Questions: Flames from exhausts

[stradale]

As I'm not very technical I've asked myself these questions a couple of times and can't really figure out the answers. Why do cars, especially high performance cars, sometimes throw flames out of their exhausts?

My guess always was it's fuel that didn't combust in the engine, but I don't know what lights the fuel. The heat of the engine or the exhausts? Usually you see it when the driver changes gear. What happens? Does the type of exhaust has any influence on this?

Apart from the technical side to this, I just love when it happens!

[TT]

I think the fuel is lighted by the exhaust system iteself, and of course you can't see backflames on standard catalytic (right word?) exhausts you'll need a "free" one

[st-anger]

Well i´m not an expert an that topic as well, but basically you`re right, the ECU injects too much fuel that can`t be burnt so there might be some flames out of the exhaust.
Combustion by-products and ultra hot exhaust temps that usually end-up being absorbed by the cat are instead pushed down to the exhaust system to the muffler tip/outlet, and every time you shifted, a little clean oxygen was sucked back into the muffler tip then when you went back on the throttle with hot turbo exhaust, suddenly you have combustibles plus a little clean O2 and 800°C exhaust temps, there are some flames. This can be seen e.g. at every rally with the torbocharged engines. the sound that can be heared when the car downshifts is not really misfiring, turbos has to be spooled to minimize lag, so basically what they do is make such a system (ALS) that when the car is one the brakes / downshifts, some fuel escapes into the exhaust manifold and ignited there. it blows up and expands, so the resulting gas spools the turbo when the driver hits the accelerator. that's why you can see fire coming out of the exhaust on downshifts.
during lag time the engine is much less responsive and its output well below nominal and to counter the effect of the turbocharger's lag time drivers used to anticipate the engine's reactions by accelerating well before they would have done in a non-turbo car. Others have used a technique, introduced by the German driver Walter Röhrl, known as "left foot braking" where the driver uses his left foot to brake the car while his right foot accelerates to keep the turbocharger in optimal load. Left foot braking is very hard on the brakes which are put into extreme stress but is very efficient in keeping the turbo spinning.
when the driver lifts his foot from the gas pedal the ignition timing is altered with sometimes 40° or more of delay (retard) and the intake air and fuel supply mixture is made richer. The inlet butterfly is kept slightly open or an air injector is used to maintain air supply to the engine. This results in air/fuel mixture that keeps getting in the combustion chambers when the driver no longer accelerates. The ignition being delayed, the air/fuel mixture reaches the exhaust tubes mostly unburned. When the spark plug fires, the exhaust valve is starting to open due to the ignition delay mentioned above. Additionally, the exhaust temperature being extremely high, the unburned fuel explodes at the contact of the exhaust tubes. Luckily the turbo sits right there and the explosion keeps it turning (otherwise it would slow down since its intake, the exhaust gases, is cut-off ) ALS was a simple idea but one that was relatively difficult to implement because of the tricky ECU stuff. Toyota were the first to use it in racing, named TCCS (Toyota Combustion Control System), Mitsubishi called it PCCS ( Post Combustion Control System) The ALS effect is mostly dependent on the air allowed into the engine, the more air supplied the more the ALS effect will be noticeable. Consequently ALS systems can be more or less aggressive. a mild ALS will maintain a 0 to 0.3 bar pressure in the inlet manifold when activated whereas, when inactive, the pressure in the inlet manifold with the throttle closed would be in the region of -2 bar. Racing ALS versions can maintain a pressure up to 1.5 bar in the inlet manifold with the throttle closed.
While the systems mounted in Toyota and Mitsubishi racing cars are relatively smooth and noiseless those fitted in Ford and Subaru cars are much more noisy and aggressive.
The bang-bang system owns its name to the loud explosion noises one hears whenever the driver lifts off. Most racing implementations have user selectable anti-lag settings depending on the terrain, usually three settings can be selected by the driver going from mild to very aggressive.

the reason race cars flame out the exhaust is because they run straight pipes so there is no muffler to block the flames escapeing the exhaust and second is that race vehicles are run rich in the engine tuning because more gas plus more O2 = MORE POWER. So with the rich setting when you let off the gas that air stops coming but the gas dosen't for just an instant so you have to much fuel in the cylinders to fully ignite so the vaporized gas escapes into the hot headers on the exhaust stroke and everyone knows what happens when you combine gas fumes and heat you get fire. Which is how even non turbo'd cars get the flash of flame.

