Methodology to Determine the Fast Burn Period Inside a Heavy-Duty Diesel Engine Converted to Natural Gas Lean-Burn Spark Ignition Operation

The conversion of existing diesel engines to natural-gas operation can reduce the dependence on petroleum imports and curtail engine-out emissions. A convenient way to perform such conversion is by adding a gas injector in the intake manifold and replacing the diesel fuel injector with a spark plug to initiate and control the combustion process. However, challenges may appear with respect to engine’s efficiency and emissions as natural-gas spark-ignition combustion inside a diesel combustion chamber is different to that in conventional spark ignition engines. For example, major difference is the phasing and duration of the fast burn, defined as the period in which the rate of heat release increases linearly with crank angle. This study presents a methodology to investigate the fast burn inside a diesel geometry using heat release data. The algorithm was applied to experimental data from a single-cylinder research engine that operated at several lean-burn conditions that changed spark timing, equivalence ratio, and engine speed. More, a 3D CFD RANS engine simulation was used to validate the developed methodology. As results showed that the fast burn definition used for conventional spark ignition engines can produce errors when applied in retrofitted diesel engines, this methodology can help optimize engine conversion.