Why no more lean burn??

[cbaber]

I did some poking around the interwebs, and found some good info. If you were to compare emissions of a standard gasoline engine to a lean burn engine straight from the exhaust with no cat, the lean burn is much cleaner. Lean-burn actually creates less NOx and CO2.

The problem comes with the catalytic converter. Normal gas engines operating at 14:7 produce the right "mix" of gasses that react inside the catalytic converter. This reaction significantly reduces NOx emissions. With lean burn, the mixture of gases entering the cat is not ideal for oxidation because of the already lower NOx and CO2. Therefore special cats and other emissions control systems are required to further reduce emissions inside the cat.

So lean burn isn't the problem, it's that catalytic converters are more efficient at cleaning up the dirtier air from regular burn than they are from lean burn, so you end up with worse emissions after the cat on lean burn engines unless expensive new cats and emissions systems are implemented.

[RustyLugNut]

Originally Posted by Daox

In that graph, NOx starts coming down, but HC (hydrocarbons, or gasoline) start going up because of misfiring.

I'm not exactly sure where that graph comes from. I believe its from a Toyota paper. Its obviously from some engine, and an engine designed for lean burn obviously isn't going to misfire at 18:1. So, you may be right, but we can't really know for sure because we don't have details on it.

NOx is reduced after 18:1 AFR and beyond, but HC starts climbing. In vehicles designed for lean burn ( Honda lean burn engines as an example ), the HC line can be held flat up until 22:1 or so. Liberal use of EGR and combustion chamber turbulence allow Honda engines to minimize partial combustion and misfires. At some point however, the HC starts climbing again as partial/mis fires become dominant. Honda engines can be run lean up to just under 26:1 AFR but the unburned hydrocarbons is unacceptable as pollutants and as wasted fuel. 28:1 AFR is the "magic" number as NOx is almost non existent at that fuel mix but producing useable torque is the issue.

[RustyLugNut]

Originally Posted by cbaber

I did some poking around the interwebs, and found some good info. If you were to compare emissions of a standard gasoline engine to a lean burn engine straight from the exhaust with no cat, the lean burn is much cleaner. Lean-burn actually creates less NOx and CO2.

The problem comes with the catalytic converter. Normal gas engines operating at 14:7 produce the right "mix" of gasses that react inside the catalytic converter. This reaction significantly reduces NOx emissions. With lean burn, the mixture of gases entering the cat is not ideal for oxidation because of the already lower NOx and CO2. Therefore special cats and other emissions control systems are required to further reduce emissions inside the cat.

So lean burn isn't the problem, it's that catalytic converters are more efficient at cleaning up the dirtier air from regular burn than they are from lean burn, so you end up with worse emissions after the cat on lean burn engines unless expensive new cats and emissions systems are implemented.

The emission levels dictate the use of Three Way Catalytic Converters (TWCC) to reach allowable outputs.

Lean burn engines can reach those same output levels but with after engine clean up technology that mirrors clean diesel tech. And, those technologies are not cheap. Particulate filter traps or oxidation catalysts and NOx absorption traps as well as the required purging circuits would mean a relatively expensive engine. The market would not bear this in small passenger applications.

[Bindegal]

So when running lean - as I´m planning to do 20-22 AFR... how mugh ignition timing is needed compared to stoich 14.7?
A friend of mine says that more timing is needed because the combustion at such lean mixtures is actually slower than stoich.
I´ve had a hard time figuring out how much timing is used on the standard engine... no logging software available for my stock 1998 ECU

/Allan

[RustyLugNut]

Originally Posted by Bindegal

So when running lean - as I´m planning to do 20-22 AFR... how mugh ignition timing is needed compared to stoich 14.7?
A friend of mine says that more timing is needed because the combustion at such lean mixtures is actually slower than stoich.
I´ve had a hard time figuring out how much timing is used on the standard engine... no logging software available for my stock 1998 ECU

/Allan

Lean burning AFR engines have relatively slow combustion and require considerably more ignition lead time. What that actual advance is going to be is dependent on the engine and other factors.

[tony_2018]

Where can one find such a cat convertor, besides Honda dealership.

[CrazyLee]

Originally Posted by RustyLugNut

Lean Burning engines is under much scrutiny. Along with improved controls, there will be applications with larger drive cycle regimes and much more seamless transitions. Of course, if a driver still insists on heavy footed driving, no technology is going to help them.

Now that about every car has a throttle by wire I wouldn't be surprised if a really true cruise control, or a monitored throttle to try to achieve the best mileage to occur.

The engineering wouldn't be difficult as the engineers have everything that goes into getting good mileage that can be used to achieve this goal. Not to say that it would totally destroy all your fun in driving in eco mode.

Maybe if we all SCREAM as loudly as possible. (Not like we aren't already doing just that!) [Nah, we will let all the lead foots do that.]

When driving The Beast (My Motor home) I have found that I have trained my foot to automatically lift the gas at the right moments. I think the computer (ECU) could do even better, since it won't get distracted.

[RustyLugNut]

Originally Posted by tony_2018

Where can one find such a cat convertor, besides Honda dealership.

The emissions profile for your engine would have to be determined along with the application drive cycle. Then, a strategy to reduce emissions via oxidation catalysts ( HC and CO ) and a NOx trap can be decided upon.

Mercedes, Audi, BMW and VW all have the required catalysts and traps but they have widely varying strategies.

Also, aftermarket dealers for catalysts and traps are numerous, but they generally build for large diesel engines. Their prices are as large as their parts.

[eco_generator]

Originally Posted by RustyLugNut

Also, aftermarket dealers for catalysts and traps are numerous, but they generally build for large diesel engines. Their prices are as large as their parts.

Ehh, I think there are two sides to that coin. I was able to get a universal 3" I.D. cat for my track car for under $100.

But I guess that would be a small cat from a diesel truck perspective, lol.

[cbaber]

I bought a magnaflow high flow cat for about $70. They make many sizes for many applications. I'm not sure if there is any difference in emissions control between OEM or aftermarket. But I don't get CELs and I have had no problems with quality.