Stoichiometric ratio and self-adaptability

Stoichiometric ratio and self-adaptability

Stoichiometric ratio is defined and indicated by the symbolα st as follows

α st= theoretical quantity of air required to burn all the fuel injected / quantity of fuel injected

Mixture concentration is defined and indicated by the Greek letterλ as the ratio::

λ = quantity of air taken in by engine / theoretical quantity of air required to burn all the fuel injected

Thus we easily find that α/α st = λ.

Stoichiometric ratio depends on fuel type. For unleaded fuels in current use, this figure is 14.7 - 14.8, which corresponds to lambda = 1. (a ratio of 14.8:1 means that 14.8 parts of air are required to burn 1 part of fuel).

The mixture is termed rich (or heavy) when the quantity if air is less than the stoichiometric level. In this case, lambda equals≤ 1;

the mixture is termed lean (or light) when the quantity of air exceeds the stoichiometric level. In this case, lambda equals≥ 1;

The strategy is designed to correct base pulse constants so that mixture concentration oscillates continually and at a high rate between 0.98 and 1.02.

This oscillation rate varies according to engine load and speed. It is in the order of tens of Hertz.

NOTE

1 Hz = 1 oscillation per second

Under conditions of:

• cut-off,
• throttle opening greater than 70%
• engine temperature less than 25 °C

the strategy is de-activated.

Self-adaptability

The control unit features a self-adaptive function able to memorise deviations between basic maps and corrections imposed by the lambda probe that occur persistently during operation. Such deviations (due to system and engine component ageing) are saved permanently so that system operation can be adapted to gradual changes in engine and components with regard to original specifications.

The strategy is de-activated while the carbon filter flushing solenoid is open. If the ECU is replaced, carry out a road test to enable the engine to warm up and the ECU autoadaptive strategy to come into play (particularly important when idling at a standstill).