807929A 1 Mercruiser CARRIER ASSEMBLY

Illustration 1 g02166133
Fuel system schematic
(1) Fuel gallery
(2) Mechanical electronic unit injectors
(3) Fuel pressure regulators
(4) Fuel return line
(5) Fuel cooler (if equipped)
(6) Tank breather filter
(7) Fuel tank
(8) Fuel supply line
(9) Primary fuel filter
(10) Electric fuel priming pump
(11) Fuel transfer pump
(12) Check valve
(13) Electronic control module (ECM)
(14) Fuel temperature sensor
(15) Pressure relief valve
(16) Secondary fuel filter
(17) Differential pressure switch (If equipped)
(18) Tertiary fuel filter
(19) Fuel pressure sensor (If equipped)
(20) Secondary fuel filter baseThe fuel supply circuit is a conventional design for unit injector diesel engines. The system consists of the following major components that are used to deliver low-pressure fuel to the unit injectors:Fuel tank - The fuel tank is used to store the fuel.Fuel priming pump - The fuel priming pump is used to evacuate the air from the fuel system, as the air is removed the system fills with fuel.Fuel filters - The primary fuel filter is used to remove abrasive material and contamination from the fuel system that may be large enough to damage the fuel transfer pump. The secondary fuel filter is used to remove abrasive material and contamination as small as 4 microns that could damage the injectors. The tertiary fuel filter is located on the secondary fuel filter base. The tertiary fuel filter is used to remove abrasive materials and contamination as small as 4 microns. A differential pressure switch is utilized on the secondary fuel filter base to indicate the life of the fuel filter.Supply lines and return lines - Supply lines and return lines are used to deliver the fuel to the different components.The purpose of the low-pressure fuel supply circuit is to supply fuel that has been filtered to the fuel injectors at a rate that is constant and a pressure that is constant. The fuel system is also utilized to cool components such as the fuel injectors.Once the injectors receive the low-pressure fuel, the fuel is pressurized again before the fuel is injected into the cylinder. The unit injector uses mechanical energy that is provided by the camshaft to achieve pressures that can be more than 200000 kPa (30000 psi).Control of the fuel delivery is managed by the engines Electronic Control Module (ECM). Data from several of the engine systems is collected by the ECM and processed to manage these aspects of fuel injection control:
Injection timing
Fuel injection timing advance
Injection duration
Engine cold mode statusThe mechanical electronic fuel system relies on a large amount of data from the other engine systems. The data that is collected by the ECM will be used to provide optimum performance of the engine.Low-Pressure Fuel Supply Circuit
The flow of fuel through the system begins at fuel tank (7). Fuel is drawn from the tank by fuel transfer pump (11). The fuel transfer pump incorporates a check valve (12) that will allow fuel to flow around the gears of the pump during priming of the fuel system. The fuel transfer pump also incorporates a pressure relief valve (15). The pressure relief valve is used to protect the fuel system from extreme pressure.The fuel transfer pump is engineered to produce an excess fuel flow throughout the fuel system. The excess fuel flow is used by the system to cool the fuel system components. The excess fuel flow also purges any air from the fuel system during operation. Air that can become trapped in the fuel system can cause cavitation that may damage the components of the unit injector.The fuel priming pump (10) is located on the fuel filter base. The fuel filter base and primary fuel filter (9) also incorporates a siphon break that prevents fuel from draining from the fuel system when the engine is not in operation. The priming pump is an electric pump that directs the flow of fuel during the priming pump operation. The filtered fuel then flows out of the fuel filter and returns to the passages in the fuel filter base. Prior to exiting the fuel filter base, the fuel pressure and the fuel temperature are sampled by the fuel pressure sensor and by the fuel temperature sensor. The signals that are generated by the sensors are used by the engine control to monitor the condition of the engines components. The fuel flows through a secondary fuel filter (16) which is a 2 micron fuel filter. The tertiary filter (18) is a 4 micron filter that is used to filter out fine particles from the fuel supply. The fuel is filtered to remove small abrasive particles that will cause premature wear to fuel system components. This information is also used to adjust the fuel delivery of the engine to optimize efficiency.The fuel is transferred by fuel supply lines (8) to the fuel gallery (1) in the cylinder head. Only a portion of the fuel that is supplied to the fuel injectors is used for engine operation. This unused fuel is discharged into the return passages of the fuel gallery. The fuel is returned to the fuel tank by the fuel return lines (4). A continuous flow of fuel is experienced within the low-pressure fuel system.During engine operation, fuel injectors (2) receive fuel from the low-pressure fuel system. The injector pressurizes the fuel to high pressure. The fuel is then injected into the cylinder. The excess fuel is returned to the tank. Refer to Systems Operation, "Unit Injector" for a complete explanation of the injection process.A pressure regulating valve (3) is located in the fuel return. The pressure regulating valve allows the low-pressure fuel system to maintain a constant pressure. A flow control orifice is also located in the fuel return. The flow control orifice maintains a system back pressure that is constant. The orifice allows the flow of fuel through the system to be constant. This prevents excessive heating of the fuel.Fuel Heaters
Fuel heaters prevent the waxing of the fuel, and the plugging of the fuel filters in cold weather. The engine does not dissipate enough heat to prevent waxing during cold-weather conditions. There are two types of fuel heaters that can be used: