Fuel Density Compensator
engine operates on a variety of fuels that have a broad range of viscosities and heat
values. These variations in the fuels affect engine output. Because it is unacceptable for
the power output of the engine to vary with fuel changes, the multifuel engine is fitted
with a device known as a fuel density compensator (fig. 5-38). The fuel density compensator is a device that
serves to vary the quantity of fuel injected to the engine by regulating the full-load
stop of the fuel pump. The characteristics of the fuels show that their heat values
decrease almost inversely proportional to their viscosities. The fuel density compensator
uses viscosity as the indicator for regulating fuel flow. Its operation is as follows:
fuel enters the compensator through the fuel pressure regulator where the fuel pressure is
regulated to a constant 20 psi regardless of engine speed and load range.
pressure-regulated fuel then passes through a series of two orifices. The two orifices, by
offering greatly different resistances to flow, form a system that is sensitive to
viscosity changes. The first orifice is annular, formed by the clearance between the servo
piston and its cylinder. This orifice is sensitive to viscosity. The second orifice is
formed by an adjustable needle valve and is not viscosity sensitive.
higher the viscosity of the fuel, the more trouble that it will have passing through the
first orifice. Because of this, the fuel pressure under the servo piston will rise
proportionally with viscosity. Because the second orifice is not viscosity sensitive, the
pressure over the servo piston will remain constant. This will cause a pressure
differential that increases proportional with viscosity, in turn, causing the piston to
seek a position in its bore that becomes higher as viscosity increases.
upward movement of the servo piston will move a wedge-shaped moveable plate, which will
decrease fuel delivery. A lower viscosity fuel will cause the piston to move downward,
causing the pump to increase fuel delivery.
the fuel passes through the two orifices, it leaves the compensator through an outlet
port. From here the fuel passes back to the pump.