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

Recent development in hydraulic fracking made natural gas (NG) to be a promising alternative gaseous fuel for heavy-duty diesel engines. The existing compression ignition (CI) engine can be retrofitted to NG spark ignition (SI) operation by replacing the diesel injector with a spark plug and fumigating NG into the intake manifold. However, the original diesel piston geometry (flat head and bowl-in-piston chamber) was usually retained to reduce modification cost. The goal of this study was to increase the understanding of the NG lean-burn characteristics in a diesel-like, fast-burn SI combustion chamber. The experimental platform can operate in conventional (i.e., all engine parts are metal) or in optical configuration (i.e., the stock piston and cylinder block are replaced with a see-through piston and an extended cylinder block). The optical data indicated a fast-propagated flame inside the piston bowl. However, this rapid-burning process did not shorten the combustion duration, which can be explained by an important fuel mass trapped in the squish that burned slowly during the expansion stroke. Steady-state experiments that operated at the metal engine configuration suggested that operating conditions controlled the phasing difference between inside- and outside-bowl burn processes. Advancing the spark timing can increase the phasing separation between these two combustion events to a point that a secondary peak will appear in the heat release rate. Moreover, changing the spark timing had a negligible effect on the end of combustion, probably due to the complex combustion of the charged squish. Overall, natural gas combustion in such retrofitted engines presents differences compared to that in conventional spark ignited engines.