Flames and Popping sounds from exhaust of IC engines [reasons]
Flames and Popping sounds from exhaust of IC engines [reasons]
(OP)
Hello everybody,
I've searched for this in previous threads but didn't find anything fully thorough.
The question is about the different situations in which a "post"-combustion event occours in the exhaust system; so this comprehend all different conditions (rpm and load) in which anybody has experienced this event regarding different engine types: SI four stroke engine with carburettors or EFI.
I'd like to create a summary of different situations as thorough as possible.
Flame from exhaust pipe - EFI engines - release (idle) from high rpm (e.g. braking):
What I know (very little) is about the flame (a proper, nearly continuous flame) from the exhaust pipe that I knew was due to the rich mix during release (braking). The mix is enriched to cool down the temperature of the engine through two factors: fuel is cold so more fuel means more cooling (of the valve for example) and secondly because of the increase of the amount of H2O produced during the combustion of a richer mix.
But this happens only in high performance engines with EFI through which it's possible to modify the amount of fuel in this way.
Are you with me??
Popping sound and little flames/explosions from exhaust pipe - EFI and carburettor engines - same conditions as before:
It's said to be due to lean mix. Ok, but why? Why there should be explosions in the exhaust if there is less fuel in the mix? And, related to this, what about the modifications to the exhaust system?
Thanks very much and sorry if this thread is out of place.
Bye!
I've searched for this in previous threads but didn't find anything fully thorough.
The question is about the different situations in which a "post"-combustion event occours in the exhaust system; so this comprehend all different conditions (rpm and load) in which anybody has experienced this event regarding different engine types: SI four stroke engine with carburettors or EFI.
I'd like to create a summary of different situations as thorough as possible.
Flame from exhaust pipe - EFI engines - release (idle) from high rpm (e.g. braking):
What I know (very little) is about the flame (a proper, nearly continuous flame) from the exhaust pipe that I knew was due to the rich mix during release (braking). The mix is enriched to cool down the temperature of the engine through two factors: fuel is cold so more fuel means more cooling (of the valve for example) and secondly because of the increase of the amount of H2O produced during the combustion of a richer mix.
But this happens only in high performance engines with EFI through which it's possible to modify the amount of fuel in this way.
Are you with me??
Popping sound and little flames/explosions from exhaust pipe - EFI and carburettor engines - same conditions as before:
It's said to be due to lean mix. Ok, but why? Why there should be explosions in the exhaust if there is less fuel in the mix? And, related to this, what about the modifications to the exhaust system?
Thanks very much and sorry if this thread is out of place.
Bye!





RE: Flames and Popping sounds from exhaust of IC engines [reasons]
Cam timing with a lot of overlap will make this worse. Air-injection systems - intentional or otherwise - that let extra air into the exhaust right at the exhaust ports will make this worse.
RE: Flames and Popping sounds from exhaust of IC engines [reasons]
Right?
What do you think about this:
Have you ever heard anything about carbs with some devices to provide more fuel at closed throttle to cool down the engine? I think it's possible to do this only with the control provided by EFI...
RE: Flames and Popping sounds from exhaust of IC engines [reasons]
The result would be high emissions ( HC and CO?).
In those carefree days Expected idle A/F was Something like this -
http://www.iwt.com.au/images/air.gif
Whatever made the engine idle happy and steady.
Even when fuel injection came on the scene something like 3% CO was the target.
I have a hard time believing intentional cooling on part throttle is important or useful.
The dangerous time for the engine is the 10 seconds or much longer operation at heavy throttle. For road use I think I better tune so that operation can be sustained, rather than hanging any hope on a close throttle rest period either before or after.
RE: Flames and Popping sounds from exhaust of IC engines [reasons]
http://www.rallycars.com/Cars/bangbang.html
RE: Flames and Popping sounds from exhaust of IC engines [reasons]
Even if the amount of fuel is not strictly limited, for longer races in particular, it's in your interest to at least pay some heed to efficiency so that you can either (1) be lighter by carrying less fuel on board, or (2) go longer between fuel stops. I've had some involvement with motorcycle endurance racing, and (2) is pretty important. Our team is just on the threshold of being able to do a one-stop in a three hour race ... we are not quite there; we are at one main stop (needed for rider change anyhow) plus a splash about 20 - 30 min from the end.
Modern fuel injection systems are more likely to shut off fuel completely under conditions where the ECU would expect questionable ignition on over-run conditions. (Ours do - OEM Honda PGM-FI on the endurance bike - so does my OEM Kawasaki)
If the deceleration fuel cut happens right after slightly-rich operation on the last few engine revs before cut-off kicked in (common!) you now have air being pumped through the engine into the hot exhaust pipe that still has left-over fuel from the previous cycle before deceleration fuel cut ... Bang.
Even after deceleration fuel cut, normally the surfaces of the intake runner are wetted with fuel, so although it might nominally be "fresh air", in reality it will have fuel in it for a few engine revolutions afterward. Too lean for the spark plug to fire, but pump that into aforementioned hot exhaust pipe ... Bang.
RE: Flames and Popping sounds from exhaust of IC engines [reasons]
Almost invariably this used to be due to an air leak in the exhaust system.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Flames and Popping sounds from exhaust of IC engines [reasons]
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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
RE: Flames and Popping sounds from exhaust of IC engines [reasons]
RE: Flames and Popping sounds from exhaust of IC engines [reasons]
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Flames and Popping sounds from exhaust of IC engines [reasons]
Anyway, my two bits on a Sunday afternoon!
Jim K.
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