A question about detonation

[donkeyking]

Regarding detonation, why are there more chances to occur detonation in lean mixture than rich mixture when the engine uses the same fuel?
For example Avgas 100/130, it is obvious this kind of fuel has more detonation chances in lean mixture than rich mixture.

Many thanks



Donkeyking

[Colin1]

A rich mixture is 'wetter' (ie a greater fuel bias in the fuel:air mixture) than a lean mixture and will cool the cylinders by evaporation.

[tail end charlie]

Regarding detonation, why are there more chances to occur detonation in lean mixture than rich mixture when the engine uses the same fule?
For example Avgas 100/130, it is obvious this kind of fuel has more detonation chance in lean mixture than rich mixture.

Many thanks

Donkeyking

There are many causes of detonation apart from the fuel, ignition timing, compression ratio shape of combustion chamber and amount of carbon build up to name a few. The change of a fuel from liquid to gas has a cooling effect so a rich mixture runs cooler than a lean mixture (as colin said). An engine uses heat to create motion so the bigger the explosion the more power it produces however with detonation it is getting out of control which can result in the engine completely "running away" and burning the pistons as well as the fuel...like it used to do on my crappy suzuki GT185.

I used avgas 100 in my racing motorcycle not because it had more or less chance of detonation but because it had better fuel quality control, it was easier to set the jetting and in 2 yrs racing I never burned a piston.

[Aaron Brooks Wolters]

A lean fuel mixture causes causes cylinder temperature to go up. Fuel helps keep the cylinder cool. If you have ever notice a lean engine when it is running the header tubes will glow. I'm not sure about aircraft engines but most automotive engines like about a 14 to one air to fuel ratio.

[mikewint]

let's try this. liquid gasoline will not burn. if you are really brave a lit match can be plunged QUICKLY into a pool of gasoline and the match will extinguish. pure vapor will also not burn the same for hydrogen gas. combustion requires fuel PLUS oxygen. rich fuel mistures burn poorly, no oxygen. lean have more oxygen and burn quickly

[tail end charlie]

let's try this. liquid gasoline will not burn. if you are really brave a lit match can be plunged QUICKLY into a pool of gasoline and the match will extinguish. pure vapor will also not burn the same for hydrogen gas. combustion requires fuel PLUS oxygen. rich fuel mistures burn poorly, no oxygen. lean have more oxygen and burn quickly

Mike that sounds like great science, can I watch you do a demo lol


seriously what you say is theoretically correct only you put the match in the gasoline eh?

[Aaron Brooks Wolters]

It is true. As long as the flame does not ignite the fumes the match will go out in the liquid. I have seen this done. I'm still not sure that I will ever try it because Murphy and I are on first name basis.

[mikewint]

i have actually done this, the container is capped and cooled to keep vapors condensed. remove the cap quickly and plunge the match quickly. it is only the vapors which are explosive
flooded engine will not start and in the cold it's much worse

[Aaron Brooks Wolters]

When you flood an engine the same thing happens. The spark will not light off the dense fuel mixture. And I believe you Mike. I wasn't joking when I said that I had seen it done. I'm just a little gun shy when it comes to open flames and gasoline. I've been caught on fire with it so I tend to keep my distance.

[tail end charlie]

i have actually done this, the container is capped and cooled to keep vapors condensed. remove the cap quickly and plunge the match quickly. it is only the vapors which are explosive

The opposite in a way is flour and wood dust/shavings they are notoriously explosive in a warehouse wheras few people would consider them dangerous.

an empty petrol tanker in a harbour is much more dangerous than a full one, full it burns empty it explodes.

[mikewint]

all are just ways of saying that combustion requires both fuel and oxygen. any combustable substance surrounded by oxygen will burn. a steel bar will not burn but fine grade 0000 steel wool burns quite nicely

[donkeyking]

I have another question.

Does turbofan or turbojet engine has detonation problem?

thanks

[FLYBOYJ]

I have another question.

Does turbofan or turbojet engine has detonation problem?

thanks

No - the combustion cycle of a turbine engine is entirely different from a reciprocating engine.

[mikewint]

FB, i may be wrong but i would say that there is no detontion at all just very rapid continous combustion. would that be correct?

[engguy]

Super lean ratios do not have the tendancy to detonate because of the cooling effect of the excess air.
Aircraft engines tend to have ineffcient combustion chambers so that doesn't help either.