DD15 Engine

Section 1 The Fuel System

Section 1
The Fuel System

The fuel supply ensures that the fuel required for combustion is available under all operating conditions in sufficient quantity, in the correct sequence and at the required pressure. Fuel to the individual cylinders is supplied from the Amplified Pressure Common Rail System. Fuel is supplied to the (APCRS) from the fuel low pressure circuit.‪

The fuel low pressure circuit ensures that the fuel is cleaned and is provided in sufficient quantities and at the required pressure to the APCRS.‪

The following diagram and flow description are a Version Five fuel system.‪

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The low pressure fuel pump (4), which is a gear pump, supplies fuel to the fuel low pressure circuit. When the engine is started, the fuel pump (4) pulls fuel from the fuel tank (16) and delivers it through the shutoff valve in the fuel feed (1) line to the fuel filter module (24). The shutoff valve in the fuel feed (1) prevents fuel leaking out when disassembling the fuel line between the fuel tank (16) and the fuel filter module (24). The fuel flows from the fuel filter module (24) to the fuel heat exchanger (2) located on the Motor Control Module (MCM) (A6.1). The electronic components of the MCM (A6.1) are cooled via the fuel heat exchanger (2). The fuel in the fuel filter module (24) first passes into the fuel prefilter (3) where large dirt particles greater than 100 micron are filtered out. A check ball which is located at the fuel inlet of the fuel prefilter (3) prevents emptying of the suction line to the fuel tank (16). Below fuel prefilter (3) an optional fuel preheater (R23) can be installed. The fuel preheater (R23) warms the fuel during low outside temperatures ensuring the flowability of the fuel.‪

The fuel which has been cleaned by the fuel prefilter (3) passes out of the fuel filter module (24) to the low pressure fuel pump (4). From there the fuel is delivered back into the fuel filter module (24) and to the water separator (5). The water inside the fuel is separated out by the water separator (5) and begins to fall into the water accumulator. If the water level in the water accumulator reaches its maximum level, this will be detected by the water level sensor (B150). The MCM (A6.1) transmits an appropriate message for illuminating the indicator lamp. At this point, the indicator lamp will inform the driver that the separated water needs to be drained off. After leaving the water separator (5) the fuel flows through the final filter (6). Both the water separator (5) and final filter (6) elements filter out fine-grained contaminants greater than 2 microns. ‪

A Supply Fuel Temperature Sensor (B10) located on the bottom of the fuel filter module detects the current fuel temperature. The MCM interrupts this information and uses it for fuel mass calculations. ‪

Designed into the water separator (5) and final filter (6) elements is a ventilation bore that allows trapped air to escape. If air is trapped in the water separator (5) or the final filter (6), for example due to replacement of a filter element, the trapped air will escape through the ventilation bore and vent lines (18.1, 18.2) into the fuel accumulator (14). From there the air is then dissipated into the fuel tank (16). Once all of the contaminants have been filtered out by the final filter (6), the fuel is delivered out of the fuel filter module (24) to the quantity control valve (Y134). The high pressure fuel pump (7) pressurizes the fuel up to 900 bar (13,053 psi) and is then fed to the fuel rail (8). The excess fuel from the quantity control valve (Y134) and high pressure fuel pump (7) is routed past the 2-stage valve (10). The excess fuel is then routed to the fuel accumulator (14).‪

The two-stage valve (10) has a number of tasks:‪

  • It is closed at pressures in the low pressure circuit below 3 bar relative to high pressure pump backflow pressure to ensure best filling under cranking speed where low pressure pump flow is low.
  • It guarantees lubrication of the high pressure fuel pump components if the pressure in the low pressure circuit is above 3 bar relative to high pressure pump backflow pressure. The lubrication path is fully open at 3.5 bar.
  • It ensures that in overrun conditions the pressure at the inlet to the high pressure pump is limited by opening of the backflow path. This function avoids pressures above burst pressure level of the filter module. If the pressure at the high pressure pump inlet exceeds 4.7 bar relative to backpressure, excess fuel is routed to the return path.

If the maximum rail pressure is exceeded (leakage at 1075 bar, open at 1250 bar), the pressure limiting valve (9) opens and passes the excess fuel to the fuel return. If the system pressure drops below 3 bar (43 psi) due to the opened pressure limiting valve (9), part of the excess fuel is taken from the fuel return from the pressure limiting valve (23) to be used for lubrication of the high pressure fuel pump. The fuel taken for lubrication is fed through the emergency lubrication line (20) and the regulator (13) to the fuel high pressure pump (7). The head pressure required to do this is generated by the throttle valve (12) which is located upstream of the fuel accumulator (14). The excess and warmed fuel from the amplifier circuits of each individual injector is routed through a passage within the cylinder head. This fuel is fed via the amplifier fuel return (21) line through the fuel cooler (11), throttle (12), and finally a regulator (13). This fuel is feed to the fuel filter module in front of the prefiler (3). The fuel cooler (11) serves to lower the temperature of the fuel to about 120°C. The throttle valve (12) in the amplifier fuel return (21) line dampens pressure peaks which occur during activation of the injector amplifiers.‪

The excess fuel for the injector needle valves is also routed through a bore in the cylinder head. The fuel return from the injector needle valves (22) flows through a regulator (13) and into the fuel accumulator (14). The regulator (13) has the task of regulating back pressure of about 1 bar (14.5 psi) relative to back pressure while the engine is running. From the fuel accumulator (14) part of the returned fuel is delivered directly upstream of the fuel prefilter (3) in the fuel low pressure circuit. ‪

The fuel quantity is determined by the throttle valve (12) and the regulator (13) in the fuel return (17) bypass. This returned fuel leads to a lower suction load for the low pressure fuel pump (4). The remaining fuel from the fuel accumulator (14) is led back to the fuel tank (16). The shutoff valve in the fuel return (15) prevents fuel leaking out when disassembling the fuel line between the fuel tank (16) and the fuel filter module (24).‪

1.1 Amplified Pressure Common-Rail System


DD15 Fuel System Technician's Guide - DDC-SVC-MAN-0037
Generated on 10-13-2008

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