Dieseling
Encyclopedia
Dieseling 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.
Dieseling is so-named because it is similar in appearance to how diesel engine
s operate: by firing without a spark. The ignition source in a diesel is the compression of the fuel in the cylinder, rather than a spark as in gasoline engines. The dieseling phenomenon occurs not because the compression ratio is sufficient to cause auto-ignition of the fuel, but a hot spot inside the cylinder starts combustion. An automobile
engine that is dieseling will typically sputter, then gradually stop.
Dieseling is not nearly as common as it once was, because it most commonly occurs in engines equipped with carburetor
s. The vast majority of American, European, and Japanese vehicles manufactured after 1990 are fuel-injected: The injectors and high-pressure fuel pump immediately cease supplying fuel to the cylinders when the ignition is switched off. If the injector is damaged or malfunctioning, a small amount of fuel can enter the chamber and be ignited, causing a sputter or two after the engine is switched off.
Dieseling (in the sense of engine run-on, and disregarding combustible gaseous mixtures via the air intake) can also occur in Diesel engines, when the piston or seals fail due to overheating, admitting engine oil into the cylinder. A structurally failing Diesel engine will often accelerate when the throttle is released, even after fuel injection is switched off.
Some carburetted engines have low-pressure fuel pumps. They are typically designed only to overcome a loss of suction in the fuel line near the engine due to fuel evaporation in hot weather, to supply sufficient fuel to maintain stoichiometric combustion under heavy load with wide-open throttle, or a combination of the two. Fuel demand is low at idle and there is more than enough manifold vacuum to draw sufficient fuel for combustion, even if the fuel pump is switched off.
Gasoline engines that are much smaller than the typical automotive engine are usually carburetted for economic and engineering reasons. Dieseling can occur in such engines. These engines include those installed in mopeds, scooters, small motorcycles, all-terrain vehicle
s, and most lawn-and-garden power tools.
Spark plug
A 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...
, gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...
powered internal combustion engines, whereby the engine keeps running for a short period after being turned off, due to fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
igniting without a spark.
Dieseling is so-named because it is similar in appearance to how diesel engine
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
s operate: by firing without a spark. The ignition source in a diesel is the compression of the fuel in the cylinder, rather than a spark as in gasoline engines. The dieseling phenomenon occurs not because the compression ratio is sufficient to cause auto-ignition of the fuel, but a hot spot inside the cylinder starts combustion. An automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
engine that is dieseling will typically sputter, then gradually stop.
Dieseling is not nearly as common as it once was, because it most commonly occurs in engines equipped with 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....
s. The vast majority of American, European, and Japanese vehicles manufactured after 1990 are fuel-injected: The injectors and high-pressure fuel pump immediately cease supplying fuel to the cylinders when the ignition is switched off. If the injector is damaged or malfunctioning, a small amount of fuel can enter the chamber and be ignited, causing a sputter or two after the engine is switched off.
Dieseling (in the sense of engine run-on, and disregarding combustible gaseous mixtures via the air intake) can also occur in Diesel engines, when the piston or seals fail due to overheating, admitting engine oil into the cylinder. A structurally failing Diesel engine will often accelerate when the throttle is released, even after fuel injection is switched off.
Some carburetted engines have low-pressure fuel pumps. They are typically designed only to overcome a loss of suction in the fuel line near the engine due to fuel evaporation in hot weather, to supply sufficient fuel to maintain stoichiometric combustion under heavy load with wide-open throttle, or a combination of the two. Fuel demand is low at idle and there is more than enough manifold vacuum to draw sufficient fuel for combustion, even if the fuel pump is switched off.
Gasoline engines that are much smaller than the typical automotive engine are usually carburetted for economic and engineering reasons. Dieseling can occur in such engines. These engines include those installed in mopeds, scooters, small motorcycles, all-terrain vehicle
All-terrain vehicle
An all-terrain vehicle , also known as a quad, quad bike, three wheeler, or four wheeler, is defined by the American National Standards Institute as a vehicle that travels on low pressure tires, with a seat that is straddled by the operator, along with handlebars for steering control...
s, and most lawn-and-garden power tools.
Potential causes
Dieseling can occur for several reasons:- Built-up carbonCarbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
in the ignition chamber can glow red after the engine is off, providing a mechanism for igniting unburnt fuel. Such a thing can happen when the engine runs very rich, depositing unspent fuel and particles on the pistons and valvesPoppet valveA poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...
. Similarly, rough metal regions within the pistonPistonA piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
chamber can cause this same problem, since they can glow red. It has also been suggested that an improperly rated spark plug can retain heat and cause the same problem. - A carburetorCarburetorA 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....
that does not completely close can contribute to running once the engine is off, since the extra fuelFuelFuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
and oxygenOxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
mixture can combust easily in the warm piston chamber. Similarly, hot vaporized oil gases from the engine crankcaseCrankcaseIn an internal combustion engine of the reciprocating type, the crankcase is the housing for the crankshaft. The enclosure forms the largest cavity in the engine and is located below the cylinder, which in a multicylinder engine are usually integrated into one or several cylinder blocks...
can provide ample fuel for dieseling. - Incorrect timing.
- An engine that runs too hot or too lean may produce an environment conducive to allowing unspent fuel to burn.
- An idle speedIdle speedIdle speed is the rotational speed the engine runs on when the engine is uncoupled to the drivetrain and the throttle pedal isn't depressed of a combustion engine...
that is too fast can leave the engine with too much angular momentumAngular momentumIn physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...
upon shutdown, raising the chances that the engine can turn over and burn more fuel and lock itself into a cycle of continuous running.