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