What causes black soot on a gas engine car?

[Demo24]

Yesterday on my drive home I ended up behind a douche in civic si who proceeded to tailgate a line of cars going up the on-ramp then 'blast' over immediately to the left lane before the merging area and got behind yet another line of cars. While I was watching this guy attempt to be a badass in a civic si I noticed that on his full throttle exploits a whole lot of black soot coming out the tail pipe. It occurred to me that I had no idea what causes that on a gas engine car, maybe direct injection and carbon buildup? It was a previous gen si coupe, so not brand new but only a few years at most, dunno if they had DI then or not.

 

[OVerLoRDI]

Burning oil?

 

[Atty]

My '12 2.0T GTI has a massive amount of black soot around my exhaust pipes. Completely stock with less than 2k miles on it. Hope its not burning oil and just lots of powaaaaaa.

 

[Demo24]

Burning oil?

This was solid black and looked 'sooty' not smokey, and it only occurred at full throttle.

 

[Ferzerp]

Unburned/not fully burned gasoline appears black just as diesel does, it's just that diesel is far more prone to it.

 

[dawp]

possibly a badly done chip? or an extremely dirty air filter.

if it was a carbed car, the mix would be off.

 

[SyndromeOCZ]

Burning alot of oil can do it as well. My brothers 05 gmc canyon goes through a quart every couple hundred of miles and it soots more than some diesels.

 

[wirednuts]

its not burning oil, thats a blue-ish white smoke.

its because at full throttle, gas is being dumped into the engine at his max levels, which inevitably some of it doesnt burn. that goes to your cat which it smolders and spits out as black smoke.

if you can, just stay under WOT. if you continue to see black soot, then check your o2 sensors.

 

[Pacfanweb]

Yep, it's gas. Happens to every car when you floor it.

 

[KentState]

This happens under rich conditions in any car which typically at WOT. A/F is a ratio so a lot more un-burnt gas is seen if you are pushing 50+ psi per cylinder of fuel into the engine as compared to cruising down the road. The catalytic converter can only do so much in these conditions as well.

 

[LTC8K6]

I have never had a fuel injected car show any signs of running rich. Never had one get dirty near the tailpipe tip, either.

My 2008 Jeep GC 5.7L, which I floor quite often, has never had any soot on it.

 

[iamwiz82]

Direct Injection causes soot on new cars, not sure what year, or even if the Civic has DI.

 

[Ferzerp]

I have never had a fuel injected car show any signs of running rich. Never had one get dirty near the tailpipe tip, either.

My 2008 Jeep GC 5.7L, which I floor quite often, has never had any soot on it.

They shouldn't if working properly, but it's by no means impossible.

 

[phucheneh]

This happens under rich conditions in any car which typically at WOT. A/F is a ratio so a lot more un-burnt gas is seen if you are pushing 50+ psi per cylinder of fuel into the engine as compared to cruising down the road. The catalytic converter can only do so much in these conditions as well.

Um...what?

No, 'any car' does not emit black smoke. The engine has to be running VERY rich to putting out black smoke. Hell, on newer cars, you could disconnect the O2's and the ECT and I still doubt you would get any black smoke.

While the catalyst does trigger an oxidation reaction with the small amounts of hydrocarbons (and carbon monoxide) that are present in the exhaust, it is not intended for cleaning black soot out of the exhaust gas.

This is why diesel trucks have separate particulate filters. Gas engines can and do emit particulates, but they are so fine as to not be readily visible. Some people want particulate filters on gas cars, too- because the catalytic converter doesn't catch them.

And finally, fuel mixture in EFI cars is not controlled by fuel pressure. Fuel pressure is intended to remain relatively constant...fueling at various levels of RPM and load is controlled by duty cycle (how long and how often the injector solenoids are energized).

Christ, and people nitpick me...

 

[Ferzerp]

He said any car running [sufficiently] rich could do it. Technically he's correct.

If you want to go off on someone, go off on the silly person who said *every* car does it.

 

[phucheneh]

...I thought that was him?

This happens under rich conditions in any car which typically at WOT.

Kinda Engrishy, but I took that to essentially mean 'this happens at WOT because the car is using more fuel.'

Even if the car is heavily overfuelling, WOT should actually smoke the least, shouldn't it? I'm not sure because an EFI car spitting black smoke just isn't something you have to deal with very often...the only instances I can remember seeing any significant amount of smoke involve forced induction blowing charge pipes off. Or just really awful 'custom' tunes.

Oh, you meant this guy-

its not burning oil, thats a blue-ish white smoke.

its because at full throttle, gas is being dumped into the engine at his max levels, which inevitably some of it doesnt burn. that goes to your cat which it smolders and spits out as black smoke.

if you can, just stay under WOT. if you continue to see black soot, then check your o2 sensors.

Some of the OXYGEN doesn't burn. Ideally, all of the fuel (relatively speaking) should burn. Hence...oxygen sensors. Not hydrocarbon sensors.

