Overview

During transient conditions such as rapid opening of the throttle there is a corresponding increase in the manifold pressure. This results in some of the fuel in the mixture to come out of suspension and adhere to the walls of the manifold causing a temporary lean condition.

The ECU accel enrichment function is used to provide additional fuel to the engine during such transient conditions. The ECU measures the opening rate of the throttle and calculates the amount of additional fuel to be added to the base injection time.

Criteria

Before the accel function is activated the accel rate must first be above the Min. Accel Limit.
If the accel rate is greater than the Max. Accel Limit this value is used in the calculation.
The accel rate is decremented by the Accel Decay constant once per engine revolution after a transient event has been detected until it returns back to zero.
A scaling constant of 100 is used to convert the numbers to engineering units of microseconds.

Two tables are provided in the engine map to control the amount of accel fuel. The main table is a correction factor vs engine rpm. As engine rpm increases the amount of accel fuel required typically decreases because the change in pressure during a transient is less. The second table is a correction factor vs water tempersature and can be used to provide even more accel fuel during engine warm up if necessary.

Calculation

Accel Fuel = Accel Rate x 100 x Accel f(Water) x Accel f(RPM)

Assuming an accel rate of 20 bits and modifiers of 1.0 and 1.2 for each of the correction tables we would calculate the accel fuel for the first engine revolution as follows :

Accel Fuel = (20 x 100 x 1.0 x 1.2) = 2400 uS

Each subsequent engine revolution the accel fuel is re-calculated using the new value as decremented by the decay constant. The calculation continues until the accel value returns back to zero or if a new accel event is detected the process starts over.

Notes

Negative throttle rate values are not used by the accel enrichment function.
The throttle accel rate can be logged by the ECU internal logger to help with tuning this function.

Winning Edge Electronics