Computers and Control Systems: Description and Operation
Motronic System "Schematic Overview":
View - DME Connector:
The Basic Motronic (Dual Sensor Motronic) system was introduced in 1982. It combines the ignition and fuel injection systems into a single control unit called the Motronic Control Unit or MCU. This made possible improvements in the overall combustion process with the flexibility to select the ideal ignition timing and fuel mixture for all operation conditions.
Control of the evaporative system for the fuel tank to reduce vehicle HC emissions was also added to the MCU along with other functions.
The Motronic Control Unit (MCU) represented a further advancement in the system control known as Digital Motor Electronics (DME) because of the use of a microprocessor to handle the large quantity of information regarding system operation.
The Dual Sensor System refers to the two separate sensors for "Engine Speed Input" and "TDC Reference Input." Both of these sensors are identical, inductive type sensors. They are mounted on the bell housing of the engine. The engine speed sensor scans the flywheel starter ring teeth and the reference point signal is produced by a plate on the flywheel.
The basic operation of all the systems remains the same. The fuel injection system is the Lambda controlled L-Jetronic, and the ignition system is map controlled by the M.C.U.
INPUT SIGNALS:
Engine Speed
The engine rpm is determined by means of an inductive pulse generator, scanning the teeth of the starter ring gear. The magnetic flux change induces an alternating current to be evaluated by the control unit.
Reference Point
A small plate on the flywheel produces, in a second identical impulse generator a reference point signal. In the control unit, the crank shaft position is computed from engine speed and reference point.
Intake Air Flow
The ambient air drawn in through the air filter and air flow meter by the engine acts upon the sensor flap of the latter. A potentiometer connected to the sensor flap supplies a voltage which is proportional to the air quantity per time unit.
Intake Air Temperature
A temperature sensor integrated into the air flow meter transmits a signal to the control unit. This signal corresponds to the intake air temperature at the sensor flap.
Engine Coolant Temperature
This is transmitted to the control unit from a sensor, located in the engine coolant.
Battery Voltage
Another input value to be acquired is the battery voltage.
Throttle Valve Switch
It transmits two signals to the control unit, one for closed throttle and one for wide open throttle.
Start Signal
The start signal is supplied from the starter motor connection terminal "50".
Lambda-Signal
The oxygen sensor provides a signal corresponding to the oxygen content in the exhaust gas.
FUEL INJECTION:
An electrically operated roller cell pump supplies, via a fuel filter, fuel to the six electromagnetic injection valves. The fuel pressure is controlled by a regulator, connected to the intake manifold vacuum, to maintain a constant pressure differential at the injection valves. Thus, the injection time serves as a measure for the fuel quantity injected. Excess fuel flows from the pressure regulator back into the fuel tank.
The injection valves are wired in parallel. Consequently, they all inject at the same time, however, at different working cycle phases of the individual cylinders. Since there is one injection per crankshaft rotation, there are two injections per total working cycle of each cylinder, and the resulting average gives a sufficiently uniform combustion in all six cylinders.
For cold starts which necessitate an additional fuel supply, a cold start valve is provided. It is also connected to the fuel line being held under constant pressure and is located in the intake manifold.
The cold start valve operates independently from the electronic control unit. During a start, it is actuated by a thermo-time-switch located in the engine coolant. The operating period increases proportionally to the decreasing engine temperature. There is no valve actuation above a certain engine coolant temperature.
FUEL SUPPLY:
The control is of the fuel pump represents a further function of the MCU. It is switched on over a relay upon cranking of the engine.
Moreover, a speed check control assures that it is only in operation with the engine running and switched off directly after the engine stops operating.
IDLE CONTROL SYSTEMS:
Idle Control Valve:
The Idle Control Valve is a solenoid plunger type valve. Air bypasses the throttle valve through the idle control valve to increase or maintain the idle speed. The amount of bypass air depends on the amount of current through the valve. The current varies between 0 and 500 MA. The valve is open when the current is off.
The idle control unit controls the amount of current through the valve to regulate the idle speed.
