1-13. ENGINE GOVERNOR AND THROTTLE CONTROL. The fuel metering in the injectors is adjusted by the engine
governor and throttle control via a mechanical linkage (fuel rack). The governor controls the engine idle and maximum
speed. When the engine is shut off, the fuel injectors are automatically positioned to the advanced fuel position for
starting. Once started, the governor immediately moves the fuel rack to the idle position. To stop the engine, the
governor stop control lever is actuated. This lever pulls the fuel rack back and shuts off the fuel supply. The stop control
lever is engaged by the electrically-operated fuel shut-off solenoid which is controlled via the ignition switch. The
solenoid is always energized when the engine is running.
a. Throttle Control - Driving Mode To control the engine speed, an air-operated throttle control air chamber is
provided. This air chamber is attached to the governor housing and linked via the governor speed control shaft to the
injector fuel rack, (see fig. 1-21). Air pressure is provided to the throttle air chamber via the treadle valve/accelerator
pedal. By depressing the accelerator pedal, the operator controls the air flow and thus the engine speed. An internal
return spring in the control air chamber will retract the linkage and reduce the engine speed to idle when the accelerator
pedal is released. An external return spring provides a similar function. It is provided to ensure positive retraction of the
linkage should the internal spring fall.
b. Throttle Control - Firefighting Mode.
(1) Variable, stabilized engine speed is required to provide infinite control of the water pump pressure. To
achieve this, a pressure-regulated throttle control governor is mechanically linked to the engine speed governor. The
throttle control governor consists of two pressure chambers mechanically connected to the governor speed control shaft.
The pressure provided in the air chamber will increase the engine speed. This will result in increased water pressure
which, via the water pressure chamber, will tend to retard the speed. Stable speed is obtained when the force from the
water pressure equalizes the force from the air pressure.
The area of the air chamber is three times larger than the area of the water chamber. This feature
enables control of water pressures that are three times higher than the controlling air pressures.
(2) When the firefighting equipment is engaged in CFR mode, air pressure, preset at 80 psi (475 kPa), is
provided to the air pressure chamber. The counterbalance water pressure, therefore, is 240 psi (1630 kPa). This is
obtained at an engine speed of about 1200 rpm.
(3) In the structural firefighting mode, engine speed and thus water pressure is manually controlled. Throttle
control air pressure is provided to the air pressure chamber from the hand throttle (air regulator) on the structural control