Cyclic combustion pressure dynamics of a diesel engine operating on waste frying oil biodiesel blends using wavelets

This paper investigates the cyclic combustion pressure dynamics of a single-cylinder diesel engine burning waste frying oil biodiesel blends, employing statistical analysis and continuous wavelet transform (CWT) to evaluate combustion stability. Experiments were conducted at a constant engine load (90%) and speed (1500 rpm), with in-cylinder pressure data collected over 400 consecutive cycles for conventional diesel (RD) and biodiesel blends (UB10, UB30, UB50). Results demonstrate that biodiesel blends marginally reduce average peak cylinder pressure (PP) by 0.3–3.8% while lowering PP variability, evidenced by declining coefficients of variation (COV-PP: 1.27–1.48%), attributed to biodiesel’s elevated viscosity promoting consistent fuel-air mixing. Conversely, mean indicated pressure (MIP) variability (COV-MIP: 3.15–4.64%) increases with biodiesel content, though remaining below the 10% threshold for operational instability. Wavelet analysis reveals distinct combustion dynamics: diesel exhibits high-frequency, intermittent oscillations, whereas biodiesel blends display sustained low-frequency periodicities linked to prolonged combustion phasing. Global wavelet spectrum (GWS) values rise progressively (7.01–12.9) with biodiesel content, quantifying amplified cyclic variability. These findings validate waste-derived biodiesel as a sustainable alternative, balancing environmental benefits with acceptable engine performance. The integration of wavelet-based signal processing advances combustion diagnostics, offering insights for optimizing alternative fuels in compression ignition engines