Tighter several, state, and local emissions regulations have led to a search for an alternative fuel. Many things are considered when determining the viability of an alternative fuel, including emissions, cost, fuel availability, fuel consumption, safety, engine life, fueling facilities, weight and space requirements of fuel tanks, and the range of a fully fueled vehicle. Currently, the major competing alternative fuels include ethanol, methanol, propane, and natural gas.

Ethanol and methanol were presented earlier under other gasoline additives. Propane is a petroleum-based pressurized fuel used as a liquid. It is a constituent of natural gas. Natural gas comes in two forms: compressed natural gas (CNG) and liquefied natural gas (LNG).

LP-Gas

LP-gas (LP stands for liquefied petroleum) is a byproduct of crude oil refining, and it is also found in natural gas wells. Fuel grade LP-gas is almost pure propane with a little butane and propylene usually present. Because of its high propane content, many people simply refer to LP-gas as propane.

Propane burns clean in the engine and can be precisely controlled. Because it vaporizes at atmospheric temperatures and pressures, it does not puddle in the intake manifold. This means it emits less hydrocarbons and carbon monoxide. Emissions controls on the engine can be simpler. Cold starting is easy, down to much below zero. At normal cold temperatures, the propane engine fires easily and produces power without surge or stumble.

One of the most noticeable differences between propane and gasoline is that propane is a dry fuel. It enters the engine as vapor. Gasoline, on the other hand, enters the engine as tiny droplets of liquid. whether it flows through a carburetor or is sprayed in through a fuel injector.

The propane fuel system is a completely closed system that contains a supply of pressurized LP-gas. Since the fuel is already under pressure, no fuel pump is needed. From the pressurized fuel tanks, the fuel flows to a vacuum filter fuel lock. This serves as a filter and a control allowing fuel to flow to the engine. Fuel flows to a converter or heat exchanger where it changes from a liquid to a gas. When the propane flows through the converter, it expands as it changes into a gas. The carburetor mixes gaseous propane with the gaseous air. Airflow into the engine is controlled by a butterfly valve in the venturi. Mixture is controlled by a fuel metering valve operated by a diaphragm, which is controlled by intake manifold pressure. The idle system is an air bleed, similar to a gasoline engine. In fact, except for the fact that the propane carburetor does not require a fuel bowl, the two carburetor types are basically the same.

Natural Gas

Vehicles have been designed with gasoline/CNG, diesel/CNG, and dedicated (single-fuel) CNG engine applications. Compressed natural gas (CNG) vehicles offer several advantages over gasoline.

- The fuel costs less.

- It is the cleanest alternative fuel, generating up to 99 percent less carbon monoxide than gasoline, no particulates, almost no sulfur dioxide, and 85 percent less reactive hydrocarbons than gasoline.

- Natural gas vehicles are safer. The fuel tanks used for CNG are aluminum or steel cylinders with walls that are 1/2 to 3/4 inch thick. They can withstand severe crash tests, direct gunfire, dynamite explosions, and burning beyond any standard sheet metal gasoline tank. Because it is lighter than air, natural gas dissipates quickly. It also has a higher ignition temperature.

- It generally reduces vehicle maintenance since it burns cleanly. Oil changes may not be needed before 12,000 miles and spark plugs could last as long as 75,000 miles.

- Natural gas in abundant and readily available in the United States.

The chief disadvantage of CNG at present is its nonavailability to most users. Fuel facilities are needed in greater numbers than are currently in existence due to the relatively shorter range of CNG  vehicles. The space taken by the CNG cylinders and their weight, about 300 pounds, also would be considered disadvantages in most applications.