Optical diagnostics of fuel induced soot formation: color ratio pyrometry of in-cylinder soot in a DISI engine

The rapid diversification of gasoline-derived fuels demands a deep understanding of in-cylinder soot formation mechanisms, especially under the transient and low-temperature conditions characteristic of cold-start operation. Reliable, high-resolution diagnostics enable spatially resolved investigation of soot formation and allow a detailed analysis of mixture formation and soot-generation processes. This study evaluates Color-Ratio-Pyrometry (CRP) as an optical soot-measurement technique in a state of the art direct-injection spark-ignition (DI-SI) engine operating at steady-state load points for both cold-start (engine coolant temperature 20 °C) and warm-engine (95 °C) conditions. Three fuel families were examined: (i) a worst-case low-volatility gasoline (LV-G) representing a high-soot baseline, (ii) a low-soot, alkylate gasoline (ALK-G), and (iii) binary blends of LV-G or ALK-G with ethanol or methanol in mass fractions of up to 30 %. CRP signals were acquired using high speed imaging through an optical access at cylinder one, synchronized with cylinder pressure and heat release data to validate the temperature estimation. The results of soot volume fractions are compared to particle concentration measurements in the raw exhaust gas using an electric mobility particle spectrometer.