This phenomenon is called dieseling
Dieselingor enginerun-on is a conditionthatcan occur in spark plug , gasolinepoweredinternal combustionengines , wherebythe enginekeeps running for a short period after beingturnedoff, dueto
fuel igniting without a spark.
Dieseling is so-named becauseit is similar in appearanceto how diesel enginesoperate:by firing without a spark. The ignition sourcein a diesel is the compressionof the fuel in the cylinder, ratherthan a spark as in gasolineengines.The dieseling phenomenonoccursnotbecausethe compressionratio is sufficientto causeauto-ignition of the fuel,buta hotspotinsidethe cylinder starts combustion.An automobileengine thatis dieselingwill typically sputter, thengraduallystop.
Dieseling is notnearly as commonas it oncewas, becauseit most commonlyoccursin enginesequipped with carburetors . The vast majorityof vehiclesmanufacturedafter 1990 are fuel-injected:The injectorsand high-pressurefuel pumpimmediately cease supplyingfuel to the cylinders when the ignition is switchedoff. If the injectoris damagedor malfunctioning,a small amount of fuel can enterthe chamberand be ignited,causinga sputteror two after the engineis switchedoff.
Dieseling (in the senseof enginerun-on, and disregardingcombustible gaseousmixturesvia the air intake) can also occur in Diesel engines, when the pistonor sealsfail dueto overheating,admittingengineoil into the cylinder. A structurallyfailing Diesel enginewill oftenaccelerate when the throttleis released,even after fuel injectionis switchedoff.
Somecarburettedengineshave low-pressurefuel pumps.They are typically designedonly to overcomea lossof suctionin the fuel line near the enginedueto fuel evaporationin hotweather,to supplysufficientfuel to maintain stoichiometric combustionunderheavy load with wide-open throttle,or a combination of the two. Fuel demand is low at idle and thereis more than enough manifold vacuumto draw sufficient fuel for combustion,even if the fuel pumpis switchedoff.
Gasoline enginesthatare much smaller than the typicalautomotive engineare usually carburettedfor economicand engineeringreasons. Dieseling can occur in suchengines. These enginesincludethoseinstalled in mopeds,scooters,small motorcycles,all-terrain vehicles , and mostlawn-and-garden powertools.
Potentialcauses
Dieseling can occur for several reasons:
Built-up carbon in the ignition chambercan glow red after the engineis off, providinga mechanism for igniting unburntfuel. Sucha thing can happen when the engineruns very rich, depositingunspentfuel and particleson the pistonsand valves . Similarly, rough metal regionswithin the pistonchambercan causethis sameproblem,sincethey can glow red. It has also beensuggestedthat an improperlyrated spark plugcan retainheatand causethe same problem.
A carburetorthatdoesnot completelyclosecan contributeto running oncethe engineis off, since the extra fuel and oxygen mixturecan combusteasily in the warm piston chamber. Similarly, hotvaporizedoil gasesfrom the enginecrankcasecan provide amplefuel for dieseling.
Incorrecttiming.
An enginethatruns toohotor too lean may producean environment conduciveto allowingunspentfuel to burn.
An idle speedthatis toofast can leave the enginewith toomuch
angular momentumuponshutdown, raisingthe chancesthatthe engine can turn over and burnmore fuel and lockitselfinto a cycle of continuous running.
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