Concerns over rising pollution, especially in congested urban environments, and energy security have prompted the federal government to try and wean industry and the public from its reliance on fossil fuels. Two complex and overlapping pieces of legislation, the Clean Air Act Amendments of 1990 and the Energy Policy Act of 1992, set the stage for doing just that -- mandating the shift to alternative fuels such as alcohol, electricity, natural gas, and propane. (For a comparison of the two laws, see chart on page 54).

According to the Environmental Protection Agency (EPA), gasoline and diesel fuel power 99% of the country's fleet population. That places the nation's motor carriers, particularly those operating in and around the nation's most polluted cities, squarely in the sights of federal policy makers.

That's because gasoline and diesel fuels, when burned, emit complex mixtures of compounds that lead to the formation of smog and other potentially toxic pollutants. Although much has been done to decrease vehicular emissions, past efforts "took petroleum fuels as a given and focused on the development of sophisticated engine and vehicle emissions control systems involving catalytic converters, on-board computers, and other hardware," EPA says.

With ever-increasing traffic volumes, so-called clean fuel technology has emerged as a key strategy in meeting federal clean air goals. Generally, these fuels emit up to 90% fewer hydrocarbons, and the hydrocarbons they do emit are less reactive (slower to form ozone) and less toxic.

Alternative fuels could also help slow the buildup of carbon dioxide, a "greenhouse gas" that contributes to global warming. Combustion of any carbon-based fuel produces carbon dioxide. Fuels produced from biomass (crops, trees, etc.) and from natural gas result in less carbon dioxide buildup than fuels made from petroleum or coal.

Clean fuels have benefits that reach beyond their air quality advantages; these fuels could decrease the country's dependence on imported oil, proponents say.

Strictly defined, alternative fuels are those that can be derived from non-crude oil resources. That means all vehicular fuels other than gasoline and diesel, although reformulated gasoline and clean diesel fuels are sometimes considered to be alternative fuels.

At present, these fuels are used primarily by fleets in mass transit applications and federal fleets. Use in heavy-duty applications is sporadic, mainly because of the lack of availability of cleanfuel engines and fueling sites. In fact, that was the reason given for EPA's one-year delay in implementing the Clean Fuel Fleet Program. Beginning with the 1999 model year, 30% of light-duty vehicles (up to 8,500 lb. GVWR) and 50% of heavy-duty vehicles (8,501-26,000 lb. GVWR) must be low-emissions vehicles in certain areas.

Let's look at Pierce Transit, which began converting its bus fleet to CNG in 1986. Today, the Tacoma, Wash.-based transit authority has 58 of its 193 buses powered by CNG. Capital costs run $30,000 to $50,000 more than for diesel counterparts, most of which is attributed to the higher cost of CNG engines and natural gas storage cylinders, according to Ron Shipley, director of maintenance. And in 1992, Pierce had to build a CNG fueling facility at a cost of $847,000. Upgrades of its maintenance facilities to provide enhanced ventilation systems and natural gas detectors increased outlays by more than $500,000.

Maintenance costs for running the agency's diesel and CNG fleets are nearly identical, Shipley says. "There are still problems with the ignition system, specifically spark plugs and wires," he says, "but electronics are making the engines more reliable, which translates into lower maintenance costs." The number of road calls per 1,000 miles is the same for the fleet's diesel and CNG buses.

Shipley reports that CNG engines are about 20% less fuel-efficient than their diesel counterparts. He attributes this to the lower compression ratios and throttling losses of the CNG engines, plus the extra weight inherent in the fuel tanks.

Turning to truck fleets, Ag Processing (AGP), an Omaha, Neb.-based cooperative, has been running on alternative fuels since 1995. Six of the firm's over-the-road heavy-duty trucks are using a blend of soybean methyl esters (biodiesel) and petroleum diesel.

Biodiesel smells more like vegetable oil than traditional diesel fumes, and runs more quietly because of the lubricating effect of the vegetable oil on the fuel pump and injectors.

