High Pressure Pump
The high pressure pump is, in principle, the same as the one used on the N63 engine. The only difference is that the fuel lines are positioned at a different angle.
N74 High Pressure Pump with Quantity Control Valve
| Index | Explanation |
|---|---|
| A | Low-pressure connection |
| B | High-pressure connection |
| 1 | Compensating chamber |
| 2 | High-pressure non-return valve |
| 3 | Pressure limiting valve |
| 4 | Pistons |
| 5 | Quantity control valve |
| 6 | Electrical connection of the quantity control valve |
The fuel is delivered to the high-pressure pump via the inlet with delivery pressure generated by the electric fuel pump. The fuel is then fed via the volume control valve and into the compression chamber of the pump element. In this pump element, the fuel is placed pressurized by a plunger and supplied via the high-pressure non-return valve to the high-pressure connection. The high-pressure pump is bolted onto the cylinder head and is driven by the camshaft by a triple cam. This means that, as soon as the engine is running, the triple cam continuously moves the plunger. Fuel is pressurized until new fuel is delivered via the volume control valve into the high-pressure pump. The volume control valve is activated by the engine management system; it specifies the delivered volume of fuel. Pressure regulation takes place via the volume control valve in that it is opened or closed by the pump element towards the fuel feed. When the quantity control valve is opened, most of the fuel drawn in by the piston is pressed back into the fuel feed.
The maximum pressure in the high-pressure area is restricted to 245 bar. If the maximum high pressure is reached, the high-pressure circuit is relaxed to the low-pressure area by a pressure limiting valve. In this case the pressure peak in the low pressure area is compensated for by the fluid volume in the area and pressure damper in the compensating chamber. The compensating chamber is integrated into the inlet towards the high pressure pump. This ensures that pressure peaks are lowered by connecting and disconnecting the high and low-pressure areas. When the piston generates pressure, fuel flows between the piston and its guide. This is deliberate, as it lubricates the pair of sliding elements. On downward movement of the pressure piston, a high pressure would arise at its rear side. This would lead to danger if the fuel is pressed through the sealing of the piston from the pump into the oil circuit of the engine. The connection to the compensating chamber means that there is never a higher pressure behind the piston than in the fuel feed. This prevents pressure fluctuations from being transferred into the low pressure fuel system, as the volume changes in front of and behind the piston are balanced.