AFR / Lambda Question

[Autoholic]

Hello all,

I have a question for the tuners and engineers on here. It stems from a disagreement. Are there any conditions in which lambda should never reach 1 or go above 1? At least if you don't want the engine to experience some form of failure. My position is that there are conditions in which the mixture needs to be rich, specifically acceleration at least. The disagreement is that the other person thinks it's technically perfectly fine to run the engine at stoich under any condition / situation for any length of time. I don't mind if I'm wrong and he's right, but he likes to argue for the sake of arguing and I really can't believe his side. So, all the wise men on here, please give me your opinions and knowledge. I'm pretty sure that there are situations which stoich would end up in damaged engine parts, from too much heat.

[cammerfe]

The stoichiometric number is an ideal arrived-at by calculation based on a chemical equation. This is significantly different than the dynamics to be found in the infinitely changing situation in an engine under acceleration. And that's entirely different if one is looking for such things as ideal cruise conditions and concomitant fuel economy.

Stated simply, some 'rich' is good under acceleration and some 'lean' is good when cruising. Stoichiometric is 'just a number'.

KS

[WConley]

This may seem counter-intuitive, but most modern catalyst-equipped cars need to run rich at WOT.  This lowers exhaust gas temperatures and reduces the thermal load on the cats.  (Yeah I know, some of the unburned fuel just ends up getting burned in the cats, but it actually results in lower temperatures overall...)

Wide-open throttle typically lasts a short time, and there is no wide-open event in the EPA emissions drive cycle.  This lets the manufacturers get away with it, under the rule that you're allowed to exceed emissions in order to protect the powertrain from damage.

Next time you see an AMG Mercedes or 5.0 Mustang romp on it, notice the smoke!  That's the reason.  The general rule of thumb is that rich = cool and lean = hot.

[jayb]

Tuning an engine for perfect stoich at wide open throttle will usually cause a reduction in power, but whether or not it will damage the engine depends on the engine, I think.  Certainly in a supercharged or turbocharged application the heat could cause serious engine damage.  On the dyno we strive for a 12.5:1 to 13.0:1 A/F for best power, and lower in power adder applications.  Getting above 13.5:1 is considered pretty dangerous on a high performance engine.  All of these A/F numbers are well below stoich (14.7:1).  I would say, at least with respect to high performance engines, that your friend is misinformed - Jay

[cammerfe]

My WIFE drives a car with cats on it. I don't tend to think in those terms.

KS

[Autoholic]

Tuning an engine for perfect stoich at wide open throttle will usually cause a reduction in power, but whether or not it will damage the engine depends on the engine, I think.  Certainly in a supercharged or turbocharged application the heat could cause serious engine damage.  On the dyno we strive for a 12.5:1 to 13.0:1 A/F for best power, and lower in power adder applications.  Getting above 13.5:1 is considered pretty dangerous on a high performance engine.  All of these A/F numbers are well below stoich (14.7:1).  I would say, at least with respect to high performance engines, that your friend is misinformed - Jay

This is more or less my position. When you are running an engine near stoich (or even lean), you are potentially at the hottest point for combustion. Under WOT, the engine is going to be constantly getting hotter until you're at red line. The friction will be at it's highest amount, and a rich mixture will help cool down the cylinders and be lower in temp normally than say at 14.7. The friend also is named Jay LOL, he's a master's student in Washington working on a mechanical engineering post grad and works on the Formula SAE team, does a lot with engines. However, I think he has a dangerous viewpoint on AFR just being a number that doesn't have anything to do with engine durability. That you can run an engine however you want, under any conditions and you won't regret it later.

[WConley]

As Jay mentioned, different engines can take different heat loads.  I remember during my stint at the Ford Dyno Building that we ran some Lima 7.5 (460) truck engines through.  Due to the cylinder head design and the cam timing on those, the exhaust manifolds got HOT!!!  Ford had to cast stainless steel exhaust manifolds for the later truck 7.5's keep them from cracking.

Those things would glow yellow-orange on the dyno when pulling any sort of power.  You bet they had to fatten the mixture at WOT to keep them alive during the expected long trailer pulls up the Rockies...

[Autoholic]

True, but I could never see an engine surviving for long under WOT with the AFR at 14.7 (assuming pure gas), regardless of engine design or type of induction system. That makes for a pretty hot combustion chamber, under full load. I would expect overheating to happen rather quickly, and knocking to become easily noticed. The only way I could see stoich being useful for WOT combustion, is if the weather was like 40 below. Does that make sense to you guys? Intentionally running an engine hot in order to fight back deadly cold temps?

[WConley]

The exhaust glows the same color orange-yellow even if it's 20 below outside.  We had dyno tests at Ford called "Deep Thermal Shock" where you would maintain a tank of coolant at -20 degrees next to an engine running WOT at max torque rpm. 

Once the engine was stable at speed, a valve was opened and the -20 coolant rushed into the engine.  Poor thing!  Sure the hoses and the water pump would frost over, but the exhaust color didn't change.  BTW - this test is an extreme durability challenge for head gasket sealing.  It's brutal!  On an aluminum head / iron block engine the head gasket surface would be worn out from differential expansion - scrubbing.

Coolant flow/ heat transfer is how you manage extremely cold weather.  Putting a piece of cardboard in front of the radiator would have 10x the effect of tweaking the A/F ratio.  You just want to get all of the coolant to the right operating temperature...

[Falcon67]

Not an engineer and prefer Comfort Suites or La Quinta - And I usually tune for power and don't give a rats about cruise as long as WOT is covered.  What I have found over the years is that motors tend to be more consistent and happy when a little fat.  Technically, you should be able to run at 14.7.  Practically, no thanks.  A recent example is the tunnel ram/edelbrock 600s on the 302.  I used an Innovate sensor and their software on a laptop to see what was up.  Stock, it was in the 13s just running along maybe dip into the 12s at wider throttle openings, pretty safe. For best and most consistent MPH using a time slip and not using any software, I ended up with 4% more fuel tuned into both ends of both carbs.  I have no idea what the Lamda or A/F is, all I care about is that it runs 81.xx MPH every stinkin' pass.