Too workmanlike to ever be seen as sexy alternatives to diesel power — certainly not in the way plug-in electric drives currently are — natural gas and propane have nevertheless helped launch the greening of America's truck fleets and their numbers continue to steadily mount.

Why not? Right now, electric trucks are limited to light- to medium-duty local delivery applications due to the extra weight carried from onboard batteries as well as the limited range between full charge-ups for both battery-equipped and plug-in types. And biofuels (biodiesel and ethanol) as well as diesel-electric hybrid drives do not suit as many applications across the full GVW range of trucking as the gaseous alternatives do. Still other alternative power sources, such as fuel cells and dimethyl ether (DME), hold promise but are not yet even on the market.

What's more, the total cost of fueling natural gas- and propane-powered trucks may be less than that of diesel- or gasoline-powered vehicles, depending on specific fleet applications and duty cycles as well as the regions in which they operate.

Yet the main advantage that these gaseous fuels offer is a highly viable alternative to fleets operating in federally defined emissions “non-attainment areas” that make it unfeasible to run diesel- or gasoline-fueled trucks.

And that's okay because engines designed to run on CNG (compressed natural gas), LNG (liquefied natural gas) and LPG (liquefied petroleum gas, a.k.a. propane) now cover applications ranging from light to heavy duty — encompassing everything from passenger cars and SUVs to Class 8 over-the-road rigs and a virtually unlimited number of vocational truck types.

These alternatives, according to the U.S. government, continue to rein as the cleanest fuel options compared to diesel — both in terms of lower tailpipe emissions and less required engine maintenance.

The biggest drawback to using gaseous fuels is they are still practically limited primarily to fleets that centrally fuel their trucks, whether at their own pumps or at the relatively small number of public fueling sites around the country.

Natural gas, be it in its CNG or LNG form, enjoys a long and successful track record as a clean, efficient and safe alternative fuel. Vehicles powered by either type are usually referred to as natural gas vehicles (NGVs).

GAS 'ER UP

According to Natural Gas Vehicles for America (NGVA), a lobbying group sponsored by several natural gas utilities as well as other firms including Honda, these are the latest key facts potential and existing NGV buyers should bear in mind:

  • About 110,000 NGVs are on U.S. roads today and more than 12 million worldwide.

  • About 1,000 NGV fueling stations are in the U.S. with about half of them open to the public.

  • In the U.S., some 30 manufacturers produce 100 models of natural-gas-fueled light-, medium- and heavy-duty vehicles and engines.

  • Natural gas costs, on average, one-third less than conventional gasoline at the pump.

  • The U.S. Energy Information Agency (EIA) reports that natural gas, on average, costs 42% less than diesel fuel on an energy-equivalent basis and is expected to cost 50% less by 2035.

  • In 2009, about 318,600 million cu. ft. of natural gas was used for vehicle fuel, also according to EIA.

  • Industry data shows that vehicular natural gas nearly doubled between 2003 and 2009, now displacing more than 300 million diesel gallon equivalents.

  • Waste collection and transfer vehicles, which account for about 11% of total vehicular natural gas use, are the fastest growing NGV segment.

  • The successful clean port transportation initiative in Southern California is spurring adoption of similar policies in other ports on both coasts.

As for the green credentials of natural gas, NGVA points out that replacing what it terms only a “typical older in-use vehicle with a new NGV” produces these reductions in exhaust emissions:

  • Carbon monoxide (CO) by 70 to 90%
  • Non-methane organic gas (NMOG) by 50 to 75%
  • Nitrogen oxides (NOx) by 75 to 95%
  • Carbon dioxide (CO2) by 20 to 30%

Jeff Clarke, NGVA's general counsel & director of regulatory affairs, says these exhaust-reduction figures “were generated using Argonne National Laboratory's AirCred Model. They made it available to help Clean Cities organizations around the country quantify the benefits of stimulus projects that were replacing older vehicles with new alternative-fueled vehicles.

