Back-fire
Encyclopedia
- For other meanings of the term see backfireBackfireThe word backfire in general use usually refers to a plan where the opposite of the desired effect happens or the perpetrator is directly affected as opposed to their intended target.In economics, backfire is described in terms of:* the Jevons paradox...
.
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
that occurs in the air intake or exhaust system rather than inside the combustion chamber
Combustion chamber
A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...
. The same term is used when unburned fuel or hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s are 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. A backfire is a separate phenomenon from the fire produced by Top Fuel
Top Fuel
Top Fuel racing is a class of drag racing in which the cars are run on a mix of approximately 90% nitromethane and 10% methanol rather than gasoline or simply methanol. The cars are purpose-built for drag racing, with an exaggerated layout that in some ways resembles open-wheel circuit racing...
dragsters.
Also, an explosion in the inlet manifold
Inlet manifold
In automotive engineering, an inlet manifold or intake manifold is the part of an engine that supplies the fuel/air mixture to the cylinders...
, carburetor
Carburetor
A carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....
/throttle body, or air cleaner of an internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
can occur when the intake valves are not shut prior to fuel combustion.
The term was derived from experiences with early unreliable firearm
Firearm
A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...
s or ammunition, in which the explosive force was directed out 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
Unintended consequence
In the social sciences, unintended consequences are outcomes that are not the outcomes intended by a purposeful action. The concept has long existed but was named and popularised in the 20th century by American sociologist Robert K. Merton...
.
Explanation
Backfire in an automobile engine typically results from various malfunctions related to the air to fuel ratio. Backfiring can occur in carbureted engineEngine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
s that are running lean where the air-fuel mixture has insufficient fuel and whenever the timing is too advanced. As the engine runs leaner or if there is less time for the fuel to burn in the combustion chamber, there is a tendency for incomplete combustion. The condition that causes this is a misfire. The result of a misfire or incomplete combustion is that unburned fuel or flammable hydrocarbons are delivered to the exhaust manifold where it may ignite unpredictably. Another backfire situation occurs when the engine is running rich (with excess fuel) and there is incomplete combustion during the Otto cycle
Otto cycle
An Otto cycle is an idealized thermodynamic cycle which describes the functioning of a typical reciprocating piston engine, the thermodynamic cycle most commonly found in automobile engines....
, with similar results.
Popularly the term is used to describe a sharp report produced by almost any type of engine. However, among engine professionals, "afterfire" is the term used to describe ignition of fuel within the engine exhaust system and "backfire" is the term used to describe this same process taking place in the induction system, primarily in internal combustion engines. The separate terms are useful when troubleshooting running problems.
When starting an engine, timing that is too advanced will fire the spark plug before the intake valve is closed. The flame front will travel back in to the intake manifold, igniting all of that air and fuel as well. The resulting explosion then travels out of the carburetor and air cleaner. A common air filter will allow the gases to escape, but will block the flame front. On many small marine engines, no air filter is used, but 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 and causing a fire or explosion. 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.
Causes
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 converterCatalytic converter
A catalytic converter is a device used to convert toxic exhaust emissions from an internal combustion engine into non-toxic substances. Inside a catalytic converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustion are converted to less toxic substances by dint...
has been removed. In some high-performance vehicles, when a driver shifts
Gear shift
The gear shift is the part of the gearbox which has the shift forks and allows the contact from the driver to the synchronization. Most of the time they are so much like the gear counter plus the reverse gear. And they make it possible to choose the gear and to switch this in or out...
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
Fuel injection
Fuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....
component such as an air-flow sensor is defective.
Common causes of backfires are:
- Poor or unregulated engine timing is often a cause of intake backfires, but can also be responsible for exhaust backfires
- Improper wiringElectrical wiringElectrical wiring in general refers to insulated conductors used to carry electricity, and associated devices. This article describes general aspects of electrical wiring as used to provide power in buildings and structures, commonly referred to as building wiring. This article is intended to...
in the ignitionIgnition systemAn ignition system is a system for igniting a fuel-air mixture. Ignition systems are well known in the field of internal combustion engines such as those used in petrol engines used to power the majority of motor vehicles, but they are also used in many other applications such as in oil-fired and...
can also lead to timing issues and backfires - Low fuel pressure, clogged fuel filterFuel filterA fuel filter is a filter in the fuel line that screens out dirt and rust particles from the fuel, normally made into cartridges containing a filter paper. They are found in most internal combustion engines....
s, and weak fuel pumpFuel pumpA fuel pump is a frequently essential component on a car or other internal combustion engined device. Many engines do not require any fuel pump at all, requiring only gravity to feed fuel from the fuel tank through a line or hose to the engine...
s could cause a severe lean air-to-fuel ratio during fuel injectionFuel injectionFuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s.... - Missing or damaged catalytic converterCatalytic converterA catalytic converter is a device used to convert toxic exhaust emissions from an internal combustion engine into non-toxic substances. Inside a catalytic converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustion are converted to less toxic substances by dint...
can result in backfires out the tailpipe
Applications
With older engine designs, backfiring can be common or unavoidable. Backfire is rare in modern vehicles with fuel-injection and computer-controlled fuel mixtures.In drag racing
Drag racing
Drag racing is a competition in which specially prepared automobiles or motorcycles compete two at a time to be the first to cross a set finish line, from a standing start, in a straight line, over a measured distance, most commonly a ¼-mile straight track....
, backfires in the intake usually result in the complete destruction of the intake manifold, carburetors, supercharger
Supercharger
A supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...
, and sometimes engine.
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
TVR Cerbera
The TVR Cerbera is a sports car manufactured by TVR between 1996 and 2003. The name is derived from Cerberus the three-headed beast of Greek legend that guarded the entrance of Hades....
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
Rolls-Royce Merlin
The Rolls-Royce Merlin is a British liquid-cooled, V-12, piston aero engine, of 27-litre capacity. Rolls-Royce Limited designed and built the engine which was initially known as the PV-12: the PV-12 became known as the Merlin following the company convention of naming its piston aero engines after...
and Griffon
Rolls-Royce Griffon
The Rolls-Royce Griffon is a British 37-litre capacity, 60-degree V-12, liquid-cooled aero engine designed and built by Rolls-Royce Limited...
, 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.
Purposely made
TankTank
A tank is a tracked, armoured fighting vehicle designed for front-line combat which combines operational mobility, tactical offensive, and defensive capabilities...
s and naval
Navy
A navy is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake- or ocean-borne combat operations and related functions...
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 fuel-air explosion
Fuel-air explosive
A fuel-air explosive device consists of a container of fuel and two separate explosive charges. After the munition is dropped or fired, the first explosive charge bursts open the container at a predetermined height and disperses the fuel in a cloud that mixes with atmospheric oxygen...
.
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.
See also
- DieselingDieselingDieseling or engine run-on is a condition that can occur in spark plug, gasoline powered internal combustion engines, whereby the engine keeps running for a short period after being turned off, due to fuel igniting without a spark....
, an after-run condition in which an engine continues to run without the spark plugSpark plugA spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...
s firing - Ignition timingIgnition timingIgnition timing, in a spark ignition internal combustion engine , is the process of setting the angle relative to piston position and crankshaft angular velocity that a spark will occur in the combustion chamber near the end of the compression stroke...
- Valve timingValve timingIn a piston engine, the valve timing is the precise timing of the opening and closing of the valves.In four-stroke cycle engines and some two-stroke cycle engines, the valve timing is controlled by the camshaft. It can be varied by modifying the camshaft, or it can be varied during engine operation...