Fuel Trim Control: Overview

FUEL TRIM CONTROL - OVERVIEW





Fuel trim is intended to reduce exhaust emissions. Fuel trim reduces emissions of nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HC).

Theoretically, fuel from the combustion process can be converted to water (H2O) and carbon dioxide (CO2) if the correct amount of oxygen is mixed with it. Exhaust gases would then be completely innocuous.

In practice, however, a certain amount of hydrocarbons (HC) and varying quantities of carbon monoxide (CO) and carbon dioxide (CO2) are present. Nitrogen oxides, such as NO and NO2 are also formed because of the high temperatures. These are normally referred to by the common term nitrogen oxides (NO).

Accelerating the reaction between the residual constituents using a three-way catalytic converter (TWC) enables this to be converted to water (H2O), carbon dioxide (CO2) and nitrogen (N2). However, this is only possible if there is the exact balance of hydrocarbons (HC), carbon monoxide (CO), oxygen (O2) and nitrogen oxides (NO) in the exhaust gases. This occurs when the fuel/air mixture prior to combustion is 14.7 kg air per 1 kg fuel. This is said to represent a value of lambda equals one, (A=1).

A basic program in the engine control module (ECM) computes the injection period based on information regarding the engine load i.e. the mass air flow and engine speed. The resulting injection period is not used directly as it is integrated in a circuit with the heated oxygen sensor (HO2S) signal, fine-tuning it to achieve A=1. The integrator is a circuit which fine-tunes the injection period to optimize the fuel/air mixture (A=1). The engine control module (ECM) uses both the front and rear heated oxygen sensor (HO2S) signals to correct the integrator and injection period. This achieves more accurate fuel trim. Fuel trim is rapid and is carried out several times per second. Fuel trim can adjust the injection period calculated by the basic program by approx. 25%.