Pis­ton en­gine com­bus­tion

ALL ABOUT DET­O­NA­TION, PRE- IG­NI­TION AND BACKFIRING

Flying - - CONTENTS - By Richie Len­gel

REL­E­VANT DIS­CUS­SION:

Not so many years ago, be­fore com­put­ers got in­volved, au­to­mo­bile en­gines ran us­ing all sorts of an­cient tech­nol­ogy that made them prone to det­o­na­tion and/or pre-ig­ni­tion when the tim­ing and fuel mix­ture got out of whack. These symp­toms, usu­ally called “en­gine knock,” were quite com­mon and rel­a­tively easy to de­tect at au­to­mo­bile speeds, but not so easy to de­tect at air­plane speeds and noise lev­els. Of course, the ma­jor­ity of the gen­eral avi­a­tion fleet still re­lies on some of that same an­cient tech­nol­ogy and even throws in some parts from a farm trac­tor (mag­ne­tos). Here’s a primer on how these con­di­tions re­late to air­craft en­gines.

NOR­MAL COM­BUS­TION:

Oc­curs when the fuel/air mix­ture ignites in the cylin­der and burns pro­gres­sively at a fairly uni­form rate across the com­bus­tion cham­ber. When ig­ni­tion is prop­erly timed, max­i­mum pres­sure is built up just af­ter the pis­ton has passed top dead cen­ter at the end of the com­pres­sion stroke. The flame fronts start at each spark plug and burn in more or less wave­like forms. The ve­loc­ity of the flame travel is in­flu­enced by the type of fuel, the ra­tio of the fuel/air mix­ture and the pres­sure and tem­per­a­ture of the fuel mix­ture.

DET­O­NA­TION:

There is a limit to the amount of com­pres­sion and the de­gree of tem­per­a­ture rise that can be tol­er­ated within an en­gine cylin­der and still per­mit nor­mal com­bus­tion. All fu­els have crit­i­cal lim­its of tem­per­a­ture and com­pres­sion. Be­yond this limit, they ig­nite spon­ta­neously and burn with ex­plo­sive vi­o­lence. If the to­tal com­pres­sion ex­ceeds the crit­i­cal point, det­o­na­tion oc­curs. The ex­plo­sive burn­ing dur­ing det­o­na­tion re­sults in an ex­tremely rapid pres­sure rise. This rapid pres­sure rise and the high in­stan­ta­neous tem­per­a­ture, com­bined with the high tur­bu­lence gen­er­ated, cause a scrub­bing ac­tion on the cylin­der and pis­ton. This can burn a hole com­pletely through the pis­ton. The det­o­na­tion char­ac­ter­is­tic of the mix­ture can be con­trolled by vary­ing the fuel-to-air ra­tio. At high power out­put, com­bus­tion pres­sures and tem­per­a­tures are higher than they are at low or medium power. There­fore, at high power set­tings, the fuel-to-air ra­tio is made richer than is needed for good com­bus­tion at medium or low power out­put. This is done be­cause, in gen­eral, a rich mix­ture does not det­o­nate as read­ily as a lean mix­ture. Un­less det­o­na­tion is heavy, there is no flight-deck ev­i­dence of its pres­ence. Light to medium det­o­na­tion does not cause no­tice­able rough­ness, tem­per­a­ture in­crease or loss of power. As a re­sult, it can be present dur­ing take­off and high-power climb with­out be­ing known to the flight crew.

PRE-IG­NI­TION:

As the name im­plies, it means com­bus­tion takes place within the cylin­der be­fore the timed spark jumps across the spark-plug ter­mi­nals. This con­di­tion can of­ten be traced to ex­ces­sive car­bon or other de­posits that cause lo­cal hot spots. Det­o­na­tion of­ten leads to pre-ig­ni­tion. How­ever, pre-ig­ni­tion may also be caused by high-power op­er­a­tion with ex­ces­sively lean mix­tures. Pre-ig­ni­tion is usu­ally in­di­cated in the cock­pit by en­gine rough­ness, backfiring and a sud­den

in­crease in cylin­der-head tem­per­a­ture. Any area within the com­bus­tion cham­ber that be­comes in­can­des­cent serves as an ig­niter in ad­vance of nor­mal timed ig­ni­tion and causes com­bus­tion ear­lier than de­sired. Pre-ig­ni­tion may be caused by an area rough­ened and heated by det­o­na­tion ero­sion. A cracked valve or pis­ton, or a bro­ken spark-plug in­su­la­tor, may fur­nish a hot point that serves as a glow plug. The hot spot can be caused by de­posits on the cham­ber sur­faces re­sult­ing from the use of leaded fu­els. Nor­mal car­bon de­posits can also cause pre-ig­ni­tion. Pre-ig­ni­tion is caused by a hot spot in the com­bus­tion cham­ber, not by in­cor­rect ig­ni­tion tim­ing. The hot spot may be due to either an over­heated cylin­der or a de­fect within the cylin­der. The most ob­vi­ous method of cor­rect­ing pre-ig­ni­tion is to re­duce the cylin­der tem­per­a­ture. The im­me­di­ate step is to re­tard the throt­tle. This re­duces the amount of fuel charge and the amount of heat gen­er­ated, af­ter which the mix­ture should be en­riched to lower the com­bus­tion tem­per­a­ture. If the en­gine is at high power when pre-ig­ni­tion oc­curs, re­tard­ing the throt­tle for a few sec­onds may pro­vide enough cool­ing to chip off some of the lead or other de­posits within the com­bus­tion cham­ber. These chipped-off par­ti­cles pass out through the ex­haust.

BACKFIRING:

When a fuel/air mix­ture does not con­tain enough fuel to con­sume all the oxy­gen, it is called a lean mix­ture. Con­versely, a charge that con­tains more fuel than re­quired is called a rich mix­ture. An ex­tremely lean mix­ture either does not burn at all or burns so slowly that com­bus­tion is not com­plete at the end of the ex­haust stroke. The flame lingers in the cylin­der and then ignites the con­tents in the in­take man­i­fold or the in­duc­tion sys­tem when the in­take valve opens. This causes an ex­plo­sion known as backfiring, which can dam­age the car­bu­re­tor and other parts of the in­duc­tion sys­tem. In­cor­rect ig­ni­tion tim­ing, or faulty ig­ni­tion wires, can cause the cylin­der to fire at the wrong time, al­low­ing the cylin­der to fire when the in­take valve is open, which can cause backfiring. Backfiring rarely in­volves the whole en­gine. There­fore, it is sel­dom the fault of the car­bu­re­tor. In prac­ti­cally all cases, backfiring is lim­ited to one or two cylin­ders. Usu­ally, it is the re­sult of a faulty valve clear­ance set­ting, de­fec­tive fuel-in­jec­tor noz­zles or other con­di­tions that cause these cylin­ders to op­er­ate leaner than the en­gine as a whole. In some in­stances, an en­gine back­fires in the idle range but op­er­ates sat­is­fac­to­rily at medium and high power set­tings. The most likely cause in this case is an ex­ces­sively lean idle mix­ture.

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