Influence of Engine Speed on Gasoline Compression Ignition (GCI) Combustion in a Single-Cylinder Light-Duty Diesel Engine

Paper #:
  • 2017-01-0742

  • 2017-03-28
Gasoline compression ignition (GCI) combustion in which a low ignition quality fuel is used to form the partially premixed charge during the extended ignition delay period has demonstrated significantly improved engine efficiency and lower smoke/NOx emissions in many previous investigations. The major advantage of GCI combustion over other alternative regimes achieving similar goals such as kinetics-controlled HCCI is a close coupling between the fuel injection event and the combustion phasing, a needed characteristic for practical engine applications. The present study aims to address key questions about fuel injection strategies and their effects on the efficiency and emissions of GCI engines. Of particular interest is the double injection strategy implementing early, near-BDC first injection for the formation of premixed charge followed by near-TDC second injection for the combustion phasing control. The engine performance and emissions testing has been conducted in a naturally aspirated, single-cylinder light-duty diesel engine equipped with a common-rail fuel injection system. From the results, it is found that the double injection has lower ignition delay time than the single injection executed at around 20°CA bTDC, leading to lower in-cylinder pressure, pressure rise rate, and heat release rate. While keeping the stable engine operation evidenced by lower than 3% CoV of IMEP, the net indicated efficiency of the double injection is increased by 93% and the indicated specific fuel consumption (ISFC) is decreased by 48%. The double injection strategy was also tested for various engine speeds ranging 1200~2000 rpm and the second injection timings between 12°CA bTDC and 3°CA aTDC. With increase in engine speed, all of the in-cylinder pressure, pressure rise rate and heat release rate show a decreasing trend due to the reduced pre-combustion mixing time. This leads to the decreased engine efficiency/increased ISFC and increased smoke emissions; however, the NOx emissions show a decreasing trend. The combustion phasing is largely unchanged with the engine speed variations but shows a high dependency on the second injection timing. The advanced second injection timing is found to cause the advanced combustion phasing, extended ignition delay, and increased engine efficiency/decreased ISFC. This leads to the reduced smoke/uHC/CO emissions but increased NOx emissions, similar to the smoke-NOx trade-off characteristics of diesel combustion.
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