Effects of injection timing on exhaust particle size and nanostructure on a diesel engine at different loads
Introduction
Diesel particles appear to be stretched as chain-like agglomerates which are composed of spherical or nearly spherical primary nucleus. By using a TEM, primary soot particle size and fractal dimension of exhaust diesel particles have been quantified with either solvent extraction or thermophoretic sampling method under different engine operating conditions (Boehman et al., 2005, Kook and Pickett, 2011, Vander Wal et al., 2007). The mean diameter of primary soot particles has a narrow Gaussian distribution, while the agglomerate size and morphology change broadly with varied engine operating conditions (Müller et al., 2005, Park et al., 2004). Further, the detailed nanostructure of primary soot particles has also been investigated extensively after the first observation of the inner core and outer shell structure by Ishiguro et al. (1997). Vander Wal and Mueller (2006) analyzed the nanostructure of soot particles produced by the combustion of three different fuels in a modern diesel engine. Their results indicated that increasing fuel oxygenation produced soot with less graphitic structure. Lu et al. (2012) compared the nanostructure of diesel soot particles generated under different engine speed and load levels. They concluded that the graphitization of primary particles was affected by engine load rather than engine speed. Al-Qurashi and Boehman (2008) investigated the impact of exhaust gas recirculation (EGR) on the nanostructure of diesel soot, and found that soot particles with highly disordered nanostructure could be produced via EGR. Seong and Boehman (2011) also found soot could become less ordered in its crystalline structure with intake oxygen enrichment. In general, the nanostructure has been found to highly depend upon the parent fuel properties and synthesis conditions including combustion temperature, reaction time and oxygen content (Song et al., 2007, Vander Wal and Tomasek, 2004). In addition, the relationship between the soot nanostructure and oxidation reactivity has been well established (Vander Wal & Tomasek, 2003). Amorphous soot particles with shorter, curved and randomly oriented carbon layers have been confirmed to be more reactive to oxidation.
With engine technological evolution, common rail injection system has been considered to be a superior enabler to reduce engine-out particle emissions. Fuel injection timing as one of the major parameters affects both the combustion process and exhaust emissions. The influence of injection timing on engine performance and regular gas emissions has been extensively studied (Bari et al., 2004, Nwafor et al., 2000, Sayin et al., 2009), and a few researches have focused on particle size distribution variations under different injection timing conditions (Agarwal et al., 2013, Kweon et al., 2003, Niemi et al., 2004). However, there is no further study focusing on the effect of injection timing on soot particle nanostructure except a recent paper by Yehliu et al. (2013). A comprehensive understanding of the effects of injection timing on exhaust particle physical characteristics is still lacking. This paper investigated both the size and nanostructure of particulate samples collected from different injection timings at two engine loads. The experimental results will contribute to understanding the correlation between engine combustion characteristics and particle formation/oxidation processes.
Section snippets
Test engine and fuel
This study was conducted on a 2002 model-year four-cylinder diesel engine which meets the Chinese national stage III emission standards. The main specifications of the engine are described in Table 1. An eddy-current dynamometer was connected with the engine to measure and adjust the speed and torque. A single main injection was adopted in this experiment, and the start of injection (SOI) and injection pressure (IP) was precisely controlled through an open access electronic control unit (ECU).
Combustion analysis
The calculated heat release rate (HRR) and gas mean temperature (GMT) are shown in Fig. 2. For a given engine load condition, the start of combustion (SOC, which refers to the crank angle at the 10% heat release point) is advanced with early SOI. Therefore, the bulk-gas temperature in the cylinder is maintained at a high level for a longer period of time, while no significant variations in the peak value of the GMT are observed as the injection timing is advanced. The maximum GMTs are around
Conclusion
In this study, physical characteristics of diesel exhaust particles including size distribution and nanosturcture were investigated using SMPS and TEM/HRTEM respectively.
As the SMPS results shown, the effects of injection timing on particle size distribution are varied with different engine load conditions. Due to the improve premixed combustion with injection timing retardation, the accumulation mode particles are reduced slightly at the low load condition. However, retarding the injection
Acknowledgment
The authors would like to thank the Projects of the International Cooperation and Exchanges National Natural Science Foundation of China (No. 51210010), the National Natural Science Foundation of China (No. 51006067) and the Ministry of Science and Technology of People’s Republic of China (No. 2010DFA72760) for financial support to this study.
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