To satisfy a single-fuel mandate, the U.S. DOD has a need for engines in the 20 to 50 hp range to both power midsized UAVs and operate on JP-8, or heavy fuel.
Powerplants for some UAVs and other compact applications rely on two-stroke engine technology due to their high power-to-weight ratio and high reliability. UAVs require high power-to-weight ratios to maximize payload capacity, and the high value of UAV payloads demands a highly reliable powerplant to ensure mission success. Compact applications such as auxiliary power units (APUs) also require high power-to-weight ratios to ensure their footprint is minimized for easy transport.
Disadvantages of two-stroke engines, as compared to four-stroke engines, include shorter service life, greater emissions, and lower fuel efficiency. Although these disadvantages have reduced the use of two-stroke engines in many commercial applications, they have not been major roadblocks to their implementation for military applications such as UAVs and APUs because the advantages outweigh the shortcomings.
Researchers from Rochester Institute of Technology set about converting two aircraft engines from gasoline to JP-8 using the Sonex Combustion System (SCS), a patented technology developed by Sonex Research Inc., a developer of spark-ignited heavy fuel engines for military applications. The SCS has been used successfully to convert small engines to heavy fuel, for example the 1.9-hp engine that powers the ScanEagle UAV system. The focus of this research project was to determine the feasibility of converting two larger engines, in the 20 hp and 40 hp range, to operate on heavy fuel using the SCS.
In two-stroke spark-ignition (SI) engines, the SCS achieves ignition of the heavy fuel by vaporizing the fuel near the spark plug late in the compression stroke. The fuel is vaporized by energy retained from the previous combustion cycle. The energy management required to vaporize the fuel around the spark plug is achieved through careful use of geometry and materials.
A bowl in the head of the cylinder surrounds the spark plug. The bowl accommodates a fraction of the incoming charge to be segregated from the bulk of the charge. Since the segregated portion of the charge has a low mass, it is quickly vaporized in the high temperature environment of the cylinder. In addition, the vaporized portion of the charge surrounds the electrode of the spark plug so it will ignite when the spark plug is energized.
Aluminum and mild steel are the materials used in the SCS. Mild steel is used for the bowl around the spark plug because the thermal capacity of mild steel enables sufficient energy from the previous combustion event to be stored in the cylinder head to vaporize the incoming fuel. In a traditional engine design, the energy used to vaporize the incoming charge in the SCS would need to be transferred out of the cylinder either by engine coolant or air, depending on the cooling scheme.
Aluminum is used in the remainder of the cylinder because it is lightweight, strong, and an effective heat transfer material that is commonly used in air-cooled engines. A typical embodiment of the SCS consists of a mild steel cylinder head, or insert, encased in an aluminum cylinder.
A feature of two-stroke engines that aids the SCS in enabling an SI engine to be operated on heavy fuel is the exhaust products that are retained in the cylinder after the expansion stroke. Energy in the hot exhaust products helps to heat and vaporize the incoming charge. Also, the exhaust products contain vaporized unburned hydrocarbons that rapidly combust and energize the combustion reaction.
These features of the SCS combine to effectively manage the waste energy and products of the previous combustion event to enable the effective use of heavy fuel in an SI engine. The result of these features is a heavy-fuel-powered two-stroke engine that has equivalent performance and fuel consumption as a gasoline-fueled two-stroke engine of equivalent size.
A typical engine conversion to heavy fuel using the SCS maintains the gasoline engine's stock carburetion or fuel-injection system, intake and exhaust systems, spark ignition system, and compression ratio. No modifications are made to the moving parts, including the piston. Two-stroke engines converted with the SCS running on JP-5, JP-8, and D-2 diesel (with lubricant additive) retain the ignition precision of the SI process and provide knock-free combustion.
For this project, a design was implemented that replaced the existing cylinder head with a new cast aluminum head assembly. This new casting was designed with a “pocket” or cavity that would accept a steel insert. The insert fits inside the head and maintains the compression ratio of the stock engines. Installation of these new components required the head of the existing cylinder jug to be removed. A ridge was machined into the top of the resulting cylinder to aid in sealing the aluminum head casting/steel insert/cylinder assembly. The volume of the new cylinder assembly was modified by removing additional material from the top of the cylinder until it matches the volume of the original cylinder jug. This is done to maintain the same compression ratio as the original engine design.
To start the converted engines, Bosch glow plugs were employed to heat the cylinder assembly to a sufficient temperature to ignite the heavy fuel. Mounting provisions were designed into the aluminum casting for two glow plugs to be installed in each cylinder assembly.
With the objective of obtaining a heavy fuel engine suitable for integration into a medium-sized UAV platform, RIT researchers selected Limbach L275E/550E engines to install the SCS. The 275E is a horizontally opposed two-cylinder two-stroke offering 20 hp. The L550E is a four-cylinder version of the two-cylinder L275E and is rated at 40 hp. It has twice the displacement as the L275E.
The L275E engine was the “proof of concept” platform to prototype the SCS heavy fuel conversion system on the Limbach series engines. Once the L275E was successfully run on heavy fuel, the conversion design, with minimal changes, was migrated to the L550E engine. The primary difference of the cylinder configuration of the L275E, as compared to the L550E cylinder configuration, is the orientation of the cooling fins of the cylinder heads. The cylinder heads of the L275E are horizontal to make the most use of the wind from the propeller. By contrast, the side-by-side configuration of the L550E cylinders requires the cooling air to pass around the cylinders vertically to maintain temperature uniformity of all cylinders. Therefore, the fins on the cylinder heads must be vertical as well.
The existing cylinder jugs of the stock engines were modified to accept the SCS cylinder head assemblies that consist of a cast aluminum head with a mild steel insert. Also, the glow plugs were installed in the cast aluminum heads to heat the cylinder heads for cold start above 75°C. The glow plugs were connected to an independent battery.
A choke system was fabricated and installed on each carburetor to assist in starting. An independent carburetor was mounted to each cylinder of the original engine.
According to RIT, conversion of the engines showed no power loss as a result of operation on heavy fuel, while special attention should be given to the cylinder head temperatures to ensure proper combustion. This may require an extensive engine cooling system to balance the airflow and maintain an acceptable temperature during static testing.
The glow plugs provided sufficient heating for engine start but required a significant warm-up time (10-25 minutes depending on ambient temperatures and local wind velocity) as well as battery power. This warm-up should be factored into engine start procedures. For reduced weight and complexity for UAV applications, RIT says consideration should be given to an external starter.