Onboard optical fuel sensing for enhanced combustion control in a multi-fuel compression-ignition engine

Compression-ignition (CI) engines with an ignition assistant can be made to operate on a wide range of fuels. The operation of such engines is most efficient and reliable if engine controls capable of real-time adjustments based on fuel cetane number (CN) are implemented. This study demonstrates a closed-loop engine control system using optical fuel sensing during fuel switches between two jet fuel blends with derived cetane numbers (DCN) of 45.80 and 32.79. Three fuel sensors utilizing Raman, dispersive near-infrared (NIR), and attenuated total reflectance Fourier-Transform Infrared (ATR-FTIR) spectroscopy were integrated in-line in a CI engine test cell to infer the DCN of the fuel upstream of the high-pressure common-rail pump. The engine control utilized the NIR sensor measurements to set CN estimation bounds. These bounds served as control inputs for regulating start of combustion (SOC) by adjusting start of injection and power delivered to an ignition assistant. All three fuel sensors provided accurate DCN estimates throughout the fuel switches, with errors of at most 6%, as validated by samples collected and tested using an Ignition Quality Tester (IQT). The engine control effectively utilized the fuel sensor measurements to maintain SOC within ±1 crank-angle degree of the desired value. Without the implemented system, misfires in engine operation are expected.