Lean burn-thoughts, theorys, and discussion

[frank316]

Originally Posted by amcpacer

I tried running lean with my 85 Chrysler Laser turbo by programming the computer to ignore O2 feedback and run lean under light cruising conditions. I have a LC-1 wideband readout. It would only function as low as 17:1 and anything leaner would cause misfire. I did however get best fuel economy running 16:1 at idle at hot idle and light cruise until +2 psi manifold pressure where it descends to 11.5:1 all the way to 19psi of boost.

I always wonder why this engine won't run lean like the rest of your guys engines.

A cylinder head thats designed for power(being the car is boosted) guessing it has excessively large ports with poor low rpm flow and therfore low rpm fuel atomization, and also being boosted its probably low compression, which is another factor which makes firing off a lean mixture difficult..

My other car is a high 11 sec turbo civic, its low compression with a large aftermarket cam, and it won't fire a mixture any leaner than about 16-1.

Ben

[Meph]

I just pulled my 1jz (turbo) back to 15.87:1 from 15.3:1, it seems to show positive results, ice yet to try my new ignition map with it (couple degrees more) and hopefully see more benefit. Hopefully ive yet to reach the cross over point where im no longer gaining ground, still wanna see 40 mpg one day

[mwebb]

in your car in your system
lean condition DID NOT CAUSE high combustion temperature .

your tampering with MAP voltage DID

by reducing the LOAD signal, the ECM provided ignition timing based on a lesser than REAL / ACTUAL LOAD so ignition timing was ADVANCED , TOO ADVANCED for actual conditions

if the ECM had been provided with a feedback signal from the AFR or front 02 sensor , the ECM would have attempted to compensate for the lean condition caused by reduced load signal .
BUT
the ECM would NOT have attempted correction to IGNITION TIMING advance .
peak combustion pressure was happening TOO soon , the engine was "pinging" and you either did not hear it or chose to ignore it .

...."I removed the factory o2 sensor to put the car into open loop and to use my wideband, but I was tuning beyond it's capabilities. Timing was stock.

>>>> Map voltage was reduced by 30% <<<<

last wideband reading was 19:1 before reducing a bit more fuel and maxing out the o2 sensor. Average RPMs were 2200-2600. Average MAP pressure was -19.7inHG and maximum was -11inHG.

Took it for a test run and car started smoking, and stumbling. Wouldn't hold idle. I got it home, checked the plugs and found my problem. This being my daily, I decided to forgo any extreme tuning as such until I can find a wideband that is capable of accurate readings at > 18:1"....

[Tygen1]

My experience with lean burn is not exactly the same as you fellas, but I feel compelled to chime in.
I just put a simple voltage divider on my MAF, and used a LM-1 wideband to monitor AFR. When I do this the motor believes it is at a lower than actual load and the ignition timing is advanced to it's maximum available, 40 degrees. I typically run around 17:1 AFR, but because it is a voltage divider, the AFR varies by load...ie. High Load=Leaner, Low Load=Richer. It works pretty well, but the PCM grows wise to my tom-foolery and kicks into limp/open loop after just a few miles. However it is easy to reset it back by using DFCO. This compromise method returns a benefit of approximatly 2-3mpg. I only engage this lean burn under light load cruising, but have found it is still able to pull up hills just fine. I have been considering using the LM-1 to set an offset, so I can keep the motor at 17:1 and switch between the standard O2 sensor and the LM-1 that is offset for cruising, however I would loose my ignition advance because the PCM would not know it was lean, the normal ignition advance is 32-34 degrees while curising.
Just throwing some thoughts out there...

[cmj]

Originally Posted by mwebb

in your car in your system
lean condition DID NOT CAUSE high combustion temperature .

your tampering with MAP voltage DID

by reducing the LOAD signal, the ECM provided ignition timing based on a lesser than REAL / ACTUAL LOAD so ignition timing was ADVANCED , TOO ADVANCED for actual conditions

if the ECM had been provided with a feedback signal from the AFR or front 02 sensor , the ECM would have attempted to compensate for the lean condition caused by reduced load signal .
BUT
the ECM would NOT have attempted correction to IGNITION TIMING advance .
peak combustion pressure was happening TOO soon , the engine was "pinging" and you either did not hear it or chose to ignore it .

...."I removed the factory o2 sensor to put the car into open loop and to use my wideband, but I was tuning beyond it's capabilities. Timing was stock.

