Digital signal processing of cylinder pressure data for combustion diagnostics of HCCI engine

Diagnosis of combustion is necessary for the estimation of the combustion quality, and control of combustion timing in advanced combustion concepts like HCCI. Combustion diagnostics is often performed using digital processing of pressure signals measured using piezoelectric sensor installed in the combustion chamber of the engine. Four-step pressure signal processing consisting of (i) absolute pressure correction, (ii) phasing w.r.t. crank angle, (iii) cycle averaging and (iv) smoothening is used to get cylinder pressure data from the engine experiments, which is further analyzed to get information about combustion characteristics. This study focuses on various aspect of signal processing (cycle averaging and smoothing) of in-cylinder pressure signal from a HCCI engine acquired using a piezoelectric pressure sensor. Experimental investigations are conducted on a HCCI combustion engine operating at different engine speed/load/air–fuel ratio conditions. The cylinder pressure history of 3000 consecutive engine cycles is acquired for analysis using piezoelectric pressure sensor. This study determines the optimum number of engine cycles to be acquired for reasonably good pressure signals based on standard deviation of in-cylinder pressure, rate of pressure rise and rate of heat release signals. Different signal smoothening methods (using various digital filters) are also analyzed and their results are compared. This study also presents effect of signal processing methods on pressure, pressure rise rate and rate of heat release curves at different engine operating conditions.

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► Diagnosis is necessary for the estimation of combustion quality, and control of combustion timing.
► 4 step pressure signal processing (i) absolute pressure correction (ii) phasing w.r.t. CAD (iii) cycle averaging (iv) smoothening.
► Oscillations in pressure during compression stroke are smaller compared to the combustion stroke.
► Without smoothening pressure signal, the optimum number of cycles used is higher than 1000 cycles for tested operating conditions.
► Leaner operating condition requires lower number of cycles for averaging as compared to richer mixtures.