Exploration of Semi-Volatile Particulate Matter Emissions from Low Temperature Combustion in a Light-Duty Diesel Engine

Diesel low temperature combustion (LTC) is an operational strategy that is effective at reducing soot and oxides of Nitrogen (NOx) emissions at low engine loads in-cylinder. A downside to LTC in diesel engines is increased hydrocarbon (HC) emissions. This study shows that semi-volatile species from LTC form the bulk of particulate matter (PM) upon dilution in the atmosphere. The nature of gas-to-particle conversion from high HC operating modes like LTC has not been well characterized. In this work, we explore engine-out PM and HC emissions from LTC and conventional diffusion combustion (CC) operation for two different engine load and speed modes using a modern light-duty diesel engine. An experimental method to investigate PM volatility was implemented. Raw exhaust was diluted under two dilution conditions. A tandem differential mobility analyzer (TDMA) was used to identify differences in volatility between particle sizes. The study revealed that LTC PM mass and number concentration showed a greater dependence on dilution conditions than PM from CC. There was also evidence of differences in particle volatility as a function of particle size for PM from LTC, with PM from CC having more consistent volatility characteristics. The results of this study show that significant semi-volatile PM emissions are present in LTC exhaust compared to CC operation though they are highly dependent on dilution conditions. This indicates that gas-to-particle conversion processes require additional study to identify the clear impact of LTC implementation on real-world PM emissions.