An Analysis of Dual-Fuel Combustion of Diesel with Compressed Natural Gas in a Single-Cylinder Engine 2018-01-0248

The recent increase in natural gas availability has made compressed natural gas (CNG) an option for fueling the transportation sector of the United States economy. In particular, CNG is advantageous in dual-fuel operation alongside ultra low sulfur diesel (ULSD) for compression ignition (CI) engines. This work investigates the usage of natural gas mixtures at varying Energy Substitution Rates (ESRs) within a high compression ratio single-cylinder CI engine, including performance and heat release modeling of dual-fuel combustion. Results demonstrate the differing behavior of utilizing CNG at various substitution rates. In particular, low ESRs (0-18%) find that operation is not too dissimilar from that of neat ULSD, moderate ESRs (40-60%) show that combustion begins to change but significant modifications to engine operation (i.e., ULSD injection timing) are not required, and high ESRs (75-85%) demonstrate relatively large changes to ULSD injection behavior in order to achieve optimized operation.

In addition, this work examines the differing engine emissions profiles with varying ESRs, highlighting the potential for CNG usage to lower Particulate Matter (PM) emissions, while alternatively increasing or decreasing NO_x production through changes to in-cylinder temperatures and heat release rates. Dual-fuel combustion is noted for significant decreases in combustion efficiency, and a rise in emissions of methane and non-methane hydrocarbons, showcasing an increasing likelihood of flame quenching from cooler in-cylinder temperatures. This lowered combustion efficiency also affects brake-specific engine efficiency, requiring slightly more fuel energy to be expended as ESR increases in comparison to operation with neat ULSD. However, this may be offset from an operational perspective due to the lowered costs of CNG as a fuel. Finally, there may exist the potential to achieve high thermal efficiencies just below peak engine operation for relatively large substitution rates of CNG without potentially increasing NO_x emissions over operation with neat ULSD.