The recent availability of fast response (< 1.0 seconds) exhaust gas temperature sensors for exhaust gas temperature measurement enables a new method for closed loop feedback engine control in small engines and other non-stoichiometric applications (V-8 marine fuel injected engines for example). Conventional closed loop stoichiometric A/F control with traditional switching type oxygen sensors is often not applicable because these engines rarely if ever operate at stoichiometric A/F ratios. Richer A/F ratio control is necessary for maximum power and combustion temperature cooling. Wide range UEGO A/F sensors are an alternate solution but the cost of these sensors may be too high. Additionally, the very short exhaust systems found on many small engines can allow ambient air to back flow into the exhaust due to pressure pulsations and corrupt the signal from an oxygen concentration type sensor. In marine engines, water in the exhaust can crack the heated sensing element in the UEGO sensor.
The positive relationship between A/F ratio and exhaust gas temperature is well known. In the range of A/F ratios between 11:1 and 14.5:1, this relationship is almost linear. Most modern automotive engine management systems currently use exhaust gas temperature models based on engine speed, engine load, ignition timing, and A/F ratio (EGR is either constant or closed). Based on these relationships and empirically derived engine mapping, it is possible to calculate A/F ratio from measured exhaust gas temperature along with measured engine speed, engine load, and ignition timing.
This paper will describe the development of a calculated A/F ratio signal based on measured exhaust gas temperature on a 500cc four stroke engine. Additionally, the results of a comparison between the calculated A/F signal and a measured A/F signal from a UEGO sensor during steady state engine operation with three different fuels will be presented.