The origin of the heavy fuel engines is the military logistical need of “a single fuel on the battlefield”. This concept benefits both logistics and security. Today the technical conditions allow their application on small UAV engines. The idea here is to use a single fuel for all types of vehicles, both ground and air. The choice for heavy-fuel drones is the same used on manned aircrafts: kerosene-based fuel like JP-5, JP-8 or Jet-A1.
Small spark ignition engines that use heavy fuels are called HFE (Heavy Fuel Engines) or multi-fuel engines because they can work with a wide range of fuels, from gasoline to diesel. They are usually two-stroke engines due to their simple construction and low weight.
UAV engine manufacturers offer different solutions for the use of heavy fuels in small unmanned vehicles powerplants. They can be classified into two main groups: direct injection and indirect injection.
Direct injection in multi fuel engines
The direct injection consists of injecting the fuel directly into the combustion chamber. The Australian company Orbital is leading the development of this technology and many manufacturers use the Orbital technology which is known today as Air Assisted Direct Injection (AADI). Orbital applied this system to their own FlexDI engines.
Other companies are developing direct injection HFE systems. For example the firm XRDi, which works on lightweight, fuel efficient and low emission propulsion systems, developed multiple fuel (MFE) engines where proprietary construction technology has been applied.
Fuel and Air Mixtures (MCDI)
The MCDI system uses a small compressor in each cylinder, this compressor injects the fuel and air into the combustion chamber through a nozzle.
Other examples of direct injection are the engines developed by Cosworth and Ricardo. Both use mechanical injection systems similar to those used in automotive mechanical diesel injection.
Ricardo Wolverine 3 HFE
The Cosworth AG heavy fuel engine
These engines are high priced solutions and their application is limited to UAVs for military applications like the Insitu EagleScan (Boeing).
The fact is that the technical complexity of a direct injection system is not necessary. Many other manufacturers have opted for indirect injection.
Indirect injection in multi fuel engines
Indirect injection is the injection system commonly used in spark ignition (gasoline) vehicles. It is a much more affordable solution than those previously exposed.
Most manufacturers of heavy fuel engines have opted for indirect injection: 3W, SKY POWER, Sonex Research or Northwest UAV, just to mention a few.
The main problem of heavy fuel engines with indirect injection systems is the cold start. Fuel atomization is not as good as in direct injection and the mixture ignition is hardest on indirect engines. For this reason, some manufacturers have developed their own systems to make starting easier. They heat the fuel or the combustion chamber to facilitate the vaporization of the fuel.
For example, Sonex Research developed its own technology of heavy fuel engines (HFE). The Sonex Combustion System (SCS) in conjunction with the Patented Cold Start System (CSS) enables converted engines to start reliably at low temperatures with JP-5, JP-8 or Jet-A1. CSS consists of a heated fuel vaporizer and a combustion chamber heater.
Another example of indirect heavy fuel injection is the company Advanced Innovative Engineering, in this test they start the engine with gasoline and make a transition to JP8 with the engine started.
I find it impractical for a customer but in practice once the engine is started and warm there are no changes in operation except that you have to use a slightly different map.
Conclusion
Although manufacturers strive to make specific developments for heavy fuel, the fact is that no engine modification is required except the fuel table to run an engine on kerosene-based fuels such as JP-5, JP-8 or Jet-A1. The motor without any modification needs to be heated above 60ºC to facilitate starting. Once the engine is started, the performance is identical to that of the gasoline-operated engine and it does not have visible smokes.