I have read so much about it being a terrible thing, but what exactly is happening when ours, or any motor, runs lean. I understand that there is a lack of fuel (or too much air for the amount of fuel, ie: high air / fuel ratio) but what exactly is the danger of it.

I have heard of a story where a piston engine generated a lot of heat because it was running lean and burned up some valves and warped the head. Please be as exact / technically descriptive as possible.

Thank you

 

A lean engine runs with less fuel, fuel acts as a cooling agent. Less fuel = more heat. This is what you heard.

The second part of this is that less fuel = higher chance of igniting before TDC. This is called Detonation. This is known to take out apex seals in our motors right quickly. Especially if you have a turbo motor.

 

Detonation, and then the consequence of popped apex seals.

In NA engines it's hard to pop apex seals running lean, but you get other bizarre damage like flattened apex/corner seal springs and eroded seals.

 

You can also have some bizarre behavior in a normally-aspirated engine if it is run really hot, due to a loss of sealing in the engine overall. When one of these engines is overheated, if it is severe enough, all of the components in it will expand while it is hot, then contract when the engine cools off. Remember, the engine is made up of two aluminum housings (rotor housings) sandwiched between three cast iron housings. Aluminum expands more than cast iron at the same temperature, which could cause a "crushing" effect on the aluminum housings. When the whole engine cools down (especially if this were to occur in winter), the contraction of the engine housings can lead to leaks between the cast iron and aluminum housings, especially if the tension bolts holding the engine together are not torqued properly or evenly, or if the engine is just old.

 

pretty much why i busted every engine i busted in the mid-90s.

 

As stated running lean leads to detonation. Detonation is when you have two flame fronts that collide. This results in a massive spike in pressure. Because the burn is no longer controlled you lose power as well as cause damage to important parts and combustion temps increase. The increase in combustion temp can lead to preignition which will kill a motor fast.

 

Maybe I am going on a tangent here, but is this possibly the reason why those with high power applications utilize water injection systems - to keep the temps down? Just a thought...do you happen to know?

 

Ok, I appreciate this - because this helps me with my understanding of detonation. But can you expand on this...

My understanding of detonation is this: Being that our engines are rotary, they must spin in a particular direction. Thus, spark/ignition must occur beyond TDC or beyond0° or else it would send the rotor in the wrong direction (ie: detonation) because you basically have two ignition sources (on separate rotor faces?) contradicting each other, instead of complimenting each other. Do you follow my thinking - and is this correct?

Sorry about the fonts......

 

not a tangent. it's related, and yes, it's to keep temps down in order to deter spontaneous mixture ignition.

 

yes, then engine only spins clockwise. and yes, ignition occurs well beyond TDC.

this is the way i picture it. spontaneous ignition (non-spark-plug-initiated) will, in a sense, try to oppose rotation as the mixture ignites before it's fully compressed - it's trying to expand while the rotor is trying to compress. unbelievable pressures! ... all of this plays hell on the seals and can even lead to cracked housings. however, it only happens in the chamber that's into the compression/ignition stroke.

 

The reason running lean causes detonation is that the act of compressing air causes it to heat up. The fuel that is mixed with the intake charge will absorb some of this heat energy. The less fuel you have as well as the amount of surface area that the fuel has will dictate how much heat it will absorb. If the mixture absorbs too much heat it will reach it's autoignition point and ignite. Detonation typically occurs right before tdc when the mixture is compressed the most.

Under proper operation the goal is to ignite the mixture at the ideal time in order to transfer the released energy into the forward motion of the rotor. When detonation occurs you now have two or more ignition sources, the spark plug and the spontaneous ignition. These flame fronts collide and cause the metalic pinging sound you hear.

Let me see if I can explain it in a real world type of example.... plus I just like a good story

Lets say you and Fred are in a field with a can of gas and a kite. You draw a long line of gas with you standing at one end, Fred standing in the middle and the kite at the other end which will represent our rotor . For our story you are the spark plug and Fred is detonation and the kite magically floats on it's own.

If you light your end of the gas trail, the flame will travel towards the kite (rotor) in a controlled manner. As the heat from the flame nears the kite, the kite will be lifted off the ground.

Now if you and fred both light the gas trail at the same time the flame will follow the gas trail it three directions. From you to Fred, from Fred to you and from Fred to the kite. The kite will still be exposed to some of the heat energy, just not as much. The flamefronts between you and Fred will collide somewhere in the middle of you. This is what happens inside your engine, except the mixture is much more fine tuned and explosive.

Now lets say as you and Fred were pouring the gas Tom (preignition) came along with a lighter and ignited the gas. There would only be one flame front and it would kill both you, Fred and the kite. Pre ignition can happen at any time and is a result of running too hot, which can be a result of prolonged detonation. One of the reasons it can be so destructive is that if it happens at BDC and the momentum of the engine will try to compress the explosion.

 

This is exactly it. Water has a much better heat absorption rate then gasoline. Water doesn't burn so it can't actually make any power directly, it simply allows you to remove heat from the combustion chamber .

 

same Idea with meth injection

 

understood - because pre-ignition would cause a spike in pressure before the rotor has completed its compression turn. so instead of driving the rotor, we're blowing out the seals.

 

My understanding is that, although it may not be nearly as high of a value as a gasoline ignition, the change from water to steam / vapor will directly drive the the piston / rotor as well.

I have read of a 6-stroke engine (versus a 4-stroke engine) which would eliminate the cooling system, and add a 2-stroke cycle which would add the injection of only water during compression, pull heat from the system (thus cooling the engine) to vaporize the water & drive the piston, be exhausted out, then return to the "normal" 4-strokes. An engine of this cycle would not be as powerful per cylinder versus a 4-stroke engine, however it would be much more efficient since you are using the wasted heat energy from the gasoline to expand the water and drive the pistons.

 

Wow - this topic brings up a lot of great reading which I believe is going to prove to be extremely helpful. Thank you so much for the input.

I am planning to experiment with some alternative fuels (beyond those typically considered) in a rotary and know that nothing will prove to be more successful than that with the right information and proper planning. I knew that detonation and pre-ignition are of particular concern and that running lean would cause this and result in a blown engine.