Automotive Systems

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Fuel Density Compensator

Fuel Density Compensator

The multifuel 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:

  • The 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.
  • The 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.
  • The 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.
  • The 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.

After the fuel passes through the two orifices, it leaves the compensator through an outlet port. From here the fuel passes back to the pump.

Figure 5-38.—Fuel density compensator.
Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

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