Tuesday, 12 February 2013

Digital Motor Electronics

The Digital Motor Electronics (DME) are the computers that control the engine in the M5. They are made by Robert Bosch GMbH.

Bosch has a long history of supplying engine electronics. Recall from the Fuel Delivery post that they were the first to make pulsed fuel injection practical in their D-Jetronic system in 1967, and subsequently were the first to control spark timing and fuel injection by means of a programmable microprocessor in their Motronic system first used by BMW in 1979. The system in the F10 M5 is a direct descendant of the first Motronic system, the M1.0. Since then, the system evolved into the ME line (which added throttle control), the MED (which added direct fuel injection) line, and now the MEVD17.2.8 which handles everything the M5 has: Valvetronic (variable intake valve lift), Double-VANOS (variable intake and exhaust valve timing), gasoline direct injection, and turbochargers. Bosch and BMW have always worked very closely with one another, and the MEVD17.2.8 is tailor-made for the M5.

The electronics that control the motor are shown below. Fuses and relays are located at (1), DME I (2) is the control unit for the first cylinder bank and is the master, and DME II (3) is the control unit for the second cylinder bank. These DMEs are liquid cooled by a separate low temperature circuit (shared with the charge air coolers) embedded in the aluminum DME housing base.

The MEVD17.2.8 DME units are each as follows.

There are separate connectors for connection to other vehicle computers (1), sensors and actuators (2,3), Valvetronic servo motors (4), power supply (5), and fuel injection and ignition (6). There are two sensors on the circuit boards themselves, one for temperature and one for ambient pressure.

There are many sensors feeding input into each DME, and many actuators that control various things in and around the engine. Furthermore, the DME's talk to other computers on the M5 as well.

The sensors include ones for speed, the mass of air entering the cylinders, oxygen sensors before and after the catalytic converters, charge air pressure sensor before the throttle, air temperature and intake manifold pressure after the throttle, camshaft position, crankshaft position, accelerator pedal position, engine temperature, oil temperature, oil level, battery, knock sensors, fuel rail pressure, and a few more.

The actuators include the fans, thermostats,  fuel tank valves, VANOS solenoids, coolant pump for the turbos, fuel injectors, ignition coils, throttle, oil pressure switch, Valvetronic motor, coolant pump, and a few more.

The units talk to one another and other computers in the car (there are quite a few which we shall cover in a later post), by means of various different "busses". In computer language, a "bus" is a defined method for computers to talk to one another. Over the years, various standards have come (but few have gone!). When BMW assembles a car, all the various components that they are integrating together operate using different busses, and so the M5 is filled with a multitude of such busses and has methods of transferring information from one bus to another (we shall cover all the bus standards in the M5 in a separate post).

The particular busses that the DME speaks to are PT-CAN and PT-CAN2 (both Bosch-developed standards), a local controller area network between the two DME's, and FlexRay, which is the newest bus standard defined by a consortium in which BMW and Bosch play prominent roles. The FlexRay bus is used to interface with the chassis management that keeps the car from spinning out, keep it flat around the corners, and control the torque distribution to the rear wheels. These chassis control systems need to operate at very high speeds, hence they speak the newest bus standard, FlexRay, to get that speed, and the DME's must keep up to tell it what it needs to know, and to react to its commands, both in a guaranteed timely fashion.

Some of the function of the DME are summarized below.

As you can see, the DMEs perform a multitude of tasks to keep the engine running smoothly. The DMEs are very much a system onto themselves, and can keep running even when other computers in the car that it does not rely upon stop functioning.

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