The downside lies in the areas of storage and performance. Since biodiesel has a higher gelling temperature than regular diesel, precautions must be taken in storage. AGP research indicates that cold filter plugging takes place in soybean oil at 30 deg F; in No. 2 diesel at 0 deg F; and in No. 1 diesel at -40 deg F. Solids form even when biodiesel is blended with other fuel. To return to solution, the temperature has to be raised above 100 deg F.

To prevent gelling, AGP installed submersible heaters and a circulation pump into aboveground storage tanks. In electronic engines, the return fuel flow from the injectors is enough to keep the biodiesel in solution.

In deciding which fuel to choose, fleets must balance a number of issues, including vehicle range, refueling options, cost of conversion or acquisition, and performance.

A number of fuels meet the definition of alternative fuels. Here's a brief look at their performance capabilities:

Electric. Battery-powered vehicles give off virtually no pollution and offer one of the best options for reducing motor vehicle emissions in polluted cities. Power is generated from stationary sources (or on-board fuel cells) using a variety of common feedstocks (coal, oil, gas, nuclear, hydro), and is stored in batteries.

While greater advances have occurred in generating stationary power, technology on the vehicle side is advancing rapidly. Within the last year, researchers report a tenfold increase in power production per unit of weight and unit of volume, opening the door to wider transportation options.

Despite this progress, the performance and range of today's electric vehicles is limited by the amount of power the battery can provide. Plus, concerns remain about durability, manufacturing, materials, and infrastructure. In addition, the cost remains prohibitively high. However, with the recent 90% reduction in the catalysts' need for fuel cells, costs keep falling. Another downside is the fact that batteries take hours to recharge.

Biodiesel. Biodiesel can be manufactured from a variety of vegetable oils. In Europe, the most common type of biodiesel is rapeseed methyl ester (RME), which is manufactured from rapeseed oil by a simple trans-esterification process. Methanol is added to the oil, produced by crushing rapeseed, and heated in the presence of a catalyst to produce RME and the by-product glycerin.

Ethanol. A liquid produced from agricultural products, ethanol, or ethyl alcohol, is blended in small amounts (10%) with gasoline to produce gasohol. As an alternative fuel, ethanol is mixed with a 15% blend of gasoline to produce a fuel called E85. This product can be used in flexible fuel vehicles, which also operate on straight gasoline. Heavy-duty vehicles use almost pure methanol fuel, or M95.

>From a performance perspective, power and acceleration are comparable to gasoline. As a cleaner burning fuel, M85 forms less carbon deposits in the engine, resulting in reduced wear and maintenance. Plus, its 102 octane rating helps prevent engine knock damage. Its range is 70% that of gasoline

While ethanol requires a relatively inexpensive conversion, the fuel costs more than gasoline and requires the use of more expensive lubricants. Public fueling stations are limited, and the price tag for building your own runs $35,000-$105,000.

Pure ethanol fuel offers low hydrocarbon and toxic emissions, and can be produced domestically from corn or other crops, as well as from cellulose materials such as wood and paper wastes, potentially minimizing the accumulation of greenhouse gases.

Methanol. Like ethanol, methanol (or wood alcohol) is a high-performance liquid fuel that emits low levels of toxic and ozone-forming compounds. It is blended with a small amount of gasoline (15%) to produce M85. The gasoline blend helps boost cold weather starting. M85 can be used in flexible-fuel vehicles that also operate on straight gasoline. Heavy-duty vehicles use pure methanol fuel, or M100.

Methanol can be produced at prices comparable to gasoline from natural gas and can also be produced from coal and wood. All major auto manufacturers have produced cars that run on M85 and some have even developed advanced M100 prototypes.

While methanol requires a lower equipment conversion cost, it costs more than gasoline and, like ethanol, requires the use of more expensive lubricants.

Although it offers a range just 60% that of gasoline, its power and acceleration are comparable to gasoline. As a cleaner burning fuel, M85 forms less carbon deposits in the engine, resulting in a reduction of wear and maintenance. Plus, its 102-octane rating helps prevent engine knock damage.