The control unit receives as inputs:
Engine Speed
Coolant Temperature (below/above 45°C)
Ambient Air Temperature
A/C on/off
Throttle Value Switch (idle contact)
Automatic Transmission Selector Position
IDLE CONTROL SPEEDS:
Automatic Transmission
1) In drive, 1 or 2 700 rpm
2) Park or Neutral
a) engine cold <45°C 950 rpm
b) engine warm >45°C
i) ambient air temp. <0°C 850 rpm
ii) ambient air tempt. >0°C
- A/C on 850 rpm
- A/C off 700 rpm
Manual Transmission
1) Engine cold <45°C 950 rpm
2) Engine warm >45°C
a) ambient air tempt. <0°C 850 rpm
b) ambient air tempt. >0°C
- A/C on 850 rpm
- A/C off 700 rpm
DECELERATION FUEL CUT-OFF:
To reduce fuel consumption during coasting, a temporary fuel cut-off to the injection valves is provided for under the following engine speed conditions:
Engine Cold - Above 1800 rpm
Engine at Operating Temp. - Above 1360 rpm
Fuel supply to the Injection Valves is resumed at an engine speed of 400 rpm less than the cut-off speed.
ENGINE SPEED LIMIT:
To limit the maximum engine speed, a fuel cut-off takes place:
- Above 5,000 for the M20 (ETA) Engine.
CLOSED LOOP CONTROL:
An oxygen sensor is installed in the engine exhaust pipe, in front of the catalyst, so that it is fully immersed in the flow of exhaust gases leaving the engine. An opening connects the interior of the sensor with the ambient air
With varying partial oxygen pressure inside and outside the sensor, a voltage is generated at the terminals. This signal is fed into the MCU-lambda control.
As the passing exhaust gas stream changes from slightly rich to slightly lean, there is a big change in oxygen partial pressure. In response to this marked change in partial pressure, the electrical potential changes from about 625 millivolts on the rich side of the stoichiometric point to less than 175 millivolts under lean conditions. (Stoichiometric point means lambda = 1.) Mixture variations confined to either side of the stoichiometric point cause only slight changes in oxygen concentration and hence the sensor produces an essential on-off signal.
IGNITION:
The primary circuit of the ignition coil is connected to the power output stage of the MCU without pre-resistor:
The secondary voltage is maintained constant, independent of engine speed and battery voltage. It flows from the ignition coil via the high-tension distributor to the spark plugs.
The distributor consists merely of a rotor: rigidly attached to the camshaft with a cap and dust-protection cover: Since the ignition timing is controlled by the MCU, the high-tension distributor serves exclusively for the high-voltage distribution. The system is non-adjustable and requires (with the exception of spark plug changes) no maintenance.
To provide interference suppression of the ignition system, suppression resistors are located in all high-voltage plugs. Moreover: the distributor cap is provided with a conductive hood which is connected to the engine mass and thereby lowers the interference radiation of the distributor.
IGNITION CHARACTERISTICS:
A basic injection 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" was developed with the objective of optimizing the ignition for the most favorable exhaust gas and fuel consumption performance for any given load/speed condition.
When the throttle valve is closed, the bottom line of the "map" is selected as the idle/coast characteristic.
When the engine speed drops below the nominal values, the ignition timing is advanced to obtain idle stabilization. For coasting, the ignition timing is programmed to obtain an optimum exhaust gas and driveablity performance.
On wide open throttle, the top line of the "map" is selected.
While cranking, an ignition timing of 10 degrees BTDC, independent of the basic ignition map, is programmed. It is retarded to 5 degrees BTDC when the engine is at operating temperature.
In addition, the ignition characteristics are modified on the basic ignition map, due to changes in engine coolant temperature and intake air temperature.
EVAPORATIVE CONTROL SYSTEM:
The evaporative control system is designed to prevent hydrocarbon vapors from escaping to the atmosphere. The system consists of a carbon canister, purge control valve and lines connecting the canister to the fuel tank and intake manifold.
When the engine is not running, hydrocarbon vapors that are not contained in the vapor storage tank pass through the line and are trapped in the canister.
When the engine is running, and a vacuum is applied to the purge line, fresh air enters the canister and purges the vapors. The air also passes through the line to the fuel tank.
Due to several modifications of the purge system, operation of the system varies with different models.
For system modification and operation, refer to the appropriate Service Information Bulletins and Workshop manuals.