Brief description of function
The M57 is the very first BMW diesel engine to be equipped with a high pressure accumulator fuel injection system (common rail).
With this new fuel injection process, a high pressure pump delivers a uniform level of pressure to the shared fuel line – the common rail – which serves all the fuel injection valves. Pressure develops to the optimum level for smooth operation. This means that each injector nozzle is capable of delivering fuel at spray pressures of up to 1350 bar.
The common rail system disconnects fuel injection and pressure generation functions. Fuel injection pressure is generated independently of engine speed and fuel injection volume and is available in the “Rail” (high pressure fuel accumulator) for injection to the cylinders.
The fuel injection timing and volume are calculated in the DDE and delivered to each engine cylinder by the injectors, each of which is actuated by energizing the appropriate solenoid valve.
Requirements and objectives
Demanding requirements and challenging objectives were set for the fuel system. Here are the most important ones:
• Fuel injection pressure
• Adjustment of fuel injection timing
• and formation of fuel injection characteristics.
Maximum fuel injection pressure should be kept as high as possible to reflect the desired emission and performance
characteristics. High fuel injection pressures reduce the size of fuel droplets and shorten the fuel injection period.
It must be possible, within broad limits, to make the injection timing dependent on load, engine speed and temperature (> 20o crankshaft angle).
Ideally, the fuel injection process should start slowly and end abruptly without any slowing in delivery speed. Preliminary fuel injection with a small volume of fuel (1 – 2 % of full throttle volume) helps to reduce combustion noise.
System structure
The fuel system is sub-divided into 2 sub-systems:
• Low pressure system
• High pressure system
The low pressure system features the following components:
• Fuel tank
• Advance delivery pump
• Outlet protection valves
• Auxiliary delivery pump
• Fuel filter with inlet pressure sensor
• Pressure relief valve (LP system) and in the fuel return line
• Fuel heating (bimetal valve)
• Fuel cooler
• Distributor unit with throttle
The high pressure system features the following components:
• High pressure pump (HPP)
• High pressure fuel accumulator (Rail)
• Pressure control valve
• Rail pressure sensor
• Injector
The system pressure is approx.
• in the LP system
– inlet end 1.5 bar < p < 5 bar (relative)
– return end p < 0.6 bar (relative)
• in the HP system 200 bar < p < 1350 bar
The following section describes the components in the direction of fuel flow.
