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Pick Important: Small-power aero piston engines are the main power type of micro-UAV systems, and the use of heavy oil for a single fuel is an inevitable trend in the future development of UAV power systems. Based on the development trends of drones at home and abroad and the selection of power system types, the application requirements characteristics of low-power aviation heavy oil piston engines were analyzed; according to the trend of single-use aviation fuel development, the development trend of low-power aviation piston engines and the technical bottlenecks faced by small-power aviation piston engines were pointed out; different innovative ideas and technical bottlenecks at this stage in domestic and foreign countries in small-power aviation heavy oil technology andand implementation types, the application characteristics and technical difficulties of different technical routes were analyzed and evaluated.

introduction

Since the 1980s, drones have developed very rapidly. Almost all early drone power plants used aviation piston engines. As the use of drones expanded, the weight increased, the lift limit increased, the speed increased and the battery life increased, turbojet, turboprop, turboshaft and turbofan engines also began to be used on drones; however, because the piston engine has many advantages such as small size, light weight, relatively high power hike, simple structure, and easy operation and maintenance, it still has an absolute advantage overall.

A small-power aero piston engine is an important part of this. Aviation heavy oil refers to the aviation fuel with fractions between aviation kerosene and diesel. In 1994, NASA began implementing a general aviation propulsion program to power general light aircraft that are safe and comfortable, easy to operate and inexpensive in the future. In this report, for the GAP (General Aviation Propulsion) and UAV Power Plan after 2010, the development of heavy fuel engine HFE (Heavy fuel engine) was proposed for the first time. This concept was proposed based on meeting the requirements of the new US airworthiness safety standards (FAA specifications) and the need for cost of use and simplified military logistics support. Since heavy oil engines have many aspects compared to existing engines

Advantages, and at the same time, it can simplify the logistics support of the troops and improve the efficiency of the guarantee. Therefore, heavy oil is an inevitable result for the fuel use of aviation piston engines in the future.

trend.

In view of the development trend of heavy oil adoption of small-power aero piston engines, this paper organizes and analyzes the innovative ideas and different implementation types of heavy oil technology at home and abroad; combined with the current development status and technical level of my country's drone, it proposes the technical entry points and ideas for the development of heavy oil technology of low-power aero piston engines in my country.

1 Difficulties in heavy oil technology of low-power aviation piston engines

The current low-power aero piston engine is basically not completely designed, but is improved from model aircraft engines and through-machines. Therefore, the structural type is basically a two-stroke method, and the fuel supply method is basically a carburetor method. The reason for adopting the two-stroke method is to meet the power-to-weight ratio requirements of the aircraft engine, and the carburetor oil supply method is to directly adopt the existing technical solutions, or make minor improvements according to the needs of the UAV power system. It is very reasonable to use a two-stroke type for a low-power aerospace piston engine. The two-stroke engine itself is simple in structure, has few moving parts, has a large power density and is also small in weight, which meets the lightweight, power-to-weight ratio requirements of the UAV power system. However, using carburetors for the oil supply method brings great problems. The two-stroke engine uses a carburetor fuel supply method. Since the short circuit loss in the scavenging process cannot be avoided, it will inevitably lead to relatively high fuel consumption and poor economicality, which will inevitably fail to meet the needs of long-term flights. In addition, the application of aviation heavy oil poses a greater challenge to the oil supply method of carburetors. The viscosity of aviation heavy oil is high and the atomization effect is worse than that of gasoline. It is basically difficult to directly use existing carburetors to rely on atomization and reasonable combustion for fuel.

Therefore, the two-stroke working type, good scavenging system and new fuel injection system are the development trends of small-power aviation piston engines. Among them, new fuel injection systems are more difficult to implement and are the technical bottleneck of small-power aviation heavy oil piston engines.

2 New low-power aviation piston engine heavy oil technology model

In order to solve the problem of heavy oil atomization at home and abroad, there are two innovative solutions for small-power piston engines. One is to improve the existing carburetors, and the other is to adopt a new oil supply method. Among them, the existing carburetor improvement method still uses carburetor to supply oil, but the design of the intake system, carburetor, and ignition system is changed according to the characteristics of heavy oil fuel, and an auxiliary starting preheating system is added. A typical system is the heavy oil engine solution of Germany 3W. Figure 1 shows a 28mL model airplane gasoline engine, and Figure 2 shows a 28mL aviation heavy oil engine. In order to achieve the trusted atomization and combustion structure of heavy oil, the heavy oil engine adopts a number of improvement measures, as follows: the intake system adopts acceleration tube; the pump membrane carburetor is improved, and the working method is close to that of the mechanical injection system; the crankcase preheating; the compression ratio is reduced; the preheating plug is added to the starting point; the ignition system is changed, and the energy is increased.

Figure 1 3W company 28mL model airplane gasoline engine

Figure 2 3W Company 28mL heavy oil engine

From the comparison between the two, we can see that after using heavy oil, the engine design changes are very large, and there are many changes, so we still use it.

There are many factors that change the heavy oil piston engine with the oil supply method of the carburetor, and there are many additional accessories, which is more complex in design, and it is more difficult to improve and implement. In addition to changing the design of the carburetor and engine to achieve heavy oil atomization and combustion, adopting a new oil supply method is also an important solution to the problem of direct heavy oil atomization, and it is also a future development trend. Generally speaking, the engine oil supply method is divided into mechanical injection system and electronically controlled fuel injection system according to the driving method. Among them, the mechanical injection system is directly driven by the engine's auxiliary mechanical mechanism to complete the fuel injection and adjustment functions, and all control and adjustment are realized through machinery. A typical system is XRDI's heavy oil solution. The biggest feature of this solution is the adoption of a self-designed fast MCDI mechanical fuel direct injection system, which directly injects fuel into the engine cylinder. At the same time, the ignition method of this solution still adopts the ignition method, and in conjunction with the direct fuel injection system, can achieve reliable starting performance.OriginalAt -30 degrees, start-up is reliable without the need for auxiliary preheating devices. Figure 3 is the exterior diagram of the XRDI heavy oil engine.

