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

Errr no. Excess fuel will still cool things down (preventing the occurance) and leaning out towards stoich will make it more likely. Same effect, just more dramatic.

And pre-ignition is NOT detonation. one can lead to the other but they are different.

Pre-ignition: A hot spot causes the mixture to ignite early

Detonation: the burn goes from controlled to uncontrolled (read explosion, rather than flame front)

Hmmm...I dunno what this was in reference to. :(


Yes, the definition of detonation is uncontrolled combustion, right?
Using the old square versus rectangle logic, pre-ignition is detonation but not vice versa?



-Ted

You can have pre-ignition without detonation. Its often referred to as pinking in the piston world. In extremis the pre-ignition can cause detonation, but you can also have detonation without pre-ignition.

So, for all rednecks out there:

Pre-ignition = pop

detonation = pop

normal = purrrrrrrrrr

:D

Bill, pinking is what we say instead of pinging in the UK. And as you said, it means the same as pre-ignition, it's just shorter to write!

When I hear the word "pinking" I think of pinking scissors.
How many of you guys know what that is??? :)
Yes, I'm secure with my feminine side. :D


-Ted

That's probably more information than ANY of us wanted...

:D

i just bought a 90 rx7 GTU and i am running lean how can i fix that?


-=scad=-

^^ What he asked.

no, all you guys are wrong. when the motor starts running lean, the hamster gets too thin to keep traction on the shaft. at this point power output starts going down and the friction from his feet slipping helps the egt's rise. if this continues, you'll blow the hamster. of course the easy fix to this is a concoction of atf, mmo, and whiskey. also run one tank of E85 with a 3point octane booster.

Can I get that in English? :D

sorry, i'm american. only thing i can tell you is not to put it in the boot.

I'm American also... Hence the Knoxville, TN (YES, tennesseeans ARE Americans ;) )

However, I am basically a newb. So the whole hamster thing lost me...

Anyone with a better explanation?

first, how do you know you are running lean. s5's are known for running rich. i know mine does.

i have a 90 tII and mine i sware runs toorich,cuz when shes cold until warm up it runs perfect.but when it gets hot it starts to bog just a tad bit if im just sitting reving it. but then i look at the end of my turbo at night its a light red like it is running a little hot in the exhaust... new down pipe so thats not it , after the down pipe no cats. so theres nothing holding the heat in??????? i am going crazy trying to get to the bottom of the problem for weeks still drive it too. just cant figure it out , dont want it to hurt my low mile engine eather... i only have origianal 73,000...on it im wondering maybe tuneup or is it my oxygen sensor took a shit from to much heat??? or will to hot melt my turbine finns on the inside of my turbo ?? hope not please someone help soon dont want to hurt anything .....................

i have a 90 tII and mine i sware runs toorich,cuz when shes cold until warm up it runs perfect.but when it gets hot it starts to bog just a tad bit if im just sitting reving it. but then i look at the end of my turbo at night its a light red like it is running a little hot in the exhaust... new down pipe so thats not it , after the down pipe no cats. so ther

you ever installed any gauges? a/f gauge, boost gauge, etc, so you know whats going on? i don't have a turbo so i'm not dead sure whats normal and whats not. thats the best i have for you.

Re: How does lean kill the engine?
 

ok
#1 LEAN in conbustion terms more air than fuel..kinda like ur engine took a diet eating more air than fuel.when the fuel mixture is lean it burns faster,before the exhaust cycle is reached..Usually we would want the exhaust to fully combust right after the port opens up..this happens when the fuel mixture is perfect..not rich or not lean..when lean it will burn before the exhaust ports open up and causes excess heat to stay inside the motor..and putting more HEAT and Pressure on the apex seals and other engine components..
Usually it will warp ur apex seals or even crack the housings..depending on how much you lean out and how long you run it that way..usually the results of a lean mixture are deadly to the motor..