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Lean burn on automotive natural gas engines
3

Lean burn on automotive natural gas engines

Lean burn on automotive natural gas engines

(OP)
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.

RE: Lean burn on automotive natural gas engines

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

 

RE: Lean burn on automotive natural gas engines



Is this off-highway on on-highway?

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

j2bprometheus

RE: Lean burn on automotive natural gas engines

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

RE: Lean burn on automotive natural gas engines

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.

 

RE: Lean burn on automotive natural gas engines

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.

RE: Lean burn on automotive natural gas engines

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

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Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Lean burn on automotive natural gas engines

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.

 

RE: Lean burn on automotive natural gas engines

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

RE: Lean burn on automotive natural gas engines

"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.

RE: Lean burn on automotive natural gas engines

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.

RE: Lean burn on automotive natural gas engines

(OP)
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.

RE: Lean burn on automotive natural gas engines

- 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.
 

RE: Lean burn on automotive natural gas engines

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.

RE: Lean burn on automotive natural gas engines

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

RE: Lean burn on automotive natural gas engines

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.

RE: Lean burn on automotive natural gas engines

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



 

RE: Lean burn on automotive natural gas engines

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.

RE: Lean burn on automotive natural gas engines

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.

RE: Lean burn on automotive natural gas engines

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)
  

 

RE: Lean burn on automotive natural gas engines

I rummaged thru some old notes from that era.

Our major issues were,

Getting enough air into the engine to get the required excess air, as Ivymike pointed out above, to get a true lean burn and meet the emissions targets we were shooting for we needed about 8% O2 in the exhaust.  Finding turbo matches was very difficult.

Fuel management, we did most of our work around Impco mixers, a lot of custom fuel valve shapes got made and tried.  Continental Controls was new about then, we tried some of their early stuff, never met our needs.  We also played around a lot with gas regulators and trim valves, idle air bleed valves, and some other things that got a bit crazy.

Ignition systems, trying to fire a plug in a lean mixture takes a lot of energy, the Altronic DISN came along and helped that out pretty good, but we had terrible trouble with spark plug life, either due to electrode erosion or flash over at the insulator.  Plus it was a pricey fix at the time.

With some Ford engines we got some help from a local race shop, one of the guys there came up with a cam grind that helped, but overall the engine got real expensive to keep running as a lean burn.

Air fuel ratio controls, mostly issues with early exhaust oxygen sensors.  CAT had one, but it was REAL expensive and had poor service life, plus it needed a fairly complex signal conversion.  Woodward came out with the GECO Control near the end of the project, we worked thru quite a few bugs with them and got a couple working quite well, but the overall price for a unit at 75 kW was awfully high at that time.

We did bust up a couple of engines pretty good, they had high boost pressures and high compression ratios, and at 8% O2 ran great, but if they got below 5% O2 they detonated and hammered themselves apart fairly quick.

Mechanically if we could keep the engines out of the detonation areas life was good, bottom ends were never a problem during this exercise, top ends had good wear rates compared to running rich burn.  Some cam and lifter combinations didn't like the ultra low or nil ash oils (scuffing and spalling), some piston and ring pack conbinations didn't like medium ash oils (deposit rates).

The project wasn't a total bust, we learned alot about natural gas engine performance, and about ignitions and AFRC's that we turned around and applied to the larger engines with some benefits.  But the objective to setup a viable and cost effective lean burn auto derived engine for generator or chiller service in our required emissions levels was never met our targets.

RE: Lean burn on automotive natural gas engines

It is extremely difficult to have an engine start and run at low and part throttle modes during lean burn, most apps shift to lean burn at cruise and WOT but start and part throttle ramp to load at stoich.  This is what is experienced in transit applications.

If I were to guess the number of lean burn applications on an over the road use, they would be only a small fraction of what I see in stationary steady-state genset use.

Early spark plug failure, piston damage, and exhaust valve torching are common lean burn problems.  I seldom see this occur in steady-state, but much more frequently in transient mode applications.  The damage can happen very quickly, often too fast to stop the engine.  Interestingly, similar damage can occur from overfueling as from lean burn.

In the early 90's, I participated in several lean burn projects, some worked well, others ate up too many pistons.

Franz
 

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

RE: Lean burn on automotive natural gas engines

When I visited customers who were working on lean burn LD engines using the first few iterations of cng injectors we were developing often I would get to see what did not work or deep six'd.  Lean burn, like steroids offer some gains but at a higher economic risk.  A decade + ago we sponsored an individual in Ontario working on an unthrottled chrysler 4.7 V8 with one intercooled solenoid electronic EGR per cylinder mfg'd by another division of the company I worked for at the time.  Using our cng injector and egr valves along with a cam ground to help reduce cycle to cycle deviation and a post cat exhaust throttling valve (normally open) used only to provide more egr at higher stoich loads, the guy was able to demonstrate a range of loads ranging from lean to just slightly rich of stoich idle as well as mid range loads at most rpms with up to 70% egr.  Full load required stoich but he was able to maintain 15% egr @ WOT and kept precat nox remarkably well under control except at cold part load.  If I read the above I would question it, as I did and my boss did but we visited and took a weekend of measurements.  