>>>> Map voltage was reduced by 30% <<<<

last wideband reading was 19:1 before reducing a bit more fuel and maxing out the o2 sensor. Average RPMs were 2200-2600. Average MAP pressure was -19.7inHG and maximum was -11inHG.

Took it for a test run and car started smoking, and stumbling. Wouldn't hold idle. I got it home, checked the plugs and found my problem. This being my daily, I decided to forgo any extreme tuning as such until I can find a wideband that is capable of accurate readings at > 18:1"....

I'm completely dumbfounded.. I can't believe I didn't think of ignition timing and ecu control. Thank you mrwebb!

I'm honestly embarrassed.

[frank316]

Ok I have been busy this weekend, went and got me an EGT gauge and got the bung tig welded in the header and have did *some* testing. Haven't had time to do any changes. Just been driving on current tune. for reference the sensor is in the #4 cylinder about 1.5 inches from the head.
So far what i've noticed-
Idle's around 18-1 afr egt-750
55mph cruise 19-20 afr egt 1150
80mph cruise 19-20 afr egt 1250
flat out wot 14.7 afr egt 1450 (cold weather has caused the wot portion of the map to lean out slightly, intake temp sensor is keeping the low speed lean burn sections of my map at about the same afrs.)

I have also included the current timing map that i'm running.

Keep up the thoughts and ideas guys!

Ben

[saand]

Frank316 looks like some great data there.
Seems like your car is already running a bit lean since it is up at 20:1 afr (air fuel ratio). I am interested to see what results you get when you start doing some changes.

For other peoples reference i assume the acronims mean the following (since i had to look some of them up)
AFR: air fuel ratio
WOT: wide open throttle
EGT: exhaust gas temperature

[Nate R]

You can't create REAL lean burn in a combustion chamber/piston crown design that wasn't intended for it. If you read the simple Wikipedia entry on it, you'll see the mention of stratified charges. At the really lean lean A/F ratios used in lean burn engines, you can't support a flame, if those mixtures were homogeneous through the cylinder. Lean burn engines, through various methods, concentrate fuel around the spark plug, generally. You end up with an area that is say 15:1, w/ the remainder of the chamber all air. You end up with a TOTAL ratio of 24:1, for example.

If it were just as simple as the right combination of timing, throttle opening, and fuel mapping, etc, all cars would've been set up for it!

[frank316]

REAL lean burn? anything leaner than a factory "optimized" 14.7-1 is lean burn... The reason lean burns not all the rage.. EPA, and $.. Lean burn does create more NOx emissions, which can be curbed but with a MUCH more expensive catalytic converter. the 3 way cats in use today NEED 14.7-1 so they have the right mix of chemicals to get rid of polluntants and not get plugged up.

Hondas solution to a homogonous mixture was to use swirl ports on the vtec-e motors, all new heads since 96 use similar swirl ports because the mixing of the fuel and air to estabilsh the "lean burn" also promote better emissions at richer air fuel ratios.

Anyways been doing some more testing, although colder winter weather, snow tires, winter gas, all have a pretty detrimental effect on mpg, car still does a pretty regular 32-36mpg better than epa average of 28 and thats about 80% city/back county driving.. and I don't really hypermile, i still drive like an idiot.. i know if i slowed down i'd get better fuel milage but ??

I have found out that there is a limit to how lean you can go.. not so much as it missing, or maybe it is and its just to hard to tell, but it doesn't seem to do as well if i lean it WAY out(past 20-1) and have to apply more throttle, than if i leave it slightly richer.

The other thing is that at around a 20-1 af at a 55mph cruise, since the temp is below 30 outside, simply turning on the heater causes the coolant temp to plummet, I really think the thermostat isn't even opening on the highway the motors running cool enough to not have too. The egts at 20-1 are lucky to be over 1050, at 14.7 its over 1300. I also have the start up tables leaned out too, but it still has to be slightly richer when the motors dead cold to get it to warm up. I need to go a grill block, but like i said its been below 30 and i'm not that hardcore..

For those still reading i'm gonna put some of my warm up air fuels and what not just for reference..
Start up dead cold from 25* or so it idles around a 14.7-1
Slowly leans out to about 18-1 at idle when warm
Crusing at part throttle low load is 18-1
Cruising at 70-75 it dips down around 14.7-1
Full throttle drops to 13.6-14 for peak power

[Ptero]

>at around a 20-1 af at a 55mph cruise, since the temp is below 30 outside, simply turning on the heater causes the coolant temp to plummet

Block off part of the front your radiator with cardboard or sheetmetal.