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Cummins Westport Completes First Commercial Sale of 2007 ISL G Natural Gas Engine

The new ISL G engine.

Cummins Westport Inc (CWI), a leading provider of high-performance alternative fuel engines for the global market, announced its first commercial order for the new ISL G natural gas engine. Sacramento (California) Regional Transit has ordered 91 ISL G engines to be installed in 40’ Low-Floor Buses in 2007.

The 8.9-liter ISL G (earlier post) will surpass EPA and CARB 2007 phase-in levels and meet 2010 emission standards of 0.2 g/bhp-hr NOx and 0.01 g/bhp-hr PM at launch in 2007. Based on the Cummins ISL, the ISL G leverages Exhaust Gas Recirculation (EGR) and stoichiometric combustion to allow the use of a maintenance-free passive three-way catalyst such as those used in passenger cars since the 1970s.

In addition to delivering ultra low emissions, the ISL G, with ratings from 250 to 320 horsepower, will deliver increased thermal efficiency and over 30% higher low-speed torque compared with the current models. The ISL G engines offer peak torque ranging from 660-1000 lb ft (895-1356 Nm).

Development of the ISL G—which received almost US$1.3 million in funding support from the US Department of Energy and the California South Coast Air Quality Management District (SCAQMD)—began three and one-half years ago.

The Sacramento Regional Transit District operates 97 bus routes and 36.87 miles of light rail covering a 418 square-mile service area. Buses and light rail run 365 days a year using 76 light rail vehicles, 254 buses powered by compressed natural gas (CNG) and 17 shuttle vans. Sacramento Regional Transit has been operating natural gas vehicles for more than 12 years.




Sounds great. Can anyone tell me the relative fuel storage penalty for natural gas vs. diesel, on an energy unit basis? I'm wondering if these might be viable for local freight delivery, reducing emissions in urban areas with pollution problems.

Also, what is the equivalent cetane of natural gas when used in a diesel engine?

Rafael Seidl

Zach -

natural gas engines of this size are spark ignition engines and are operated stoichiometrically so a standard three-way catalyst can be used to clean up the exhaust gas. Thermodynamic efficiency is worse than for diesels, compensating for the superior H-to-C ratio of the natural gas fuel. Bottom line: CO2 emissions are more or less unchanged, but NOx and PM are eliminated. From an air quality perspective, this makes sense for HDVs that are operated in population centers, i.e. city buses, garbage trucks, delivery vans etc.

Typically, a diesel engine block is modified because nobody builds large-displacement gasoline engines any more. The compression ratio is reduced from ~20 to ~12-14 by using shorter connrods, to prevent engine knock. Direct injection of the natural gas would be ideal, but the state of the art is PFI; this means less oxygen gets into the chamber (leading to reduced specific power), unless the boost pressure is also raised (leading to greater turbo lag or the need for a fuel-guzzling supercharger).

CNG buses usually feature roof-mounted tanks. I'm not sure what pressures they are rated for (LDV CNG tanks support 200-300 bar depending on design). To date, no regulatory agency has approved the storage of CNG fuel in the cavities of suitably designed structural members.


Usually the cylinders used on buses with Westport spark ignited Nat Gas engines are 5,000 PSI. One major supplier, Dynetek Industries, with mfging in Calgary, Alberta and Ratingen, Germany does make lightweight tanks up to 10,000 PSI, but I guess they are quite a bit more expensive and maybe another limitation is the pressure available at the Nat Gas refueling stations.


We have NG powered city busses in Southern California and they are nice and clean. It would be good to see trucks that are used in the city also be NG fueled. It might clean up the air a bit. From what I have observed, the busses seem to have the NG stored on the roof, which it takes up some space. This might be an issue with trucks that haul freight.


How does cost of NG power compare to diesel power at today's prices?

Anyone know?

Scott V

Diesel engines have the most efficient conversion of btu's to work of any portable internal combustion engine based on total cost (cost of fuel, fueling stations, logistics, safety considerations, etc.) The emissions standards of 2007 & 2010 will keep diesel in the lead. Transit authorities that enjoy government subsidies, NG fueling stations currently in place and experience with NG power have too much invested to change back. Alternative fuels such as NG definitely have a place in certain applications.


As per the Clean Energy Fuels (largest distributor of CNG in the U.S.) web site --

"Public price for CNG in Oct. 2006 across Southern California: $2.199 per 'gallon'. "

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