Motronics system description (DME)

The Motronics unit used in the E30 325, M3 and late 318's is based on the L-Jetronic system of earlier cars such as the M10 E30 318

The major difference is that timing advance no longer relies on mechanical function, but is instead controlled electronically via controlling of the coil output.

The Motronics relies on the following input signals to determine spark and fuel supply :

Engine speed

This is determined through a crank sensor reading the teeth of the starter ring gear. The inductive pulse generator (crank sensor) produces an AC voltage signal which is sent to the DME for evaluation of engine speed.

Reference signal

The reference signal is determined by an identical inductive pulse generator as the engine speed. The difference is the Reference sensor is looking at one large pin only, located on the flywheel slightly behind the starter ring gear. The signal to the control unit indicates Top Dead Center.

Airflow

Incoming air volume is measured by the Airflow Meter. The Airflow Meter determines air volume via a potentiometer linked to a door. As the throttle is opened, the engine will draw air through he Airflow Meter, sucking open the door, and the potentiometer reports the doors position to the DME. The door position is compared to an imbedded map that provides the exact calculation of air volume from the door opening position. This information is calculated against the engine speed signal to determine what load the engine is under. The DME then supplies the appropriate amount of fuel and timing curve to match the situation.

Intake Air Temp.

An NTC (Negative Temperature Co-efficient) Sensor is placed in the air duct for the Airflow Meter, it measures air temperature coming into the system to allow compensation for air density at various temperatures. An NTC sensor gains resistance as the temperature drops. It is the same type of sensor used for ambient temperature readings on the On Board Converter. If the sensor is unplugged, there is maximum resistance, and the reading would be the coldest specified for the sensor.
Temperature increases are determined from a control signal of 5 volts. The higher the temp, the less resistance, and so, the higher the returning voltage. The lower the temp, the higher the resistance, and so the lower the return voltage. This is how the DME determines temperature.

Engine temperature.

An NTC sensor located in the thermostat housing is used in the above way to monitor and report engine coolant temperature.

Battery Voltage

The battery voltage must be monitored to allow compensation to and for all components. An injector opening time must be increased slightly to compensate for low voltage, as an example.

Throttle position

Throttle position is not constantly monitored. The throttle position sensor consists of two micro-switches, one at idle position and the other at full throttle.

Start Information

During starting , a slightly richer fuel mixture is enacted by the DME. In order to do this, the DME is given a signal from the start system, while the starter is being operated, via what is called Terminal 50. Any reference to terminal 50 in BMW information , refers to the key in the start position.

Lambda sensor (Oxygen sensor)

The oxygen sensor provides a reading of the oxygen content in the exhaust gas. This is used in comparison to determine how much unburned or poorly burnt fuel is being expelled from the engine. Fuel delivery is altered to compensate for mixture reports supplied by the oxygen sensor

Mixture preparation

These vehicles are equipped with two fuel pumps, a pre-pump located inside the fuel tank, which sucks fuel from the tank and delivers it to the main pump. The main pump is located just forward of the left side of the fuel tank. This system allows the main pump to use its power to build and maintain the correct fuel pressure in the fuel rail without the extra strain of having to draw fuel out of the tank.
The pre-pump is a vane style pump , while the main pump is a roller cell style.

The fuel pressure is maintained in the fuel rail with a pressure regulator located on the forward end of the rail. This results in only one variable in amount of fuel delivery, and that is injector opening time. The system uses a map to decide how much fuel to inject, based on the specified fuel pressure. If fuel pressure is increased or decreased due to faulty components, the amount of fuel injected will be incorrect. The result would be a lean or rich report from the oxygen sensor, resulting in an increase or decrease, respectively, of the injector opening time.

To lessen the complexity of the Motronic system, the fuel injectors are programmed to inject 50 % of the required fuel twice every revolution of the camshaft. This corresponds to a full charge of fuel for every revolution of the crankshaft for each cylinder.

Cold starts require more fuel, this is determined from a Terminal 50 signal, combined with the engine temperature signal. If engine temp is below a fixed value, when Terminal 50 is reported, the cold start valve will actuate, placing extra fuel in the intake system. Later cars did away with the cold start injector and, instead, use longer injector opening times on the regular injectors to inject the extra fuel required. As the engine temperature sensor sees the temperature increase, the extra fuel amount is lowered, until, at normal operating temperature, there is no additional fuel being added.

Idle control system

An electronically governed idle control valve keeps the idle speed stable under the various engine operating conditions. Measured intake air, which bypasses the airflow meter, is measured by the idle valve, and fuel supply is compensated accordingly.

The opening of the valve is controlled by the Idle Control Unit (ICU) which uses the signal from the engine speed sensor to determine rpm and compensate it according to engine load.

