Overview

Cosworth’s involvement in unmanned aerial vehicle programmes began in 2005 in response to the US military’s policy to use jet fuel for UAVs. Our single-cylinder, naturally aspirated compression-ignition AE unit impressed the US Navy sufficiently for them to fund a larger engine, the 10bhp AG.

Using Cosworth’s in-house engineering expertise and test facilities, development into the AG continued with renewed funding in 2015, leading to a second-generation unit: a 250cc two-stroke boxer twin that runs on jet fuel, and set new standards for fuel efficiency and time between overhauls (TBO).

Challenges

Initially the AG was designed for simplicity and dependability. The US Navy required higher propulsion performance, more electric power, and longer time between overhauls, which led to the AG2.

With learnings from the AG, we started tackling the demands brought by high cylinder pressure, namely high mechanical loading that typically has weightier structural requirements.

Compromises were needed between weight and strength, and challenges with fuel delivery, combustion and lubrication needed to be overcome.

Solution

Our engineers excel when undertaking projects from scratch. Rather than modifying existing engines to fit a brief, they take a holistic approach – the art of the possible.

The general arrangement of the AG2 is that of a horizontally opposed ‘flat’ twin with the crankshaft directly driving the propeller and alternator.

Our mechanical expertise produced a light engine despite higher mechanical loads. A spark ignition engine could be lighter still, but would need to take into account greater fuel consumption due to lower efficiency. Over a mission, the combined weight of the fuel and AG2 is less than that of a spark ignition engine.

The AG2 has fuel delivered to its common injection rail by a custom-made, lightweight, mechanically driven high-pressure pump integrated into the crankcase. The quantity of fuel is precisely controlled by a custom ECU to deliver the aircraft’s speed and power demands.

Inside the engine, carefully designed parts ensure oil is targeted at critical lubrication areas,,with the system developed to minimise oil flow to reduce the oil mass on a long mission.

Crankcase, barrels and heads are billet productions, machined in our Advanced Manufacturing Centre. To counter friction and enhance wear, AG2 pistons are coated with Diamond-like Carbon (DLC). Conventional piston rings have detail development to cope with the marginal lubrication of a heavy-fuel, two-stroke engine, and provide sufficient heat transfer.

The engine was TBO-validated on our in-house dynos, running to 10bhp at 5,000rpm on representative mission profiling. An overhaul is required at 500 hours. During which pistons, piston rings and bearings are inspected, and again at 1000 and 1500 hours. We have demonstrated TBO and overall life of the unit to 2000 hours (far exceeding the targets outlined by the US Navy) as well as unrivalled fuel efficiency.

With AG2, we have demonstrated the transfer of Cosworth’s expertise into UAV technology often exceeding the performance of alternative manufacturers.

The Cosworth Advantage

Our skilled in-house engineering team and facilities has transformed Unmanned Aerial Vehicle engine technology.

We used GT Power 1D models to define the boundary conditions for the combustion CFD. The innovative combustion system developed for the AE through to the AG2, was optimised at the design stage with the assistance of 3D CFD code.

Predictive combustion with detailed chemistry provided insights into the influence of altitude and fuel type on combustion, helping to identify a robust combination of injector and combustion chamber. The result ensured high efficiency and a wide range of ignitability, leading to wider range of operability.

CFD was applied to study the cooling flows over the heads and barrels. Thermal Finite Element Analysis used input from the 1D modelling to help define cooling fin geometry and understand its impact on piston and cylinder temperatures. 1D and 3D analysis extended to the exhaust system.

Thanks to our state-of-the-art facilities, we generated excellent correlation between analysis models and subsequent dyno testing of the actual engine.

We converted one of our ten engine dynos to run small engines. The dyno cell allows us to run the engine at different intake and exhaust pressures, and temperatures, to replicate mission flight profiles and altitude conditions.

Engine Spec:

Capacity: 243cc
Cooling: Air cooled with active ECU control of cooling ducts
Combustion system: Common rail direct fuel injection with electronic control. Compression ignition
Fuel types: JP-5, JP-8, DS2
Power Output: 10.1hp (7.5kW) @ 5000rpm
Fuel consumption:  <247g/kW.hr minimum BSFC
Engine speed range: 2250 to 5000 rpm
Electrical power generation: 1250W at 28Vdc from 3000 to 5000rpm
Mass: 22.9lb (10.4kg) engine, alternator, prop extension, ECU, wiring loom, air intake, oil pipes, fuel pipes, fuel filter, muffler and exhaust system
TBO: 500hrs demonstrated
Life:  2000hrs demonstrated
Cold start: <30 seconds at -18°C with no preheating