Assessment of different included spray cone angles and injection strategies for PCCI diesel engine combustion

Paper #:
  • 2017-01-0717

  • 2017-03-28
In the foreseeable future, the transportation sector will continue to rely on internal combustion engines. Therefore, the reduction of engine-out emissions combined with increased engine efficiency are important goals to meet future legislative regulations and restricted fossil fuel resources. One viable option, which provides lower peak temperatures and increased mixture homogeneity and thus simultaneously reduces nitric oxides (NOx) as well as soot, is the low-temperature combustion (LTC) concept. Applied to a Diesel engine, the LTC concept is frequently called as premixed charge compression ignition (PCCI). The main idea behind this approach is to increase the ignition delay time in order to promote the mixture formation. This goal is achieved by the combination of early injection timings and high amounts of EGR. Early injection timings can lead to higher combustion sound level and lower engine efficiency, due to early combustion phasing before top dead center (bTDC). In addition, spray-wall interactions and over leaning of the mixture might result in an increase of total hydrocarbons (THC) and carbon monoxide (CO). Numerous studies showed possibilities to counter act this 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. Experiments in this study are carried out in four-cylinder Diesel engine with a displacement of 1.9 l and a compression ratio of 17.5:1. The different approaches are investigated for different loads (2.5-5 bar) and different engine speeds (1500-2500 rpm). The applied injection strategies are a single injection with conventional timing, and three multiple injection approaches with varying timings for the pilot injection and fuel mass distribution. The actual main injection timings for the multiple injections are controlled such that the combustion center is kept constant at 8° after top dead center (aTDC) Comparison of experimental data for an EGR variation shows that the split injection increases indicated efficiency. This effect is more pronounced for narrower included spray angles. As a result, applying the 100° nozzle with split 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 air entrainment, which becomes more important when external EGR is increased. Furthermore, the combination of advanced injection timings and nozzle geometry lead to simultaneous reduction of NOx, soot, and CSL, while fuel efficiency remains constant. No remarkable rise in unburned hydrocarbons as well as carbon monoxide emissions is observed and Euro 6 emission regulations are achieved, if the efficiency of the catalyst is assumed to be 95 %.
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