Chapter 3: Analysis of the current dynamic status of unmanned combat platforms

With the development of newer technologies, unmanned combat platforms will be more and more used in actual combat and their scope will be wider and wider. my country started late in the research on power equipment of unmanned combat platforms. my country's engines basically rely on imports to meet the needs of the company. Domestic drones, unmanned vehicles, and unmanned boats are subject to power on all battlefields of sea, land and air.

▇Heavy oil usage is limited

The United States’ "UAV System Roadmap (2005-2030)" points out: “Propulsion technology and processor technology are the two key technologies of drones.”

Modern aero piston engines are originally used as power plants for primary trainers, administrative aircraft, small sports aircraft and drones. Most of the fuel of these engines is aviation gasoline, but aviation gasoline has some shortcomings in actual use:

The first problem is that aviation gasoline generally contains lead, to be precise, Tetra-ethyllead (abbreviation TEL). Tetraethyl lead can increase the octane number of fuel and increase the flash point of fuel, prevent knocking in the engine, allow the engine to use a higher compression ratio, and thus improve the efficiency and power of the automobile engine. However, excessive removal of tetraethyl lead can cause systemic diseases caused by neurological poisoning in humans. In severe cases, it can cause coma to death, and is suspected of cancer, which is harmful to the human body. Even though lead-containing additives are declining, lead-containing gasoline, an increasingly important environmental protection, is naturally unpopular in today's society. European and American countries have clearly proposed a period of prohibiting the use of leaded fuel, and also prohibiting the equip of leaded gasoline on military facilities such as warships, and many large routes are not equipped with aviation gasoline. This is one of the biggest problems with aviation gasoline.

One of the solutions is to switch to automobile unleaded gasoline instead of aviation leaded gasoline. However, automotive gasoline has relatively saturated steam pressure and additive corrosion. If the fuel system of the aircraft engine is not fully designed, if the fuel material is gasified into bubbles in the fuel pipeline, it will hinder or even block the engine fuel supply, posing safety hazards to the aircraft. This "VaporLock" effect is the second problem brought by gasoline.

The third problem is that aviation gasoline is volatile and has a low flash point (a lower temperature when a certain ignition source is used to cause liquid vaporization and ignition under specified test conditions) and gasoline mixtures are prone to burning and explosion when exposed to open flames at room temperature. It is not safe enough for dangerous military scenarios.

Due to these reasons, the focus of drone fuel has gradually shifted to aviation heavy fuel.

Air heavy oil

Aviation heavy oil refers to the aviation fuel with fractions between aviation kerosene and diesel. In practical applications, aviation heavy oil generally refers to aviation kerosene or light aviation diesel. Aviation heavy oil does not have lead additives containing lead gasoline, nor does it have the problem of unleaded gasoline gas locks. The flash point and ignition point are relatively higher than gasoline, and the probability of combustion and explosion after an unexpected situation is greatly reduced. Therefore, safety is greatly improved compared with aviation gasoline, and storage and transportation are more convenient. The use of heavy oil as fuel for small and medium-sized drones is also beneficial to reducing the burden of military logistics support. In addition, compared with jet engines with the same power, piston engines have many advantages such as small size, light weight, relatively high power hike, simple structure, and easy operation and maintenance. As the power of small and medium-sized drones, it still has advantages overall.

With the increasing demand for a single fuel, it has become particularly important for small and medium-sized drone engines to use heavy oil (aviation kerosene, diesel, etc.) as fuel to achieve sharing a fuel with equipment powers such as manned fighter jets to effectively reduce logistics support costs. At the same time, heavy oil has a high density and a relatively high flash point, making transportation and storage safer.

UAVs require power equipment to have relatively high power density, lightweight and low vibration characteristics, which are the technical advantages of traditional spark plug ignition aerial gasoline engines (Otto cycle). Therefore, various two-stroke and four-stroke (naturally aspirated and turbocharged, etc.) ignited aviation gasoline piston engines have been widely used in early military and civilian aircraft. However, as demand escalates, aircraft's requirements for power are constantly increasing. In terms of fuel economy, safety, low-income costs, especially under the requirements of unified power oil products in military products, traditional gasoline engines are difficult to meet the requirements.