“They used six years [in service] as average for light-duty vehicle replacements and twelve years [of original use] for heavy-duty replacements,” he continues.

NGVA also cautions that the “actual emissions benefits of introducing natural gas vehicles into a fleet will vary depending on the type of NGVs used and whether the emissions comparison is based on the emissions of the vehicles being replaced or new motor vehicles.”

Fleets that replace in-use medium- and heavy-duty diesel vehicles with new natural gas vehicles will see the most significant reductions in emissions, the group notes, “since medium- and heavy-duty trucks put out much more emissions than light-duty vehicles.”

The fundamentals of NGVs are fairly straightforward: They operate on the same basic principles as gasoline-powered vehicles. Fuel is mixed with air and fed into the cylinder where it is then ignited by a spark plug to move a piston up and down.

CNG AND LNG

Natural gas can power vehicles currently powered by gasoline and diesel. But because natural gas is a gas rather than a liquid, at standard pressure and temperature, some vehicle modifications are required. These changes mainly involve the fuel storage tank, fueling receptacle/nozzle and the engine.

Most NGVs run on CNG, which is stored onboard under high pressure in tube-shaped cylinders attached to the rear, top or undercarriage of the vehicle. These cylinders meet rigorous safety standards, states NGVA, and are made of high-strength materials designed to withstand impact and puncture. In the case of fire, they are fitted with pressure relief devices for controlled venting of the gas, rather than letting the pressure build up in the tank.

At CNG stations, explains NGVA, the gas is typically drawn from a local utility's line at low pressure, compressed and then stored in the vehicle's storage tanks at high pressure.

Fueling equipment for CNG vehicles can be either “fast fill” or “time fill.” In fast fill, a large compressor coupled with a high-pressure storage tank (called a cascade) fills the tank in about the same amount of time it takes to fuel a gasoline or diesel vehicle.

With time fill, there is no storage system and a much smaller compressor, and vehicles are typically refueled overnight at a rate of about a gallon an hour.

Natural gas can also be liquefied for use as a vehicle fuel. LNG requires only 30% of the space of CNG to store the same amount of energy and is typically only used with heavy-duty vehicles. To keep the natural gas cold and thus liquefied, LNG is stored on vehicles in double-walled, vacuum-insulated pressure vessels, which are essentially Thermos bottles.

NGVA points out that while LNG can be produced on-site from available natural gas, it is typically delivered to a fueling station via tanker truck. LNG is then stored on-site in special cryogenic storage tanks. To fuel vehicles, LNG is pumped into the vehicles much like other liquid fuels, although the cryogenic fueling equipment is technically more sophisticated.

There are also LCNG fueling stations. These use LNG to fuel both LNG and CNG vehicles. LNG vehicles are fueled as described above and for CNG vehicles, the LNG is compressed as a liquid and then gasified. From there, the high-pressure gas is stored on the vehicle just as it is at a fast-fill CNG station. Since it takes less energy to compress a liquid than a gas, once the LNG is available, these “dual” stations are less expensive to operate, according to NGVA.

On the NGVA website (www.ngvc.org), an extensive guide is posted that lists all the light-, medium- and heavy-duty vehicles and/or engine combinations available directly from original equipment manufacturers or via qualified system retrofitters using EPA-/CARB-certified systems from small volume OEMs.

BEYOND BARBECUE

Whether you call it LPG or propane, besides its familiar home uses, this gaseous fuel is the earliest practical alternative fuel embraced by trucking. Back in the early 1980s, propane was even offered as a factory option on some medium-duty trucks. Today, it is available only via retrofitting, but it can power everything from a pickup to a Class 8 rig.

This fuel is a hydrocarbon and is typically produced as a by-product of both natural gas processing and crude oil refining. It is nontoxic, colorless and virtually odorless and, as with natural gas, an identifying odor is added so the gas can be readily detected.

