Small and miniature aircraft, also known as UAVs (unmanned aerial vehicles), require reliable propulsion systems in order to achieve extended flight times with minimal quantities of fuel. Previously, the only option available for fuel delivery to the engine was a carburetor.
The latter, however, is not able to provide an optimum fuel mix for all of the various phases of flight. A newly developed miniaturized electronic fuel injection (EFI) system created especially for small combustion engines provides the benefits of fuel injection, long enjoyed in larger engines, for small UAVs. This system combines reduced consumption and considerably increased reliability. Microdrives are used to power the fuel pump to ensure the correct pressure in the fuel system.
Considerably cheaper to purchase and maintain than manned flights, UAVs are compact and need small internal combustion engines for power; yet, they are fitted with carburetors for fuel delivery purposes. An EFI system, developed by Currawong Engineering Pty Ltd from Kingston, Tasmania, Australia, delivers an optimum fuel mix at all stages of flight, even for miniature aircraft. The fuel, which is under pressure within the system, is injected into the intake manifold using special fuel injectors; while, the fuel/air ratio is electronically controlled by a multi function electronic control unit (ECU). A low-weight DCMicromotor ensures consistent fuel pressure, regardless of height, fuel levels and flight maneuvers. Auxiliary energy is required to build up the necessary fuel pressure.
The problem of aircraft engines with carburetors is that the mixture is too rich at altitude if the carburetor is perfectly tuned at sea level. If the mixture is ideal at altitude, it is too weak on the ground.
Aircraft engines are demanding when it comes to controlling the fuel/air ratio. Fuel and air will only ignite within specific limits, with an even smaller window if the engine needs to run to specific performance and consumption levels. Efficient consumption is especially important in small aircrafts. Every gram saved extends flight time or increases the usable load. One solution is electronic fuel injection.
Because fuel is under several bar of pressure, the vaporization of fuel is ruled out even at high altitudes. Injectors spray the fuel into the inlet pipe, allowing a free, aerodynamic inlet path with no Venturi nozzles. The engine can “breathe” freely, unleashing more performance. This effort is most noticeable at greater altitudes where the air is thinner. Based on electronic engine data such as inlet temperature and air pressure, the amount of fuel injected is constantly recalculated. The injection point and duration are coordinated with the crankshaft angle. In conjunction with the shape of the injectors, the pressure in the fuel system ensures extremely fine vaporization of the fuel. These injectors deliver an optimum mixture of fuel, not only in the ignition where there is practically no flow at all, but also in thin air while in high altitudes, or at full power on the ground in thick air. Overall use of an injection system translates into 15 to 30% better consumption, which increases performance and improves engine reliability.
Although the principle is simple, designing an injection system for miniature engines is complex. Specialists have managed to supply engines of between 10 and 250 ccm for compact systems. As well as the controller, the sensors for the crankshaft position and inlet and cylinder head temperature incorporates an ignition module, injector, fuel pressure accumulator and a fuel pump. The precision mechanical components weigh less than 200 g and take up just 74 x 58 x 39 mm of space. The area of application of the certified components covers a temperature range of – 30 to + 50 °C and an altitude of 6000 m. Non-stop test runs over 1500 hours. Once beyond 700 hours, the test confirms the components to be extremely reliable.
A piston pump ensures the essential pressure build-up. A downstream pressure accumulator, with pressure controller, has a constant pressure of 2.9 to 3.2 bar in the system. In the fuel pump, a microdrive works with a bevel gear system on a crankshaft, powering the actual pump pistons. The 90° force transfer in the gearheads allows the DCMicromotor to be positioned behind the pump cylinder to save space. Controlled by pulse width modulation, the pump unit measures 65 x 22 x 34 mm. To increase torque and enable optimum transfer of the 2.5 W output to the pump crankshaft, planetary gearheads are connected downstream of the engine. The torque, for a two-speed plastic gear weighing 5 g, can be up to 200 mNm. The input ratio reduces 19:1 for a 15 mm diameter. Designed to work in a temperature range of – 30 to + 65 °C, the piston pump is more than adequate for UAV flights anywhere in the world.
Filed Under: Design World articles, Fluid power, Motion control • motor controls, Motors • dc, Pumps
Charles Dohogne says
Nice tech– for the 70s!! Direct Cylinder injection would give much better fuel econimy. Motor boat engines have used it for years.
Motorcycles have had EFI for a long time. Are aircraft engineers just in a world by themselves?
Sounds simple, but in a motorcycle or boat if the engine stops you just stop in one side of the road or continue floating (if the ECU fails…) in an airplane the whole project crash. That means an ultra safe unique system or a redundant system (2 ECU’s, more weight)… IMHO…
Um… isn’t that a picture of a turbine based UAV? Anyone want to collaborate with me on a redundant ECU port injected Wankel based UAV power plant?
It’s called TESTING. You test several different systems in extreme environments. A back up ECU would weigh what, a pound?
William K. says
It is interesting that the assertion is made about a lack of altitude compensation for carburetors. An old manual for an engine used in WW2 fighter planes has a section on adjusting the altitude compensation system, and this was in the early 1940’s. In the mid 1970’s, Carted carburetors often had altitude compensation devices, to meet emission standards. So the technology has existed for a while.
But it does seem that EFI could indeed provide better fuel economy with less effort than is needed to do it with a carburetor, and it may be more reliable than a fully compensated mechanical system. Of course the major advantage of the EFI system is the ease of adjustment of the fuel/air ratio to compensate for different conditions and requirements. It is important to be accurate in explaining the real reasons for making the changes, rather than misrepresenting the state of current technology. Fuel system engineers have done very remarkable things with purely mechanical systems for a long time. The fact that new technology is available does not make the previous achievements invalid.
Maybe they could use a simple carburetor as the back-up. No ECU? No problem! Also, Charles Jones @ Curtis Wright tried to turn it into an aircraft engine. Great power to weight, but not great fuel economy.