How does lean kill the engine?
 

I know when the car runs lean, the egt rise and the engine dies. What i'm curious is about the process. What exactly happens that kills the engine? if someone knows, thanks :)

-Carlos

As the engine compresses the air/fuel mixture heat is created. If too much heat is created the fuel is ignited prematurely which is VERY bad for a rotary. This is what detonation or preignition is.

Another way of saying the same thing would be that when the air/fuel mixture is ignited, it's ignited before the rotor comes to TDC.

When you have more oxygen than fuel, the fuel burns extremely rappidly, so much in fact, that it reaches it's peak expansion rate/pressure before the rotor even passes TDC. In other words, it tries to force the enigne to run backwards.

Conversely, the goal of tuning an aftermarket EMS is to get the fuel mixtue just lean enough so that it doesn't detonate, but at the same time doesn't burn too slowly.

Running too lean is a good way to blast an apex seal out your tailpipe.

and shred your turbine

that makes sense. Thanks for the explenation :)

An engine is dependent on CONTROLLED combustion.
If the combustion is not controlled, you get engine damage IF the load is great.

Lean conditions elevate EGT's AND combustion pressures.  This is the definition of uncontrolled combustion.

Detonation leads to elevated combustion pressures.  These pressures are typically 10x greater (normal cumbustion pressure are about 1000psi), and this causes internal engine damage.


-Ted

NB peak EGT occurs at a stoichometric AFR. If you are running NA and go leaner than that, then EGTs will drop. Emmissions will go way out, but that's another story. You should be able to cruise at 16:1 or lower with no adverse effects. I have seen results that would indicate the lean limit with certain aviation apps of the 13B is nearly 20:1, but no corroboration.

Yeh, something about the centerfugal force throwing the richer part of the mixture twards the spark plugs :p

Uh, that's counter to what I've seen.
Leaning on a rotary engine tends to skyrocket EGT's.
Leaning on a piston engine kills power, but there's no way the EGT will come down.
Of course, we're talking under heavy load - i.e. WOT.


-Ted

interesting

and now for the redneck explanation:

Lean = too much air, the fuel EXPLODES inside the engine

Controlled = fuel burns steadily, and expands steadily

Are you sure on that? I swear that everything I've read (and tried) on a piston engine, you will get more power with a slight lean condition, but risk detonation.

When we were trying to tune a Haltech on a Honda H22A motor, leaning the fuel caused power to noticable drop on a DynoJet.


-Ted

How far did you lean it out though? A slight lean condition will provide more power than a rich condition (for reasons stated above, more explosive fuel).

It's really a balancing act. Knowing what's lean enought to create more power but yet not so lean that it results in detonation. Go too lean and you go KA-POW. Just a little lean and you're making more par :)

And you just contradicted yourself there. If you lean it out you get more explosive fuel? Leaner makes it explode easier, but teh more fuel you can ram in, teh more power you can make. But if you put in to much fuel, there isn't enough o2 to burn all the fuel anymore.

The generally worldwide accepted "optimum" mixture is 14.7:1 So you can go leaner for greater fuel-efficiency, or richer for more power

Not as in you get more fuel in, but the fuel you do have in there is more explosive, you read what I wrote wrong I think.

If you put in more fuel without more air, you're killing power. If you put in slightly less fuel than you need, the fuel becomes more explosive and grants you a slight power boost. It's usually not worth the risk though.

We were doing course fuel adjustment, which at those RPM's would loosely translate into 0.5% decrease.


-Ted

AS you go from a best power mixture (say 13:1) towards stoich, then EGTs will go up. All the text books show that. You peak at stoich, then drop. If you read all the aviation tuning guides they all talk about tuning for '50 degrees lean of peak' for cruise.

This goes counter to the way most of us think. However it only works at cruise, as you loose a lot of power running lean, and eventually start to misfire. However, on a non boosted engine, then you won't harm things to lean way out on cruise. Its also easier to do on an aircraft as you lean off, then open the throttle a bit and lean off some more. This allows you to reduce pumping losses as well. You can do it in cars, as mitsubishi do in the GDI engine, but it is ermmm, non trivial

At WOT you need to add excess fuel to cool things down. Water injection will do the same job.

That explains everything - the conditions I was talking about is for WOT and heavy load conditions, which is basically opposite of "cruise" conditions. :)


-Ted

Umm, sort of. Aviation engines at "cruise" are at a major percentage of their potential power output. It's very difficult to come up with blanket rules concerning the AF/R-EGT relationship. The dynamics Bill was referring to above, as in the EGT dropping when lean of stoich is well documented by every investigator from SAE on down. I thinbk when people say "leaner = higher EGT" they're really meaning "I started quite rich and leaner than this = higher EGT" Run a boosted rotary at 14/1 and watch it detonate and spit apex seals. It's still rich of stoich but clearly too lean for proper best power operation not to mention reliability. It's all relative, as Einstein was fond of saying...

Understood.
I think the loads on the powerplant is different than an automotive engine running at WOT.  Unless I'm totally off-base here?


Hmmm...even this isn't right.  Once the AFR's get close to ideal power ratios, the EGT should come down.


I don't think we can ultimately say 14.7:1 is the idea stoic for all load conditions?



-Ted

Think of it this way. When you have exactly the right amount of fuel for the air, you will get the maximum heat. More fuel and you will not burn all the fuel, so some of it just absorbs heat. Less fuel and there is incomplete combustion, so oxygen is left begind all lonely and unreacted, so the exhaust gas must be cooler, as less energy has been released.

14.7:1 is only good for 1 thing. Catalytic converters. It neither gives best power nor best economy.

With one exception...

With the elevated heat in the combustion chamber, that triggers pre-ignition which is essentially detonation.  Detonation is uncontrolled combustion, which elevates combustion temps and pressures.  This is how skyrocketing EGT's are triggered in a (boosted) engine under heavy load (i.e. WOT).



-Ted