Description of the controller operation (BMW 5 Series E12)

Engine idle speed control

The engine idle speed is regulated regardless of the engine running-in stage (values of friction moment), intake air temperature and atmospheric pressure and is constantly maintained at a given level. To do this, the controller constantly compares the measured engine speed with the nominal value entered into its memory. If there is a deviation from the norm, the controller sends a signal to the servo drive to correct the position of the air damper.

In addition to the idle mode, the controller also takes into account the coolant temperature, which ensures idle speed regulation when starting a cold engine at low ambient temperatures.

Enrichment of the combustible mixture during starting, warming up and increasing the engine speed

The controller determines the required degree of enrichment of the combustible mixture during start-up, warm-up and increase in engine speed depending on the crankshaft speed, position of the throttle valve of the 1st chamber, duration of engine operation after a cold start, coolant temperature and opening speed of the throttle valve of the 1st chamber. The corresponding controller output signal is sent to the carburetor during the movement of the air damper in the closing direction. In addition, the controller provides the ability to issue a command for a short-term movement of the air damper in the closing direction when the engine speed increases. The moment of closing, its duration and repeated opening of the air damper are calculated by the controller depending on the operating parameters.

Controlling the amount of combustible mixture at start-up and at fast idle speed

Depending on the coolant temperature and engine speed, the controller determines the amount of combustible mixture required for reliable engine starting and instant and smooth transition to accelerated idle mode, and sends a corresponding signal to the throttle valve servo drive of the 1st chamber. As soon as the crankshaft speed at idle speed is higher than the nominal value, the controller's idle speed control circuit takes over control of the cylinder filling.

Control of fuel cut-off when engine speed drops and when engine stops

When the engine speed decreases, the forced idle system is activated: the fuel supply to the engine is stopped, but additional air is still supplied to the cylinders. The fuel supply is stopped and the filling of the cylinders is increased during engine braking by closing the throttle valve of the first chamber when the crankshaft speed exceeds the threshold value of 1400 rpm. As soon as the speed during engine braking decreases to the threshold value, the controller output signals the resumption of fuel supply to the servo drives of the throttle valve of the first chamber and the air damper, thereby ensuring a relatively smooth (without failures) resumption of the combustion process of the combustible mixture. If the speed increases again during engine braking, the fuel supply and, consequently, the combustion of the combustible mixture are immediately resumed.

When the ignition is turned off, the throttle valve of the first chamber closes, the fuel supply stops and the engine stops immediately. After some time, the throttle valve of the first chamber returns to the starting position to avoid it jamming when in contact with the diffuser surface.

Additional controller functions

In addition to controlling the carburetor operation in the above modes, the controller performs the following tasks:

• based on operating parameters, it sends a fuel consumption signal to the economizer;
• controls the ignition timing at idle speed depending on the intake manifold temperature and the position of the idle switch.

When the idle switch is in the pressed position (its contacts are open), and the temperature of the intake manifold walls exceeds 15°C, atmospheric pressure is supplied to the vacuum regulator of the ignition distributor sensor via the electromagnetic valve and it does not affect the ignition timing.