Engine Electronics MEVD
High-Pressure Fuel System
High-pressure fuel system
The N55 6-cylinder spark-ignition engine relies on direct fuel injection. The direct fuel injection increases the performance. The maximum fuel pressure is 200 bar (idle: 50 bar, WOT full load: 200 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). A stoichiometric air-fuel mixture contains a ratio of 14.7 kilograms of air to 1 kilogram of fuel. Homogeneous mixture formation renders it possible to use a conventional emissions control system. Fully-sequential multipoint injection with selective control for each individual cylinder offers the following advantages:
- Optimal fuel mixtures for each individual cylinder
- The injection duration is precisely adapted to the engine's instantaneous operating conditions (engine speed, load factor and temperature)
- Responds to changes in load by correcting the injection duration specifically for the individual cylinder (during the intake stroke the injection duration can be corrected with a supplementary post-injection discharge as well as by extending or reducing the injection period)
- Selective deactivation of individual cylinders is also available (for instance, in response to a defective ignition coil)
- Allows individual diagnosis of each fuel injector.
Brief component description
This section describes the following components within the high-pressure fuel system:
- High-pressure pump with fuel quantity control valve
- Rail pressure sensor
- Solenoid injection valve.
High-pressure pump with fuel quantity control valve
The high pressure pump is a fuel pump with 3 pistons and the following tasks:
- Increase fuel pressure (range from 50 to 200 bar)
- Deliver fuel to rail.
The high pressure pump is bolted onto the rear end of the vacuum pump. The high pressure pump drive shaft is connected to the vacuum pump drive shaft, which is driven by the timing chain.
The fuel-quantity control valve is mounted on the high-pressure pump. The fuel-quantity control valve is controlled using a pulse -width-modulated signal. While the fuel-quantity control system is active the DME digital engine electronics system uses various pulse-duty factors to regulate operation of the fuel-quantity control valve. The rail pressure is adjusted to the specified level calculated in the DME digital engine electronics ECU.
The high-pressure pump generates pressure to maintain a constant level in the rail. Two valves are employed to maintain the pressure within the fuel rail at the required level:
- Fuel quantity control valve
- Rail pressure regulating valve.
The correct rail pressure is set depending on the situation at the time using one of the following three possible control types:
- Volumetric flow regulation by the flow regulating valve:
The flow regulating valve only allows the amount of fuel to flow into the high pressure pump from the low-pressure side that is required in order to generate the required fuel rail pressure. In this scenario, the cylinders in the high pressure pump are not completely filled with fuel. Increasing the current results in progressively higher levels of rail pressure. The rail pressure regulating valve is supplied with so much current that it stays closed.
- Pressure regulation by the rail pressure regulating valve (e.g. in the overrun phase):
The high-pressure pump provides a constant supply of highly-pressurised fuel to the rail. The rail pressure regulating valve reduces the fuel quantity in the low pressure fuel system if too much is delivered to the rail. Increasing the current results in progressively higher levels of rail pressure. The fuel quantity control valve is thereby closed.
- Combined closed-loop control with simultaneous regulation from the rail pressure regulating valve and the fuel-quantity control valve:
For very low fuel injection rates of less than 4 mg (when idling), it is necessary for the rail pressure regulating valve to remove some fuel from the high-pressure fuel system. The reason is that the high-pressure pump cannot run with zero delivery. This means that the high-pressure pump continues to supply the high-pressure fuel system with fuel, even when the fuel-quantity control valve is closed. This would lead to excessive rail pressure and thus to a control deviation. The excess rail pressure is counteracted by continuously changing between the "fuel quantity control" and "pressure regulation" control types.
These control strategies come into play under the following conditions:
- During engine starts: Full delivery
- When the engine is running: Load-dependent change between 3 control types
The rail pressure is set by the 3 control types depending on the load. As a result the high pressure pump delivers the exact fuel quantity required by the engine. The high-pressure pump's output is reduced, simultaneously reducing the engine's fuel consumption.
Rail pressure sensor
The rail-pressure sensor is threaded into the end of the fuel-distribution rail.
