12. Cyclic Combustion Variations in Diesel–Natural Gas Dual Fuel Engines

Dual fuel combustion is achieved by using a combination of two fuels with extremely different ignition characteristics. For instance, a low-reactivity fuel such as natural gas is compression-ignited using a calibrated amount of appropriately timed, high-pressure, high-reactivity diesel spray. The ensuing combustion occurs at predominantly fuel-lean conditions and is therefore devoid of soot emissions, and the relatively small amount of diesel fuel used also results in the simultaneous reduction in nitrogen oxide emissions. In addition, the use of natural gas, which is predominantly composed of methane, offers the necessary fuel flexibility required to reduce carbon dioxide emissions from conventional neat diesel fired power trains in transportation and power generation applications. The greatest reductions in carbon dioxide emissions are achieved with highest natural gas substitution. However, this also causes problems with high cyclic combustion variations leading to an increased propensity to misfire and high engine-out hydrocarbon emissions. This chapter reviews the current state of the art in strategies to mitigate cyclic combustion variations in dual fuel natural gas engines and provides substantial insights gleaned from past experimental dual fuel combustion research conducted by the authors. In particular, the chapter discusses opportunities and challenges associated with low-temperature dual fuel combustion engines.