Comparisons of Particulate Size Distributions from Multiple Combustion Strategies

Zhang, Y. Comparisons of Particulate Size Distributions from Multiple Combustion Strategies. University of Wisconsin-Madison, 2017.

Particulate matter emissions from advanced combustion engines is an important topic for engine research. Research shows that the particle emissions from internal combustion engines, especially small nanoparticles, can be harmful to human health, as well as the environment. In this study, a comparison of particle size distribution (PSD) measurements from eight different combustion strategies was conducted at four different load-speed points. The particle size distributions are measured using a scanning mobility particle sizer (SMPS) together with a condensation particle counter (CPC). To study the influence of volatile particles, PSD measurements were performed with and without a volatile particle remover (thermodenuder, TD) at both low and high dilution ratios. The common engine platform utilized in the experiment helps eliminating the influence of background particulate and ensures similarity in dilution conditions.

The results show a large number of volatile particles were present under low dilution ratio (LDR) sample conditions for most of the operating conditions. The use of TD, especially when coupled with high dilution ratio (HDR), was demonstrated to be very effective at removing volatile particles and provided consistent measurements across all combustion strategies. PSD comparisons among all combustion strategies were made using HDR with the TD. The results showed that gasoline premixed combustion strategies such as HCCI and GCI generally showed low PSD magnitudes for particle sizes greater than the Particle Measurement Programme (PMP) cutoffii diameter (23 nm), and the PSDs were highly nuclei-mode particle dominated. The strategies using diesel as the only fuel (DLTC and CDC) generally showed the highest particle number emissions for particles larger than 23 nm, and highly accumulation mode particle dominated PSDs were observed. A consistent correlation between the increase of the direct-injection of diesel fuel or the non-uniformity of the in-cylinder fuel distribution and a higher fraction of accumulation-mode particles was observed over all combustion strategies.

A DI fuel substitution study and injector nozzle geometry study were conducted to understand the correlation between the PSD and DI fueling. It was found that DI fuel properties has a clear impact on PSD behavior for CDC and NG DPI. Fuel with lower density and lower sooting tendency led to a nuclei-mode particle dominated PSD shape. For NG RCCI, accumulation-mode particle concentration was found to be insensitive to DI fuel properties. Similar PSD behavior of increased nuclei-mode particle fraction was also observed when a smaller orifice nozzle was used for CDC and NG DPI operation. For NG DPI, a reduction of DI fuel fraction generally led to a reduction in accumulation-mode particles.