Puff of smoke on acceleration from nearly-new cars?

[Howard-]

This thread is purely out of curiosity, really.

Recently I've seen a couple of very new (i.e. 0-2 years old) cars (Audi RS4 and Golf GTI) emit a little puff of what was maybe white but probably grey/black smoke when the driver has clearly mashed his right foot down.

Is there a simple explanation for this that I'm missing? I am presuming the piston rings and valve seals and head gaskets aren't knackered just yet

[kambites]

I believe modern cars always over-fuel slightly when the throttle is opened past a certain point. I suppose it's just unburned fuel?

[s m]

If it's grey/black, then as per above, it will be fuel

[datum77]

There are ONLY 3 colours of smoke that can come out of the rear of any car.

Black:- This is unburnt fuel, either petrol or diesel, and means that the fuel is not being burnt properly.

White:- This is water vapour. ALL engines suck in air from outside and that air has water vapour in it which is then burnt and comes out as dripping water out of the exhaust or, on cold mornings, white smoke. Perfectly normal. It can also come from a head gasket failure which means that LOTS of water is getting into the combustion chamber and is being burnt off.

Blue:- This is oil vapour. This can be caused by several different things. It means that oil is getting into the combustion chamber, which it shouldn't do under ANY circumstances. Usually caused by broken piston/rings or worn valve guides, etc.

[Oldandslow]

datum77 said:

There are ONLY 3 colours of smoke that can come out of the rear of any car.

Black:- This is unburnt fuel, either petrol or diesel, and means that the fuel is not being burnt properly.

White:- This is water vapour. ALL engines suck in air from outside and that air has water vapour in it which is then burnt and comes out as dripping water out of the exhaust or, on cold mornings, white smoke. Perfectly normal. It can also come from a head gasket failure which means that LOTS of water is getting into the combustion chamber and is being burnt off.

Blue:- This is oil vapour. This can be caused by several different things. It means that oil is getting into the combustion chamber, which it shouldn't do under ANY circumstances. Usually caused by broken piston/rings or worn valve guides, etc.

Pedant's note - water is also produced by the combustion process along with CO2 etc.

[ChrisNic]

After a DPF regen my new shape Leon FR (184 BHP) can chuck out a surprising amount of white/grey "ash", sadly not the tailgater annoying amount of soot my old Fabia VRS used to create though.

[Mopar440]

Oldandslow said:

datum77 said:

There are ONLY 3 colours of smoke that can come out of the rear of any car.

Black:- This is unburnt fuel, either petrol or diesel, and means that the fuel is not being burnt properly.

White:- This is water vapour. ALL engines suck in air from outside and that air has water vapour in it which is then burnt and comes out as dripping water out of the exhaust or, on cold mornings, white smoke. Perfectly normal. It can also come from a head gasket failure which means that LOTS of water is getting into the combustion chamber and is being burnt off.

Blue:- This is oil vapour. This can be caused by several different things. It means that oil is getting into the combustion chamber, which it shouldn't do under ANY circumstances. Usually caused by broken piston/rings or worn valve guides, etc.

Pedant's note - water is also produced by the combustion process along with CO2 etc.

Water isn't produced by the combustion process, it's already present in the charge of air (as clearly explained by Datum77 in his post).

There is also burnt brake fluid smoke, which is also white.

[kambites]

Mopar440 said:

Water isn't produced by the combustion process,

Where do you think the hydrogen in the hydro-carbon you're burning goes, then?

[anonymous-user]

Burning a litre of petrol produces around a litre of water as a by-product.

[gamefreaks]

Mopar440 said:

Water isn't produced by the combustion process, it's already present in the charge of air (as clearly explained by Datum77 in his post).

There is also burnt brake fluid smoke, which is also white.

Umm... Fuel + Oxygen -> Water + CO2. The water comes form the Hydrogen in the fuel and the Oxygen in the air.

[eldar]

kambites said:

I believe modern cars always over-fuel slightly when the throttle is opened past a certain point. I suppose it's just unburned fuel?

Not just modern cars. Carbs usually have an accelerator pump linker to the accelerator.

[kambites]

eldar said:

Not just modern cars. Carbs usually have an accelerator pump linker to the accelerator.

In my experience, Carbs overfuel all of the time, although maybe that's just because I'm crap at setting them up.

[ferrariF50lover]

rovermorris999 said:

Burning a litre of petrol produces around a litre of water as a by-product.

This is why I love PH. Incredible stat about which I had absolutely no idea. You've brightened my day.

[kambites]

It's why things like Propane burners aren't a very good idea in garages unless they're externally vented - they chuck out a huge amount of water.

[aw51 121565]

datum77 said:

... ALL engines suck in air from outside and that air has water vapour in it which is then burnt and comes out as dripping water out of the exhaust or, on cold mornings, white smoke. Perfectly normal. ...

Burning water vapour to, errm, give liquid water or (on cold mornings) white smoke?

That's the energy crisis sorted then...

Sorry, I am being very facetious - but earlier posters, despite their "Fuel + Oxygen --> Carbon Dioxide + Water" comments, didn't drill this down enough pedantically .

[JonnyVTEC]

Direct injection petrols tend to be a bit sootier aswell due to the formation of larger particulates during combustion. You can see this on the EU6 petrol emission requirments for PM. Potentially we might see GPFs appearing on cars.