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Internal combustion engine-powered generatorsets ueled by natural gas are commandingmore attention these days as interest growsin on-site power production equipment that isboth ecient and environmentally riendly.

In response to this interest, manuacturers haveintroduced natural gas engine-powered generator setsthat eature lean-burn technology. The combustionis considered lean when excess air is introduced into

the engine along with the uel.This produces two positive eects. First, the excessair reduces the temperature o the combustionprocess and this reduces the amount o oxides o

nitrogen (NOx) produced by nearly hal, compared to aconventional natural gas engine. Second, since there isalso excess oxygen available, the combustion processis more ecient and more power is produced rom thesame amount o uel.

Combustion process

Any air/uel reaction requires an energy source toinitiate combustion. In natural gas engines, the sparkplug perorms this unction. In lean-burn engines, the

combustion process is enhanced by pre-mixing the airand uel upstream o the turbocharger beore introduc-tion into the cylinder. This creates a more homogenousmixture in the combustion chamber and reduces theoccurrence o knocking or detonation. To preventeither knocking or misring, the combustion processmust be controlled within a narrow operating window.Charge air temperatures and volume, together with airto uel ratio, are constantly monitored. The micropro-cessor-based engine controller regulates the uel fowand air/gas mixture and ignition timing.

New lean-burn engines rom Cummins are designed

to operate at a lean air/gas ratio o Lambda = 1.7.(Traditional stoichiometric natural gas engines have anair/gas ratio o Lambda = 1.0). In the chart (at let) thatplots Break Mean Eective Pressure (BMEP) against

Air Excess (Lambda), the operating window is a verynarrow band where eciency peaks and NOx is nearits minimum. A richer mixture (stoichiometric) can po-tentially produce knocking and higher NOx emissions;a leaner mixture than Lambda 1.7 may not combustreliably and cause misring, which raises HC emissions.Full-authority electronic engines, sensors and micro-processors in the new lean-burn engines are critical ormaintaining combustion within these boundaries.

> White paper

ByKeithPackham, GasApplicationsManager,EnergySolutions

Lean-burn engine technologyincreases eciency,reduces NOx emissions

Power topic #7009 | Technical inormation rom Cummins Power Generation

0.8 1.0 1.2 1.4 1.6 1.8 2.0

Detonation

OperatingWindow

Air Excess (lambda)

NOx Eiciency

Misfire

DETONATION CHART

20

18

16

14

12

10

8

0.6

BMEP(bar)

1 bar = 14.5 PSI

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The design o the lean-burn engine incorporates asimple open combustion chamber housed in thepiston crown. The shape o the piston crown intro-duces turbulence in the incoming air/uel mixture thatpromotes more complete combustion by thoroughlyexposing it to the advancing fame ront. The fameplate o the cylinder head is regular (fat) and thespark plug is centrally located. The air and gas uelare correctly mixed under the control o the enginemanagement system.

Reduced emissions

One o the results o this technology is signicantlyreduced emissions in the exhaust. Cummins newlean-burn gas engine generators have NOx emissionsas low as .85 grams/BHP-hr, and produce low amountso hydrocarbons (HC), carbon monoxide (CO) andparticulate matter (PM). This allows the generatorsets to meet the most stringent air quality regulationswithout ater-treatment devices in the exhaust stream.

One o the results o this technology issignicantly reduced emissions in the exhaust.

For even lower emissions, lean-burn gas enginegenerator sets are requently coupled with integratedater-treatment options such as Selective CatalyticReduction (SCR) and Oxidation Catalysts, resulting inNOx levels at or below 0.15 grams/BHP-hr. With theseater-treatment options, the gas engine generators have

been shown to meet the most stringent prime poweremissions regulations anywhere in the world.

Fuel fexibility

Another advantage o the lean-burn technology withull-authority electronic engine controls is the abilityto operate on gas with a wide range o quality. Ameasurement called the Methane Number (MN) isused to determine uel gas suitability as an engine uel.Most natural gas has an MN rom 70 to 97, and pipeline

quality gas typically has an MN o about 75. Resourcerecovery gas rom landlls or sewage treatment acili-ties is typically o lower quality, but is oten suitableor use in lean-burn engines. Cummins lean-burn gasengine generators will operate on gas with an MN o 50or greater, providing excellent uel fexibility. However,gas with a MN below 70 may require derating o thegenerator output.

Lean-burn gas engine generator sets are setting a newstandard or uel eciency, high power output or theirsize, and or low emissions. In regions with supplies onatural gas, these generator sets are providing highly

reliable electric power or utility peaking, distributedgeneration, prime power and or combined heat andpower systems.

For additional technical support, please contact yourlocal Cummins Power Generation distributor. They canbe ound at www.cumminspower.com.

About the author

KeithPackhamisGasApplications

ManagerorCumminsPower

Generation,andprovidesexpert

technicaladviceandsupporttothe

globalEnergySolutionsBusinessteam

onthedesign,installationandoperation

ogasuelledenergyplants.Hehasbeeninvolvedwith

thedesign,installation,maintenance,andoperational

aspectsocombinedheatandpower(CHP)plants,

boilerplantsorsteamorhotwater,powergeneration

anddistribution,rerigeration,waterandefuent

treatmentplantsandtheiroptimumperormance.He

holdsaBachelorsdegreeinEnergyEngineeringrom

SouthbankUniversityinEngland.

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PT-7009 (05/07) Formerly PT-40