A couple additives say that their product eliminates this...

 

Ive always understood tha tto mean run-on... when you shut the engine off and it continues running untillit burns all the fuel i teh system up..
older engines and alot of tractor/industrial engines will do this... if your gas engines do it, its considered bad..

 

Diesels use heat ignition - very high cylinder temperatures ignite the fuel - cetane is the rating for self-ignition

Gassers (patooie!) use spark ignition - oil in the combustion chamber carbonizes at combustion chamber temps, as do other combustion by-products, which deposit on the piston crowns and heads - if those deposits become hot enough, gasoline will ignite long B4 the timed spark is delivered - that is ping, or Dieseling - octane is the rating for resistance to self-ignition.

That additive stuff dissolves and burns off the deposits during the normal spark-ignited combustion cycle - in theory, of course

 

Many things can cause this in a gasser. Advanced timing, hot carbon spots in the combustion chambers etc.

 

Yes indeed. Essentially trying to run like a diesel does.

Many have argued that too much timing or too low an octane are also to blame. Those are different scenarios all together, as they are still a somewhat controlled combustion.

Dieseling, like described above, is uncontrolled combustion, can happen at any time due to the fuel/air mix entering at the same time, and can do much more damage in very little time.

 

Similar can happen in a Diesel engine - the combustion flame-front never actually contacts the piston crown in normal operation - as combustion progresses, cylinder pressure increases, pushing the piston ahead of the flame-front - only the hot gasses contact the crown - leaky injectors with bad spray pattern can fire a stream of fuel directly on the piston crown, eventually destroying the anodized surface - you would hear the 'ping', but the normally loud Diesel combustion rattle masks it - which is why it is easy to hole a Diesel piston - you cannot hear the increasing ping which indicates it is happening - high EGT's can do the same, but the EGT guage tells that tale - leaky injectors are sneaky damage

 

I'm sure it's just semantics but pre-detonation is "pinging" in a gas engine. The CC (combustion chamber) gets too hot or fuel octane rating is too low for the compression ratio (higher octane slows down the burn rate). Thus like stated above the combustion actually occurs BEFORE the spark ignites the air/fuel mixture(and before TDC as well). Err....TDC is top dead center. This will happen silently as well with the only real way of finding it is by tiny black dots (carbon deposits) on the spark plug insulator. If it's REALLY bad then you can actually hear the pre-detonation and everyone seems to call that "pinging". But that's kind of a mis-nomer as well because you can get "pinging" from being too advanced on your spark as well. That's not pre-detonation because the spark is causing the explosion.

Dieseling is when a gas engine continues to run AFTER the ignition system has been shut down. You've seen the old movies....the guy turns the car off and it keeps chuggin along for a few seconds. This is usually associated with residual very high temps from carbon build up and the carburetor throttle plates staying slightly open (for normal idleing). The combination causes a small amount of air/fuel to get to the CC and ignite from just the residual heat from the last combustion staying in the built up carbon. Most stock carbs will actually have an anti-dieseling device on them which if memory serves, it's just an electronic selonoid that closes the throttle plates fully when the ignition is turned off, thus not letting any fuel/air into the CC. This can't happen in a CR diesel because the fuel isn't injected until the combustion event is scheduled. The squeeze on the air heats it to the piont that all is needed is a little squirt of good ole number 2....and bang! For that matter injected gassers can't diesel either because there is no fuel in the mixture....just air. So I think the only gasser that can diesel is a carbureted engine or possibly a mechanically injected setup. But any gasser can get pre-detonation....no matter how it's fed.

clear as mud?

 

Well said Deesil. That's what I was taught back in the day. Dieseling (or run on) was pretty common on many of the cars I had when I was young. Concepts and perceptions change though and what we learned on carbureted engines may seem different today. I was taught to set timing on a old one lunger with make or break ignition. Somehow I can't see that happening anymore. I still have an old 3 HP Fairbanks-Morse engine, built in 1916, that I fire up now and then, just to listen and remember the old days.


Dan

 

Think of "dieseling" as pinging with the ignition off. Usually though, pinging is when fuel autoignites early enough to cause a HUGE rise in cylinder pressure that can damage pistons and take out headgaskets.

Dieseling is usually a later-timing type of autoignition where there's just enough residual heat to light the mixture without a spark plug.

jmo

 

That's true - been so long since I messed with bad-running gassers (patooie!) that I had forgotten Dieseling run-on after ignition power is off - ping is where the flame actually touches the piston crown, resulting in localized sudden drastic increase in temperature.

 

The flame won't necessarily touch the piston crown when pinging, but it will get a lot closer and sometimes touch. The main thing is that the pressure spike increases the heat penetration to the piston crown, and can melt pistons and melt or crack ring lands.

 

Heavy acceleration when passing a Prius on the left, causing a black exhaust cloud to said Prius