Len Hamilton United States Navai Academy, 121 BiakeRoad, Annapoiis,MD 21402 e-mail: ljhamiit@usnaedu Dianne Luning Prak United States Navai Academy, 121 BiakeRoad, Annapoiis,MD 21402 e-maii: prak@usna.edu Jim Cowart United States Navai Academy, 121 BiakeRoad, Annapolis, MD 21402 e-mail: cowart@usna.edu Predicting tiie Piiysicai and Ciiemicai Ignition Delays in a Military Diesel Engine Running n-Hexadecane Fuel There are currently numerous efforts to create renewable fuels that have similar properties to conventional diesel fuels. One major future challenge is evaluating how these new fuels will function in older legacy diesel engines. It is desired to have physically based modeling tools that will predict new fuei performance without extensive full scale engine testing. This study evaluates two modeling tools that are used together to predict ignition delay in a militaiy diesel engine running n-hexadecane as a fuel across the engine's speed-load range. AVL-FIRFÍ^ is used to predict the physical delay of the fuel from the start of injec- tion until the formation of a combustible mixture. Then a detailed Lawrence Livermore National Laboratory (LLNL) chemical kinetic mechanism is used to predict the chemical ignition delay. This total model predicted ignition delay is then compared to the experi- mental engine data. The combined model predicted results show good agreement to tliat of the experimental data across the engine operating range with the chemical delay being a larger fraction of the total ignition delay. This study shows that predictive tools have the potential to evaluate new fuel combustion performance. [DOI: 10.1115/1.4026657] Introduction With a widespread motivation to produce new diesel-like fuels from various renewable feedstocks, one open question revolves around how these fuels with different fuel structures will behave in an old engine. Hydro-treated (or hydro-processed) renewable fuels for vegetable oil feedstocks, in general, have a strongly par- affinic nature, comprised of straight chain alkanes as well as branched alkane fuel components. The specific nature of each fuel is a function of the detailed feedstock as well as the processing methods. The US Navy is considering two new hydroprocessed fuels, one from algae sources and another from nonedible vegetable oil sources [1,2]. How might these new fuels perform in a wide vari- ety of engines? What methods might be employed to make such an evaluation? These are challenging questions that motivated this current study. While diesel combustion is a complex multiphase, transient, and turbulent reacting flow, a more straightforward focus may provide the means to evaluate a new fuel. Ignition delay (IGD) is the time from the start of injection (SOI) to the start of combustion (SOC). The authors have seen in a number of previous studies that if IGD occurs within a reasonable window, then good engine operation is likely [3,4]. Certainly the resulting diesel combustion after combustion is very complex; however, the start of combus- tion (SOC = SOI -f IGD) is conceptually more straightforward and may provide the metric to focus on for new fuels [5]. IGD has long been a focus of diesel engine research. One popu- lar correlation is the Arrhenius form of IGD, which includes an apparent activation energy term, a linear pressure term, and the activation energy, gas constant, and temperature in the exponen- tial exponent [6]. The specific correlations have been seen to be engine and fuel dependent; thus, it is believed that a significant fuel change would not be well characterized by this approach. Another popular IGD correlation by Hardenberg and Hase has Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received January 15, 2014; final manuscript received January 23, 2014; published online February 20, 2014. Editor; David Wisler. been widely used for many decades [7]. This correlation included engine speed and charge temperature and pressure effects. The activation energy term was additionally correlated to cetane num- ber (CN) [7]. These empirical studies characterize the overall IGD, which is comprised of both the physical delay due to the fuel spray breakup and mixing with combustion chamber air as well as the chemical delay due to the kinetic behavior of the fuel. A new renewable fuel with different physical and chemical prop- erties will likely not have similar physical and chemical behavior as compared to conventional petroleum based hydrocarbon fuels. The challenge with many new diesel-like fuels is that aromatic and cycloparaffinic hydrocarbon compounds are not part of the new fuel composition; thus, IGD behavior is likely to be very dif- ferent (e.g., higher CN). Physical properties are also different with the absence of aromatics; thus, the resulting diesel spray behavior is likely to be different. In an effort to evaluate a new fuel in a leg- acy engine, this study seeks to evaluate the IGD performance of a pure fuel, n-hexadecane (nC16) in a legacy military engine across the entire speed-load operating map. Two widely available model- ing tools are used. These were chosen for their availability and reasonable computing cost. AVL-EIRE will be used to evaluate the physical delay of the IGD, and the LLNL detailed chemical ki- netic mechanism for nC16 will also be applied for the chemical delay period. Experimental Results Summary The engine used in this study is a military AM General HIVIMWV ("Humvee") engine. Numerous other papers by the authors outline the engine and experimental setup, principally looking at new fuels in this legacy military engine [1,2,5]. The Humvee engine is a mechanically injected IDI V8 diesel engine (6.5 L displacement). The engine used in this study is turbo- charged (up to 0.5 bar gauge boost). The fuel used in this study is the relatively simple pure compo- nent n-hexadecane (e.g., cetane) with a cetane number (by defini- tion) of 100. Key measured physical properties of this fuel are included in the Appendix. The physical properties are important Journai of Engineering for Gas Turbines and Power Copyright © 2014 by ASME JULY2014, Vol. 136 / 071505-1