The potential of using the ion-current signal for optimizing engine stability - Comparisons of lean and EGR (stoichiometric) operation

Ion current measurements can give information useful for controlling the combustion stability in a multi-cylinder engine. Operation near the dilution limit (air or EGR) can be achieved and it can be optimized individually for the cylinders, resulting in a system with better engine stability for highly diluted mixtures. This method will also compensate for engine wear, e.g. changes in volumetric efficiency and fuel injector characteristics. Especially in a port injected engine, changes in fuel injector characteristics can lead to increased emissions and deteriorated engine performance when operating with a closed-loop lambda control system. One problem using the ion-current signal to control engine stability near the lean limit is the weak signal resulting in low signal to noise ratio. Measurements presented in this paper were made on a turbocharged 9.6 liter six cylinder natural gas engine with port injection. Each cylinder was individually controlled by a cylinder control module (CCM). A high turbulence combustion chamber was used to be able to operate with highly diluted mixtures. Comparisons between lean and EGR (stoichiometric) operation were made to investigate the potential of using the ion-current signal to control engine stability (cylinder to cylinder and cycle to cycle variations). A much stronger ion-current signal was found with EGR compared to lean operation, for the same load and comparable emissions.