Theory of Operation - PT Fuel System
The following is a short description of the theory of op-
eration of the Cummins PT Fuel System.
The PT, or Pressure Time, fuel system derives its name
from the two main variables that affect the amount of fuel
metered per cycle in the Cummins fuel system. ‘‘P’’
stands for pressure at the inlet of the injectors. It is con-
trolled by the fuel pump. ‘‘T’’ refers to the time that is
available for fuel to flow into the injectors. It is controlled
by engine speed through the camshaft and injection train.
In the PT system, the amount of fuel burned by the engine
is the amount that is metered into the injector cup and
injected. The quantity metered is dependent on the fuel
pressure at the injector, the flow area of the injector and
the time the fuel is allowed to flow into the metering
chamber of the cup.
The fuel pressure at the injector is controlled by the PT
fuel pump. The pressure supplied by the pump varies
according to different operating conditions.
P=Fuel Pressure at the
Injector Inlet
T=Time Fuel Is Allowed
to Flow
Fuel System General Information Section 5 - Fuel System
N14Page 5-4
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