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A-Primary Filter, B-Final Filter, C-Diagnostic Port, D-Transfer Pump, E-High Press. Pump, F-Overflow Valve,
G-Common Rail, H-Relief Valve, I-Fuel Return Line, J-Electronic Injector, L-Flow Limiter, M-Control Valves,
N-Check Valves, O-Hand Primer Pump, P-Transfer Pump-Out, and Q-Transfer Pump-In.

Common Rail Fuel System Operation
The common rail fuel system enables engines to produce power efficiently while still meeting ever more stringent emission regulations. The common rail system works because it makes the extremely high fuel pressure that is needed to more finely atomize the fuel droplets. Smaller droplet size permits better combustion in the cylinder.

Here’s How It Works: Fuel is drawn from the vented fuel tank and through the boat’s fuel/water separator filter, and then through the primary fuel filter (A) by the fuel transfer pump (D). A pressure relief valve on the final filter allows fuel to bypass the final filter if the filter becomes plugged. Bypass fuel is directed back to the tank to prevent both filter rupture and unfiltered fuel from entering the high-pressure fuel system. Check valves (N) are used to prevent fuel from draining out of the fuel filter and the high-pressure fuel pump when the engine is not running.

The fuel exits the final filter and flows to the high-pressure fuel pump (E). The high-pressure fuel pump begins raising the pressure of the fuel to prepare for injection. Pump control valves  (M) control when fuel enters the pump. These valves are controlled by the engines’ computer (ECU).  When the necessary volume of fuel is in the pumping chamber to maintain the correct fuel pressure in the high-pressure common rail (HPCR)  (G), the ECU will shut the valves. When the fuel pressure in the pump exceeds the pump’s delivery valve opening pressure, the high-pressure fuel is allowed into the (HPCR) which evenly distributes fuel to all of the electronic injectors (J). The HPCR uses flow limiters (L) to maintain a constant pressure to the injectors. The ECU sends a signal to the two-way valve, inside the injector body to control the volume of fuel, the timing of delivery, and the rate of delivery for each injector. Excess fuel from the nozzles travels through the fuel rail return line.

A pressure relief valve (H) will allow excess fuel in the HPCR to flow into the low-pressure fuel rail return line (I). An overflow valve (F) on the high-pressure fuel pump will also release excess fuel into the fuel rail return line and back to the tank.  The fuel transfer pump is mounted on the high-pressure fuel pump and is driven by the high-pressure fuel pump camshaft. The transfer pump draws fuel from the fuel tank, through the primary filter, and into the transfer pump inlet (Q).  Fuel is then pressurized, exits the transfer pump (P), and travels to the final filter. The hand primer (O) is provided for bleeding air from the fuel system. The final filter is a 2-micron filter. Fuel enters the final filter at the fuel inlet, and flows through the filter element, and exits through the outlet to the high pressure fuel pump.

The John Deere 6081 high-pressure common rail engine uses the Denso ECD-U2 high-pressure fuel pump. The main components of the ECD-U2  pump are the driveshaft, two 3-lobed cams, timing wheel, two pumping plungers, pump control valves, delivery valves, and the pump position sensor. Filtered fuel from the primary filter fills the high-pressure fuel pump at the fuel inlet. Both cam lobes and the timing wheel are attached to the drive shaft.  As the driveshaft rotates, both cam lobes operate their respective plungers to increase the pressure of the fuel. The timing wheel is used to keep the high-pressure fuel pump and the engine timing in sync with each other.

The timing wheel has 6 equally spaced notches plus 1 additional notch. The ECU uses the pump position sensor to detect each notch on the gear as it rotates past the sensor. The ECU uses the additional seventh notch to determine when cylinder #1 is approaching Top-Dead-Center (TDC). The transfer pump and the high-pressure fuel pump come as an assembled unit. The entire pump must be replaced if a failure occurs.

Inside The Common Rail
High-pressure fuel is delivered to the high-pressure common rail (HPCR) at the fuel inlets from the high-pressure fuel pump. The HPCR distributes high-pressure fuel to the Electronic Injectors (EIs).
The fuel rail pressure sensor, flow limiter, and pressure relief valve work together to regulate fuel distribution. The fuel rail pressure sensor detects the fuel pressure inside the rail. The engine control unit (ECU) uses this sensor to monitor the fuel pressure to determine the timing of the pump control valves on the high-pressure fuel pump. The flow limiters (L) use a piston and ball valve to reduce pressure pulsations. This ensures a steady pressure in the fuel lines to the electronic injectors. The flow limiters are also used to limit the maximum fuel flow to the injectors to prevent engine damage due to a failed injector or a high-pressure leak by shutting off fuel to that particular injector. This is done by moving the ball valve until it seats closing the valve.

If a fuel pressure of 200 MPa (2000 bar) (29,000 psi) is generated within the HPCR, the pressure relief valve (H) will release the excess pressure and drain fuel back to the tank. Electronic Injector Operation: The electronic injectors (EIs) are located inside the engine’s cylinder head and are electronically controlled by the ECU. The amount of fuel delivered to the cylinder is in direct proportion with the length of time current is supplied to the two-way electromagnetic valve on each injector.