This sensor furnishes the DME digital engine electronics system with a measurement of the fuel pressure downstream from the high-pressure pump.
The rail-pressure sensor supports the rail-pressure control process. The rail-pressure sensor's signal serves as a vital input signal for the DME, which employs it in defining how it controls the quantity-control valve. The quantity-control valve is a component within the high-pressure pump.
Strain gauges are employed to monitor the rail pressure. A membrane equipped with strain-gauge strips is deflected by the applied pressure. The changes in resistance in the strain gauge are electronically detected by a Wheatstone bridge and evaluated. The measured voltage is then included as an actual value for closed-loop control of rail pressure.
A signal wire relays the rail-pressure information back to the DME digital engine electronics control unit. The effective signal for rail pressure fluctuates according to pressure levels. The measurement range of approximately 0.5 - 4.5 volts corresponds to rail pressures extending from 0 MPa (0 bar) to 25 MPa (250 bar).
Injector
The solenoid-controlled injection valve discharges a high-pressure spray into the combustion chamber. The solenoid-controlled injection valve is a retracting-plunger valve characterised by a high level of flexibility in its spray-propagation pattern (angle and shape). The discharge orifices form the high-precision injection pattern. This provides even and consistent propagation.
The retracting-plunger solenoid-controlled injection valve continues to maintain stable propagation properties, even at the temperatures and pressure levels encountered within the combustion chamber. Fuel is injected into the combustion chambers under high pressure (between 50 and 200 bar) during the intake and compression strokes. During the warm-up phase yet another, minute quantity of fuel is injected to assist the catalytic converter reach its effective operating temperature more quickly (catalytic converter heating). During cold starting the fuel is sprayed during the compression stroke in multiple pulses.
This strategy provides reliable cold-starts along with substantive improvements in the areas of pollutant emissions and fuel consumption.
The solenoid-controlled injection valve injects the quantity of fuel required under the instantaneous conditions into the combustion chamber. The mass can be adapted based on 2 control variables:
- Rail pressure
- Injector opening period.
The opening period is controlled by the fuel injection signal. The opening period is specified by the Digital Engine Electronics (DME). The opening stroke is determined by the activation duration of the injector. Generally, the stroke is always a maximum stroke.
The DME controls activation of the solenoid-controlled injection valve on the earth side.
The activation can be divided into 4 phases:
- Opening phase
- Intake phase
- Holding phase
- Deactivation phase.
System overview
System functions
The following system functions are described:
- High-pressure control
High-pressure control
The quantity-control valve regulates fuel delivery between the low-pressure and high-pressure sides of the high-pressure pump. The desired rail pressure is set by the 3 control types. The signal from the rail pressure sensor serves as an input signal for the digital engine electronics (DME) to activate the quantity control valve.
If the rail pressure sensor or high pressure pump fails, the quantity control valve is not longer activated by the Digital Engine Electronics (DME). An integrated bypass valve in the high pressure pump allows continued but restricted driving.
Notes for Service department
General notes
NOTICE: The quantity-control valve cannot be replaced as an individual part.
Owing to the potential contamination hazard, the entire high-pressure pump must be replaced.
NOTICE: Allow the engine to cool down.
Never start repair work on the fuel system without allowing the engine to cool down first. The coolant temperature must not exceed 40 °C. Compliance with this instruction is absolutely vital, as otherwise residual pressure within the high-pressure fuel system could result in uncontrolled fuel spray.
NOTICE: Cleanliness during repair operations.
Special attention should always be devoted to maintaining clean conditions and to carefully following the repair instructions during all repair work. Even minute contamination and minor damage to the threaded connections on the high-pressure lines can result in leaks. Remove the ignition coils before installing the solenoid-controlled fuel injectors.
NOTICE: Protect ignition coils against contamination.
When carrying out repair work on the N55 always ensure that the ignition coils are not contaminated by fuel. Contact with fuel substantially reduces the ability of silicone to provide effective sealing. The result would be arcing between the spark plugs and the cylinder head, leading to ignition miss. Prior to working on the fuel system always remove the spark plugs and seal off the spark plug wells with shop towels to protect them from fuel.
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.