Comparison of Particulate Size Distributions from Advanced and Conventional Combustion - Part I: CDC, HCCI, and RCCI

Comparison of particulate size distribution measurements from different combustion strategies was conducted with a four-stroke single-cylinder diesel engine. Measurements were performed at four different load-speed points with matched combustion phasing. Particle size distributions were measured using a scanning mobility particle sizer (SMPS). To study the influence of volatile particles, measurements were performed with and without a volatile particle remover (thermodenuder) at low and high dilution ratios. The use of a single testing platform enables quantitative comparison between combustion strategies since background sources of particulate are held constant. A large number of volatile particles were present under low dilution ratio sample conditions for most of the operating conditions. To avoid the impact of volatile particles, comparisons were made based on the high dilution ratio measurements with the thermodenuder. As anticipated, CDC had the highest particle number emissions for all operating conditions for particle sizes greater than the 23 nm PMP cutoff. Somewhat surprisingly, the RCCI data show significantly higher particle numbers than the HCCI data for all operating conditions. The higher RCCI particle number emissions indicate that the direct-injection of fuel and non-uniformity of the in-cylinder fuel distribution may contribute to particulate generated from RCCI. Both the RCCI and HCCI results show bimodal behavior with a large number of sub-23 nm particles present, even when high-dilution is used in conjunction with the thermodenuder. Some of these particles may be volatile particles that the thermodenuder is unable to eliminate. The nature of these particles needs to be further investigated in future work.