What is the relationship between lean burn, timing and octane?

[rmay635703]

After a long and unfruitful search for a 3d BSFC chart that includes AFR in it I have now turned my sights on a question I have long had.

What affect does running leaner than stoich have on timing and octane?

When I look on this subject I get conflicting information,

One article says timeing has to be advanced because of the slower flamefront, others say that it causes knock and needs to have timing retarded.
Maybe its both?

Odd afront to this is that many seem to believe lower octane fuel helps with lean burn but if you get knock how can this be true.

So what is it?

Thanx
Ryan

Interesting read

http://energy.gov/sites/prod/files/2...dge_2013_o.pdf

http://www.google.com/url?q=http://g...J5StsssBp-10_A

[pgfpro]

Heres a chart from Internal Combustion Engine Fundamentals by John B. Heywood.

Its showing ISFC instead of BSFC but this will give you and idea on one of the reasons lean burn is more efficient.

When running leaner then stoich the flame speed will decrease so you will have to increase your timing advance. Now there are ways to increase flame speed when running lean burn, swirl, squish, and turbulent flow etc.

The second part of your question on octane is more dependent on a lot of variables. At light load you don't need a high octane fuel due to low amount of fuel and heat being produced. At mid load you now have to increase your octane because there is now enough fuel to make enough heat that the engine becomes heat soaked. Plus as a example say you went from a A/F ratio of 13.5:1 to 20.0:1 you just lost a lot of fuel that was pulling heat out (around 35% less fuel) during the intake and compression stroke through phase change liquid to vapor. What I have been doing on my Talon is using the turbo and inter-cooler to be able to run mid load while staying in lean burn.
The extra cool air helps cool combustion and keeps the engine from knocking.

[RustyLugNut]

The subject is a complex one and that is why there is seemingly contradictory information.

One area of misunderstanding is the area just lean of stoich or 15-18:1 air fuel ratio (AFR). Someone had already posted a graph on another thread showing the classic increase in combustion temperature in this lean combustion mixture. It goes against logic that combustion temperature would increase when there is an obvious reduction in fuel. But, we must not forget that N2 is present and will oxidize given enough endothermic energy and will then release energy adding to the combustion heat.

Beyond 18:1 AFR, the heat capacity of the lean mixture reduces the formation of NOx.

The above is not to be confused by the "racer speak" of "lean mixtures". Racers run their gasoline engines about 12:1 AFR to provide extra fuel to cool combustion via extra heat capacity and thermochemical formation of CO and HC. Running "lean" could still mean 13-14:1 AFR and detonation conditions occurring.

Running lean beyond 18:1 AFR will result in reduced heat and pressure output until the limits of flammability are reached.

Also, most high performance testing is done at wide open throttle and maximum heat production.

[rmay635703]

In my application the motor always runs WOT at around stoich, I am attempting to improve this by nudging the AFR somewhere between 15-16:1 AFR

The motor calls for premium and only runs WOT but at a variety of RPMs from 1200-3600rpm.

Based on what you are stating to move to peak heat I will need to keep a similar or higher octane (calls for 91, will likely need 93 or 91 with more ethanol)
and may also need water injection of some type.

I wanted to combine this with a hot air intake since I am running the motor lightly loaded (only in terms of peak output at peak RPM) but based on what you state this will possibly cause knock and again possibly require water injection.

Ah well, if I ever get off my arse we will see how this works, maybe start with hot intake (WAI) and leave AFR alone.

Cheers
Ryan

[Baltothewolf]

I think EGR has a huge play in lean burn as well. I believe that under lean burn, the introduction of exhaust gas brings down the oxygen content in the mixture. This does 3 things, 1. Makes the mixture less volatile, 2. Allows greater resistance to detonation, 3. Allows more timing to be applied to compensate for the slow burning of the mixture. I believe this is also the reason that, when an EGR is failing on an insight, lean burn (as well as timing in lean burn) will randomly drop out.

[redpoint5]

Originally Posted by pgfpro

At light load you don't need a high octane fuel due to low amount of fuel and heat being produced. At mid load you now have to increase your octane because there is now enough fuel to make enough heat that the engine becomes heat soaked.

This gives me an idea. Octane booster could be held separately from low-octane fuel, and mixed as conditions dictate. The majority of the time that an engine operates, it doesn't require the full octane rating.

Colder air or higher elevations at light load would reduce the octane mixture. Heavier loads at lower elevations would increase the octane mixture.

Perhaps there would be cost and performance improvements by having continuously adjustable octane?

[pgfpro]

Originally Posted by redpoint5

This gives me an idea. Octane booster could be held separately from low-octane fuel, and mixed as conditions dictate. The majority of the time that an engine operates, it doesn't require the full octane rating.

Colder air or higher elevations at light load would reduce the octane mixture. Heavier loads at lower elevations would increase the octane mixture.

Perhaps there would be cost and performance improvements by having continuously adjustable octane?

One of my fellow DSM guys did just what your talking about. He ran a secondary fuel pump, fuel cell, fuel regulator and a secondary set of injectors for race fuel. It work really well!!!

[LeanMeanDSM]

You can make power with lean burn without detonation, egr and water meth, cold air intake, and more octane all hurt lean burn performance, when your on the lean burn flammability limit, you want the hottest most explosive mix you can get, or the mix will not burn or not burn fast enough to be usable. It is possible to go past mbt with lean burn and have a mix that burns so slowly that your timing has to be advanced to the point of knocking and not making any power, in that case you need to add a hotter air intake or less octain in your fuel, so that you can speed up the burn and retard the timing and still make the same or more power.