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 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
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
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
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.
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
- 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
- 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.