Assessment of Different Included Spray Cone Angles and Injection Strategies for PCCI Diesel Engine Combustion 2017-01-0717

For compliance with legislative regulations as well as restricted resources of fossil fuel, it is essential to further reduce engine-out emissions and increase engine efficiency. As a result of lower peak temperatures and increased homogeneity, premixed Low-Temperature Combustion (LTC) has the potential to simultaneously reduce nitrogen oxides (BSNO_x) and soot. However, LTC can lead to higher emissions of unburnt total hydrocarbons (BSTHC) and carbon monoxide (BSCO). Furthermore, losses in efficiency are often observed, due to early combustion phasing (CA50) before top dead center (bTDC).

Various studies have shown possibilities to counteract these drawbacks, such as split-injection strategies or different nozzle geometries. In this work, the combination of both is investigated. Three different nozzle geometries with included spray angles of 100°, 120°, and 148° and four injection strategies are applied to investigate the engine performance. The applied injection strategies are a single-injection with conventional timing, two double-injection approaches with different pilot-injection timings, and a triple-injection strategy. The main injection timings are controlled such that the combustion center is kept constant at 8 °CA after top dead center (aTDC). Comparison of experimental data for an exhaust gas recirculation (EGR) variation shows that the double-injection strategies increase indicated efficiency. This effect is more pronounced for narrower spray angles. As a result, applying the 100° nozzle with double-injection achieves the highest indicated efficiency. With increasing EGR, however, the narrower included spray angles lead to high amounts of soot emissions while the 148° nozzle maintains reasonable levels. Further investigation reveals that the overall better performance of the 148° nozzle is mainly attributed to increased air entrainment, which becomes more important when external EGR is increased. Furthermore, the combination of the triple-injection strategy and the wide nozzle geometry (148°) leads to simultaneous reduction of BSNO_x, soot, and combustion sound level (CSL), while fuel efficiency remains constant and no remarkable rise in BSTHC or BSCO is observed.