs, this doesn't change - the injectors are not told to open.
Latent heat of evaporation is significant, but with a closed throttle, there is too little of anything. Important is not the actual times, but the duty cycles.

I've another question to add to my first (where does the unburnt fuel come from?) - How long does it take for a cat to cool below lightoff?

Regards
Sutripta
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Old 15th March 2011, 23:01   #53
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Originally Posted by Sutripta View Post
... Latent heat of evaporation is significant, ...
That has already been taken care of in the first few cm after the jet in carb engine. No effect by the time it reaches cylinder. Even in GDI it hardly has any effect in the cylinder.
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Originally Posted by Sutripta View Post
... Important is ...
For what?

In the end it all boils down to actual time available for cooling or heating - duty cycle is an abstraction.
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Originally Posted by Sutripta View Post
... where does the unburnt fuel come from? ...
One of those infernal cycles from Mech Engg. . If, for the circumstances of work extraction there is not enough time to burn (rapid temperature fall within cycle due to work extraction) the fire dies down too soon, and then one gets ... (Oy, where have all the Mechies vanished in this hour of need?)
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... How long does it take for a cat to cool below lightoff? ...
Other than that image in Stardust (mag), never knew a feline could light up/light off to chill out!

PS: Sutripta - we are seriously OT wrt the thread title. Shouldn't we move all the OT stuff to a new thread?

Last edited by DerAlte : 15th March 2011 at 23:05.
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Old 16th March 2011, 09:55   #54
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Default re: RPM shoots up when changing gears

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We were waiting for you! Welcome. (Given your relative absence, I take it that your 'Mad March' starts before ours.)
My 'Mad March' hopefully ends after 15 April.

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Originally Posted by Sutripta View Post
I've another question to add to my first (where does the unburnt fuel come from?) - How long does it take for a cat to cool below lightoff?
So is this the unburnt fuel that's making the revs shoot up when changing gears? Is this an injector calibration fault?
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With 2-strokes this would be slightly better, not very significantly.
But what about the lubrication being supplied through the pre-mix? Once petrol supply is cut off in a 2-stroke without the auto-lube system, no petrol = no lube oil = burnt engine? Let's say, a Yezdi or Bajaj Chetak coming downhill from Shimla to Parwanoo in gear, throttle closed and using engine braking, can the engine burn out in such a case due to lack of lubrication?
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With ECUs, this doesn't change - the injectors are not told to open.
With the trottle closed fully during engine overrun, is the ECM asking the injectors to deliver zero fuel, or is there enough fuel supplied to maintain idling?

An additional observation that needs to be explained - in a carb engine, when coming to a stop and shifting to neutral, the engine returns to idling revs quickly and linearly. OTOH, in a FI system, the idlling speed comes down close to idling and maintains it for a second or two, before settling lower into idling revs. For e.g., if the idling is set at 700 rpm, immediately after stopping and shifting to neutral, the engine would turn over at, say, 850-900 rpm, before settling down to 700. Is this a function that's built into the ECU, or is this an unintentional delay between the ECU reducing fuel and the injectors responding?

New thread or change in thread title? Say, "Behaviour of fuel-injected petrol engines"?

Last edited by SS-Traveller : 16th March 2011 at 09:59.
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Old 16th March 2011, 11:35   #55
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... So is this the unburnt fuel that's making the revs shoot up when changing gears? Is this an injector calibration fault?
How can it? Unburnt fuel would have already exited at the end of the exhaust stroke, and cannot contribute any more to the engine cycle. Injector calibration faulty? No way - in over-run injector is not opening. This thread's problem has nothing to do with over-run or injectors.

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Originally Posted by SS-Traveller View Post
... But what about the lubrication being supplied through the pre-mix? ...
Sir, aren't we mixing up lubrication (pre-mix) and cooling (burnout) here?

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Originally Posted by SS-Traveller View Post
... With the trottle closed fully during engine overrun, is the ECM asking the injectors to deliver zero fuel, or is there enough fuel supplied to maintain idling? ...
Zero fuel (injector is not opened). Injector is not an intelligent entity - ECU opens it for the appropriate length of time.

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Originally Posted by SS-Traveller View Post
... For e.g., if the idling is set at 700 rpm, immediately after stopping and shifting to neutral, the engine would turn over at, say, 850-900 rpm, before settling down to 700. Is this a function that's built into the ECU, or is this an unintentional delay between the ECU reducing fuel and the injectors responding? ...
Deliberate ramp-down profile to prevent stalling! Could be (other) load sensing, i.e. a/c compressor still drawing load.

There is no delay (virtually, since there may be a few microseconds delay) between ECU intending and injector responding.

Quote:
Originally Posted by SS-Traveller View Post
... New thread or change in thread title? Say, "Behaviour of fuel-injected petrol engines"?
OMG, that would be a sweeping topic for what we are discussing OT here!

Last edited by DerAlte : 16th March 2011 at 11:39.
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Old 16th March 2011, 12:53   #56
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Sir, aren't we mixing up lubrication (pre-mix) and cooling (burnout) here?
No, no, this query is OT, but would like it answered if possible.
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Zero fuel (injector is not opened). Injector is not an intelligent entity - ECU opens it for the appropriate length of time.
Understood.
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Originally Posted by DerAlte View Post
Deliberate ramp-down profile to prevent stalling!
There may also be a deliberate ramp-up programmed into the ECU when declutching & shutting down throttle during gearshifts? If so, the explanation by nitrogary is one of the reasons...
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...when the throttle closes. the closure can happen much earlier if we just move the foot sideways to our right and just let the throttle snap back. if fuelling reduces at the same rate, the engine would stall. so it must be reduced gradually.
so when you change gears, generally you would snap the throttle close and depress the clutch.
This removes the load off the engine and makes it rev up.
...and this is what I found additionally. Does this apply to all cars, or just Hyundai?
Quote:
...if a throttle body closes immediately, pressure inside the engine(crankcase) becomes so great, it caused more oil vaporization than usual. This is quite normal for any turbo charged engines, but Hyundai wanted even better emissions and didn't want to deal with the "oil loss" issues. Thus by partially keeping the throttle body open even after gas pedal has been completely released, pressure can escape from engine.

