Air Fuel Ratio vs. Power Output

For the average MPEFI car, what differences in mixture are implemented by the ECU when comparing part throttle light-load cruising, to added throttle and acceleration? To accelerate from one RPM to another, certainly you need more charge in the cylinders for each combustion event - so more fuel, more air. But also under load, the mixture is usually richened, is it not?

On another note, but along the same lines: would it be possible to run intentionally lean (slightly) during part throttle cruising for the sake of fuel conservation? Would power output fall as soon as the mixture leans out at all, or is there some measure of extra fuel in the OEM fuel map, which is essentially being wasted?

Jim

[xenon]

Good question.

I was wondering the same thing yesterday. I installed my EMS and the base map apparently does not need much tweaking. I get rock solid (10.5-11.5 AFR) at wide open throttle but I am getting very lean (16.5-18 AFR) while cruising around. :roll:

Does the car need to be tuned or is it safe to cruise at these lean AFR?

[whiplash]

Hey guys,

Xenon, in terms of your AFRs at different points, it is usually dependent on your car's open/closed loop operation. Generally, open and closed loop operation is dependent on throttle position where open loop is at "high throttle," and closed at "low throttle." During low throttle, the car runs stoichiometrically, generally trying to keep the same AFRs that you would see at idle, maybe 15-18. In open loop, the ecu realizes that you require acceleration and adds fuel to prevent engine damage from detonation, etc. as high AFRs bring higher exhaust temps, which could induce early combustion of the fuel in the cylinders when pistons are in a less-than-desirable position =)
In terms of my car being turbo from factory, Toyota decided to be extra safe and tuned AFRs of around 9.0-9.5 for open loop operation, while tuning for about 15.0-15.5 in closed loop. 9.0 may seem extremely low, but detonation is very much dependent on the octane of gas being used, with higher octane ensuring the gas does not prematurely combust. This is especially important in a turbocharged car as the air is inherently hotter already due to turbocharger operation. However, knowing that everyone does not use high octane gas, they tuned very low to ensure people using lower octane gas would not see detonation upon acceleration. Because I know a little about what I'm doing, ie. using proper octane and the like, I have now tuned for 11.0-11.5 on open loop, while you really can't change closed loop levels. Thereby you'll achieve slightly more power and save gas by raising AFRs levels at high throttle, but I wouldn't suggest over 12.0 on street gas as detonation becomes an issue with any octane at this level.

I was going to answer JR next, but it seems I think I covered most of it, it is relatively advantageous to run "leaner" at cruising speeds, which is why cruising throttle being very low, put you in closed-loop operation, thus running stoichiometrically. However, accelerating drops AFRs to prevent detonation as ecu moves into open-loop operation, but adjusting them slightly (as manufacturers set these very low on purpose due to people using cheap gas) can also be advantageous in terms of fuel consumption and power.

Hope you guys can decipher all this,

Allan.

I was going to answer JR next, but it seems I think I covered most of it, it is relatively advantageous to run "leaner" at cruising speeds, which is why cruising throttle being very low, put you in closed-loop operation, thus running stoichiometrically. However, accelerating drops AFRs to prevent detonation as ecu moves into open-loop operation, but adjusting them slightly (as manufacturers set these very low on purpose due to people using cheap gas) can also be advantageous in terms of fuel consumption and power.


Good writeup Allan, just a few minor corrections. Supras stock should not be running in the 9:0 AFR range. Most engines including their inline 6's have a lot of problems running this rich without misfiring. Certainly if they did fuel/oil contamination would be a big problem. I've tuned a number of supras over the years and have found them to run in the low 11:1 range dipping into the 10's at higher RPMs. If you're registerring richer than this then I would wonder if the fuel system or the WBO2 is working correctly. Indeed more WBO2's at least the bosch LSU units won't read richer than 10:1. My old horiba did and so did my techedge and they both used the NTK L1H1.

The main reason OEM's target stoich is for emissions. Running leaner than stoich causes an increase - off the top of my head I think it was NOX but I'd have to look it up. You can run most engines lean to the point of misfire during lower RPM cruise. At a point it will cause a "flat spot" in the feel of the engine but shouldn't be harmful. If you run them lean like this at high RPMs you're quite a bit more likely to suffer burned exhaust valves as it does run a lot hotter and extra heat plus residual oxygen are not things you want around your exhaust valves so do be careful.