The idle control system consists of the idle control valve (ICV) and the idle control unit (ICU) up to 1988 models. From 1988 models on, the idle control system was modified. From then, the idle was controlled directly by the Motronic unit, and the valve is non adjustable.

If the A/C compressor is engaged, or at an ambient temperature below 5°C (41°F ) the ideal idle requested by the valve is 850 rpm.
At engine temperatures below 45° C (114° F) the idle is 950 rpm, and above these temperatures, at 700 rpm.
Automatic transmission in Drive will result in an idle of 700 rpm

Ignition

The ignition dwell time is modified electronically according to a map that is checked against inputs such as engine speed, battery voltage and airflow reading. The final stage for the ignition controls the coil via current supplied to Terminal 15. Terminal 15 refers to ignition key in run position, with the engine running, and is used for many BMW electrical operating tests, etc.

Function of the Motronic control unit - Fuel Injection

In the control unit, the airflow meter voltage which is proportional to the air quantity per time unit is divided by engine speed in digital processing/ This corresponds to the air quantity taken in per stroke and represents the uncorrected basic injection time.

In order to compensate the valve operating and releasing time depending on battery voltage, the injection time is extended by the battery voltage correction time. The injection valves are actuated according to the corrected basic injection time.

An adjustable map which is controlled by the oxygen sensor is superimposed on this basic anticipatory control signal. Now by interpolation of the basic injection time a fuel / air mixture which is optimized for low exhaust emission and fuel consumption at every engine speed / load point can be obtained.

With the throttle valve closed during idling or coasting, the lowest oxygen value line is chosen. This enables precise enrichment in coasting operation at low engine speed in order to achieve good combustion.

At full load, a speed dependant full load operation becomes effective which allows the minimum enrichment required for maximum performance.

Due to the small, strongly pulsating air quantity available during starting, a fixed value instead of the air quantity signal is set to determine that injection time. A correction depending on coolant temperature increases the injection time with a cold engine in order to supply the additional quantity of fuel required. At very low starting temperature, the aforementioned cold start valve starts to operate in addition to the regular injectors. In order to achieve a combustion improvement with a cold engine directly after the start, a warm up enrichment depending on coolant temperature and time is provided.

Additionally, an enrichment which depends on coolant temperature is operating for adaptation of the mixture during the warm up phase.

In order to compensate for variations of air density at different air temperatures, the intake air temperature measured in the airflow sensor is used as correction factor for the injection time.

An acceleration enrichment depending on engine temperature is made in order to improve the gas intake and to reduce NOx peaks during acceleration phases.

A deceleration cut off serves to provide fuel economy during coasting. Due to the idle speed rise with a cold engine, the injection is interrupted above 1800 rpm and with a warm engine, above 1360 rpm , while coasting.

To achieve engine speed limitation , the fuel injection is switched off at the maximum permissible engine speed.

The control of the fuel pump represents a further function of the control unit. It is switched on over a relay when the Terminal 50 (starter) is activated. An engine speed control assures that the fuel pump operates only with the engine running.

Function of the Motronic control unit - Ignition

The uncorrected basic injection time for injection is used as load signal for ignition. A 3 dimensional ignition map is sup-opposed on signal and engine speed. So the most favorable ignition timing in respect to exhaust gas emissions and fuel consumption is selected by the control unit at each engine speed / load point.

With the throttle valve closed, the lowest line of the map is selected as the idle/ coasting characteristic line. For engine speed below the required idle speed the ignition timing is advanced in order to obtain idle stabilization. For deceleration operation , the ignition timing is programmed in adaptation to exhaust emission and driveability.

At full load, the uppermost line of the ignition map is selected, where the most favorable ignition value pertaining to the individual injection for full load increase is programmed with the knock limit being taken into consideration.

For starting an ignition time of 10° BTDC independent from the ignition map is programmed which is retarded down to 5° BTDC with a hot engine.

Moreover, an ignition timing correction depending on engine temperature is effective in the different operating ranges of the ignition map.

During idle and thrust operation of the cold engine, the ignition timing is advanced up to 5° BTDC beyond the idle and thrust characteristic line to achieve quicker warm up by increased engine speed.

During partial full load with a cold engine, the ignition timing is advanced up to 8° BTDC in order to match the demands in these operating conditions. With increasing temperature of the intake air and to prevent pinging at partial and full load, the ignition timing is retarded about 8° in comparison to the ignition map.

When the engine is at rest, the electronic control unit prevents the primary circuit of the ignition coil from being closed by means of an open circuit current switch off.

Emission control system

The purpose of the emission control system is to reduce or to eliminate the harmful components in the exhaust gas and other emissions and to prevent contamination of the environment.

