Interest is high in natural gas as a heavy-duty truck fuel. With a dollar per diesel-equivalent gallon differential, why wouldn't it be? Natural gas is likely to remain less expensive than diesel for the foreseeable future, and domestic supplies of the stuff are plentiful. Is natural gas a savior-in-waiting for beleaguered diesel buyers?

Maybe, but here are six questions you need to ask before taking the plunge.

"Everybody is interested in saving money, but is natural gas a practical alternative?" asks Andy Douglas, national sales manager, specialty trucks at Kenworth. "Certainly it will work for some, but not everyone. There are six basic factors customers should consider when determining if natural gas will work in their application. The first and foremost being their proximity to a fuel supply."

1) Where is your fuel source?

As the nation's natural gas fueling network expands, that question becomes less of a concern. Presently, there are pockets of availability on the east and west coasts as well as in areas of Texas. More filling sites are emerging, but they are by no means common, yet. The map link above reveals an expanding network of filling stations that will supply fuel on certain corridors. Much of that network shown is already in development and will be in place later this year. More development is planned for 2013 and beyond.

2) Which fuel type; LNG or CNG?

The next question is whether you want to use compressed (CNG) or liquified (LNG) natural gas. There are several angles to this decision, including availability of a fuel source, the truck's planned operating range and your tolerance for cost.

"CNG is currently more prevalent, but LNG is gaining traction as more of it becomes available," Douglas says. "The over-the-road segment is driving the proliferation of LNG because they need the extra range provided by the greater on-truck storage offered by LNG tanks."

All available natural gas engines burn the fuel in a gaseous state, so they will run with either CNG or LNG tanks. With the CNG tank, fuel flows directly to the engine from the tank. With LNG tanks, the liquid fuel passes through a heat exchanger where it is 'gasified' prior to combustion.

For a given amount of space on the frame rails, liquid storage tanks offer roughly twice the operating range of compressed gas storage tanks, but the compressed tanks are more costly than the liquid tanks, and their range is limited. The liquid tanks are also quite heavy. They can, in some cases, add about 600 lb to the weight of the vehicle.

As for cost, Douglas says the CNG tanks add about $30,000 to the price of the truck, while an LNG tank would add about $20,000.

3) What is your required operating range?

Natural gas has a lower energy density than diesel, so for simplicity's sake, natural gas measures are stated in diesel equivalent volumes. It also gets rather onerous converting from cubic feet to gallons, and factoring energy content.

To determine onboard storage requirements, Douglas uses a quick and simple energy and fuel consumption calculation.

"We take 90 percent of the stored amount of fuel (in diesel-equivalent volume) and assume a very conservative miles-per-gallon estimate so we're not fostering 'range anxiety'" he says. "On the other end of the spectrum, we avoid over-spec'ing the storage capacity because the tanks represent a large portion of the incremental cost in natural gas fuel systems."

For example, an 80-gallon diesel-equivalent tank would have a range of about 300-400 miles (80 gal x 90% x 5 mpg = 360 miles). The energy equivalent of 80 gallons of diesel would require two 100-inch-long CNG tanks or a single LNG tank.

CNG tanks can be frame-mounted or installed in a cabinet behind the cab in applications where frame space is limited or non-existent. The CNG cabinet installations can store the equivalent of 100 gallons of diesel, Douglas says.

4) What is the intended application?

"This industry is used to bigger-is-better, but with natural gas, there's a real need to find the right fit; the right engine platform for the application," Douglas cautions. "The 8.9-liter Cummins-Westport ISL-G does a good job in an urban or regional pickup and delivery environment on reasonably flat ground, while the 15-liter Westport GX is better suited for high GVW applications on big grades, and possibly off-road. Soon, we'll have the CWI ISX12-G coming right up the middle as an ideal engine for regional or longhaul truckload applications."

In May, Volvo Trucks announced plans to launch a natural gas version of its 13-liter D13 engine in North America. The engine will use high-pressure diesel ignition technology along the same lines as Westport's GX 15 engine, which uses an 15-liter Cummins ISX platform. Volvo says the truck should be on the market by 2014.

5) What is the gross combination weight of the vehicle?

In the various displacement categories, natural gas ratings are near or equal to their diesel counterparts. We know of plans to offer beefed up ratings on the ISX12-G and the GX engine in the near future, so the 'power' differences will be moot.

Presently, the ISL-G engine (up to 320 hp / 1,000 lb-ft) is recommended for gross combination weights up to 75,000 pounds.

The Westport GX engine (up to 475 hp / 1,750 lb-ft) is recommended for combination gross weights exceeding 80,000 pounds. We know of applications where GX engines are pulling loads in excess of 100,000 pounds GCVW.

The new kid on the block, the ISX12-G (up to 400 hp / 1,450 lb-ft) holds a great deal of promise for the truckload sector. Boasting ratings similar to its diesel counterpart, it will fit well into the regional and longhaul world once the appropriate fueling infrastructure is in place.

6) Who is your service partner?

And finally, because the technology is new, and suitable national service and repair network is in very early development, you need to ensure the infrastructure exists where you plan to run the truck.

"Because natural gas systems require specialized tooling and training, it's very important to partner with a dealer that has a history with natural gas and the commitment to get its techs trained," says Douglas. "At Kenworth, before taking an order, we make sure there's a service plan in place."

With diesel, service capability is foregone conclusion. With natural gas, customers need to take the extra step to make sure the dealer is ready to service the trucks over the long term.

Other Considerations

Of course, none of these questions can be answered in isolation. They are mutually dependant, and the answer to one question can determine the answer to another. At the end of the day, the business model has to work.

There are many incentive programs in place today, with California and Texas leading the country on that front, says Douglas. "But we think natural gas works even without the incentives. We believe the economics of natural gas stand on their own today.

"With a typical customer running 80,000 or more miles per year, we can show a very strong payback in somewhere between 12 to 18 months," he says. "And most of that cost is in the tankage. That's prompting a lot of interest in developing new tanks that will help drive cost out of the technology."

While the partnership between trucking and natural gas is still young, it seems like the best years are still ahead. Ask the right questions and you'll have a better understanding of what you're getting into.