Browse Publications Technical Papers 2019-24-0124
2019-09-09

Experimental Investigation of Combustion Characteristics in a Heavy-Duty Compression-Ignition Engine Retrofitted to Natural-Gas Spark-Ignition Operation 2019-24-0124

The conversion of existing diesel engines to natural gas operation can reduce U.S. dependence on petroleum imports and curtail engine-out emissions. Diesel compression ignition engines can be modified to NG spark ignition, by replacing the diesel injector with a NG spark plug and by fumigating NG in the intake manifold, to increase utilization of natural gas heavy-duty transportation sector. As the original diesel piston is maintained during conversion to decrease engine modification cost, the major of this study was to investigate the lean-burn characteristic of natural gas burning in this bowl-in-piston combustion chamber, which can accelerate the introduction of heavy-duty natural gas vehicles. Data analysis from engine experiments that changed spark timing indicated a two-stage combustion process in such retrofitted engines, which is different from traditional spark ignition engines. Inside-bowl burning is one combustion stage which experienced fast flame propagation and burned majority of fuel mass. An important mass trapped in the squish region but burned more slowly formed the other combustion stage. This combustion feature contributed to an almost identical end of combustion in spite of the difference in spark timing. As a result, the conventional Wiebe function cannot be used to predict the combustion process. In addition, combustion phasing controlled the overlap between these two combustion events. Advanced spark timing operation separated more the phasing difference of these two stages and thus a second heat release peak appeared during the late combusting stage. However, the COV of IMEP for were below 2%, despite the over-weighted late combustion and lean combustion operation. The stable combustion process and lack of knocking showed promise for heavy-duty CI engines converted to natural gas spark ignition operation.

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