Air-Fuel Ratio Measurement Diagnostics During Cranking and Startup in a Port-Fuel-Injected Spark-Ignition Engine

Cranking and startup fuel control has become increasingly important due to ever tightening emission requirements. Additionally, engine-off strategies during idle will require substantially more engine startup events with the associated need for very clean starts. Thus, knowledge of an engine's Air-Fuel Ratio (AFR) during its early cycles is necessary in order to optimize cranking and startup fueling.

This paper examines and compares two methods of measuring an engine's AFR during engine startup (approximately the first second of operation); an in-cylinder technique using a Fast Flame Ionization Detector (FFID) and the conventional exhaust based Universal Exhaust Gas Oxygen (UEGO) sensor method. Engine starts using a Ford Zetec engine were performed at three different temperatures (0, 20 and 90 C) as well as different initial engine starting positions. The results show that, as expected, the UEGO sensor's response time affects how soon it provides reliable AFR data after start, however, the delay is a number of cycles, and not a number of seconds. It is seen that the UEGO sensor provides accurate cycle resolved AFR data starting with the third cycle to seventh cycle after start. This variation in cycle number after start at which the UEGO sensor becomes accurate is a result of the differences in real time between cycles for the different types of starts as well as the fundamental response time of the sensor. Additionally, the above results are independent of starting temperature. Thus, the UEGO sensor can be used as an accurate AFR measurement device approximately one-half second after cranking begins over a wide range of temperatures.