[stradale]

Thanks very much st-anger! I have to let that sink in for a while, but I understand most of it. A compelling read, I must say... 8)

You also answered a question that popped up in my head, but that I forgot asking here. At first I thought only cars with turbos did it, but later I noticed also some NA cars threw flames. I was puzzled about whether it had to do something with the turbos or not. Both yes and no it seems. Thanks for clearing that up as well.

[TT]

Well i´m not an expert an that topic as well

LOL! If you would have been an expert you had to type for 5 days I guess

[st-anger]

Originally Posted by TT Well i´m not an expert an that topic as well LOL! If you would have been an expert you had to type for 5 days I guess

i´ll take that as a compliment
...LOL...5 days....

[TT]

It was dude that was a GOOD explanation and if I always enjoy reading your posts, always interesting!
But try not to abuse because if you'll really type for 5 days before posting, I'd never survive the reading

[stradale]

Hahaha, I think st-anger had to restrain himself in explaining it because he's on holiday. Good thing, because otherwise my brain would suffer from an input overload as well! I wouldn't want any flames out of my ears...

Anyway, it sounded like an explanation with plenty of expertise to me.

BTW, how's your holiday, st-anger?

[st-anger]

Originally Posted by stradale Hahaha, I think st-anger had to restrain himself in explaining it because he's on holiday. Good thing, because otherwise my brain would suffer from an input overload as well! I wouldn't want any flames out of my ears... Anyway, it sounded like an explanation with plenty of expertise to me. BTW, how's your holiday, st-anger?

well i´m back home since yesterday, business`s calling...
although i´m more the winter holiday guy ( typical for a Austrian ), this holiday has to be one of the best holidays so far, everything was just excellent and so relaxing, so we both had a really good time...
i can highely recommend you a stay in one of the villas on Northern Island, although it`s kinda expensive, but IMO it`s worth every cent...

just look for yourself:




http://www.eyesonafrica.net/Producti...s/northisl.htm

[stradale]

:shock: :shock:

Very... *cough*...nice...

Wow, that's definately on my wishlist now...

[graywolf624]

You can also make a car throw flames by a quick modification regardless of the power of the engine. Basically it involves some trick instalation of spark plugs into your exhaust which ignites the fumes. Dont do this unless your sure your exhaust has no holes, otherwise you could go up like a roman candle.

[maalox]

I'm pretty sure misfiring systems/anti-lag systems (aka "bang bang" exhausts) have since been outmoded by exhaust gas recirculation and other techniques in WRC use. For one thing they cause extensive damage to the exhaust systems and turbos, necessitating a rebuild after every race. They also were just too damn loud leading to complaints and new regulations.

Any Initial D fans who've seen the Evo III with bang bang ought to know that a street car's exhaust would sadly never survive when fitted with such a system .

EDIT: further investigation reveals a pretty good website: http://www.rallycars.com/Cars/bangbang.html

[scubywrxr]

Decatting a subaru impreza turbo opens the wastegate and this results in the dumping of fuel into the exhaust, which can result in pops and bangs and flames.. a special flamer kit like the one mentioned above can also be installed for similar effect.

normally aspirated cars that are fuelled rich will flame on the overrun..

[jakaracman]

Originally Posted by stradale Thanks very much st-anger! I have to let that sink in for a while, but I understand most of it. A compelling read, I must say... 8) You also answered a question that popped up in my head, but that I forgot asking here. At first I thought only cars with turbos did it, but later I noticed also some NA cars threw flames. I was puzzled about whether it had to do something with the turbos or not. Both yes and no it seems. Thanks for clearing that up as well.

Non turbo cars: usually it is because driver hits rev limiter or on upchange. Most racing cars have the rev limiter that cuts spark. When engine hits rev limiter (on cars with sequential gearbox it's usually fixed so tahat it cuts ignition when you upshift, so you can do it without clutch and gas pedal flat to thre floor), ther's no spark but enough fuel, which ignites in hot exhaust.