Figure 3 XRDI 12.5kW heavy oil aviation piston engine

Engines using mechanical injection systems require separate mechanical injection adjustment and drive devices, and all of these functions are realized by mechanical structures, so the overall design is relatively complex, basically a brand new engine design is required, with a large investment, and the mechanical adjustment system has limited adjustment range, poor freedom and flexibility, and a wide range of adaptation. The electronically controlled fuel injection system itself is relatively mature in automotive large-displacement engines and has more experience to learn from. At the same time, because the electronically controlled fuel injection system is flexible in control and easy to adjust, there are many innovations in the electronically controlled fuel injection system. The basic idea is to improve the electrically controlled fuel injection system of traditional large-displacement engines or make new and innovative designs according to the requirements of aviation piston engines. Typical systems include the AADI (AirAssistant Direct Injection) air-assisted injection system of Orbital, the injection system based on MEMS microelectromechanical technology of NWUAV, the DFI micro-pressure direct injection system of JM Harwood, and the MFVI (MultipleFixed-VolumeInjection) small-scale micro-multiple injection system proposed by Beihang. Orbital's AADI air-assisted injection system was adopted by Hirth, Germany, to develop heavy-oil aircraft engines. The air-assisted injection system uses an integrated nozzle to achieve good atomization of fuel by the impact of the relatively pressurized air on fuel particles; by adjusting the auxiliary air pressure and the time of clamping in air, different fog beam shapes can be obtained to adapt to different combustion chamber shapes and spark plug positions. Hirth Company has achieved direct injection in the cylinder of a two-stroke heavy oil engine through the air-assisted injection system, reducing short-circuit fuel loss during the scavenging process, greatly improving economic efficiency; combined with the layered intake diversion structure, spark plugs and nozzle arrangement, the combustion conditions are improved and the combustion efficiency is greatly improved. The atomization effect of the AADI air-assisted injection system injects diesel and gasoline is shown in Figure 4.

diesel fuel gasoline

10mg/ injection 10mg/squirt

6.8μSMD          5.7μ SMD

Figure 4 Orbital AADI air-assisted jet atomization effect

NWUAV, the United States, cooperated with Oregon State University Micro-Nanomic Research Center (ONAMI) to conduct a MEMS-based micro-Fuel injection system development for electromechanical systems. NWUAV has purchased five HP patents on inkjet printers, which are concentrated in microchannel processing, inkjet atomization, flow control and other aspects. Based on the above patent, NWUAV Company has developed an electronically controlled fuel injection system based on MEMS. The injection effect and microchannel structure are shown in Figures 5 and 6.

Since this system is based on MEMS micro-electromechanical system, the injection atomization effect is very good, and it can be used for gasoline, diesel, aviation kerosene and heavy oil, and has a wide range of applications. Due to the adoption of a micro-channel injection structure, the number of injected droplets can be directly controlled to achieve rapid control of the injection volume, and the fuel economy is very good. In addition, the system has very low power consumption, no more than 10W, and is especially suitable for the application requirements of lightweight and fast powered drones. However, the system adopts MEMS processing technology, which has a complex process, high processing difficulty, and high R&D investment. At the same time, due to patent protection, it is difficult to conduct related research. In view of the difficulties of in-cylinder direct injection systems of small-power aeronautical piston engines, combined with the relatively pressure and speed driving method of piezoelectric crystals, JM Harwood has designed an in-cylinder direct injection system suitable for small-power piston engines, which can be directly suitable for aviation heavy oil. The company has implemented the design and development of prototypes, which can achieve 3000psi(20MPa) injection pressure and technical indicators of injection atomization diameter of 8μm, but the technology is still in the research and development stage and the research on engine adaptation is underway. The spray atomization effect and atomization particle size effect are shown in Figures 7 and 8.

Beihang Micro Engine and Distributed Energy Laboratory proposed a small-scale micro-multiple injection system structure based on the actual working characteristics of small-power aviation heavy oil piston engines and the previous experience in fuel injection system development. The system is a low-pressure fixed volume injection multiple times. The fuel volume inhaled by the injector is certain, and the fuel quantity adjustment is achieved through the number of injections. The fuel injector of this system is a miniature fixed-capacity plunger pump. The plunger pump has only one oil inlet and one oil outlet. The oil inlet is a one-way valve and the oil outlet is an ordinary needle valve structure and there is no oil return circuit. Its basic principle structure is shown in Figure 9.

The characteristics of this system are that the structure is very simple, belonging to a micro electromagnetic plunger pump, with very few components, only one; and the system connection is simple and does not require a return oil circuit; the driving method and control adjustment are very simple, and there is no need for complex pressure regulation control of traditional fuel injection systems; the system has low power consumption, belonging to a pulse driving working mode rather than a continuous working mode; the volume and weight are small, easy to integrate, which can achieve a more integrated degree of the engine control system; heavy oil atomization can be directly realized, and trustworthy start without auxiliary preheating devices can be realized.

3 Conclusion

1) Two-stroke low-power aerospace piston engine occupies an important position in the UAV power system.

2) Aviation heavy oil is the future development trend of piston engine fuel, and the use of heavy oil for small-power aviation piston engines requires technological breakthroughs.

3) Small-power aviation piston engines require technological breakthroughs in the use of heavy oil, and an innovative electronically controlled heavy oil injection system is the core key.