Another drawback is limited public fueling facilities. The cost for private installation runs $35,000-$105,000.

Natural gas. Although natural gas is abundant and costs less than gasoline, compressed natural gas (CNG) must be stored under pressure in heavy tanks, and the cost of accommodating these tanks must be considered. Despite its limiting effect on vehicle power, efficiency, and range, natural gas appears to have a bright future as a motor vehicle fuel because of its low emissions of ozone-depleting hydrocarbons.

Natural gas is primarily methane, along with small amounts of other hydrocarbon gases. The gas is compressed to 3,000-3,600 psi, or liquefied at very low temperatures when used in vehicles.

Compressed natural gas (CNG) results in about a 10% power loss, which is partially offset by the use of higher compression ratios to boost efficiency and power. Because it burns cleaner, it will reduce carbon deposits inside the engine. Oil-change intervals and engine life can be extended. Since the gaseous fuel doesn't need to be vaporized before entering the engine, CNG boosts cold weather starting. It offers a range that is about 30% that of gasoline.

Refueling can be accomplished through a small but growing public distribution network. While quick filling is possible, most private fueling facilities are designed for overnight fueling. Private facilities cost $25,000-$250,000 to build.

Propane. Liquefied petroleum gas (LPG), or propane, which contains butane and other hydrocarbons, is a gas that is stored and transported as a liquid under moderate pressure (125-200 psi). Vehicles currently account for less than 4% of propane demand.

Perhaps the most economical of alternative fuels, many light-duty fleets using propane report lower operating and maintenance costs. Conversion from gasoline averages about $2,000 per vehicle.

In terms of power and acceleration, propane offers performance similar to gasoline. Its higher octane number helps prevent engine knock damage and allows use of higher compression ratios to boost efficiency and power. Because it burns cleaner, propane will reduce carbon deposits inside the engine. In addition, it boosts cold weather performance since it enters the engine in a vaporized state.

A relatively dense fuel, propane offers higher fuel storage capability than CNG, making it easy to transport. But its cryogenic nature requires storage in unique vessels with limited liquid holding time.

Propane is comparable to gasoline in terms of refueling time and procedure, using a special nozzle to the vehicle's tank at a rate of 10-12 gal./min. It also boasts the longest range of any clean fuel for an equivalent volume. Propane is readily available via a network of refueling stations throughout the country.

Another benefit of propane is that it offers 80% of the range of gasoline, allowing vehicles to get by with smaller tanks than other alternative fuels. However, it must be stored in a sealed, pressure-tight system at all times.

Converting an engine to propane use requires a converter that serves as a vaporizer and pressure regulator; an air gas mixer, which mixes the propane with air; a control processor, a computer that adjusts fuel delivery; and an automatic safety lock-off valve that shuts off fuel flow when the engine is not running. Conversion costs run about $2,500 per vehicle.

Reformulated gas. The petroleum industry is beginning to market gasoline formulations that emit fewer hydrocarbons, nitrogen oxides, carbon monoxide, and toxics than conventional gasoline. These new gasolines can be introduced without major modification to existing vehicles or the fuel distribution system. The Clean Air Act requires some gasoline modifications to reduce carbon monoxide emissions beginning in 1992, and use of reformulated gasoline in certain polluted cities beginning in 1995.

Clean diesel. Clean, or city, diesel is the name generally given to a special grade of high-quality diesel fuel that exhibits significantly better environmental characteristics than the standard grade. Such fuels are characterized by very low levels of sulfur (typically 0.001-0.005% by weight), low density, low aromatics and PAH content, a narrow boiling range, and a high cetane number to minimize emissions.

The mandate for alternative fuels stems from two separate laws -- with different goals and enforcement mechanisms -- controlling the alternative fuels landscape. One is the Clean Air Act, which is aimed at reducing emissions of hazardous pollutants, many of which are linked to highway emissions. And the other is the Energy Policy Act, which attempts to lessen the country's dependence on foreign oil. (For more information, see table on page 54 of Fleet Owner's June 1998 issue.)