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65 “Turbines in general are compact relative to the amount of power they produce, and efficient for high-speed medium- and high-altitude aircraft,” explains the French company’s CEO Damien Fauvet. “The problem however is that the more you try to scale down a turbine engine, the more fuel efficiency you lose. “For example, the A400M military transport aircraft uses a 7000 hp turboprop, which achieves greater efficiency than a diesel engine thanks to its high pressure ratio and internal operating temperature. A small, 100 hp turbine with an equally small compressor can’t typically sustain the necessary pressure ratio or integrate enough cooling to manage the heat as well as a big turboprop. “Such an engine would be doomed to low efficiency, perhaps extracting only around 10% of the energy contained in its fuel. By comparison, a decent automotive engine outputs around 25% of its fuel’s energy, while a good diesel engine extracts 30-35%. The key to advancing ‘microturbine’ engines therefore is to solve the problem of fuel efficiency.” Ordinarily, turbine engines do not achieve comparable fuel efficiencies with reciprocating engines unless they produce 2000 hp (1492 kW) or more. However, Fauvet and his team have designed their engines to operate at 26-30% efficiency when producing 50-100 kW. The heat exchanger adds a new stage and section to the traditional turbine operating cycles. A typical turbine engine consists of three main parts, or stages. First is a compressor, which feeds pressurised air to a combustor. That burns a fuel-air mixture, which drives a turbine wheel, from which propulsion is generated (most often through a jet of charge air or, in the case of a turboprop, by the rotation of a driveshaft via a gearbox to a propeller).  These components are generally designed annularly (in cylindrical or ring-shaped stacks) with air travelling through the engine in a straight line from the compressor at the front to the exhaust at the back. As Fauvet explains, that means a lot of heat escapes and is wasted via the exhaust jet, which exits the turbine at 700 C. In Turbotech’s patented ‘regenerative cycle heat exchanger’ though, the intake air travels in an extra back-to-front loop before combustion, during which it is pre-heated by the exhaust gas. By heating the air before it enters the combustor, the amount of fuel needed to produce the necessary charge for driving the turbine is much reduced. Therefore, the fuel efficiency of the small turbine engine is roughly doubled compared with designs without a heat exchanger, while emissions and noise are also significantly reduced. The TP-R90 can therefore achieve fuel efficiency and brake-specific fuel consumption comparable to that in a reciprocating engine but with a higher power-to-weight ratio, while the TG- R55 (when coupled with its fuel tank and associated equipment) can supply electrical energy and power at far higher densities than battery packs. Both systems also come with the benefits of TBOs extending into Turbotech TP-R90 and TG-R55 | Dossier A small turbine can’t typically sustain the necessary pressure ratio, and would have an efficiency of only around 10% Unmanned Systems Technology | April/May 2020 Both the TG-R55 (pictured) and the TP-R90 are designed as recuperated microturbine engine systems. The solution shown here mounts a generator rather than a gearbox and propeller (Author’s image)