on a pressure-time principle. A gear-driven positive
displacement low-pressure fuel pump naturally
supplies fuel pressure. The time for metering is
determined by the interval that the metering orifice in
the injector remains open. This interval is established
and controlled by the engine speed, which determines
the rate of camshaft rotation and consequently the
injector plunger movement.
Since Cummins engines are all four-cycle, the
camshaft is driven from the crankshaft gear at one-half
of engine speed. The fuel pump turns at engine speed.
Because of this relationship, additional governing of
fuel flow is necessary in the fuel pump.
A flyball type mechanical governor controls fuel
pressure and engine torque throughout the entire
operating range. It also controls the idling speed of the
engine and prevents engine overspeeding in the high-
speed range. The throttle shaft is simply a shaft with a
hole; therefore, the alignment of this hole with the fuel
passages determines pressure at the injectors.
A single low-pressure fuel line from the fuel pump
serves all injectors; therefore, the pressure and the
amount of metered fuel to each cylinder are equal.
The fuel-metering process in the IT fuel system has
three main advantages:
The injector accomplishes all metering and
The injector injects a finely atomized fuel spray
into the combustion chamber at spray-in-
pressures exceeding 20,000 psi.
A low-pressure common-rail system is used,
with the pressure being developed in a gear-type
pump. This eliminates the need for high-
pressure fuel lines running from the fuel pump
to each injector.
FUEL PUMP.The fuel pump (fig. 5-29)
commonly used in the pressure-time system is the PTG-
AFC pump (PT pump with a governor and an air-fuel
control attachment). The "P" in the name refers to the
actual fuel pressure that is produced by the gear pump
Figure 5-29.Pressure-time (PT) gear pump.