You see those flames because the engine is not firing when the car is braking. Nonetheless, gasoline is injected on purpose into the cylinder to cool it.
You don't need to have an injector for this to happen: you can get the same effect in carburated engines. Either injected or aspired, the un-fired gasoline goes to the exhaust where it creates the flames when it comes in contact with the hot exhaust tube.
If you wish (I hope you don't) you can buy a kit that does the same thing for your car. As the exhaust of your car, tipically, doesn't reach such high temperatures as the ones reached in an F1 exhaust tube, this thingie has an electrical spark to inflame the unburned gas.
This same effect happens in almost all kind of racecars: the thing here is that
the intake and exhaust valves are open at the same time, to make the engine more efficient. The higher the RPMs the engine reach, the larger the overlap between exhaust and intake is (I mean, the longer the time that the exhaust an intake valves are open at the same time).
This is a "recycling" of an older post we wrote back in 2008 or so, between some of the guys here:
1. In any engine there is valve overlap, as Belatti, Riff_Raff and Fridge13 explained in detail here:
Exhaust flames on overrun
In the following pictures, the blue arc shows the time the intake is open, the red arc shows the time the exhaust is open.
Notice the arcs overlap in a regular car and overlap a lot in a racing engine.
Standard Camshaft: the overlap is small (red and blue segments of circle on the top)
Performance Camshaft: huge overlap. Both valves are open for a long time.
If you wish, you can watch the animation where I took the previous pictures from here:
http://static.howstuffworks.com/flash/camshaft-cam.swf (you need a Flash plugin in your browser to watch it).
Why do you need to have overlapping valve openings? Because
air has inertia. I copy and paste an explanation I already gave here, in line with Fridge13 and Riff Raff:
If you close the intake valve just when the piston reaches the bottom of its movement (what is called "BDC" or "Bottom Dead Center"),
you are going to stop the inrush of air right when is entering the cylinder at its highest speed.
The air has inertia, like almost anything in this world. So, you left the valve open a little more time, with the final effect that,
even while the cylinder is moving up, on the compression stroke, the air is still entering the cylinder because of that inertia, giving you a little extra mixture inside the cylinder.
Piston at the bottom of its movement: the air is entering at high speed, the piston barely has started to move upwards, so air continues to enter
If you close the intake at BDC, you'll shut the intake when the air is at top speed. So, you left the valve open for a little, because even if the piston starts to move upwards after the BDC, the air is not going to "feel" that movement instantly: the "compression wave" has to move from the piston head towards the opening for the air to stop entering.
Under that circumstances, even when the piston is moving upwards, air continues to enter the cylinder for some milliseconds. So, you left the intake open a little beyond BDC.
Now, imagine the opposite: the piston is at the top of its movement, in the exhaust stroke, air is exiting.
The air exits faster when the cylinder reaches TDC (top dead center), that is, the topmost point of its movement, right?
Piston at the top dead center: air is still exiting at high speed, so it pulls some air from the intake (if you open the intake valve some milliseconds before TDC)
So, if you open the intake valve a little BEFORE TDC, the fumes, that are exiting from the cylinder through the exhaust valve at top speed, "pull" some air from the intake valve, helping some extra air to enter the cylinder. This is called "scavenging".
So, a valve camshaft is a compromise: at low rpm you do not want overlap, the air is not rushing in and out at high speeds.
Finally,
when you overlap the exit and intake of air, the exiting air helps to "pull" in the intake air.
This effect of the "inertia of the air" is more noticeable at high rpms, simply because the air is moving faster. This is why the overlap of the valves is greater in race engines, that develop ultra-high rpms.
2. I copy another of my posts verbatim:
When you close the throttle, the engine manifold vacuum increases. I think this is the same thing that happens when you block the pipe of a vacuum cleaner with your hand, for example.
On carburated cars, like NASCAR (ehem...), the 'idle port', (below the throttle plates), is then subjected to high intake manifold vacuum throwing extra fuel into the combustion chamber.