With the use of a 3 way catalyst which reduces and oxidizes the fuel / air ratio must be given in a manner so that a reaction balance exists between the exhaust gas and the oxygen. This small range can only be kept with the so called Lambda control.

Oxygen sensor

An electrically heated oxygen sensor is installed in front of the catalytic converter. It is fully immersed in the exhaust gas flow. This results in a lower ambient temperature for the oxygen sensor which increases its life considerably. In addition , the measuring accuracy is more precise since the measuring of exhaust gas for all cylinders is guaranteed. The heating of the oxygen sensor is actuated once the starter is turning and the fuel pump relay is energized.

The oxygen sensor acts due to a chemical and physical process as a DC generator. It emits a voltage between 200mV and 800mV. The generated voltage is directed to the electronic control unit of the fuel injection system as a signal. The oxygen sensor compares the partial oxygen pressure in the exhaust gas with the partial oxygen pressure in the ambient air. The difference is transformed into a voltage signal and given to the electronic control unit.

Generally the emissions of a four stroke engine reach a minimum when the prepared fuel / air ratio is 14:1. This ratio is called the stoichiometric ratio , or Lambda 1. If Lambda is less than 1, i.e. the fuel/ air ratio may be 13:1, the engine is running rich, whereas it operates lean when the Lambda is more than 1.

For low pollution as well as for correct functioning of the catalyst, the fuel / air ratio must be very close to Lambda 1. This is possible by means of a continuous regulation in accordance with the composition of the exhaust gases. The signal from the oxygen sensor causes the ECU of the fuel injection system to vary the injected fuel amount so that the pollution reaches a minimum.

At Lambda 1 a certain voltage is emitted, approximately 450mV. If the voltage signal changes from this value, indicating that the fuel / air ratio is out of Lambda 1, the ECU re-adjusts the amount of injected fuel until Lambda 1 is reached. The oxygen sensor is very sensitive in a range around Lambda 1.

The oxygen sensor is designated to operate in the exhaust gases of unleaded fuel. If fuel contains lead , the oxygen sensor will be damaged and must be replaced.

The oxygen sensor must also be renewed every 50,000 miles to ensure accurate performance.

Catalytic converter

The purpose of the catalytic converter is to finish the burning process of the mixture which has not been fully burned in the combustion chamber.

The catalyst removes CO and HC contamination by oxidation to CO2 and H2O, and NOx contamination by reduction to N2 and O2.

Due to the capability to reduce three harmful contamination components, this type of catalyst is called a 3 way catalyst.

The catalytic converter is integrated into the exhaust system and installed below the vehicles floor in the area of the front seats.

For the correct functioning of the catalytic converter, it is essential that only unleaded fuel is used,. Use of leaded fuel destroys the catalyst.

The catalyst is maintenance free. The spark plug change interval of every 30,000 miles should not be exceeded. The catalyst can suffer damage as a result of misfiring spark plugs. When misfire is detected, the engine should be shut down until repaired to prevent unnecessary damage to the catalyst.

Evaporative emission control system

The evaporative emission control system consists of a purge system leading form the fuel tank to the throttle housing. Between fuel tank and throttle housing , a charcoal canister and a vapor liquid separator are interconnected in the purge system. Emission of gases from the fuel tank is prevented by a sealed fuel cap. The fuel tank has no direct vent to the atmosphere.

When the vehicle is stopped and the engine is off, the gasoline vapors are collected in the vapor liquid separator. While parts of vapors are condensed and the condensed liquid is permitted to flow back to the fuel tank, excess vapors continue to the charcoal canister, where they are absorbed and retained.

After the engine is started, the vacuum in the throttle housing causes a suction effect at the purge line and the gasoline vapors are drawn from the canister and mixed with fresh air burnt by the engine.

The Charcoal canister is equipped with a screen in its bottom surface to permit the drawing in of fresh air for purging of vapor saturated carbon.

The vapor liquid separator (2.7 liter capacity) is also capable of compensating for fuel expansions of a completely filled gasoline tank under conditions of ambient temperatures which may fluctuate about 26°C (80° F)

This system does not employ moving parts and is considered maintenance free.

Crankcase emission control system.

The purpose of the crankcase emission control system is to prevent the escape of blow-by gases and oil vapors into the atmosphere. These gases are directed from the valve cover to the throttle valve and air collector.

This system is maintenance free , however, it is still necessary to check the connections and hoses for condition and tightness.

Service system

The oxygen sensor is monitored by a service warning system, every 50,000 miles, a warning light "Oxygen Sensor" comes on at the dashboard, indicating the emission control system needs maintenance. The oxygen sensor should be replaced and the service interval bulb on the Check Control removed. This warning light is only in place on early E30's.