Use of ionization current to estimate CO rate in a small 2-stroke SI engine

This paper presents an experimental study on a 2-stroke SI engine, used on small portable tools for gardening or agriculture, aimed to identify possible correlations between parameters related to ionization current and air/fuel mixture richness, considering different fuels and spark plug wear. This, to realize a simple system to control the engine parameters and adapt them to engine aging and fuel type changing. The engine was fed with commercial gasoline, low octane number gasoline, alkylate gasoline and a blend of 80% gasoline and 20% ethanol. In all tests carried out with varying engine speed and spark advance the ionization signal was characterized by a single peak, resulting in the impossibility of distinguishing chemical and thermal ionization. All data collected were analyzed looking for correlations between all the available data of CO emissions and several characteristic parameters obtained from the ionization signal. The correlations are targeted to estimate CO rate at exhaust so as to realize a virtual CO sensor for AFR control purposes. An effective strategy, based on the measurement of a series of ionization waveforms while the richness of the mixture is changing, normalized with respect to the maximum values and considering operating conditions where CO rate is more than 2%, is set up making feasible the control of these small 2-stroke engines using the ionization current signal.