In addition, customers also found that due to the attenuation of performance attenuation of lower altitude and higher altitude, it will lead to difficulty in taking off and landing than the original; due to insufficient power, it will lead to a reduction in payload; excessive gasoline consumption will lead to a reduction in range; poor gasoline heat dissipation, and aircraft will experience safety and stability problems such as power attenuation in warmer areas of the Middle East. The development of heavy oil aviation piston engines has become the main technical direction.

Heavy oil fuel has relatively flash points, and its atomization and combustion performance are not as good as gasoline. The temperature-induced ignition combustion generated by diesel engines through compression adopts a relatively compressive ratio, the body structure size and weight are larger, and the vibration and noise are also greater. Therefore, the thermal efficiency of the diesel engine is very high, but the work-to-weight ratio is lower than that of the gasoline engine. When used on small and medium-sized drones, the power-to-weight ratio is a key factor in engine selection and even a decisive factor. Heavy oil fuel has a high viscosity at low temperatures and is not easy to atomize and ignite. Therefore, the engine has poor starting performance at low temperatures and needs to be started using fuel or cylinder preheating. Therefore, the United States' "Drive Development Roadmap for 2005-2030" lists heavy oil engines as one of the important development goals of drone technology. It can be predicted that in special cities such as military applications, heavy oil engines will replace existing aviation gasoline engines in the future.

▇Current status and development direction of heavy oil power at home and abroad

1. Current status of development of heavy oil power abroad

According to the United States' "Drive Roadmap for 2005-2030", heavy oil engines are listed as one of the important development goals of drone technology. The United States' reconnaissance drone scanning Eagle Power has also gradually transitioned from the early "gasoline engine" to the "heavy oil engine". All of them use 3W models, and the technical solution still uses ignition heavy oil power with poor fuel consumption. Instead of the Scan Eagle's Drone Black Jack, the power supply is still powered by 3W ignition heavy oil.

Scan the Eagle

Most of the international piston aircraft engines are gasoline-powered, and most of the current products are carburetors and air-cooled structures; there are problems of poor air performance, relatively high fuel consumption, and poor environmental adaptability.

The United States is the first country to carry out research on small aviation heavy oil piston engines. After years of exploration and research, the technology of small aviation heavy oil piston engines in the United States is becoming increasingly mature. The engine has two main technical paths: one is to simply modify the gasoline engine to realize ignition combustion of heavy oil; the other is to develop a compressed ignition heavy oil engine for solutions with better physical and chemical characteristics of heavy oil.

In order to meet the requirements of fuel integration, the US military took the lead in carrying out research on small aviation heavy oil piston engines. Since 1989, the US Navy has successively carried out research on three small aviation heavy oil active engines. By the late 1990s, the U.S. military had increasingly demanded lighter and more powered engines, and at the same time, the requirements for fuel integration became increasingly strict.

2、Current status of domestic heavy oil power development

Domestic development of aviation heavy oil piston engines is still in its infancy. The research content mainly focuses on absorbing and learning from mature foreign technologies, and its independent research and development capabilities are weak. In recent years, many research institutions have carried out some technical research on heavy oil engines and have achieved certain theoretical research results. The main research directions are mainly to change to heavy oil ignition, and heavy oil ignition combustion is partially achieved. China has carried out key technologies such as relatively boosting and two-stage boosting in empty drone cities. Single-stage boosting technology has been implemented for engineering applications, and the boosting ratio has reached more than 4.0. The booster technology is currently mainly used in gasoline engines, but it can also be used in compressed-ignition heavy oil engines to increase the engine's work-to-weight ratio.

Although my country has achieved certain theoretical research results, there are currently no actual developed models for small aviation heavy oil piston engines, and it is still far from the goal of engineering. Most of China's current products are based on automotive engine modifications, not for aviation applications, and have problems such as low power-to-weight ratio, unstable quality, and low reliability. The heavy oil piston engine specially developed for aviation applications is still blank.

