Modulating air/fuel ratio

A method of controlling air to fuel ratio peak to peak amplitude in operation of an internal combustion engine producing exhaust gas which is passed through a catalyst, by modulating the peak to peak air to fuel ratio amplitude as a function of engine rpm and load, and further:

• modulating the peak to peak air to fuel ratio amplitude as a function of catalyst temperature including the steps of:
• determining whether the engine is at idle;
• if yes, setting a standard peak to peak air to fuel ratio amplitude equal to an idle calibrateable constant multiplied by a first function of catalyst temperature, said first function of catalyst temperature being an idle speed control peak to peak air to fuel multiplier determined as a function of catalyst temperature, thereby reducing variation in engine rpm and improving efficiency of operation of the catalyst;
• and
• if no, setting the standard peak to peak air to fuel ratio amplitude equal to a non-idle calibrateable constant multiplied by a second function of catalyst temperature, said second function of catalyst temperature being a peak to peak air to fuel ratio multiplier determined by the output of a lookup table which is based on inputs of catalyst temperature versus engine load, thereby improving efficiency of operation of the catalyst.