General description of the injection system

The Bosch Motronic system fitted on the 1998 5 cylinder 20valve engine belongs to the category of digital, inductive discharge, static advance electronic ignition systems integrated with sequential, timed type electronic injection.

This system has only one electronic control unit, a single set of wiring and one set of common sensors. Its function is to inject the exact quantity of petrol into the engine inlet manifold, upstream of the inlet valves, to mix with the air introduced into the cylinder so that the correct mixture strength is obtained.

The Motronic M 2.10.4 system ensures efficient operation making it possible to achieve optimum performance and fuel consumption and to reduce harmful emissions through a response to the different engine operating conditions.

General description of the injection system

The essential conditions which always have to be satisfied in the preparation of the air/fuel mixture for the smooth running of ignition engines are basically two:

1. the metering (air/fuel ratio) should be kept as constant as possible and close to the stoichiometric value, in order to ensure the rapid combustion required to prevent unnecessary fuel consumption;
2. the mixture should be composed of petrol vapours atomized as finely and as uniformly as possible in the air.

In the Bosch Motronic M 2.10.4 system the injector nozzles carry out the task of atomizing the petrol into minute droplets.

Since the absolute pressure of the air drawn may vary, it is necessary to adjust the quantity of petrol to be injected so as not to alter the weight ratio between the air and the petrol.

The constancy of this ratio is obtained by varying the supply pressure of the fuel, by means of a regulator, according to the valueof the vacuum of the air in the inlet manifold, so that the difference between the two pressures is constant at all engine operating conditions.

As far as the optimum metering, on the other hand, is concerned, it is calculated according to the following measurements:

• exact quantity of air drawn in by the flow meter (air flow meter);
• engine rotation speed by means of the rpm sensor;
• acceleration required by means of the butterfly valve position sensor;
• temperature of the engine coolant by means of the sensor on the thermostat mounting;
• measuring the oxygen content of the exhaust gases via the Lambda sensor.

This information is processed by a micro-processor in the injection/ignition electronic control unit which determines the basic injection time using values, obtained experimentally, which are stored in a special memory in the above mentioned control unit.

General description of the ignition system

The ignition system is the static advance, inductive discharge type (i.e. without a high tension distributor) with power modules inside the electronic control unit.

The system has a single coil for each spark plug; the advantages of this solution are:

• less electrical overloading
• guarantee of constant discharge at each spark plug

The electronic control unit memory contains the entire series of optimum advance values which the engine can adopt for its operating range according to the engine speed and load conditions.