Two stroke engines for UAV: info collected by BML

UK engineering company Cosworth has been selected by the US Navy for the first of three phases of the two-year ultra endurance unmanned air vehicle (UE-UAV) heavy fuel engine technology development programme.

Cosworth plans to increase the power output of the 3 HP (2.23kW) AE-1 engine, which weighs 3.5 kg, during the four month-long first phase, as well as improving its fuel efficiency and reducing its noise levels. The power output increase, to the US Navy's target of up to 10 HP, is expected to require an increase in the AE-1's number of cylinders from one to two. The three phases include extensive ground and flight test evaluations of heavy fuel engines to prove their technological readiness.

Cosworth's aerospace and defence business unit leader Simon Corbyn says: "Development of a range of heavy-fuel UAV engines is a key requirement to alleviate the military's dependency on multiple fuels. In contrast heavy fuels such as diesel and jet fuels are routinely supported and significantly reduce risk.

"If successful the engine will be used for the US Navy's Small Tactical Unmanned Aerial System, Tier Two class UAV programme. In the second and third quarters last year the AE-1, fuelled with diesel, was used to power the Arcturus T-16 UAV in test flights over California as part of the US Marine Corp's Tier Two tactical UAV concept demonstration programme. In rig tests at Cosworth's Northampton head office the AE-1 used a variety of heavy fuels.

These engines have been developed and built specifically for model flying by a team of specialists. Both engines feature multiple ballraces. The electronic ignition system with integral retard makes the engines very easy to start, and no great force is required. The G 26 is an ideal power plant for two-metre class aerobatic models. The G 57 might well be the lightest high-torque engine in its class, making it an excellent option for F3A-X models. However, it is also a fine choice for other power models weighing up to 13 kg all-up, and for aero-towing tugs of similar weight and up to 2.80 m span.

The G 57 in particular incorporates technologies and manufacturing processes developed for F-1 racing, and both engines have been subjected to an extended programme of static and practical testing. In conjunction with suitable exhaust systems these engines develop a level of power which is normally the province of engines of much greater swept capacity.

The engines are operated using a petrol / oil mix (2-stroke mixture, 1 : 50), while a 4-cell battery of at least 800 mAh capacity is required for the ignition system. The set includes operating instructions, ignition system, propeller fixings and exhaust gasket. We recommend that these engines should be run using the exhaust systems developed specially for them.

From the above plot, we deduce a linear output power scaling with engine capacity of 76 Watts per cc for the Modellmotoren engines. Note that the top three points for the Modellmotoren engines are for multi-cylinder engines (actually 2, 2 and 4 cylinder engines respectively)

Note that the largest engine is a four cylinder motor: hence the lower power-to-weight ratio. We have left off the very high revving OS Engines 91VR-DF for ducted fans with a massive power-to-weight ratio of 5.407 kWatts / kg. If we ignore the Zenoah engines with their low power-to-weight ratios, we get an average power-to-weight ratio of 2.8926 kWatts / kg.

We calculate an average engine efficiency of 0.654 J/ccfam for all of the above engines.