Characterization and rejection of noise from in-cylinder pressure traces in a diesel engine

Diesel engine in-cylinder pressure analysis is important for engine research and diagnosis. It has been a subject of interest right from the inception of internal combustion engines. Engine cylinder pressure measurements provide insight into the combustion process and the accuracy of these measurements governs the quality of analyses of different combustion modes of the engine. Since the in-cylinder pressure increases abruptly after the start of combustion, non-flush mounting of the pressure transducer creates standing/resonant waves in the access passage which severely affect the recorded pressure fidelity by introducing undesired noise. The challenge is to get rid of these pressure pulsations and characterize the unaccounted noise which can lead to erroneous determination of different combustion parameters and characteristics. This work focuses on online filtering of the noisy pressure data so as to obviate the need of any post-processing for combustion and noise analysis. An online filtering algorithm is defined which is a combination of a five-point moving average filter and a forward and reverse Butterworth digital filter. The filter is tested for its robustness over different engine operating conditions such as engine load, speed, boost etc. The cut-off frequency of the filter is determined on a cycle-by-cycle basis using an algorithm based on the power spectral density of the pressure signal. The noise component is segregated from the pressure trace by means of pressure decomposition technique and the peak noise power is attributed to the access passage resonance frequency. Further development of this approach can be used to achieve optimal combustion control by means of the development of optimal injection strategies in order to fulfill emission reduction and performance requirements in Diesel engines