Engine Braking?

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Originally Posted by JohnL

aaronng,
The throttle plate and IACV don't do anything out of the usual when the engine is on the overun. Overun fuel cut off isn't cotrolled using aiflow, though the IACV does remain momentarily open immediately after the throttle plate has been closed to (I think) assist in burning still unburnt fuel as it flows through the exhaust system (an emissions control measure).

My understanding of what happens is that the ECU detects that the throttle is at the closed position (using the TPS input) but also detects that engine rpm is above X, and using these inputs the ECU 'decides' that an overun condition must exist and as programmed cuts the injectors until either the throttle is opened again or the rpm reaches a set point somewhat above idle speed.

This feature lessens fuel consumptioon and emissions, and helps to prevent backfiring (which can damage the cat apart from anything else...).

The braking effect comes from internal engine friction (and to a much lesser degree from internal gearbox friction). Some of the braking effect comes from the compression of gas in the cylinders, but since the throttle plate is shut very little air should be able to flow into the cylinders from the intake side, so perhaps there is some reverse flowing gas coming in via the exhaust valve...?

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Thanks for explaining. If there is still air going in and the injectors are cut off, wouldn't there be a split second of lean combustion when the left over injected fuel in the manifold enters the combustion chambers after the injectors are closed ?

^^I was thinking that... coz that perfectly explains exhaust pop on throttle back off

i'm guessing this is something that can be confirmed by someone with a wideband setup?

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Originally Posted by dsp26

^^I was thinking that... coz that perfectly explains exhaust pop on throttle back off

i'm guessing this is something that can be confirmed by someone with a wideband setup?

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Exhaust pop is caused by a removal of the resonator/muffler and/or a rich situation. Not a lean one.

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Originally Posted by CRXer

john,most of the effect is compression,theres always air available to compress,due to idle passages & overlap.

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My point is that when the throttle plate is closed there is actually a very limited airflow through the induction system relative to the rpm at which the engine is turning. The flow through the bypass passages and past the slight gap around the circumference of the TP will be very restricted compared to what would be required to even remotely fill the cylinders.

So, overrun will create the highest vacuum (lowest pressure) conditions ever seen in the plenum / manifold. So, if on the overun the majority of airflow into the cylinders is coming from the induction side then there would be relatively little air available to be compressed by the rising pistons, and thus relatively little engine braking effect (at least from the compression of gas).

On the other hand, considering that the atmospheric end of the exhaust system is never closed by any type of valve as the induction system can be, if there is a substantial reversion of gas flow coming into the cylinder through the open exhaust valve, then it may be that most of the gas that is compressed in the cylinders to create the engine braking effectb is actualy entering the cylinders via the exhaust valve..?

I know from personal experience that an engine with a higher CR does create a stronger engine braking effect than a similar engine with a lower CR. A lessening of engine braking effect is also evident with engines that have lost substantial compression due to wear.

An interesting experiment to see if we can get a seat of the pants feel for any difference in engine braking effect relative to just closing the throttle, might be to switch off the engine while the throttle is still wide open (beware steering locks...).

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Originally Posted by aaronng

Thanks for explaining. If there is still air going in and the injectors are cut off, wouldn't there be a split second of lean combustion when the left over injected fuel in the manifold enters the combustion chambers after the injectors are closed ?

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Quite possibly, but I can't see it causing any problem...

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Originally Posted by aaronng

Exhaust pop is caused by a removal of the resonator/muffler and/or a rich situation. Not a lean one.

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Also, not only must unburnt fuel exist, so too must sufficient oxygen and heat. So, there may be an external backfire when the hot gasses (and hot unburnt fuel content) reaches the end of the pipe and finds oxygen. This tends to be more likely to create a backfire with a shorter exhaust because the temp is higher when the hot unburnt fuel vapour reaches oxygen at the end of the pipe.

An internal backfire (i.e. inside the exhaust system) might occur if the system has a leak (e.g. manifold gasket leak) and can 'suck' oxygen into the pipe at the 'hot end'. (A hole on the exhaust system - especially nearer the exhaust valve may allow air to be 'sucked' into the pipe due to the positive and negative pressure waves within the exhaust system).

Any backfiring is of course much less likely if the injectors are cut on the overun, which must be why backfiring is much less common in these days of ubiquitous fuel injection than it was in the days when FI was uncommon or non-existant (carbies of course not cutting fuel on the overun).