The project was quite interesting but it was risky to presume that multiple egr solenoids could be economically produced within the needed dynamic range.  And as expected there was very little misfire tolerance.

Turbo

RE: Lean burn on automotive natural gas engines

"at most rpms with up to 70% egr"
[b]turbocohen/b], that sounds a little extreme; care to elucidate?

RE: Lean burn on automotive natural gas engines

Lean low load and idle the 12:1 cr engine ran reasonably stable wot using o2 laden egr dilution.  I thought BS too until after the visit.  70% egr is not 70% manifold co2.. calibrated the stand that day myself and saw about 50%.  No, aliens did not invade my body but when I saw the um, lets say.. enhanced coil on plug ignition I could imagine they were picking up rfi.  This engine was not fit for prime time over the road but a testbed for an ME that my former employer sponsored.  I built injectors for and visited over 50 univerities with alt fuel projects and few were as out of the box as this one.  If I am not mistaken the guy working on the project went to work at Navistar or a diesel r&d outfit in colorado.  An SAE paper was in the works that I reviewed but I am certain it was not published.. if you want to know more I can let you know.

RE: Lean burn on automotive natural gas engines

OK, so we're talking cutting (bleeding) edge here...
I'm in Colorado so which R&D outfit are you referring to?
If you have more to share perhaps a new thread would be in order... smile

RE: Lean burn on automotive natural gas engines

The company Turbo was formerly associated with was indeed cutting edge which is how I became familiar with him.  Whatever one reads as published research material, there are several layers of unpublished material behind the scenes, and THAT is where the Skunk Works does their dirty work.

Even though I was a contractor at an engine R&D lab in the early 90's, some areas were off limits to me.  Windows papered over, cardboard blinders over the instrument panel, one cell even had a military guard stationed there.  Sometimes, weird sounds or smells would be a give-away, and sometimes just the absence of sound could be just as telling.

Franz

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Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Lean burn on automotive natural gas engines

In this case it was not rocket science, nothing automotive is...  The project was of interest to my employer due to the requirement for many solenoids from one mfg.  There was a cng fuel rail and an egr supply rail.  The cooled egr was delivered through the existing port fuel injector ports on the stock plastic manifold and the cng was injected through the sidewall of the egr pipe right next to the port maybe 10 mm above the stock location on the same plane.  The egr dilution strategy, I presume, was to maintain enough N2, co2 dilution to limit the %o2 and afr on an unthrottled engine without using vvt.  The toughest part was aquiring reliable feedback and the mfg cost would be higher and much riskier than throttless VVT and DI.

The vvt/di approach won for good reason and in the future I believe vvt/di stoich will work better than lean port cng as well.  We also supported a few di cng projects over a decade ago.  

Going back to the OP, a GM 6L v8 low pressure (~60 bar) DI CNG lab rat was on the road some time ago that showed some promise but thelack of funds, lower cost of port injection and little interest at the time pulled the plug.  I built a few sets of DI capable cng injectors for former clients and another group of students who were competing at an sae competition.  Mileage improvements and co/hc reductions were there in spades but Nox was horrific due I beleive a lack of proper chamber shape, inadequate spark heat and less than optimized placement of less than optimum injectors.  The port cng injectors we built then (substantially identical to todays continental cng injector) were equipped with modified armatures, coils and pole pieces shared by an earlier DI gasonline application. Opening under high inlet pressure was not an issue with enough amp turns and drivers.. the problem imo was poor control over fuel/air preparation in the cumbustion chamber.

Just my 0.02 dinars..  Respectfully, Turbo Cohen

RE: Lean burn on automotive natural gas engines

hemi, CSU has one of the largest natural gas engine research centers in the US.  

Colorado State University's Engines and Energy Conversion Laboratory (EECL)  

RE: Lean burn on automotive natural gas engines

Yes, I'm aware of CSU's lab but I haven't had the pleasure of a visit, yet.  Haven't heard of an "extreme EGR" project there, either.  Is it hush-hush?

RE: Lean burn on automotive natural gas engines

For more info feel free to read http://www.flexdi.com/flexdi-solution-gaseous-fuel.asp

The company was partners with Siemens long ago and this group broke off after I left.  They were in the same facility I worked at but they were not involved with cng until after I left..

The technology aint perfect yet, nobody has perfected gasoline or diesel injectors either, but this may be a good place to look if you are considering a lean burn gm or ford cng di application.

Good luck with your search Msantiago

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