Propane and trucking have a long history together. The first bobtail truck to transport propane was built in 1928, reports the Propane Education & Research Council (PERC), and in 1965, Chevrolet introduced four new truck engines designed to run on propane.

According to the U.S. Dept. of Energy (DOE), propane or LPG is a “clean-burning fossil fuel that can be used to power internal combustion engines.” LPG-fueled vehicles produce fewer toxic and smog-forming air pollutants, states DOE, and adds that “LPG is usually less expensive than gasoline, and most LPG used in the U.S. comes from domestic sources.”

Surprisingly, given propane's obvious green credentials, no LPG-fueled passenger cars or trucks have been produced commercially in the U.S. since the 2004 model year. But, as DOE points out, gasoline and diesel vehicles can be retrofitted to run on LPG in addition to conventional fuel in “dual fuel” vehicles. The LPG is stored in high-pressure fuel tanks and the vehicles use separate fuel systems.

Propane conversions are often completed by specialized outfitters on production vehicles, including full-size pickups, vans, chassis cabs and step vans as well as on both medium- and heavy-duty vocational trucks.

DOE says propane has a high octane rating and excellent properties for spark-ignited internal combustion engines. It is non-toxic and presents no threat to soil, surface water or groundwater, the agency adds. DOE ranks LPG as the third most commonly used engine fuel, after gasoline and diesel, in the U.S.

Propane is considered an alternative fuel under the Energy Policy Act of 1992. LPG sold as vehicle fuel can be a mixture of propane with smaller amounts of other gases, notes DOE. The Gas Processors Assn.'s HD-5 specification for propane vehicle fuel says it must consist of 90% propane, no more than 5% propylene, and 5% other gases, primarily butane and butylene.

Propane is a gas at normal temperatures and pressures. It is stored onboard a vehicle in a tank pressurized to around 300 psi, or about twice the pressure of an inflated truck tire. Under this pressure, propane becomes a liquid with an energy density 270 times greater than in gaseous form, says DOE, which means a gallon of propane has about 25% less energy than a gallon of gasoline.

DOE explains that because propane is transformed into a gaseous state before it is burned in an internal combustion engine, the engine runs more efficiently in low-speed, light-throttle conditions. The agency notes that the “introduction of liquid propane injection [LPI] engines promise higher fuel efficiency.”

According to Cleanfuel USA, a converter of LPG trucks using such engines, LPI-equipped engines are “engineered to maximize the design benefits of the gasoline engine and utilize the OEM engine computer, specifically calibrated for propane. This allows for optimal fuel economy, performance and low emissions, while leaving the OEM-developed diagnostics intact.”

PROPANE POWER

Federal, state, and local incentive programs promote the adoption of propane vehicles, points out PERC. “Financial incentives help to defray the costs of purchasing, converting, and operating a propane-fueled vehicle through tax deductions, grants or rebates,” states PERC. In addition, the council notes that many local governments also offer non-financial incentives to managers of propane fleet vehicles, such as preferential parking for their vehicles or the option to use carpool lanes.

PERC contends that the low operating costs of propane vehicles, combined with alternative-fuel tax credits, make propane a cost-effective fuel for many fleets. The group also points out that with “so many on-road propane vehicles to choose from [including the Roush-converted Ford F-250 pickup and work trucks equipped with GM's 8.1-liter engine] and so many aftermarket systems on the market, switching to propane is “easier than ever before.”

To help make the switch decision easier, a PERC website (www.autogasusa.org) offers access to a “fleet calculator” that it says contains the latest information about alternative-fuel tax credits and enables fleets to determine how much they would save yearly by using propane-fueled vehicles instead of those running on gasoline or diesel.

If perhaps out of necessity more than anything else, CNG, LNG and LPG nonetheless became key early drivers in public, private and regulatory efforts to green America's truck fleets — both to cut harmful exhaust emissions and to reduce fuel costs.

That these alternative fuels have been around for so long only attests to their viability and reliability, not to how many fleets may benefit by switching their trucks to run on them.