Fuel Delivery and Air Induction: Description and Operation
The Boxster has a modified Motronic (DME) of version M 5.2.2 to generate the injection signal, to calculate the ignition angle as well as to provide adaptive control functions for various systems and to carry out diagnostic functions.
The principal features of the Boxster's DME:
- Electronic map ignition system with permanent high- voltage distribution
- Sequential injection
- Separate fuel mixture control for each cylinder bank
- Adaptive stereo oxygen sensor control
- Potential-free oxygen sensors
- Ground-side control of oxygen sensor heating units
- Hall sensor installed in the intake camshaft for cylinder line 1 - 3
- Actuation of VarioCam valves for cylinder line 1 - 3 and 4 - 6
- New hot-film mass airflow sensor
- Intake air temperature sensor installed in hot-film airflow sensor
- Induction control with new twin-coil rotary adjuster
- Adaptive throttle potentiometer
- Adaptive knock control
- Determination of oil temperature
- New temperature sensor to determine engine temperature and to provide analog display on instrument cluster
- Engine compartment temperature sensor to control engine compartment scavenging blower
- Radiator fan control by DME controller
- Actuation and disabling of start relay
- Control of air-conditioning compressor relay
- Actuation of check-engine lamp in the case of combustion faults which may damage the catalytic converter
- Torque reduction for traction control by cancellation of ignition angle and suppression of injection
- Electronically controlled venting of active carbon canister
- Deactivation of fuel pump by airbag control module in crash
- Change of programmed idling speed for Tiptronic vehicles and selected drive position
- Reduction of torque when Tiptronic shifts gear
- Device for feedforward control when checking oxygen control loop (e.g. for exhaust-gas check)
Additional features for USA vehicles:
- Complete OBD II diagnosis with activation of check-engine lamp
- Secondary air system
- 2 additional oxygen sensors after catalytic converters
- Additional Hall sensors for camshaft for cylinder line 4-6
Fuel Supply:
Image Legend:
1 - Fuel tank
2 - Ascending pipe with fuel pump and sensor for fuel level indicator
3 - Fuel filter
4 - Fuel return
5 - Fuel supply
6 - To carbon canister
7 - Carbon canister
8 - Scavenging air pipe
Fuel Circuit:
An in-tank pump draws the fuel from the fuel tank installed at the front of the vehicle and pumps it into the fuel filter. Afterwards, the fuel then enters the fuel manifold for cylinder line 1 - 3. The inlet to this manifold is located at cylinder 3. The test connection for fuel pressure is located at the manifold for cylinder line 1 - 3, which like the manifold for cylinder line 4 - 6 is made of metal and coated yellow.
Both manifolds are linked by a Tecalan fuel pipe which must never be squashed or buckled, otherwise it would be permanently damaged. The pressure regulator is inserted into the manifold for cylinder 4 - 6 at the end of the pipe system. From here, a return pipe leads off to the fuel tank.
Fuel Pipes:
The Boxster's plastic fuel pipes have connectors which allow components to be connected quickly, simply, and precisely.
Great care must be taken when handling these pipes since damage to the sealing faces as well as dirt may cause leakage.
Fuel Pump:
An in-tank pump is used in the Boxster. This provides advantages with regard to noise behavior and residual fuel suction behavior since, on the one hand, it is installed in the fuel tank (noise damping) and, on the other hand, it is attached directly to the specially designed base of the fuel tank (suction behavior) by means of a bayonet ring.
Construction:
The pump is designed as a two-stage system. The first stage functions according to the flow-wheel principle (1). The fuel is forced-flow ventilated by a special channel (2) as it is drawn from the fuel tank to remove any gas and vapor bubbles. It is then led into the reservoir chamber (4) via an ascending pipe (3). The precisely defined cross-section of the overflow pipe (5) produces slight overpressure in the reservoir chamber which prevents the formation of a gas bubble mixture and also ensures the precharging of pressure for the main stage. The pressurization of the fuel and the preceding ventilation process are necessary to guarantee a good hot-feed behavior of the system.
The second stage of the fuel pump (main feed pump) is configured as an internal gear pump (6) and pumps the fuel out of the reservoir chamber (7), through a filter, and then into the fuel filter and the fuel manifold pipes. The returned fuel is led back to the reservoir chamber via a return pipe (8). This completes the fuel circuit. When the engine is switched off, the pressure retention valve (9) closes the pipe loop and ensures that the necessary overpressure to prevent the formation of vapor bubbles.
Actuation:
To ensure that the required fuel pressure and the necessary amount of fuel are provided, the DME control module actuates the fuel pump for approx. 1 second whenever the ignition is switched on (pump priming).
To trigger another pump priming operation, the running engine must be switched off and the ignition then switched on again.
In addition, the fuel pump is activated by the DME control unit whenever the engine speed exceeds 15 rpm.
Safety Functions:
When the engine has been started, the airbag control unit sends a signal to the DME control module, the frequency and pulse duty factor of which is defined as "airbag not triggered". When the airbag is triggered, the pulse duty factor changes this signal. This change causes the DME control unit to switch off the fuel pump.
Fuel Filter:
The fuel filter is installed in the center tunnel above the pipes leading to the radiator. Due to the construction of the filter, special attention must be paid to the fuel-flow direction. In the Boxster, an electric cable is connected to the filter housing. This cable diverts voltage potentials caused by flowing fuel and differing material characteristics (filter housing: aluminium; fuel pipe: Tecalan) to ground.
Fuel Pressure Regulator:
A miniature fuel pressure regulator installed in the fuel manifold for cylinder line 4 - 6 (level with the intake pipe for cylinder 4) is used in the Boxster. It adjusts the fuel system pressure to 3.8 ±0.2 bar, depending on the intake pipe pressure.
Injection Valves:
The injection valves are specially designed for the Boxster engine. The coils of the valves are made of brass and have an internal resistance of approx. 12 ohm. One end of the valve is inserted into the fuel manifold (secured by a clip). The other end is inserted into the intake pipe.