Control Unit (DME)

DME And EML Diagram:

NOTE: The BMW V12 engine uses two complete independent (DME) Motronic engine control systems. DME control unit 1 for cylinders 1-6, and DME control unit 2 cylinders 7-12.

The Digital Motronic Engine control units (DME) precisely controls fuel injection and ignition under variable operating conditions. Various engine sensors and other systems (ie. air conditioning, auto transmission etc.) supply each DME control unit with operating information. Each DME control unit processes the sensor input and sends output signals to adjust injector pulse width and ignition timing.

DME Control Unit Pin Designations:

Inputs signals include:

^ Engine Speed
^ Reference Mark Signal
^ Airflow Volume
^ Intake Air Temperature
^ Engine Temperature
^ Cylinder Identification
^ Throttle Position
^ Lambda Signal
^ Battery Voltage
^ Vehicle Speed
^ Gear Position (Auto Trans. only)
^ A/C Signal

Output Signals include:

^ Fuel Injection Control
^ Ignition Control
^ Fuel Pump Control
^ Evaporative Control


FUEL INJECTION CONTROL

The basic fuel injection time is calculated from the airflow volume signal and engine speed signal input. The injector opening time is then corrected from other input signals depending on the engine's operating conditions. Each DME control unit directs the fuel injectors to operate in two groups at different working cycle phases of the individual cylinders. The groups consist of cylinders 1,3 & 5 and 2,4 & 6 (DME Control Unit 1), and cylinders 7,9 & 11 and 8,10 & 12 (DME Control Unit 2). There is one injection per crankshaft rotation and group which results in one injection per total working cycle of each cylinder (half-sequential injection).
During cold start operation a coolant temperature related quantity of fuel is injected three times per crankshaft revolution for up to five crankshaft revolutions. The quantity of fuel is in excess of the normal injected quantity. After completion of five revolutions, the high initial fuel quantity is reduced on a rpm-related basis. This is accomplished in the control unit by start-up recognition. Start-up recognition is permanently programmed in each DME control unit and is temperature and speed-dependent. After the start-up recognition threshold has been reached, which is between 600 and 800 rpm, injection will take place only once per crankshaft revolution.
Each DME control unit will use the after-start warm-up function, which is mapped and controlled up to a coolant temperature of 158°F, to determine the fuel injection quantity during the warm-up phase. The control unit will compare and evaluate input signals from the coolant temperature sensor and the engine speed signal against fixed values in it's program. When these values are combined the proper fuel quantity will be injected. The system also recognizes a repeat start and reduces the injected quantity of fuel to avoid an over-rich condition in the event of a restart.
When the engine reaches normal operating temperature (above 158°F) the basic fuel injection time will be calculated from the airflow volume signal, the engine speed signal and the oxygen sensor signal. The oxygen sensor signals are used as correction factors to obtain the optimum air/fuel mixture for reduction of pollutants and maximum engine performance.

FUEL DELIVERY CONTROL

Operation of the electric fuel pumps is controlled through relays which are energized by the Motronic control units. The relays are energized only during engine cranking or while the engine is running. Each DME control unit uses the engine speed signal as an input for fuel pump control.

Ignition Timing Program Map:

IGNITION CONTROL

The primary circuit of the ignition coils are switched by the power output stage of it's respective DME control unit. The uncorrected basic ignition time is used as a load signal for the ignition. Based upon these load conditions in relation to various engine speeds a three dimensional ignition performance characteristic "Basic Ignition Map" is used to optimize the ignition to achieve the most favorable exhaust gas and fuel consumption performance for any given load or speed condition.
When the throttle valves are in the closed position, the bottom line of the basic ignition map is selected as the idle/coast characteristic. For engine speeds below the nominal values, the ignition timing is advanced to obtain idle stabilization. For coasting ignition timing is programmed to obtain minimum exhaust gas pollutants and optimum driveability performance. For the wide open throttle positions, the top line of the basic ignition map is selected. This full-load characteristic is programmed with engine knock limits taken into consideration. For very high ambient temperatures the ignition is retarded as a function of engine and intake air temperature.

EVAP EMISSION CONTROL

Each DME control unit controls operation of it's evaporative emission system by switching the electric purge valve off and on based on engine speed, load, and temperature characteristics.

ON-BOARD DIAGNOSTICS

In the event of a DME system malfunction, circuit information will be stored in the internal memory of the DME control unit. Intermittent failures are stored with a record of the failure frequency. Each DME control unit is able to store up to five malfunctions, and will give priority to those items which can cause the vehicle to exceed exhaust emission standards. In addition to being stored in memory, an emissions-related failure will cause the "Check Engine" light to illuminate on the instrument cluster. Furthermore, the "Check Engine" light is also used to display faults which are stored in the control unit's memory when the self-diagnostic procedure has been initiated. Faults may also be retrieved with the BMW service tester if available.

ADAPTIVE CONTROL

Each DME control unit has adaptive air/fuel mixture capabilities. The control units continuously compare the programmed data, for the air fuel mixture, with the input values. If a deviation occurs, the new input values are superimposed and stored in the programmed data map. As a result, new output signals are sent to the injectors. This allows the effected DME control unit to correct the injector pulse width for break-in wear or slight air leaks which might occur in the systems. If power is interrupted to the control unit, new adaptation takes place within several minutes of engine operation.