Fuel supply, function - GF07.10-P-1003MMO
ENGINE 276.9 in MODEL 212.095 up to model year 2014
Function requirements for fuel supply, general points
- Circuit 15 (ignition ON)
- Circuit 87M (engine timing ON)
Fuel supply, general points
The fuel supply supplies filtered fuel out of the fuel tank in adequate amounts under all operating conditions at an adequate pressure to the fuel injectors (Y76).
Function sequence for fuel supply
The function sequence for fuel supply is described in the following steps:
- Function sequence for fuel low-pressure circuit
- Function sequence for fuel high-pressure circuit
- Function sequence for safety fuel shutoff
- Function sequence for fuel quality monitoring (for code (929) Engine for ethanol fuel)
Function sequence for fuel low-pressure circuit
Switching on of the fuel pump (M3) takes place if the signal "fuel pump ON" is received by the fuel system control unit (N118). This signal is sent redundantly from the ME-SFI [ME] control unit (N3/10) as a CAN signal via the drive train CAN (CAN C) and as a ground signal.
The fuel system control unit also receives the CAN signal "specified pressure of the fuel" from the ME-SFI [ME] control unit.
The fuel system control unit detects the current fuel pressure by means of a voltage signal from the fuel pressure sensor (B4/7) and transmits this information via the drive train CAN to the ME-SFI [ME] control unit.
The fuel system control unit evaluates the current fuel pressure, compares it with the specified fuel pressure and actuates the fuel pump in such a way that the actual value corresponds to the specified value.
The fuel pump pressure is regulated dependent on the fuel temperature and the engine RPM between about 4.0 and a maximum of 6.7 bar.
For actuation the fuel pump suctions the fuel from the fuel feed module and pumps it through the fuel filter to the fuel system high pressure pump (single line system without return line).
The overflow valve in the fuel filter opens at a fuel pressure from about 7 to 9 bar. Fuel is removed upstream of the filter over a T-piece which drives the suction jet pump with 20 to 40 l/h.
This suction jet pump delivers the fuel out of the left fuel tank chamber into the fuel feed module (in the right fuel tank chamber) and thus prevents single-sided emptying of the fuel tank.
There is a check valve in the feed line to the fuel filter which prevents dropping of the fuel pressure (down to below 4.5 bar) for a switched off fuel pump.
Fuel low-pressure circuit
Function sequence for fuel system high-pressure circuit
In the fuel high-pressure circuit the fuel high pressure of about 200 bar required for spray guided direct injection is generated, regulated and the pressure is stored in both rails.
The ME-SFI [ME] control unit reads in the following sensor for regulation of the high pressure fuel:
- Fuel pressure and temperature sensor (B4/25)
The fuel from the fuel tank flows from the low-pressure fuel distributor to the fuel system high pressure pump. This compresses the fuel (according to the operating condition) up to 200 bar and directs it to the fuel injectors via the high-pressure lines and the rails.
The three fuel injectors per cylinder head are supplied directly from the associated rail with fuel.
At the fuel system high pressure pump there is a quantity control valve (Y94) which regulates the fuel quantity which is fed to the pump element, dependent on requirements and a characteristics map. Actuation of the quantity control valve takes place via a PWM signal from the ME-SFI [ME] control unit
The fuel pressure and temperature sensor detects the current fuel high pressure in the rail as well as the temperature of the fuel in the left rail. The operating pressure is about 200 bar. It is only at vehicle standstill and for a selector lever position "N" or "P" that the pressure drops to 150 bar in order to reduce the noise emissions from the fuel system high pressure pump.
In the case of stopping a vehicle with a hot engine, the fuel pressure can increase up to 250 bar (+17 bar) in the high-pressure circuit. Upon reaching this threshold a valve opens in the fuel system high pressure pump and the pressure is reduced. Upon starting the engine the pressure falls rapidly to the normal operating pressure of about 200 bar.
