Comparison of Air Meter Interface Strategies for Engine Management Systems

When an air meter is specified for an engine management system, air meter accuracy is given high priority. Air meter manufacturers characterize the accuracy of their products using laboratory instrumentation to measure the air meter output vs. flow characteristics. Ultimately the air meter is applied to an engine management system in a vehicle. The engine management system must use the information provided by the air meter without the benefit of laboratory instrumentation. Therefore, the entire measurement system must be considered in evaluating the effective accuracy. The most fundamental aspect to consider is the output signal format between the air meter and the engine management system. Two commonly available formats will be investigated: frequency and voltage.

This paper develops the equations relating signal resolution and accuracy to such factors as air meter flow curve shape, air flow dynamic range, analog-to-digital converter resolution, frequency range, timer period resolution, and measurement strategy. These equations will allow evaluation of the uncertainty introduced by the measurement system for both voltage-out and frequency-out air meters. Airflow measurement under steady state conditions is examined for both signal formats. Using the equations developed here, one can compare the tradeoffs in measurement accuracy of various air meter interface approaches for specific situations.