air fuel ratio during lean cruise

[Circlotron]

As above there's no point going leaner if it only causes you to put the foot down more

That would mean lower vacuum therefore less pumping losses. Even if that is the only apparent consequence of going lean it must help economy a little.

[delcowizzid]

theres no point going far away from 14.7 your just taking life out of the rings and valve seats.

I as wondering if there was a trade off to going too lean, when you do tunes delcowizzid, do you set a target of 14.7 for cruise areas and run no lean cruise at all?

i run most at 14.7 with O2 sensors and without o2 s i run as low as 14:1 so the engine can lean out over time from aging componants without getting way to lean.i never use the lean cruise function i like torque

[antus]

when you go lean exhaust temps go up so you need to keep in mind worst case scenario of how hot things might get on a 45 degree day for a road car, though generally when cruising you'll have decent airflow.

[vlad01]

aren't exhaust temps highest at 14.7?

exhaust temps are suppose to drop off as it goes leaner or richer from 14.7.

[Circlotron]

According to this graph -> http://www.team-integra.net/images/BAEC ... gChart.jpg NOx peaks at about 16:1 so that is also where peak combustion temp would occur. There is scope for exhaust temp to rise when you go still leaner because a lean mixture burns slower to the extent that the expanding cylinder volume may not be able to extract as much heat from the late temp/pressure peak so it goes out the exhaust instead. Less fuel may mean less total heat but an even greater percentage than before goes where you don't want, inlcuding to the water jacket.

[9c1vs]

Thanks for all the informative replies.

What about during decel cut? Obviously their is no fuel at all during these times, but then again no combustion happening. I have it set on my car and i think because it has high compression it has a big effect in slowing the car down. I was coming down the range the other day and hardly even need to touch the brakes and also noticed engine temps went back down to the thermostat opening temp.

Is this something that many of you use or avoid? I turned it on because i figured they had it as standard and therefore must be okay.

[vlad01]

As above there's no point going leaner if it only causes you to put the foot down more

That would mean lower vacuum therefore less pumping losses. Even if that is the only apparent consequence of going lean it must help economy a little.

that is one of the points of it.


to be honest though, when I started using lean cruise I noticed 0 difference in any way the engine operated. the only noticeable things were less fuel usage and the engine tended to run cooler on the highway.

[vlad01]

According to this graph -> http://www.team-integra.net/images/BAEC ... gChart.jpg NOx peaks at about 16:1 so that is also where peak combustion temp would occur. There is scope for exhaust temp to rise when you go still leaner because a lean mixture burns slower to the extent that the expanding cylinder volume may not be able to extract as much heat from the late temp/pressure peak so it goes out the exhaust instead. Less fuel may mean less total heat but an even greater percentage than before goes where you don't want, inlcuding to the water jacket.

yeah I sen those graphs but I dont think that reflects actual exhaust temps as the peak in nox is partly due to more free oxygen available in a leaner mixture. All EGT vs AFR graphs I have seen show the EGT peak exactly at 14.7


here is one I found, crude graph but you get the idea.

[antus]

Ive seen my mates datto idling at 18:1 with glowing headers, put more fuel in to bring it back to stoich and they stop glowing. No other changes.

[Charlescrown]

That's exactly right. Most modern engines are whats know as fast burn. The faster you can burn the fuel the more power you extract. This is shown by later ignition timing settings on some cars. Alsop the faster it burns the less chance of detonation occurring. People seem to think that high octane fuels burn slower they don't they just have better knock control. If you lean out the mixture it burns slower and more heat is wasted out the exhaust. I used to teach LPG course and we deliberately leaned out the mixture on a dyno to demonstrate the extreme increase of the exhaust temps. When you look back at Chrysler ELB it had a very tight advance curve plus changes to the combustion chamber. If you look at the ADR emission test the highway cruise parts only have a small input. I think most manufacturers stick to 14.7 for other reasons like engine preservation.