On some cars, injectors are located between the throttle plate and the intake valve. The intake engine vacuum on deceleration pulls fuel from these injectors and send it to the cylinders.
In modern cars the injectors have a tight shut-off on deceleration to avoid that, specially at normal, highway speeds, so flames don't come out of them (I think F1 cars behave in the same way).
3. There are more things: at low rpm the spark is not as energetic as at high rpm and when you close the throttle there is less air in the mixture.
Actually there are "flamethrowers kits" that cut the spark and use an ignition coil in the exhaust for you to impress your friends:
4. Many racing engines use more fuel than needed to cool the cylinders. This is notorious in NASCAR aspirated engines, but it is also widely used in F1.
On a side note, that's the reason why it is so hard to keep a racing engine running and why you can stall it so easily: the overlap that allows you to enhance the performance of the engine (after all,
a combustion engine is an air pump), gives you a very poor performance at low revolutions: racing engines don't like to be at low revs. At low revs the inertia is not that important because the air is moving at low speeds and having both valves open gives you an "unstable" air pump: it is "open" to the external atmospheric pressure when it should be closed.
The first engines that were fabricated in this world, back at the end of the XIX century had valves that closed and opened at BDC and TDC. Only when engines reached revs in excess of 1.000 rpm or so, engineers devised the overlap that gives you more efficiency.
That's also the reason why high performance cars have variable cams: the overlap is optimized depending on the regime of revs you're running.
5. if the engine has fixed cams, you can't control your overlap without changing the cams, but
the effects caused by changing the back pressure in the exhaust pipe are the same.
When you reduce back pressure, it is equivalent to increasing valve overlap, and when you increase back pressure, it is the same as decreasing the amount of valve overlap.
An engine with less overlap will give you more torque and less HP. The opposite is also true: an engine with more overlap will give you more HP and less torque.
That's why some people will say, "
you need a muffler for torque", or, "
you'll have more high-end, but less torque, if you run straight exhaust".
This is really important if you're into drag racing (or, heavens forbid, because you do street racing for money).
By reducing back pressure in an exhaust system, you increase high-end horsepower at the cost of low-end torque. For example,
low overlap cams are called "RV" because they're used in recreational vehicles, that tow large loads and need to have high torque.
RV cam kit picture: you cannot notice the difference in the lobes of the cams, but it is there. Those kits are popular among truckers that tow large loads.
On the contrary, high RPM engines with high overlap tend to have more HP and less torque.
As you can imagine,
racing engines, with almost no backpressure and no mufflers, create the same effect as valve overlap: they pull unburnt air and fuel from the cylinder and hence, flames on the exhaust.
On top of that:
- you close the throttle, because flames appear when you "coast the vehicle" or you brake, thus creating vacuum on the intake of fuel, sucking fuel from the "idle" ports (idle ports, or whatever the english name, are ports that inject fuel to mantain the engine idling when you don't push the throttle pedal).
- the spark is weak because the engine is not at top rpm (at least in some cars).
Spark jumping in a "primitive" test of the ignition coil: the higher the rpm, the spark seems to be more bluish
When I've done that, you can see that the spark that jumps it's "bluer" as rpms increase. Thus, I deduce, the spark has less "power" at low rpm. I've seen that in a model 66 car without electronic ignition, I don't think this is important is newer cars.
- the exhaust valve is open while the intake valve is also open at TDC, between exhaust and intake strokes.
- the droplets might be larger in injected cars (although I've seen flames in carburated cars).
- the engine manufacturer might have designed the engine map to throw extra fuel under those circumstances to cool the engine.
... so the engine expels unburnt fuel through the exhaust: you get flames.
Yes, it's more or less cool, but you'll deposit carbon on piston heads and increase wear of first piston ring (because of carbon on the cylinder walls).
Phew, I hope nobody falls asleep while reading this post. Sorry for the long explanation, but if you like racing (of course you do!) you need to understand this.
