High-Pressure Fuel System

High-Pressure Fuel System

High-pressure fuel system
The 4-cylinder petrol engine operates with direct fuel injection. The purpose of direct fuel injection is to increase performance. The maximum fuel pressure in this case is 120 bar (idle: 50 bar, full load: 120 bar). The use of direct fuel injection creates a homogeneous mixture preparation in the entire combustion chamber. Homogeneous mixture preparation means that the fuel air ratio is regulated stoichiometrically in the same way as for intake pipe fuel injection (Lambda = 1). "Stoichiometrically" means a ratio of 14.7 kilograms of air to 1 kilogram of fuel. Due to the homogeneous mixture preparation, conventional exhaust re-treatment can be used.







Brief component description
The following components for the high-pressure fuel system are described:

Injector
The injector is on the side of the cylinder. During fully sequential fuel injection, each injector is activated by the DME control unit via its own output stage. During this process, the injection period of the specific cylinder is adjusted to suit the operating condition (engine speed, load and engine temperature).
The fuel quantity injected depends on the rail pressure, counterpressure in the combustion chamber and the opening period of the valve.







The incoming vehicle voltage is stepped up to between 85 and 100 volts by using a pulse output stage with high-power capacitors. A current flows in the output stage up to a certain switch-off value. The switch-off produces an induction voltage, e.g. 85 volts, that charges the high-power capacitors (boosters). A current level of between 2.8 and 16 amperes is supplied via this capacitor current to the injectors. The DME activates the injectors on the ground side.
Voltage is supplied to the DME via the load-shedding relay terminal 15. This voltage is applied at the 4 injectors. Also observe the diagnosis wiring diagram.

High pressure pump with quantity control valve
2 pistons in the high pressure pump generate the required pressure in the high-pressure fuel system. The intake camshaft drives the high pressure pump mechanically. The maximum fuel pressure is 120 bar. The necessary fuel pressure is determined by the engine control depending on the engine load and engine speed.
The quantity control valve can be found at the high pressure pump. The DME control unit activates the quantity control valve.







Rail pressure sensor
The rail pressure sensor is built into the stainless steel rail. Fuel is stored under pressure temporarily in the rail, then distributed to the injectors. The fuel pressure in the rail is measured by a silicon element on a metal membrane. The rail pressure sensor supplies a proportional voltage across the entire measuring range. The rail pressure sensor is supplied with a voltage of 5 volts and ground by the DME. The signal for evaluation fluctuates depending on the pressure. The measuring range of approx. 0.5 to 4.6 volts corresponds to a rail pressure of between 0 bar and 250 bar.







If the rail-pressure sensor fails, the quantity control valve is activated in emergency operation by the DME.

System functions
The following system functions are described for the high-pressure fuel system:

High-pressure control
The quantity control valve adjusts the fuel supply from the low pressure side to the high pressure side of the high pressure pump in order to achieve the required rail pressure. The quantity control valve is hydraulically forced open once a certain pressure has been reached in the high pressure side of the high pressure pump. The quantity control valve is a component of the high pressure pump.
The signal from the rail-pressure sensor is an important input signal of the DME for activation of the quantity control valve (component of the high pressure pump). If the rail-pressure sensor fails, the quantity control valve is activated in emergency operation by the DME.

Notes for Service department

General notes

NOTICE: The quantity control valve cannot be replaced individually.

The high pressure pump must always be entirely replaced due to the risk of contamination.

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