lefuton

would someone be so kind as to explain how/why detonation occurs in a rotary when you run too lean? or too hot for that matter. i just spent the last 2 hours scouring the rx7 forum and all i got was don't do this and don't do that or you'll detonate, but no explanations as to the physics behind it.

im guessing early detonation is bad because...it pushes against the e-shaft in the wrong direction?

tia

DTECH-RX

I'll give it a shot....

By running too lean, there is less fuel in the combustion chamber than needed to control the proper timing of the air/fuel charge being ignited by the spark of the ignition system, as opposed to just the compression of the rotor (the more you compress something the higher the temperature) or a "hot spot " (high temp area of the combustion chamber or anything in it). So basically the charge is being ignited before its suppose to which would put stress on all the components in the engine including the e-shaft and the apex seals which maintain the integrity of the combustion chamber compression.

End result....BOOM!....engine damage in a number of different possible ways.

lefuton

hrm, so if you don't have enough fuel in the chamber it burns easier?

wakeech

hmm... it was my understanding that one cannot run an engine at stoich ("lean", but air is just = with fuel by the req'd ratios) as the combustion happens far too quickly: it explodes rather than burns because of some kind of something... can't remember.
when running at a healthy ratio (which means more fuel than stoich, but not "rich") the fuel and air combine slowly in a controlled reaction, with a distinct flame front, forcing the rotor around as the combustion both heats the gas and increases the number of molecules (...right? it's been a LONG time since i worked that one out), increasing the pressure in the combustion chamber driving the output shaft around, rather than a sudden BLAST jarring the whole motor.

...also, from what i can remember, if the fuel ratio is far below stoich, not too much happens: like when you're not going to seize the engine when pull starting a lawnmower, or when you're starting your car.

DTECH-RX

Actually in a way, yes. With an improper amount of fuel in the mix, the air molecules can be compressed to a point where they can ignite just from the heat of compression causing a premature ignition or "detonation". This can also happen from "overheating" or a "hot spot" as I mentioned above.

It's this violent, untimely explosion when the rotor is not in an optimum position to properly take advantage of the combustion vs. a relatively "controlled" burn of a proper air/fuel mixture at the right time, that causes stress and in severe cases, serious engine damage.

MWG

detonation is caused because part of the fuel mixture fires before the spark plug fires and cause damage to the parts because of the conficting flame front. the reason for more fuel is because the fuel and air need to be correct so that the fuel burns in the right ratio to oxygen. when you dont have enough fuel that extra oxygen burns and cause damage to the engine by creating too much heat. The fuel helps to keep the combustion chamber at the correct temp, that is best for the engine combustion. You can make more power with less fuel by slowing down the flame front. which is one of the reasons we are able to make more power using less gas in newer cars.

MWG

Alot of times people call it detonation when the timing is off. but detonation has to do with where the flame front starts at. In a perfect world we would have is start a one end of the combustion chamber in a rotary and go to the other side with out any trouble. because there is a thing such as pre ingition that is not detonation. pre ingition start from having a hot spot ingite the fuel/air mixture. There is then the detonation which is caused buy not a correct air to fuel mixture. I hope this helps.

DTECH-RX

Pre-ignition, in one way or the other is what causes detonation so they are basically directly related. You can't have detonation without some kind of pre-ignition causing an uncontrolled burn condition in the combustion chamber. But if you want to get technical, yes they are separate components of the same end result. That's why I put detonation in quotes in my previous post.



BTW...this is also the reason for using higher octane fuel. The higher the octane, the less likely it is to pre-ignite making it less likely to cause detonation.

Sputnik

It works just like with a piston engine. If the air/fuel ratio is too lean, and/or the compression is too high (too high compression alone for the grade of fuel used could produce pre-detonation), then the mixture will be too unstable, and can tend to ignite on it's own (either partially or completely).

It can simply kill power with partial ignition if it's not too extreme. But if it is too early, then it will put stresses on the rotor and e-shaft, just like it would on the pistons, rods, and crankshaft of a piston engine. In that matter, blowing apex seals on a rotary is very similar to blowing piston rings on a piston engine.

---jps