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ANTIKNOCK PROPERTIES
Natural gas has a research octane rating of about 130, making it relatively resistant to engine knock. The
antiknock property is a result of the high ignition temperature, resistance to autoignition, and the relatively
low flame speed of natural gas. Antiknock properties allow the use of engine compression ratios in the range
of 15:1 (compared to 8:1 to 10:1 for gasoline). The low flame speed of natural gas results in a longer duration
of combustion. To compensate for the lower flame speed, ignition timing is advanced. As with other fuels,
knock may occur with advanced ignition timing, prolonged combustion (i.e. too lean mixture), and exces-
sively high compression ratios.
ENERGY CONVERSIONS
•
100—125 cubic feet NG = 1.0 gallon gasoline
•
136 cubic feet NG = 1.0 gallon diesel
•
1 cubic foot NG = 1000 Btu
•
114,000 Btu = 1.0 gallon gasoline
•
83,700 Btu = 1.0 gallon LNG
VEHICLE PERFORMANCE AND EMISSIONS
NGV performance, fuel economy, and emissions can be significantly altered with vehicle tuning (e.g.,
ignition timing, air/fuel ratio). Appropriate tuning adjustments can optimize either performance, fuel economy,
or emissions. Alternatively, a compromise tuning may be effected. Tuning optimization for power generally
increases emissions. Substantial improvements in performance and emissions can be obtained using natural
gas conversion kits that are specially designed for a given vehicle make and model. Further improvements
can be achieved with factory built, dedicated NGVs.
STARTING PROCEDURE
Make sure that the main shutoff valve and at least one tank valve are open. The fuel gauge shows empty
if no fuel is available to the mixer. Check the tank and main fuel shutoff valves if no fuel is indicated on the
fuel gauge.
NOTE:
Fuel gauge takes time to register after turning the key.
Starting procedures are identical to those of a gasoline engine vehicle. With the vehicle in neutral, turn
the key until the engine catches. Do not press the accelerator. If the engine has trouble starting depress the
accelerator slightly and release when the engine catches. Pumping the accelerator in no way assists in start-
ing the engine.
Engine operation and characteristics should be similar to those of a gasoline engine.
COLD START
For cold starts in low ambient temperatures, natural gas has an advantage over liquid fuels because it is
already in the vapor phase. With correct conversion kit installation and vehicle maintenance, cold start abil-
ity is better with natural gas than gasoline.
OPERATION & MAINTENANCE of VEHICLE COMPONENTS
CNG vehicles require Department of Transportation (DOT) certified cylinders for the storage of pressur-
ized (maximum 3600 psi) natural gas. Refueling port and lines with pressure safety valves must also be
installed. High pressure fuel lines from the storage cylinder lead to a pressure regulator/reducer, which
reduces gas pressure in one or two steps. In a fuel injected converted vehicle, a mixer/carburetor must be
added for the injection of natural gas. In carbureted fuel systems, a specialized mixer/carburetor for natural
gas may be installed. LNG vehicles require insulated, pressurized (10 to 35 psi) fuel tanks. LNG is vaporized
in the fuel line and warmed in the heat exchanger generally located under the hood. Pressure is reduced by
the pressure regulator before the vapors are transported to the mixer/carburetor.
