Fuel Injection
Fuel Injection
Fuel injection
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 formation means that the air/fuel 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)
- For varying load, cylinder-specific correction of fuel injection (during an intake stroke, the injection period can be corrected by extending or shortening it through subsequent fuel injection)
- 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
The following components for the fuel injection system are described:
- Solenoid injection valve.
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 characterized 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.
Together with the spark plug, the injector is integrated centrally in the cylinder head between the intake and exhaust valves. This installation position avoids wetting the cylinder walls and the piston crown with injected fuel. Uniform formation of the fuel-air mixture is achieved with the aid of gas movement in the combustion chamber and cone-shaped fuel dispersion. The movement of gas is influenced by the geometry of the intake ports and the shape of the piston crown. The injected fuel is swirled with charge air in the combustion chamber.
System overview
Notes for Service department
General notes
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.
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