834954 Mercury COVER

Low-Pressure Fuel System
Illustration 1 g06105490
(1) Injector Cooling Circuit Back-Pressure Regulator
Illustration 2 g06306363
(2) Filter Outlet Pressure Sensor
(3) Filter Inlet Pressure Sensor
(4) Starting Priority Valve
(5) System Regulator
(6) Fuel Rail Pressure Sensor
(7) Fuel Rail Temperature Sensor
(8) Mechanical Fuel Transfer Pump
(9) Fuel Transfer Pump Relief Valve
(10) Fuel Transfer Pump Inlet Pressure Sensor
(11) High-Pressure Fuel Pump (HPFP) Inlet Temperature Sensor
(12) Secondary Fuel FiltersThe basic function of the low-pressure fuel system is to provide fuel to the high-pressure fuel pump inlet at the:
Proper Pressure
Proper Temperature
Proper Cleanliness LevelThe fuel transfer pump (8) increases the fuel pressure for the transport through the secondary filters (12). The fuel transfer pump is equipped with a 950 kPa (138 psi) relief valve to prevent overpressure.Pressure sensors are at the inlet (3) and outlet (2) of the secondary filters to monitor filter condition. A temperature sensor (11) measures the temperature of the fuel exiting the secondary filter. The fuel then reaches the high-pressure pump inlet where a system pressure regulator (5) maintains the pressure at 650 kPa (95 psi) to 950 kPa (138 psi).During an engine start, a special starting priority valve (4) prevents flow to the injector cooling flow portion of the low-pressure fuel system until 250 kPa (36 psi) is reached. This action ensures adequate flow to the high-pressure pump at low engine cranking speeds. A certain amount of fuel flow bypasses the high-pressure pump and circulates through the injector cooling circuit. The injector cooling circuit back-pressure regulator (1) maintains the pressure in the injector cooling circuit at 350 kPa (51 psi). The unused fuel return then passes through the Mono-Block on the high-pressure pump and is returned back to the fuel tank.Low-Pressure Fuel System Schematic
Illustration 3 g06306368
(1) Fuel cooled injectors
(2) High-pressure system
(3) Back-pressure regulator
(4) Mono-block
(5) Fuel cooler application depended
(6) 100 µm strainer
(7) Fuel boost pump
(8) Primary filter
(9) Fuel tank
(10) Priming manual or solenoid valve
(11) Fuel Transfer Pump (FTP)
(12) FTP pressure relief valve
(13) Fuel filter group
(14) Anit-reverse flow check
(15) Starting valve
(16) System regulator
(17) 2.3 mm (0.091 inch) Cooling supply orificeAn electric off-engine boost pump supplies fuel from the tank to the on-engine mechanical fuel transfer pump. Fuel is drawn from the tank thru a strainer to protect the boost pump from debris. A bypass valve allows fuel to flow around the boost pump to prevent an engine shutdown in the event of a boost pump failure. Fuel then passes through the primary filters. The amount of filtered fuel required by the on-engine fuel system flows to the mechanical fuel transfer pump and the rest recirculates through the priming loop check valve and priming solenoid valve back to the boost pump inlet.The fuel transfer pump provides a fuel pressure of 600 to 850 kPa (87 to 123 psi) to the secondary filter inlet. After the fuel has been filtered, the fuel flows into the high-pressure fuel system. Some of the flow that is not used by the high-pressure fuel system is used to cool the injectors. The rest is recirculated back to the fuel transfer pump inlet. The pressure regulator is used to maintain the pressure in the low-pressure fuel system to a gauge pressure of 600 to 850 kPa (87 to 123 psi). Check valves are used to prevent fuel from draining from the high-pressure fuel system.When the fuel system has air trapped in the fuel filters, lines, and other fuel system components, the engine could experience difficulty starting. The electric boost pump forces the air that is in the fuel out of the system through the injector cooling circuit.Priming is required after servicing the fuel system such as changing the fuel filter. Priming is also recommended when the engine has not run for a long time. Refer to Locomotive OEM Operation and Maintenance Manuals for instructions on how to prime the fuel system. During priming, the priming solenoid valve closes delivering full flow to the engine.Fuel Transfer Pump
Illustration 4 g06301265
(1) Fuel Transfer Pump
(2) High-Pressure Fuel Pump
Illustration 5 g06083244
(3) Fuel Transfer Pump Pressure Relief Valve
(4) Shaft Seal
(5) Weep Hole (Do Not Obstruct)The fuel transfer pump (1) is a fixed displacement gear pump that is driven by the high-pressure pump. The fuel transfer pump receives fuel from the primary fuel filtration unit. The pump sends the fuel through the secondary filters on the way to the high-pressure fuel pump. The fuel transfer pump also supplies cooling flow to the injectors, and the high-pressure pump.The fuel transfer pump is mounted on the high-pressure pump (2). A relief valve (3) in the fuel transfer pump limits the outlet pressure to 950 kPa (138 psi). Other important parts include the shaft seal (4) and the weep hole (5) which prevent fuel in oil contamination.High-Pressure Fuel System
Illustration 6 g06301284
(1) Injector
(2) Accumulator-Quill Tube
(3) Line Support Clamps
(4) High-Pressure Injection Lines
(5) Pump to Accumulator High-Pressure Lines
(6) High-Pressure junction Block
(7) High-Pressure Fuel PumpThe common rail high pressure fuel system on this engine consists of high-pressure fuel accumulator-quill tubes (2) that maintain a volume of fuel that is used to supply each cylinder as needed and high-pressure fuel lines (4) to transport fuel between the high-pressure fuel pump (7), high-pressure accumulator quill tubes (2), and the common-rail injectors (1).The main components of this system Illustration 6 are between the high-pressure fuel pump (7) and the injectors (1):
High-Pressure Fuel Pump to Junction Block Lines
High-Pre