Flames and Popping sounds from exhaust of IC engines [reasons]

[battaglione]

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!

 

[BrianPetersen]

Doesn't matter rich or lean, it can happen either way. A "weak" mixture - very little air and fuel because of the shut throttle but a lot of recirculated exhaust because it's being drawn back through the engine by intake manifold vacuum during valve overlap - doesn't get fired by the spark plug, or it does fire but doesn't burn properly, so it gets carried through to the exhaust stroke. Now that gets pumped into a 1500 F exhaust manifold and it WILL ignite.

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.

 

[battaglione]

Thanks, that's interesting: so the point is the presence of exhaust gasses in the combustion chamber that affects the combustion. This happens also at higher RPM but in this case there is a "stronger" mix... and the influence of the exhaust gasses is negligible.
Right?

What do you think about this:

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??

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...

 

[Tmoose]

In the non-emission era slowing with throttle closed would drag the relatively rich (but still pretty small volume) idle mixture into the engine.
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.

 

[moon161]

Keeps the turbo spooled when the right foot is lifted.

http://www.rallycars.com/Cars/bangbang.html

 

[BrianPetersen]

Rally car "anti-lag" systems excepted, in many forms of racing nowadays, letting extra fuel in during shut-throttle conditions for purpose of "cooling" is far less efficient than simply relying on the vehicle's normal cooling system to take care of the cooling. It doesn't accomplish anything that the normal cooling system couldn't do better ... In many forms of racing, the amount of fuel to complete the race is limited, and throwing unnecessary fuel out the exhaust is not consistent with finishing the race. Fuel that you threw out the exhaust is fuel you could have used to better effect at some other time to make power when the driver/rider was asking for more!

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.

 

[GregLocock]

"Popping sound and little flames/explosions from exhaust pipe - EFI and carburettor engines"

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

http://eng-tips.com/market.cfm?

 

[dgallup]

Quite a few modern motorcycles use a "secondary air injection" system to add extra air into the exhaust during closed throttle operation. I know Kawasaki and Triumph do this, probably others. It does cause exhaust popping and flames. Blocking off the SAI eliminates it (unless there is another exhaust leak).

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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.

 

[crerus75]

As BrianPetersen says, you will see commonly see this on race vehicles. You can also do it on an old carbureted Ford Ranger if you hollow out the catalytic converter and keep the smog pump hooked up. Just mash the throttle for a second to make sure the engine sucks in a nice shot from the accelerator pump, and get ready for a bang that will almost blow the muffler off the truck. Or so I've been told.

 

[Lou Scannon]

Then there's the old trick with carbureted vehicles of shutting off the ignition for a second or so. A little scary to do when the key is interlocked with the steering.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[Mitsos1]

I've been trying to get rid of exactly this in my ...flamethrower Alfa 75 24v trackday car. Background info: The 3liter engine and management come from the old 164Q4 4x4 car. My car is rwd. While everything is perfect fuel/ignition-wise (dyno mapped) and it makes a respectable 272hp on (almost) production cams, 3-foot flames come out the back every time I shift on the track. Just like the pic below, but I would definitely like to get rid of this. The 4x4 donor car had some additional inputs to the ecu like wheel speed and abs signal inputs. Apparently, something is missing and the ecu does not cut fuel at zero TPS when shifting. I double-checked by plugging in spare ecu's and TPS but no dice. You can imagine going wot and letting up to downshift... At least I know why it does it. I have thought of 'fixing' it myself by putting a relay in the 12v injector bus, which would interrupt supply with the throttle closed and rpm>1300 or so, but I suspect the ecu is not as dumb as to sit back and do nothing -it will see a lambda no-signal and probably see it as a problem...
Anyway, my two bits on a Sunday afternoon!
Jim K.