There are leakage lines on both rails which, in the case of a leak on the sealing rings for the fuel injectors to the rail, lead the fuel into the cylinder head. This prevents fuel escaping and thus any possible ignition on hot engine parts.
During regulation of the fuel high-pressure circuit one differentiates between the following operating conditions:
- Start
- Normal mode
- Low-pressure limp-home mode (fuel high pressure not reached)
- Stop
Start
- The quantity control valve is energized and closed, so there is full delivery from the fuel system high pressure pump and a faster pressure buildup
- Fuel pump pressure is approx. 4.5 to 6.7 bar
Normal mode
- Quantity control valve regulates fed in and required fuel quantity over the duty cycle
- The fuel pump pressure is dependent on the fuel temperature between about 3.0 to 5.5 bar.
- The fuel predelivery pressure is dependent on the engine speed and fuel temperature and varies between 4.5 and 6.7 bar (absolute)
Low-pressure limp-home mode (fuel high pressure not reached)
- The quantity control valve is deenergized and therefore opened
- Fuel pump pressure about 4.5 to 6.7 bar, fuel flows over the open quantity control valve into the rails
- Actuation of fuel injectors extended
- Reduced performance, max. speed approx. 70 km/h
Stop
- The quantity control valve is deenergized and opened
- Fuel pump not actuated
Assembly operations
The high pressure fuel lines made out of stainless steel can be reused after checking.
Fuel high-pressure circuit
Function sequence for safety fuel shutoff
The safety fuel shutoff system is designed to ensure traffic and passenger safety.
The ME-SFI [ME] control unit controls the safety fuel shutoff on the basis of the following sensors and signals:
- Crankshaft Hall sensor (B70), engine speed
- Throttle valve actuator (M16/6), throttle valve position
- Supplemental restraint system control unit (N2/10), direct crash signal
- Supplemental Restraint System control unit, indirect crash signal via chassis CAN (CAN E) (as of model year 2014 via suspension CAN 1 (CAN E1))
The safety fuel shutoff is activated by the ME-SFI [ME] control unit for the following conditions:
- Mechanical fault in the throttle valve actuator
- Absence of the engine speed signal
- Crash signal
Mechanical fault in the throttle valve actuator
When evaluating the throttle valve position, if the ME-SFI [ME] control unit detects a mechanical fault in the throttle valve actuator, engine speed is limited to about 1400 RPM at idle and about 1800 RPM in driving mode by shutting off the fuel injectors.
Absence of the engine speed signal
If the engine speed signal generated by the crankshaft Hall sensor is missing, the fuel pump is shut off via the fuel system control unit.
Crash signal
If the ME-SFI [ME] control unit receives a crash signal indirectly via chassis CAN (as of model year 2014 via suspension CAN 1) or directly from the Supplemental Restraint System control unit, it switches off the fuel pump via the fuel system control unit (directly and via the drive train CAN) and the quantity control valves in order to depressurize the fuel system.
Function sequence for fuel quality monitoring
(for code (929) Engine for ethanol fuel)
With the ever higher increase in the admixture of ethanol into the fuel it has become necessary to monitor the ethanol content or the fuel ethanol mixture. Variations in the stoichiometric fuel air/fuel ratio can occur due to the variable ethanol contents. This can lead to a loss of engine power.
Monitoring takes place by means of the fuel quality sensor (B4/31) which detects the relative conductivity of the gasoline ethanol mixture. The value of the relative conductivity of the gasoline ethanol mixture and the fuel temperature. This value is measured by the fuel quality sensor and is transmitted with an appropriate voltage signal to the FSCU (N118). There it is converted and sent as information to the ME-SFI [ME] control unit in order to undertake the appropriate adaptation of the engine timing.
The power supply of the fuel quality sensor takes place from the FSCU.
| Electrical function schematic for fuel supply | PE07.10-P-2703-97DAI | ||
| Electrical function schematic for safety fuel shutoff | PE07.10-P-2733-97DAI | ||
| Overview of system components for gasoline injection and ignition system with direct injection | GF07.70-P-9998MMO |