Comparison of Ultra-Low Sulfur Diesel and Biodiesel Combustion Characteristics in the Partially Premixed Charge Compression Ignition Regime

Abstract

Compression Ignition (CI) engines offer comparatively higher thermal efficiency. However, rising concerns over the increase in harmful emissions generated by CI engines such as nitrogen oxides (NOx) and particulate matter (PM) need to be addressed. Low temperature combustion through Partially Premixed Charged Compression Ignition (PPCI) is a promising solution proven to alleviate the NOx-PM tradeoff. Therefore, PPCI combustion using ultra-low sulfur diesel (ULSD) was attempted in previous work. With the motivation to further reduce emissions, biodiesel (BD) which has been reported to generate less incomplete combustion products and harmful emissions compared to ULSD was utilized. In addition, renewable sources can be utilized to synthesize BD; thus, reducing the dependence on fossil fuels. Therefore, experimental studies were performed to compare the combustion performance and emission characteristics of BD and ULSD while operating in the PPCI mode. Initially, Fuel Injection Timing (FIT) variation strategy was utilized to achieve PPCI; however, there was limited success in alleviating the NOx-PM tradeoff with BD. The high compression ratio of the test engine was a limiting factor in achieving PPCI due to its influence on the Ignition Delay (ID) period. Subsequently, Exhaust Gas Recirculation (EGR) in conjunction with FIT advancement was used to achieve PPCI. Simultaneous reduction of NOx and PM was achieved by utilizing high rates of EGR at intermittent FITs for both BD and ULSD. Importantly, the incomplete combustion products and NOx emissions were lower for BD at all FIT and EGR settings compared to ULSD. Additionally, the Negative Temperature Coefficient (NTC) behavior of ULSD and BD was captured during the experiments where the ID marginally declined with increasing in-cylinder temperature. Following this, a zero-dimensional combustion model simulating CI combustion was developed using engine geometry and fundamental conservation laws. Corresponding detailed reaction mechanisms of methyl decanoate and ULSD surrogates were incorporated to represent BD and ULSD combustion kinetics respectively. The results predicted peak in-cylinder pressure and temperature data reasonably for the conventional and 15.0° injection events. Moreover, the ID trends were in reasonable agreement with experimental data at all FIT settings for both reaction mechanisms. However, the predicted ID results did not provide sufficient information to state that NTC behavior was captured by the model.