Estimation of cycle-resolved in-cylinder pressure and air-fuel ratio using spark plug ionization current sensing

Abstract

In recent years, several new sensor technologies have been developed and implemented within automotive industries due to the increasing requirements for improved engine performance and emission reduction. It requires detailed and specified knowledge of the combustion process inside the engine cylinder along with a sophisticated technique in engine diagnostics and control. During the last few years, the ionization current signal detection has been the emerging technology in the new sensor developments, in which the spark plug is used as a combustion probe, to improve the performance and emissions of an automobile engine. In this paper, a novel methodology will be presented which allows the cycle-resolved as well as the mean-value estimation of the air-fuel ratio and in-cylinder pressure based on the ionization current signal measurements. The implementation details of this methodology as well as extensive results will be presented for a wide range of air-fuel ratios. The main advantage of this new approach to process the ionization signal is its strong potential for real-time estimation of the air-fuel ratio and combustion diagnostics of individual cylinders and engine cycles. All the complex physics during the actual events (combustion process, ion generation, engine dynamics, etc.) are automatically self-extracted by this technique from acquired data in an initial off-line mapping phase. Once this has been performed, the air-fuel ratio and in-cylinder pressure can easily be estimated for each individual cylinder and combustion event in real-time with few computational requirements. Hence, this methodology has a high potential for the real-time combustion diagnostics and engine control based on the air-fuel ratio and in-cylinder pressure, while eliminating the requirements for installing expensive air-fuel ratio and in-cylinder pressure sensors. The results indicate that estimation of the cycle-resolved air-fuel ratio and in-cylinder pressure is reasonably accurate and robust, despite the inherently noisy character of the ionization signals, with estimation errors typically in the order of 2 per cent or less, except for very fuel-rich conditions.