Comparison of ULSD, Used Cooking Oil Biodiesel, and JP-8 Performance and Emissions in a Single-Cylinder Compression-Ignition Engine

With the rapid growth of biodiesel production, it is prudent to research ways to improve its operation and performance in an engine, especially concerning fuel economy and exhaust emissions. This requires a thorough understanding of both the biodiesel production and engine operating processes. Completion of a published study of the impact of biodiesel fuel properties on engine operation indicated that it is difficult to draw conclusions about the exact causes of increased NO_x emissions with respect to biodiesel properties without the capability of measuring engine cylinder pressures. As improvements were made to the authors' laboratory, a system to monitor and record pressure inside a diesel engine during operation was constructed to test dissimilar fuels. In the current work, three different fuels were tested in order to investigate combustion phasing, emissions, and fuel consumption as a function of fuel properties such as density, viscosity, Cetane Number, and energy content. The fuels tested were Ultra Low-Sulfur Diesel (ULSD), Used Cooking Oil (UCO) biodiesel produced on campus, and jet propellant number 8 (JP-8). ULSD serves as a control fuel for the test since the engine and injector are designed to utilize this specific fuel. JP-8 was included in order to investigate the feasibility of using this fuel in diesel engines as a part of the United States Military's Single Fuel Forward policy. This policy involves a simplification of battlefield logistics through use of a single fuel across all combustion platforms and will result in the use of JP-8 in older engines that were not intended for this fuel.

The engine used for this study was a single-cylinder direct-injection diesel engine, originally packaged as part of an electric generator. The engine was loaded by applying resistance loads while simultaneously recording in-cylinder pressures, engine speed, torque, and air and fuel flow rates at steady-state conditions. The emissions of nitrogen oxides (NO_x), carbon monoxide (CO), and total hydrocarbons (HC) were also measured. The experimental results indicate that fuel chemistry differences between JP-8, UCO biodiesel, and ULSD result in distinctive combustion characteristics affecting both performance and emissions. Specifically, it was found that UCO biodiesel usage advances injection and combustion timing due to an increased bulk modulus and Cetane Number. This leads to improved combustion efficiency, but also higher NO_x emissions due to higher in-cylinder pressures and temperatures (among other factors). In addition, JP-8 combustion occurs later in the engine cycle than ULSD due in part to a lower bulk modulus and resultant later injection. This results in lower combustion efficiency and NO_x emissions than both UCO biodiesel and ULSD. However, the fuel consumption of ULSD and JP-8 were found to be similar across all loads, indicating a possible reduction in fuel consumption for JP-8 to levels below those of ULSD through optimization of injection timing.