Determination of cyclic air-fuel ratio and analysis of low and high-frequency variations in dual-fuel RCCI engine

Higher cyclic variations (CVs) in the engine affect the performance, emissions and drivability of the vehicle. Higher CVs are one of the challenges in dual-fuel reactivity-controlled compression ignition (RCCI) engines, mainly at lower loads. Cyclic disparities in the charge preparation, such as air-fuel ratio (AFR), result in CVs in the combustion parameters. The pressure moment method (PMM) is employed in the gasoline/methanol-diesel RCCI engine to estimate cyclic disparities in AFR. The logged cyclic in-cylinder pressure is used to calculate the cyclic AFR. After determining the cyclic AFR, statistical analysis and return maps are applied for the analyses of variations in the AFR, CA10, CA50, pmax and IMEP. For examining the low and high-frequency disparities in cyclic AFR and its relationship with CA10, Wavelet transform (WT) is further applied. A good relationship is found between the estimated mean AFR and the experimental mean AFR. Return maps signify that for the earlier start of injection (SOI), the data points of AFR are more scattered correspondingly, the data points for CA10, CA50, pmax and IMEP are more scattered. WT analysis shows that both high-frequency and low-frequency variations are present in dual-fuel RCCI combustion. It is found that high-frequency discrepancies in the AFR are consistent throughout the engine cycles for all the tested injection timings. Wavelet results confirm that high-frequency fluctuations in AFR result in disparities in CA10 and IMEP.