From Rev hang explanation...
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OMG, that would be a sweeping topic for what we are discussing OT here!
LOL! How about "FI petrol engine revs up on gearchange and idling - Is this normal?" ?
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Old 16th March 2011, 14:09   #57
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... But what about the lubrication being supplied through the pre-mix? Once petrol supply is cut off in a 2-stroke ... Yezdi or Bajaj Chetak coming downhill from Shimla to Parwanoo in gear, throttle closed and using engine braking, can the engine burn out in such a case due to lack of lubrication? ...
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Originally Posted by SS-Traveller View Post
No, no, this query is OT, but would like it answered if possible. ...
You are omitting splash lubrication and assuming all lubrication is provided by oil in the premix, and there is no cooling of the engine body.

It is all about thermodynamic equilibrium of the whole engine, including temperature of the cylinder sleeve, which is in contact with the engine body, the engine body itself, which is being cooled either water or air flow over fins, air temperature and excess heat produced every cycle (major part of the heat being taken away by work done on the piston and load).
* In over-run, actually the load is doing work on the piston. With no petrol, air compression by this 'work done' causes temperature rise, which is heating head and sleeve
* However, with no fuel being burnt, the temperature rise due to excess heat is far lesser than when firing
* Considering the heat being taken away by engine cooling, the heat input/output relationship will balance at a far lower temperature
* Without oil from pre-mix, the rings will heat up a bit more gradually, but no risk of seizure or burn-out. Difficult to prove without equation solving, but one has to consider heat is also being taken away - preventing NRZ temp rise.

In all this, actually the vehicle which started from Shimla would have stopped (by engine braking in gear) long before Parwanoo, right?

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Originally Posted by SS-Traveller View Post
... and this is what I found additionally. Does this apply to all cars, or just Hyundai? ...
Schwer zu glauben! Or may be I am not understanding the logic presented (I am not an engine-vengine person) or maybe the concept is completely non-intuitive, but ... wouldn't there be less evaporation in oil if the crankcase pressure is higher?
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Old 16th March 2011, 16:13   #58
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Default re: RPM shoots up when changing gears

Hi,
Very interesting, entertaining, waaay off topic discussions!

@SS. I have toured in the hills somewhat on an RX (autolube, not premix). This thought of lubrication (or lack of it) never entered my head then. Maybe that's why I faced no problems, either of the engine seizing, or plugs fouling. Moreover, I don't recall any of the regulars/ oldtimers mentioning this. And believe me, they would give you (uncalled for or otherwise) advice on any and every topic under the sun. Bottom line, I can conjecture with the rest of you, but I don't know.

Two strokes don't have splash lubrication. Also, the oil dissolves in the atomised petrol, and is carried to all parts. Somehow, I don't see this happening in the overrun case. All I can think is that under no load conditions, the thin existing film of oil is sufficient for roller bearings.

More detailed speculation tonight! Tomorrow, I'm off for a week.

Re: the actual thread starters problem, the best answers will come from the ECU coders (as to what has been done) and from their seniors as to why that has been done. Anything mysterious, you can blame on 'these newfangled emission c**p.'

Regards
Sutripta

Last edited by Sutripta : 16th March 2011 at 16:15.
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Old 16th March 2011, 20:20   #59
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Originally Posted by nitrogary View Post
so if you snap the throttle which often happens before taking a corner while braking, you can observe that the engine misfires because of excess fuel (rich mixture) and the fire may travel out of the exhaust pipe also which gives a marvellous glow.
Never thought about this aspect ^^, interesting.

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Employed at MM, I presume?
"Employed" as in past tense.
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Sounds like Spike, huh? .. LOL!!.
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spike and i are friends and we share a few common interests including the rx. but i did not get to work with him on the same project.. that would have been fun!
Yeah man, that would have been fun; a 140 BHP pocket rocket. Nevertheless, it is on the cards

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the actual thread starters problem, the best answers will come from the ECU coders (as to what has been done) and from their seniors as to why that has been done. Anything mysterious, you can blame on 'these newfangled emission c**p.'
Spike

PS- @Nitrogary, Sutripta, SS da, can we have a separate thread on calibrations? Would be a good topic for discussion.
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Old 16th March 2011, 20:26   #60
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Originally Posted by DerAlte View Post
Schwer zu glauben! Or may be I am not understanding the logic presented (I am not an engine-vengine person) or maybe the concept is completely non-intuitive, but ... wouldn't there be less evaporation in oil if the crankcase pressure is higher?
What, Sir, would have provoked you to "schwer" at me? That what I say sounds like Ripley?

With an increase in crankcase pressure, the existing oil vapours would be forced out through the breather more rapidly, isn't it?
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You are omitting splash lubrication and assuming all lubrication is provided by oil in the premix...
Some dentists do understand engines just a little bit, but...
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Originally Posted by Sutripta View Post
Two strokes don't have splash lubrication.
Maintenant, quel professeur devrais-je croire?
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