As for how much fuel it saves, surprisingly not as much as some people think. On a 1500km trip this summer I brought my tuning stuff along and after I swapped engines on the way back had to re-tune the car. I took the first driving shift and an entire tank lasted about 425km. Most of the time the AFR was around 13.5:1. Em took the second shift and I leaned it out to about 15.5:1. End result for the rest of the trip, a consistant 460km/tank. The RPMs were fairly high as it had a B16 tranny so that explains the relatively poor mileage.

The 12:0 limit is a good one but I'd suggest to be more safe and target 11.5:1 for a turbo car. A local "tuner" routinely tunes his turbo cars well into the 12's and I've had a few of them in for head gaskets and pistons so there is a real life example of why you shouldn't run them too lean under boost. Even so, some of them survived. I've even seen some untuned turbo'd cars run for years well into the 15:1 range under 10+ PSI of boost so even poor tuning sometimes won't kill things. Was your engine assembled on a Monday or a Wednesday?

Finally, you do produce more power richer than stoich. Some factory ECU's maintain stoich at anything less than WOT and they use timing to avoid detonation. This results in slightly better fuel efficiency and I have a feeling they add a lot of EGR as well to fight detonation. One thing you will find if you tune your own setup is that there are a number of ways to arrive at the "right" answer. Sometimes there are better ways to accomplish the same thing but often it comes down to the preference of the tuner.

-Michael

[xenon]

I don’t have open and closed loop on my interceptor-x. There is only manifold pressure and injector staging involved.

I have lean condition at the switch point between primary and secondary injectors. It is VERY difficult to get even air fuel ratio at the point where the injectors are switching because of the Int-X's coarseness. Most people just richen up the transition spot, but it still does not fully eliminate the problem.

I was just curious if the lean condition while cruising was harmful to my engine.

[Jdc]

It could be bad for your specific engine. Older rotary engines did not respond well to lean A/F ratios. Do some research and get other people advice on this that have similar mods.

Can't tell you for sure if it would hurt because I'm no rotary specialist. As I understand rotaries are very sensitive to detonation and because of the large combustion chamber size it's easy to detonate. As long as the combusion chamber pressures are relatively low the chances of detonation in any engine are quite slim.

-Michael

[FC3S-USD]

For cruising, I use to have 15.5 afrs low rpms and 5th gear, moment that I would hit 0 psi I pulled timming and fuel would go straight to 12 afr at 0 psi then at 3 psi 11.5 and 6psi up I was at 11.

It shouldnt hurt your engine, but check the egt temps, dont want them to get tooo hot, I found cruising with 16.5 afr ( right before it would start to buck) egts would go pretty high up so i tried to bring it down some.

And depends on your manifold, IF its a shortie ( im guessing it is with the greddy kit ) then all that heat stays around the block.


NOTE: If anything happens Me not liable

[Stroked-Z]

I think most cars are setup to run lean in high vacuum (crusing) situations...and then richen up as load increases (vacuum decreases)

Xenon.....10.5 AFR really is too rich, but its safe. Keep an eye on your oil....it may thin out fast (fill with fuel)...keep it clean.

If all else fails, put a carburator on it :D

-Carm

Lots of OEMs run their cars that rich out of the factory. I'd be interested to know what those carb'd cars ran from the factory but that was probably back in the middle stone age before O2 sensors were invented.

-Michael

[Lancer]

Lots of OEMs run their cars that rich out of the factory. I'd be interested to know what those carb'd cars ran from the factory but that was probably back in the middle stone age before O2 sensors were invented.

-Michael

I was shocked to see I was pegging out the WB O2 on Talon with the factory tuning even at 17 psi (11 psi stock)
I was under 10.0:1. AFter extensive tuning, the best ratio for me at 31 psi, 17 deg total advance, pump fuel, is 11.5:1
That seems to be the sweet spot. Any richer and I get a little more detonation. 12.5 and up and I get detonation.
I keep it at 11.5 to keep the EGTs lower.

At cruise I'm setup to hit around 14.7 at all times just like factory. Only when boost over 5 psi comes on does it dip under 13:1. With boost under 5 psi and light load I'm at around 13.5-14.0:1
I get excellent fuel economy and the plugs stay really nice, no detonation. EGT around 1300-1400F at cruise.

Jim as for a clear answer to your question I would say the best way to get excellent fuel economy on most cars is too avoid using more throttle. Use the right gear, don't load the motor, part throttle only. at 1/3 throttle or more that's when the mixture is richened by the ECU and you waste fuel.