Researchers at the US Army Research Laboratory’s (ARL) Combustion Research Laboratory are working to develop engines optimized to run on JP-8 for use in military ground vehicles, stationary power generators, and small unmanned aerial vehicles. JP-8 is a kerosene-base jet fuel, similar to Jet-A for commercial aviation.
In the late 1980s, the DOD issued a “Single Fuel Forward” policy calling for use of JP-8 fuel to reduce the significant logistic burden associated with managing and transporting multiple fuels on the battlefield—albeit with Commercial Off-The Shelf (COTS) internal combustion piston engines originally designed for Diesel Fuel (DF-2) operating with reduced performance with JP-8. There are three challenges to using JP-8 in these COTS engines, according to Dr. Chol-Bum “Mike” Kweon, acting team lead of the Engines Team of ARL’s Vehicle Technology Directorate at Aberdeen Proving Ground, Md.:
Fuel lubricity (which relates to mechanical wear in the system, including high pressure common rail pumps).
Cetane number variability (cetane numbers for JP-8 fuels generally overlap within the ULSD specification range, but there are several outliers below a cetane index of 40 that present special challenges for combustion in a piston engine. The resulting increase in ignition delay would be excessive and would result in reduced efficiency or even misfire.)
The fact that none of the engines that use JP-8 are designed—including their fuel systems—and calibrated for JP-8. Generally speaking, noted AVL researchers in a paper on the problem (Johnson & Hunter 2012), military-grade jet fuels have higher energy content on a mass basis (as a result of the higher hydrogen to carbon ratio), but lower energy density on a volume basis.
As a result of these factors, power and torque output from a COTS diesel engine operated on JP-8 rather than DF-2 can be reduced on the order of 5%.
(In their paper, Johnson and Hunter describe AVL’s approach to this problem: Cylinder Pressure Monitoring to provide closed-loop feedback to enable real-time control of combustion in a compression ignition engine. This makes it possible to adapt to the fuel ignition quality and energy density by adjusting the main injection quantity and the placement of the injection events. This enables he engine control system to detect fuel quality and adapt the combustion phasing quickly and robustly and without any prior knowledge of fuel properties.)
Kweon said the design gap is namely due to the fact that not enough information exists in industry and government on the specific combustion characteristics associated with JP-8’s use in intermittent combustion engines.
Usually large companies are not willing to develop engines specific for JP-8 because it requires significant effort and funding while the market for the military is relatively small and unstable. Therefore, relatively small companies have been developing JP-8-fueled engines for Unmanned Aerial Vehicles, while diesel engines are used for ground vehicle engines. Small companies do not have the capability to perform basic fundamental research.
Fuel spray liquid penetration, quenching, vaporization, and mixing characteristics must be precisely understood to properly design combustion chambers and fuel injection systems because a fundamental understanding of fuel spray and combustion is essential in optimizing combustion processes of JP-8-fueled engines to improve fuel efficiency, engine performance and reliability.—Mike Kweon
ARL’s Combustion Research Laboratory has a high-temperature and high-pressure combustion chamber that opened this summer for fuel spray and combustion research, critical areas of interest Defense-wide to facilitate the basic research and development work necessary for the successful development of JP-8-fueled combustion systems.
ARL is currently collaborating with Army Materiel Systems Analysis Activity to assess a fuel injector that is used in a Caterpillar C7 engine, which is used in the Stryker armored fighting vehicle. Results will define how fuel properties affect the performance of the fuel injection systems that are currently used in ground vehicle engines.
ARL’s vehicle technology research dates back to the early 1980s in gas turbine engines. When this research area relocated to APG in 2011 from NASA Glenn in Cleveland, Ohio, due to Base Realignment and Closure Activity, ARL broadened its vehicle technology focus to include internal combustion engines. This laboratory is accomplished with the ARL infrastructure fund that was awarded at the end of 2010.
The only combustion lab space of its kind in DoD, the Combustion Research Laboratory will also be used to facilitate the development of heavy fuel injection systems that will ultimately lead to the development of high-efficient UAV engines.
Currently, notes Kweon, who received his Masters and Ph.D. from the University of Wisconsin-Madison, there is no “robust” heavy fuel injection system for UAV engines. (Kweon was formerly employed at General Motors R&D in Warren, Mich., and at GM Powertrain in Pontiac, Mich., where he conducted research in cylinder pressure-based control. He established the Gas Technologies Institute’s engine research capability in Des Plaines, Ill. before joining ARL in 2010.)
ARL’s combustion laboratory contains a high-temperature— up to 1,000 Kelvin (K)— and high-pressure— up to 150 bar— combustion chamber that can simulate real engine operating conditions except for fluid motion. This type of combustion chamber allows the investigation and study of uninterrupted spray and combustion processes. This is the only DoD lab with this capability.
The ARL facility also has air and onsite nitrogen supply systems in which it can control oxygen concentration from zero to 21% (pure air) in the gas mixing system. Through the high-pressure compressor, air, nitrogen, or a mixture of air and nitrogen can be supplied to the combustion chamber at pressures over 300 bar(g) to study spray only, spray and combustion, or to simulate exhaust gas recirculation, or EGR, that is common in current engines.
...we use only 150 bar(g) in the combustion chamber because this represents most of turbocharged internal combustion engine operating conditions. Sandia National Laboratories, N.M., and Michigan Technological University have a different type of combustion chamber (i.e., constant volume chamber) that has similar capabilities to the one at ARL in terms of temperature and pressure
The main difference is that the one at ARL is a flow-through type combustion chamber that controls chamber pressure, temperature, and flowrate (very slow compared to fuel injection velocity) at set points in the test section, while the constant volume chamber has varying chamber temperature and pressure as it uses premixed combustion gases. And the injection frequency is much higher for a flow-through chamber than the constant volume chamber. Therefore, we can perform multiple injections per cycle and perform testing much faster in the flow-through chamber than in the constant volume chamber.—Mike Kweon
The new laboratory will also be used to assess the performance of heavy fuel injection systems for various fuels such as JP-8, diesel, bio, and synthetic fuels; investigate the impact of various fuel properties on spray and combustion processes, ultimately on engine performance and efficiency; assess the impact of the aging of fuel injection systems on the engine performance and fuel efficiency, especially for ground vehicle engines and assess JP-8 surrogate fuels that are being formulated under the various DOD programs.
This laboratory has a unique capability to assess the various JP-8 surrogate fuels and to compare the results with the combustion mechanisms developed by various universities and government laboratories.
This will help the scientists and researchers to develop a universal JP-8 combustion mechanism. This laboratory will be used to generate spray and combustion database that will be needed for the development and validation of computation fluid dynamics (CFD) models for engine spray and combustion processes to support the development of advanced concepts and practical designs. These CFD models will be used to optimize internal combustion engines for both UAS (unmanned aerial systems) and ground vehicles in terms of injector parameters and combustion chamber designs. These research efforts will enable UAS engines to efficiently run on heavy fuels such as JP-8.—Mike Kweon
Gustav Johnson and Gary Hunter (2012) How To Deal With Fuel Found In Theater: AVL Cypress - Cylinder Pressure Based Combustion Control For Consistent Performance With Varying Fuel Properties (2012 NDIA Ground Vehicle Systems Engineering And Technology Symposium)
Chol-Bum Kweon (2011) A Review of Heavy-Fueled Rotary Engine Combustion Technologies (ARL-TR-5546)