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GAS SYSTEMS l
Natural gas supplied from main transmission line is normally received at pressures not greater
than 51) pounds per square inch (psi) while with LPG, which is Butane or Propane or usually a mixture
of the two gases,
the pressure may go as high as 180-200 psi depending on the mix ratio and tempera-
ture. To be used by the engine the pressure must be reduced to.ounce values.
The reduction is nor- s
mally accomplished in two stages by a primary regulator which reduc line pressures to about 6 ounces
per square inch and a secondary low pressure regulator which further reduces the pressure in accord-
ance with engine demand. The primary regulator is often furnished by the fuel supplier. The fuel
supplier should also insure that gas pressure is sufficient to operate the primary regulator.
If pressure
drops too low, the regulator may not function at all. - While propane, for example, tends to maintain
some vaporizing pressure even at temperatures down to -20’ F. , butane returns to liquid state and
offers little or no vapor pressure when the temperature drops to freezing or 32O F. For this ‘reason,
fuel suppliers usually supply LPG of higher butane content in hot weather but will alter this to a higher
percentage of propane during cold weather. If this ratio is not changed,
starting and operating difficul-
ties may be encountered in cold weather.
Straight gas and combination gas/gasoline systems are used.
The combination system uses a gas
mixer or adapter attached to a standard gasoline carburetor.
and regulators are discussed in the following.
Some common carburetors, adapters,
GAS CARBURETORS - ADAPTERS
In a venturi type carburetor,
the gas nozzle is located at the point of greatest pressure drop inside
the venturi.
This creates a suction within that nozzle that varies with changing rate of air flow and
causes a greater volume of gas to be metered at heavier loads and a lower volume at lighter engine load.
The throttle valve regulates the rate of air flow through the carburetor.
this valve is wide open and offers ‘little restriction to air flow.
When an engine is at top speed,
Closing the throttle causes a decrease
in air flow and lowers engine speed and power.
As the valve closes, it reduces pressure through the
venturi and past the fuel nozzle.
On engines with the gasoline carburetor,
an adapter with venturi is installed between the carbure-
tor and air cleaner.
The adapter consists of a gas jet and venturi.
Accurdte metering of the gas is
accomplished by load and starting or idle adjustment screws in the body of the adapter.
Most gas carburetors are simple devices having only one moving part--the throttle shaft and
plate (or butterfly valve as it is sometimes called).
Because of this, they should require very little
attention and readjustment should seldom be needed.
If needles wear out or venturi is damaged, these
I
IDLE FUEL
BREATHER.
TUBE TO
VALVE COVER
IDLE
SPEED
ADJUSTMENT
JdAIN FUEL
.
SECONDARY
GAS REGULATOR
(MODEL UN)
-=--ii& OIL BATH
I
TYPE AIR
CLEANER
(3/4” NPT
FROM PRIMARY REGULATOR)
.
FIGURE 4-8 --
TYPICAL STRAIGHT GAS FUEL SYSTEM
46
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