Fuel Trim Control

FUEL TRIM





Overview
The purpose of EGR and fuel trim control is to reduce exhaust gas emissions. The EGR system is used to achieve a partial reduction in emissions of nitrogen oxides (NOx) in the gases, while the fuel trim function reduces the level even further and also reduces emissions of carbon monoxide (CO) and hydrocarbons (HO).

Theoretically, if the correct amounts of oxygen and chemically pure fuel are supplied, the products of combustion will be water (H2O) and carbon dioxide (CO2), and the exhaust gases will be completely innocuous.





In practice, however, a certain amount of hydrocarbons (HO) and varying quantities of carbon monoxide (CO) and carbon dioxide (CO2) are present. Nitrogen oxides, such as NO and N02, are also formed. These are normally referred to by the common term nitrogen oxides (NOx).





Accelerating the reaction between the residual constituents using a catalytic converter enables these to be converted to water (H20), carbon dioxide (CO2) and nitrogen (N2).

However, this is possible only if the exhaust gases contain an exact balance of hydrocarbons (HC), carbon monoxide (C0), oxygen (02) and nitrogen oxides (NOx), which occurs when the mixture consists of 14.7 lbs/kg of air to 1 lb/kg of fuel. This is said to represent a value of lambda = 1.





A basic program in the ECM computes the injection period based on information regarding the engine load i.e. the mass air flow and engine speed. The period computed in this manner is not used directly, but is modified by a factor (integrator) in which the HO2S signal is used for fine adjustment of the period to achieve a value of Xl, representing the optimum air/fuel ratio. In cars with two HO2S sensors (certain markets only), the ECM also uses the rear HO2S signal to correct the integrator and, as a result, the injection period. Among other purposes, this is done to achieve more accurate control.

Control is rapid and may be applied several times per second. This short term fuel trim function can modify the injection period computed by the basic program by a maximum of approx. ±25%.





Adaptive Functions
Certain factors, for instance tolerance discrepancies on certain components such as the MAF meter and injectors, air leakage in the intake section or clogged air filters, will affect the fuel/air mixture. To compensate for this, the ECM fuel trim function incorporates adaptive (or "self-learning") functions. When the engine is new, the integrator is assumed to vary cyclically about a nominal median of 1.00 (A), with a variation of perhaps 15% in the injection period when the fuel trim function is active.

In the event, for example, of an air leakage, the integrator will be displaced very rapidly to a new position (B), at which it may vary between 1.10 (+10%) and 1.20 (+20%). In other words, the variation in amplitude will still be 5%, although the new median will be displaced relative to the original median (A). Effectively, this means that the injection period has been increased to compensate for the increased air flow.

The adaptive functions will correct this change and the integrator will operate about the new median (B), with the full control range of ±25% again available.

In simple terms, the adaptive function may be regarded as a measure of the difference (C) between the original (A) and the new median (B).

The adaptive functions consist of two elements:

^ The first element (an additive term) can increase or decrease the injection period by 0.5 ms. Adaption takes place when the engine operates at idling speed, and it adjusts the integrator to the centre line within a few minutes. This is the control module's way of adjusting the CO content at idling speed.

^ The second element (a multiplicative factor) can increase or decrease the injection period by 25%. This adaption takes place when the engine operates at part load.

The adaptive adjustments to the injection period, called long term fuel trim, are stored in the ECM after the engine has been stopped. This means that the correct mixture conditions are achieved immediately on restarting, before the HO2S has become sufficiently hot to function.