Lean burn on automotive natural gas engines 3

[msantiago]

Hello All,

I would like to know if anyone has experience on the increase of efficency and emissions with natural gas automotive engines running on lean burn.

Also if any advice for vendors that can supply GM vortec 8.1 turbo or Ford 6.8 turbo on lean burn.

 

[ivymike]

I would expect that you cannot meet current US emissions limits without running stoichiometric w/three-way catalyst.

 

[j2bprometheus]

Is this off-highway on on-highway?

For off-highway, would the engine be driving a gen-set?

j2bprometheus

 

[msantiago]

It is a power generation application where the NG engine is driving a generation set with power in the 50-90 kw range

 

[ivymike]

then I take back what I said... you _might_ be able to hit 4.0g/kW-hr NOx+HC using a lean burn engine with AFRC and an oxidation catalyst on one of those little engines... maybe. I don't know of any OEM examples of a lean-burn engine meeting that limit with a displacement under 10L. Caterpillar's smallest lean burn engine that I know of is 18L.

 

[catserveng]

We tried this about 5 years ago on both GM and Ford industrial engines driving both generators and chillers on some units we had under contract, even had some help for a while from the South Coast APCD to try and improve small unit emissions reliability.

In short, it was a bust, the cost of required improvements to turbocharging, ignition and air fuel control just wasn't economical. In the end all of the units we tried went back to stoich with three way catalysts.

Since then there have been some improvements in the offering for electronic ignitions and better lean burn AFRC's, but I'm not sure the base engine designs have improved enough to make lean burn work effectively without high fuel consumption and poor part load performance.

Mike L.

 

[franzh]

There is more to running lean burn than adjusting the AF mixture lean. An engine will not easily operate in lean burn mode at all throttle settings, one reason lean burn is not seen in transit applications. A steady state gen set or irrigation system is the most common application for lean burn.

When running lean burn, the AF ratio must get past the NOx curve to be effective, but power drops off considerably, making a turbocharger almost mandatory. You MIGHT get by without it but dont expect any power miracles. When running in lean burn mode, maintaining the AF ratio is especially critical, the slightest deviation can result in almost immediate engine damage.

One company I am aware of offering lean burn fuel systems in either natural gas or propane is Continental Controls (no personal affiliation).

Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ivymike]

There are also Altronic, Emit technologies, and CleanAir Systems (the latter perhaps a reseller of Altronic stuff, w/application support). Miratech may also offer AFRC.

 

[bcs5274]

x2 on emitt, good people and they are putting systems on small engines in the gas field so i don't se an issue with generators

 

[Lou Scannon]

"An engine will not easily operate in lean burn mode at all throttle settings, one reason lean burn is not seen in transit applications."
franzh, I know you are generally well informed about gaseous fueled engines, so this statement surprises me.
Though full-size natural gas transit applications are now migrating toward stoich-EGR for emissions reasons, lean-burn engines have been the norm (Cummins-Westport, Deere, not to mention European & Asian OEMs) from the 90's through the present.

 

[Fmangas]

Woodward has a slick system using an 02 sensor, trim valve and L-series governour. We use rich burn for all of our systems but, if we were to go lean burn this would be the way we would do it.

 

[msantiago]

Trying to understand better the pros an cons of learn burn,

With the expected benefits of learn-burn providing better fuel economy and longer engine life due to the lower engine temp what reasons are preventing the use of learn-burn on Gensets.

 

[ivymike]

- in the US, NOx emissions limits are much easier to meet w/rich burn on a small engine
- block load acceptance is better with rich burn (or so I'm told)
- altitude capability is better with rich burn
- it's more difficult to get stable operation and complete combustion running lean burn in a small-displacement engine... perhaps something to do with the ratio of volume to surface area

Lean burn IS widely used on gensets 18L and up, even in the US.

 

[EngJW]

An application that I know of uses a three way catalyst and is probably running stoichiometric. It uses an electronic throttle body with an electronically controlled pressure regulator. The system is made by Econtrols in San Antonio and is expensive. The reason this system was used is that CFR 1048 (for off-road spark ignition engines over 25hp) requires transient testing and most controllers could not respond fast enough.

