Specifics of the Combustion Phenomenon Inside a Heavy-Duty Diesel Engine Converted to Natural Gas Lean-Burn Spark Ignition Operation

The conversion of existing diesel engines to natural gas with the least amount of modifications can reduce the dependence on conventional oil and enhance national energy security. This study investigated such engine conversion using an experimental platform that consisted of a single-cylinder diesel engine modified for lean-burn natural-gas spark-ignition operation through the addition of a gas injector and a spark plug. Following steady-state experiments at several operating conditions that changed spark timing, mixture equivalence ratio, and engine speed, the experimental results suggested that the combustion phenomena in diesel engines retrofitted to lean-burn natural gas spark ignition presents significant differences compared to that in a conventional stoichiometric spark ignition engine. For example, the apparent heat release rate inferred from recorded pressure data is the addition of two separate, sequential combustion events: a fast burn inside the piston bowl and a slow event inside the squish region. To model the heat release in such converted engine, each combustion event was approximated to a Gaussian curve, with the total heat release during the engine cycle being the superimposition of the two curves. While this double-peak curve fitting might not accurately capture the physics of the combustion behaviors, it supported the investigation of two distinct combustion stages in such engines.