Air Fuel Control System

Years ago, before exhaust emissions were a concern, the natural gas engines used mainly by the natural gas industry were designed to run with excess air. The air-fuel ratio controllers were mechanical devices that were not very accurate and sometimes not even used. These engines ran very well with 5% to 20% excess air. The air-fuel ratio would often vary with load and as long as the engines would carry the load and didn`t detonate or misfire, people were happy.

Later when exhaust emissions became important, it was discovered that these engines were running with very high NOx levels, sometimes at the peak of the NOx curve. Two strategies evolved to reduce the NOx while containing the CO and unburned hydrocarbons.