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Originally Posted by JohnL

My point is that when the throttle plate is closed there is actually a very limited airflow through the induction system relative to the rpm at which the engine is turning. The flow through the bypass passages and past the slight gap around the circumference of the TP will be very restricted compared to what would be required to even remotely fill the cylinders.

So, overrun will create the highest vacuum (lowest pressure) conditions ever seen in the plenum / manifold. So, if on the overun the majority of airflow into the cylinders is coming from the induction side then there would be relatively little air available to be compressed by the rising pistons, and thus relatively little engine braking effect (at least from the compression of gas).

On the other hand, considering that the atmospheric end of the exhaust system is never closed by any type of valve as the induction system can be, if there is a substantial reversion of gas flow coming into the cylinder through the open exhaust valve, then it may be that most of the gas that is compressed in the cylinders to create the engine braking effectb is actualy entering the cylinders via the exhaust valve..?

I know from personal experience that an engine with a higher CR does create a stronger engine braking effect than a similar engine with a lower CR. A lessening of engine braking effect is also evident with engines that have lost substantial compression due to wear.

An interesting experiment to see if we can get a seat of the pants feel for any difference in engine braking effect relative to just closing the throttle, might be to switch off the engine while the throttle is still wide open (beware steering locks...).

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yeh string is right,allthough obviously u wont get it all back on the power stroke,u'll get enough to negate the compression effect.
for a normal car engine,u'd prob have to go back one stroke & most of the effect would be intake trying to pull open a vacuum....

a steering lock will only pop when u pull out the key,so lets give it a go.......
somebody????.......my car is in a million little pieces atm......

aaronng,u prob drive more efficiently then your average man,i could highly imagine u getting better than the factory is willing to quote in any case ;)
also like john said,the lean condition wouldnt matter,for u now are not trying to force a piston against the drivetrain anymore.

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Originally Posted by dsp26

^^I was thinking that... coz that perfectly explains exhaust pop on throttle back off

i'm guessing this is something that can be confirmed by someone with a wideband setup?

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WB just goes instantly out of range,most dropping out at 20-22 AF

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Originally Posted by CRXer

aaronng,u prob drive more efficiently then your average man,i could highly imagine u getting better than the factory is willing to quote in any case ;)
also like john said,the lean condition wouldnt matter,for u now are not trying to force a piston against the drivetrain anymore.

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I WOT to redline in 2nd gear at least twice a day though. I drive it hard enough that I have one of the worst vibrating engine of the Euros. Difference is I don't use more than 1/4 throttle and 2500rpm until the oil temp reaches 70 ºC, so my engine's piston ring seal should still be pretty good that the efficiency has not reduced, eventhough it has been to a few track days and has 85000km. The rest of the car on the other hand.... feels very very loose, rattly and knocky.

see,your always thinking about it,average joe doesnt think at all.
what percentage of your driving do u think that coupla runs to redline makes up?
very little i presume...

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Originally Posted by CRXer

yeh string is right,allthough obviously u wont get it all back on the power stroke,u'll get enough to negate the compression effect.

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I can see that on the down stroke the compressed charge will 'give back' at least some of the energy 'stored' in the compressed charge, but minus the energy converted by the compression into heat. It seems to me that this must ultimately be what happens to the kinetic energy of the moving mass, i.e. it's converted to heat through the vector of gas compression, thus creating the engine braking effect.

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Originally Posted by CRXer

for a normal car engine,u'd prob have to go back one stroke & most of the effect would be intake trying to pull open a vacuum....

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Which would have a limitation in that even with 100% efficiency the work is actualy done not by creating a vacuum, but by the underside of the piston 'pushing' against atmospheric pressure, which has a nominal value of only 14.7psi (give or take altitude variation and crankcase fluctuations, which won't be much).

So, even with an unrealistically high efficiency the work done creating at best a partial vacuum is not going to be the same as compressing in cylinder gas to pressures much greater than atmospheric...

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It also just occurred to me that cutting the fuel injection on the overun should add to engine longevity since the bore walls aren't being rinsed with unburnt fuel (washing off the bore oil film), and less oil degradation due to less unburnt fuel finding it's way into the oil (i.e. less oil dilution).

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Originally Posted by CRXer

see,your always thinking about it,average joe doesnt think at all.
what percentage of your driving do u think that coupla runs to redline makes up?
very little i presume...

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Little in daily driving, but of course I have trackdays to satisfy my cravings. :p