Bad O2 sensors that set a CEL will make the car run in open loop (should not cause smoke). Ones that don't are just not as sensitive as they should be, which will make the car run lean, not rich- ECM sees too little oxygen, assumes the engine is running rich, and dials the fuel back.

 

[Vdubchaos]

My '12 2.0T GTI has a massive amount of black soot around my exhaust pipes. Completely stock with less than 2k miles on it. Hope its not burning oil and just lots of powaaaaaa.

hehe

you just wait buddy

 

[Ferzerp]

...I thought that was him?



Kinda Engrishy, but I took that to essentially mean 'this happens at WOT because the car is using more fuel.'

Even if the car is heavily overfuelling, WOT should actually smoke the least, shouldn't it? I'm not sure because an EFI car spitting black smoke just isn't something you have to deal with very often...the only instances I can remember seeing any significant amount of smoke involve forced induction blowing charge pipes off. Or just really awful 'custom' tunes.

Oh, you meant this guy-



Some of the OXYGEN doesn't burn. Ideally, all of the fuel (relatively speaking) should burn. Hence...oxygen sensors. Not hydrocarbon sensors.

Bad O2 sensors that set a CEL will make the car run in open loop (should not cause smoke). Ones that don't are just not as sensitive as they should be, which will make the car run lean, not rich- ECM sees too little oxygen, assumes the engine is running rich, and dials the fuel back.

Naw, I meant the "happens to every car when you floor it" guy.

 

[phucheneh]

Now, I meant the "happens to every car when you floor it" guy.

Oh...I think I missed him because his sig is three times as long as his post.

Still though...I don't think I've posted anything incorrect. But if you think so, we can totally fight about it.

And then have makeup buttsex.

 

[Ferzerp]

I'm just saying that one guy was not really wrong.

 

[Demo24]

Excess gas makes sense now I think about it, also very much so in this situation as it looked fairly modified. Had a dealer tag on it too, sucks for that guy I guess, looks like he bought a car with a crappy tune.

 

[nerp]

No car should do this at WOT unless it's old or the victim of a hackjob mod.

 

[BUTCH1]

No car should do this at WOT unless it's old or the victim of a hackjob mod.

No FI car anyway, back when "feedback" carbs were around this was common, I'd go with hackjob tune..

 

[jlee]

Um...what?

No, 'any car' does not emit black smoke. The engine has to be running VERY rich to putting out black smoke. Hell, on newer cars, you could disconnect the O2's and the ECT and I still doubt you would get any black smoke.

While the catalyst does trigger an oxidation reaction with the small amounts of hydrocarbons (and carbon monoxide) that are present in the exhaust, it is not intended for cleaning black soot out of the exhaust gas.

This is why diesel trucks have separate particulate filters. Gas engines can and do emit particulates, but they are so fine as to not be readily visible. Some people want particulate filters on gas cars, too- because the catalytic converter doesn't catch them.

And finally, fuel mixture in EFI cars is not controlled by fuel pressure. Fuel pressure is intended to remain relatively constant...[/n]fueling at various levels of RPM and load is controlled by duty cycle (how long and how often the injector solenoids are energized).

Christ, and people nitpick me...

Not necessarily. Toyota's 3S-GTE's fuel pressure varies based on boost level. Duty cycle does vary as well, of course, but fuel pressure is not constant once you're at positive manifold pressure.

 

[exdeath]

Black stuff in any combustion process is carbon as a result of an imbalanced chemical reaction leaving behind carbon with no oxygen to combine with to form CO2; eg: rich mixture.

Anything from cars to the pilot light on your natural gas water heater, to random crap you burn with a cigarette lighter.

A burst of black exhaust in a tuned car is normal immediately following a transition to WOT. This is called "accelerator pump" logic or tip in enrichment. Air flow changes in an engine are instant, while fuel metering lags behind because the computer and fuel system can only respond to what JUST happened, not predict whats GOING to happen. There is a brief opportunity for knocking and engine damage to occur if the fuel system doesn't respond fast enough to the sudden unpredictable increase in airflow, compounded by the fact that this usually occurs under the heaviest load condition at lower RPM since you are accelerating and starting a pull.

To compensate, when a sudden swing in TPS and manifold pressure occur, an extra unmeasured shot of fuel is blindly added on top of the normal load/RPM value in anticipation of the incoming air to keep the mixture on the safe side during this chaotic transition. Too rich is better than too lean. This value cannot be measured or predicted by the computer, it is a static value tweaked by a human tuner. This enrichment shot only lasts a second or two until the transition to WOT completes and the sudden change stabilizes and becomes steady state.

On any kind of high performance engine with high compression, high boost, etc, this can be very rich, especially with a factory tune where they have to account for the worst possible climates, temperatures, terrible fuel octane, owner abuse, etc.

Normally a catalyst will clean up much of this and burn the remaining fuel, but on a higher performance car with a more aggressive tip in and aftermarket catless exhaust, black smoke on hard acceleration is normal. It doesn't matter if it's gasoline, diesel, natural gas, anything involving HC + O2 combustion.