A back-fire or backfire is an explosion produced either by a running internal combustion engine that occurs in the air intake or exhaust system rather than inside the combustion chamber

A back-fire or backfire is an explosion produced either by a running internal combustion engine that occurs in the air intake or exhaust system rather than inside the combustion chamber or unburned fuel or hydrocarbons ignited somewhere in the exhaust system. 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.

Backfiring in internal combustion engines occur outside of the combustion chamber, and are 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 engine 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.

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