3、Key technologies of heavy oil engines and domestic development directions

Domestic drones, including fixed wings and helicopters, use imported gasoline engines in large quantities. Heavy oil engines are an inevitable development trend of the future power system, but engine technology is difficult. The development of heavy oil engines should be distinguished based on the flight platforms and application characteristics of different magnitudes. They cannot be generalized and different technical solutions are adopted.

For small heavy oil engines with power orders of less than 20kW, a ignition heavy oil engine solution can be adopted to improve the heavy oil fuel winding based on the existing two-stroke gasoline engine. We adopt technologies such as fuel preheating, electronic fuel injection and heat recovery to achieve rapid atomization and stable combustion of heavy oil fuels and improve low starting performance. In actual engineering applications, specific maintenance measures are taken to shorten the maintenance cycle and increase trustworthiness.

For heavy oil engines with power order of more than 50kW, reliability is a crucial factor, and traditional compression ignition engine solutions should be adopted. Solve the problem of relatively boosting technology and system matching technology, adopt a horizontally opposed engine body structure, adopt a lightweight design, and the work-to-weight ratio is close to that of the gasoline engine. However, due to the reduction of fuel consumption, the increase in weight can be compensated through system optimization matching. The engineering application technology is mature and the engine is more reliable.

▇Heavy oil power banned from selling abroad, and domestically encountered "bottleneck"

Currently, many UAVs in my country use Austrian Rotax914 piston engines, which are the main power systems of many air-long-time drones in the world. However, once the other party restricts exports, domestic UAVs will fall into a difficult situation of lacking power equipment.

"In the more important technical field of unmanned helicopter power equipment, my country's helicopter engines rely basically on imports to meet the requirements, especially power systems with a take-off weight of 200kg-300kg, and the domestic power field is all blank." Professor Yang Jing, director of the Institute of Power Engineering, School of Vehicle Engineering, Chongqing University of Technology, introduced that at present, the power devices used by unmanned helicopters can be divided into three categories, piston engines, turboshaft engines and current power systems. Among them, the piston engine is a more important power plant composition for unmanned helicopters. Because the engines for unmanned helicopters have relatively relatively demanding power-to-weight ratio requirements, their working environment is extremely harsh, and my country's unmanned helicopter development started late, especially in the field of piston engines for unmanned helicopters, there are no mature products.

Rotax914 piston engine

The external operating environment of aircraft engines is extremely strict, and it must be adapted to the ultrasonic state from the ground to the low oxygen environment of ten thousand meters, from the static state of the ground to thousands of meters per hour, and from the dry desert environment to the tropical humid environment. In short, aircraft engines need to reach a normal working life of thousands of hours in extremely harsh environments such as temperature, colder, faster, pressure, faster, faster, load, hypoxia, and vibration. This makes the development of aircraft engines extremely demanding in disciplines such as structural mechanics, materials science, gas dynamics, engineering thermodynamics, rotor dynamics, fluid mechanics, electronics, control theory, etc.

Entering the new century, China's aviation engine field is facing severe challenges and has good historic development opportunities. Accelerating the development of aviation power and curing the "heart disease" of aircraft has attracted the attention of the leaders of the Party Central Committee and the State Council. Aviation power has been included in the country's key scientific and technological projects, aviation engines and gas turbines have been listed as major national projects, China's aerospace industry has achieved remarkable achievements and achievements, but it is still a certain gap with the world's advanced level. We must focus on developing forward-looking, pioneering, exploratory and disruptive advanced technologies, step up, make more efforts to catch up and surpass, and seize the comparison of international competition system.

Carlo Kopp, founder of Air Power Australia, said, "If China cannot develop a competitive engine, the performance of Chinese aircraft and the air defense capabilities of the Air Force will be severely restricted by other countries."

（Stay tuned for Chapter 4:Haery Power—"Small and Medium-sized Aviation Engines）