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A back-fire or backfire is an explosion produced either by a running internal combustion engine that occurs in the induction system rather than inside the combustion chamber. Unburned fuel or hydrocarbons ignited from a slower burning, lean fuel air mixture that is still burning inside the cylinder when the intake valve opens. Not to be confused with afterfire which is caused by an excessively rich fuel air mixture that is not completely burned during combustion. It is then ignited in the exhaust. A visible flame may momentarily shoot out of the exhaust pipe. Either condition causes an objectionable popping noise, together with possible loss of power and forward motion. A back-fire is a separate phenomenon from the fire produced by Top Fuel dragsters.
The term derives from parallel experiences with early unreliable firearms or ammunition, in which the explosive force was directed out at the breech instead of the muzzle. From this came the use of the word "backfire" as a verb to indicate something that produces an unintended, unexpected, and undesired result.
Backfiring in internal combustion engines occurs outside of the combustion chamber, and is typically the result of an improper air to fuel ratio. An overly lean air-fuel mixture (i.e. an overabundance of air) can lead to a failure to ignite in the combustion chamber, also called a "misfire". The unburnt fuel then enters the exhaust system, where hot components can cause the fuel to ignite unpredictably. Alternatively, rich air-fuel mixtures (i.e. an overabundance of fuel) can result in incomplete combustion, again causing unburnt fuel to enter the exhaust system.
Backfires may also occur before the combustion chamber. One possible cause of this is timing. If the timing is too advanced, the spark plug fires before the intake valves close, causing the combustion to propagate into the intake manifold, further igniting the air-fuel mixture there; the resulting explosion then travels out of the carburetor and air filter. On many small marine engines, a screen is placed over the intake of the carburetor as a flame arrestor, to prevent these flames from escaping the intake and potentially igniting fuel or fuel vapors in the enclosed sump or bilge of the boat, causing a fire or explosion. Alternatively, the ignition timing may be retarded, in which case the combustion is not completed by the time the exhaust valves open, allowing the combustion to propagate into the exhaust system.
Additionally, improperly adjusted carburetors that create a lean condition during acceleration can cause the air fuel mixture to burn so slowly, that combustion is still taking place during the exhaust stroke, and even when the intake valve opens. The flame front can then travel up the intake and cause a backfire. In this situation it is conceivable that there is a backfire occurring in the intake manifold and exhaust manifold simultaneously.
In both cases (combustion occurring before and after the combustion chamber), the result is a sharp pop, which is colloquially referred to as a "backfire". However, for troubleshooting, engine mechanics more strictly define an ignition of fuel within the engine exhaust system as an "afterfire", while a "backfire" is this same process taking place in the induction system.
Exhaust system backfires occur in engines that have an emission system malfunction, like an air injection system diverter valve problem, an exhaust leak, or when the catalytic converter has been removed. In some high-performance vehicles, when a driver shifts up and lets off the accelerator, the engine has a moment of running rich. This causes an incomplete burn which causes the fumes to explode in the exhaust system along with an audible pop or bang sound. This is a result of working equipment, and is unlikely to cause damage.
A fuel-injected engine may backfire if an intake leak is present (causing the engine to run lean), or a fuel injection component such as an air-flow sensor is defective.
Common causes of backfires are:
With older engine designs, backfiring can be common or unavoidable. Backfire is rare in modern vehicles with fuel-injection and computer-controlled fuel mixtures.
Cars with sports exhausts (both factory-fitted and aftermarket) are much more likely to backfire. In some circumstances the backfire is seen as an additional perk of the car. The TVR Cerbera is an example of a car with factory-fitted sports exhausts which produce frequent backfires on engine braking.
In high-powered supercharged aircraft piston engines such as the Rolls-Royce Merlin and Griffon, backfiring into the inlet manifold is prevented with flame traps inside the manifold, the traps preventing the flame propagating into the compressed air/fuel mixture within the manifold.
Tanks and naval ships may use the injection of fuel or special "fog oil" into exhaust tract to create smoke screens. Rather than burning, the oil normally evaporates and re-condenses at a particular droplet size, but under abnormal conditions it may catch fire or even produce a thermobaric explosion.
Cars extensively modified for visual appearance and not road use (stunts, ads, movies etc.) may be fitted with gasoline injectors in their exhaust systems, or even with small flamethrowers separate from the exhaust.
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