 

[ivymike]

if it's successfully using a TWC, it has to be stoich (the TWC will quit working if excess O2 gets to about 1%)

 

[catserveng]

The pros and cons of lean burn vs rich burn are affected by a lot of things.

First, what emission target are you trying to reach? When we went thru this exercise most of the industry thought 2 grams of NOx was way low, we were trying to hit 1 gram and in some cases 0.6, it is NOT twice as hard to get half as much, it's a lot harder.

In some cases fuel consumption was actually worse with lean burn configuration.

Rich burn with three way catalyst is always a loser in the exhaust temperature part of this, in some cases we had to derate engines due to the thermal problems, mostly valve life.

Engine startablity, in general seemed we had fewer starting and getting on line with rich burns than lean burns, especially in parallel to grid applications.

Engine durability, usually the winner here is lean burn, mainly because of impacts of the higher exhaust temps, however, in some lean burns the margin to detonation was pretty close, and we seemed to have more failures in lean burns than in rich burns, especially early on. Detonation protection systems usually only came on larger engines because of cost, but we did apply some aftermarket units on a couple with mixed results.

Maintainability, I have to say, overall a rich burn was easier for most technicians to deal with, the operating margins seemed wider, and some guys could keep an engine in compliance just by watching the differential temp across the catalyst. Also the AFRC for rich burn used automotive exhaust sensors, cost and reliability was much better. As lean burn has become more common some of these issues have improved, but most lean burn engines have more complex control systems and are more affected by ambient conditions and fuel quality than the rich burn engines seemed to be.

This experience is somewhat old now, and from a tough emissions market, but in genenral I think still pretty accurate.

 

[ivymike]

well, I'd say that for the slightly larger stationary lean burn engines (ie G3516), the NOx emissions variability is more easily controlled over the long run w/lean burn than rich burn, because the slope of the NOx curve vs. AFR is very low (vs very steep near stoich). With NOx sensor feedback (vs. O2 sensing) the Cat engines control within a few (10 or 15, I think) ppm with relatively little need for adjustment. Using a TWC you often need to fiddle with the AFRC to stay in compliance, if you were going to be tested on it (which in many cases you're not).

The newer Cat G3516s can reliably hit 1.0g/hp-hr or 0.5g/hp-hr (depending on setpoint) without NOx aftertreatment. 1.0g/hp-hr will be the fed. limit in the US starting next summer.

http://catoilandgas.cat.com/cda/files/1216322/7/LEHW0002-02.pdf?m=204692&x=7

 

[catserveng]

Yes, I would agree the large industrial engines can meet the targets much more reliably, but these were also built from pretty much the ground up as lean burn engines. The 3500 series gas engines were intially introduced as lean burn, then offered as rich burn later.

NOx sensor feedback is very new, and in it's early stages, as with charge density, had lots of issues that always didn't live up to the promises. Larger engines (with larger price tags)can afford the newer and more complex controls, along with the base engine engineering to make lean burn work as well as it does. As an example, the CAT G3520 has 3 ECM's, a temperature sensing module for exhaust temps, a fuel mass flow controller (Woodward Raptor), two other Woodward actuators (throttle and compressor bypass), a detonation sensor per every two cylinders, a handful of pressure and temperature sensors, and lots of wire and connectors tying it all together. Not to mention the millions of dollars spent actually making it work in the field.

The original poster asked about auto derived NG engines, and the experience shared above was centered around those particular types of engines.

 

[dcasto]

call me nieve, but a lean burn natural gas engine from a car maker won't have the metalurgy to surive and you'll burn a hole in the piston in a NY second.

 

[ivymike]

Okay, but I'm not sure what that word means. In the combustion regime (that I assume) we're talking about, the lean burn engine sees lower combustion temps than the rich burn one. It's not the familiar gasoline engine version of "running lean" where you're a gnat's hair leaner than stoich (lambda of 1.05 or so, and combustion temps reach their max). It's waaaaay past that, where NOx doesn't form in great quantities during combustion.

The so-called "rich burn" natural gas engines run in the neighborhood I think you're thinking of. rich burn, in this case, means less than 2% excess O2 - w/catalyst, between 0%-0.5% excess O2

"lean burn" would be 6% to 10% excess O2, or a lambda of about 1.5-1.8 (1.67 for the case of 8.7% excess O2)