Quotethe engine will go lean and then fail making expensive noises.
Hmm there it is again... please is there
any evidence that these engines go bang when they run lean on fuel? Or is this simply an urban legend (oft-mentioned on here ;) ).
If they do go nuclear due to
lack of fuel then can someone please explain to me exactly what happens?
I will try to explain the reasons why from what I know.
Essentially in an engine there is excess fuel injected into the engine, this is done to help with controlling and cooling the temperatures inside the engine, if you don\'t have enough fuel goin into the engine for both combustion and cooling then temperatures inside the engine will increase, increased temperature inside an engine can cause the fuel and air mixture to burn before the spark plug has ignited the mixture.
As rotaries have higher EGTs than 4stroke piston engines due to the lack of an exhaust stroke in the 4stroke engine. A 4stroke has a power stroke every other stroke whereas a rotary has 3 power strokes per full revolution of the rotor.
Our rotors are also made of cast iron which retain heat, whereas pistons are made of alloys which cool a lot better, so if you have a really hot rotor from lack of fuel the the mixture will be ignited by the rotor.
Pre ignition or detonation in a rotary will take out one of the seals inside the engine, if it doesn\'t your lucky.
Post detonation can be just as bad in an engine as too much fuel can ignite again after the spark has ignited it. This is what happened to my engine, the engine kept on flooding and throwing too much fuel in to my engine. I was only going at 2,000rpm over a speed bump and it went bang.
You get different types of issues.
The main problem with a lean condition is really only when on boost.
If you just cut fuel at idle, the engine will just miss a beat and not \'fire the charge\'. But if you have boost and \'some\' fuel - this can cause a variety of issues:
- pre ignition - can be caused by carbon build up (usually from running too rich normally), or a dieseling affect caused by high boost.
- High combustion temps - running lean or high boost can give too fast a flame front in the chamber - causing pressure spikes - hence killing seals
- Lubrication ? Since a lot of rotary guys go premix - the lubrication is coming from the fuel - less fuel - less lubrication -but I suspect this is a gradual killer.
Nice to read this. didn\'t thought that way yet. Knew about the fact, leaner is hotter but never thought about the fact that that will be a cause of pre ignition.
When using water injection, AFRs can be taken slightly leaner but still not too low as the water is replacing the excess fuel with is put into the engine normally.
Thanks guys. Ok the issue seems to be due to (i) the higher normal operating temperature of the Wankel engine, and (ii) the presence of 3 power strokes per revolution. Running lean increases the chamber temperature (as one would expect) but whereas in an Otto engine the chamber has time to cool between power phases, the 3 power strokes per rev means another (and another) follows on in quick succession, meaning the rotary chamber doesn\'t get a chance to cool. This results in pre-ignition and a rapid runaway situation develops resulting in blown apex seals.
So whereas an Otto engine can survive running lean for quite some time (it is less susceptible to pre-ignition - although it may still get detonation but that\'s less of a problem), a Wankel engine can go nuclear in a matter of seconds.
A piston engine can tolerate detonation more but it can still cause damage. Heats spots on a piston engine get get so hot that they melt a hole in them. My dad runs an air cooled 2stroke engine and a little bit of grit was stuck in the main jet when he changed to a larger main jet, he was on his way back from Scarborough and the engine just stopped making power. He stripped the engine and there was a hole the size of a twenty pence in the top. And it was a high quality piston.
Admittedly these engines run hotter than a 4stroke or a water cooled engine but it shows you what damage a lean condition can do to a piston engine.
A non turbod rotary won\'t see the excessive heat that a turbo motor can produce, but a lean mix can still cause damage.
I think you\'re confusing detonation with pre-ignition ;) I didn\'t say piston engines are immune to detonation; obviously they are not, it\'s just that it is rarely the instantaneous meltdown which can happen with pre-ignition. When an engine starts knocking most people will instinctively lift off the throttle which can help you save it from harm.
Pre-ignition can of course cause detonation (resulting in holed pistons) but it\'s not the only cause.
My hypothesis is that it is pre-ignition rather than detonation which is the cause of the blown apex seals/springs in rotary engines.
Or it could be pre-ignition induced detonation :Giggle
QuoteA non turbod rotary won\'t see the excessive heat that a turbo motor can produce, but a lean mix can still cause damage.
True.
They are one in the same.
What is Detonation?
Definition: Detonation (generally caused by fuel with a low octane rating) is the tendency for the fuel to pre-ignite or auto-ignite in an engine\'s combustion chamber. This early (before the spark plug fires) ignition of fuel creates a shock wave throughout the cylinder as the burning and expanding fuel air mixture collides with the piston that is still traveling towards top-dead-center. The resulting knock/ping is the sound of the pistons slamming against the cylinder walls. Severe detonation can break pistons and destroy engines.
Pronunciation: det-n-ey-shuh
Also Known As: pre-ignition, auto-ignition, pinging, or knocking
Sorry no. Don\'t know where you got that definition from (since you don\'t quote a source) but it has confused pre-ignition (igntion before the spark plug fires) with detonation (explosive ignition of gas pockets - which may be before or after the spark plug has fired).
You can have pre-ignition without detonation. You can have detonation without pre-ignition.
Although pre-ignition often results in detonation hence people often think they are one and the same.
They are different and have different causes.
p.s. the clue as to the veracity of that definition lies in the statement "The resulting knock/ping is the sound of the pistons slamming against the cylinder walls" which is of course utter nonsense!
We\'re both right. You would understand me better if we were talking, I find it hard putting info in my head down into a key board.
On high boosted piston engines the bottom of the piston, or skirt, can slap against the cylinder walls, this can be caused by many things, over revving, bad bearings and detonation. It is the piston rings that create the seal on the bore not the piston itself. They are smaller than the bore.
I\'m sure you will know this anyway so I won\'t go on further as I will sound patronising.
The piston slapping/pinging on the bore is what I would consider on a rotary to be similar as an apex seal experiencing chatter. This can open the seal created by the Apex seal and cause fresh air and fuel mixture to go into the exhaust or into the next power stroke.
I do think though that on a turbod rotary the likelihood of chatter is reduced more because of the increased cylinder pressures that will actually push the seal out onto the rotor housing more.
Agreed ;)
The pinging/knocking noise is caused by the pressure wave from the detonation explosion - i.e. you are *hearing* the explosion. Consider it this way, diesel engines suffer pre-ignition most of the time (especially at idle) - this is why they clatter. If the clattering was the piston hitting the wall then the engines would nuke themselves inside a few yards. The clattering is the sound made by the detonations.
Yep you are right - skirt slap can happen, but that isn\'t what you are usually hearing when you hear detonation. So yes detonation can cause piston slap but it\'s not compulsory and the sound you hear isn\'t the sound of the slap.
I think I\'d better stop now before I bore (oops no pun intended! :D ) peeps any further :) :)
If people are bored then they shouldn\'t go on a website that is for a specific engine. People need to learn somehow and will only do so by listening reading and experiencing these things.