and maintained at the inlet to the injectors. The "T"
refers to the fact that the actual "time" available for the
fuel to flow into the injector assembly (cup) is
determined by the engine speed as a function of the
engine camshaft and injection train components.
The air-fuel control (AFC) is an acceleration
exhaust smoke control device built internally into the
pump body. The AFC unit is designed to restrict fuel
flow in direct proportion to the air intake manifold
pressure of the engine during acceleration, under load,
and during lug-down conditions.
Within the pump assembly a fuel pump bypass
button of varying sizes can be installed to control the
maximum fuel delivery pressure of the gear-type pump
before it opens and bypasses fuel back to the inlet side of
the pump. In this way the horsepower setting of the
engine can be altered fairly easily. The major functions
of the PTG-AFC fuel pump assembly are as follows:
1. To pull and transfer fuel from the tank and filter
2. To develop sufficient fuel pressure to the fuel
rail (common fuel passage) to all of the injectors
3. To provide engine idle speed control
4. To limit the maximum no-load and full-load
speed of the engine (governing)
5. To allow the operator to control the throttle
position and therefore the power output of the
6. To control exhaust smoke emissions to EPA
specifications under all operating conditions
7. To allow shutdown of the engine when desired
A major feature of the PT pump system is that there
is no need to time the pump to the engine. The pump is
designed simply to generate and supply a given flow
rate at a specified pressure setting to the rail to all
injectors. The injectors themselves are timed to ensure
that the start of injection will occur at the right time for
The basic flow of fuel into and through the PT pump
assembly will vary slightly depending on the actual
model. A simplified fuel flow is as follows:
As the operator cranks the engine, fuel is drawn
from the fuel tank by the gear pump through the
fuel supply line to the primary filter. This filter is
normally a filter/water separator.
The filter fuel then flows through a small filter
screen that is located within the PT pump
assembly, and then flows down into the internal
The position of the governor plunger determines
the fuel flow through various governor plunger
The position of the mechanically operated
throttle determines the amount of fuel that can
flow through the throttle shaft.
Fuel from the throttle shaft is then directed to the
AFC needle valve.
The position of the AFC control plunger within
the AFC barrel determines how much throttle
fuel can flow into and through the AFC unit and
on to the engine fuel rail, which feeds the fuel
The AFC plunger position is determined by the
amount of turbocharger boost pressure in the intake
manifold, which is piped through the air passage from
the intake manifold to the AFC unit. At engine start-up,
the boost pressure is very low; therefore, flow is limited.
Fuel under pressure flows through the electric solenoid
valve, which is energized by power from the ignition
switch. This fuel then flows through the fuel rail
pressure line and into the injectors.
A percentage of the fuel from both the PT pump and
the injectors is routed back to the fuel tank in order to
carry away some of the heat that was picked up cooling
and lubricating the internal components of the pump
and the injectors.
INJECTORS.A PT injector is provided at each
engine cylinder to spray the fuel into the combustion
chambers. PT injectors are of the unit type and are
operated mechanically by a plunger return spring and a
rocker arm mechanism operating off the camshaft.
There are four phases of injector operation, which are as