Turbines have been the propulsion engines of choice for large aircraft for many years, while small aircraft operators have had to be satisfied with piston engines, despite a desire for turbine power. The problem has been that low power turbines have unacceptable efficiency ratings of around 10 percent. Now, Turbotech S.A.S., a startup company in France, using ANSYS computational fluid dynamics (CFD) and structural simulation solutions, has patented a regenerative, high-temperature heat exchanger that increases the efficiency of low power turbines by a factor of 2–3. They achieved this by recycling what would normally be waste heat in the exhaust gases to preheat the air entering the combustor, resulting in less fuel required to generate the same amount of power. Turbotech views the turbogenerator as the “missing link” that will enable the future of hybrid-electric aeronautical propulsion.

While regenerative turbines are common for large land-based applications like multistage municipal power generators, compressing the design to make it compact and light enough to fit into a small aircraft without adding excessive weight was a challenge. One key to Turbotech's success was working with Le Guellec Tubes & Profilés, also in France, to develop compact, low-cost microtubes with sufficient strength and surface area to capture enough of the high-temperature waste heat and recycle it to the preheater. Another key was using ANSYS CFD and ANSYS Mechanical simulations for fluid flow, structural, and thermal analysis so they could test the designs virtually without having to build physical prototypes.

Figure 1. The TP-R90 turboprop (Photo: Turbotech)

Turbotech is a member of the ANSYS Startup Program, which provides simulation solutions to startups at steep discounts so they can use simulation technology to accelerate their development time and save costs by avoiding the step of building physical prototypes. The collaboration with ANSYS was fundamental in that it allowed a small team of engineers to access the same resources as the big aerospace companies with which it is competing.

“We needed to maintain the high performance of a very large engine while drastically reducing the costs and size,” says Jean-Michel Guimbard, co-founder and chief technology officer of turbomachinery at Turbotech. “We could only get there through simulation. This was a big challenge, but it was made possible by our collaboration with ANSYS."

Two Products for Two Markets

Turbotech took a two-pronged approach in their product development efforts: a turboprop and a turbogenerator. The turboprob, called TP-R90, can be used as a drop-in replacement, with the same center of gravity, for conventional 4-piston engines in small planes and helicopters. In the turboprop, the speed of the turbine is reduced through a standard gearbox to drive the propellers. The TP-R90 was created to fill the need for lighter, quieter, more reliable and less expensive replacement engines.

Figure 2. The TG-R55 turbogenerator (Photo:Turbotech)

The TG-R55 turbogenerator is more forward-looking, intended to power electrically driven urban air mobility applications like vertical takeoff and landing (VTOL) vehicles that will be used in robo-taxis and delivery drones in the autonomous flight sector of the future. Here, the electric machine is driven at the same speed as the turbine, eliminating the need for a gearbox to further save weight.

Billed as the “first low-fuel-burn light turboprop” by Turbotech, the 64 kg (dry weight) TP-R90 produces 90 kW of continuous power by burning a range of fuels, including Jet-A1, diesel, UL91, AVGAS and biofuels. This ultra-quiet, vibration-free engine consumes 18–25 l/hr of Jet-A1 fuel at cruising speed. Its reliability is reflected in its 3,000 hour time-between-overhaul (TBO) specification.

The TG-R55 turbogenerator is the first onboard electric genset dedicated to the hybrid-electric aircraft industry. This 55-kg (dry weight) turbogenerator provides 55 kW of continuous power running on Jet-A1, diesel and biofuels. It can also be adapted to run on hydrogen for a completely emission-free flight. It consumes 15-22 l/hr of Jet-A1 fuel at cruising speed and can operate for 3,000 hours between overhauls.

Recent demonstrations of VTOLs and robo-taxis have all been powered by batteries, with only 5–10 minutes of operating time — hardly sufficient for practical use. By recharging batteries inflight, the TG-R55 turbogenerator increases the range to several hours while improving reliability and system redundancy for increased safety.

Figure 3. A schematic of Turbotech’s TG-R55 turbogenerator

Figure 3 shows how the TG-R55 works. The heat exchanger on the right contains the microtubes that capture the hot exhaust gases and recirculate them back to the combustor; it is the key differentiating technology of Turbotech’s product. The mechanical integrity of the heat exchanger is critical to performance and service life. ANSYS Mechanical simulations found that displacements due to thermal expansion differed from those predicted by initial hand calculations. This early discovery of potential reliability or performance problems gave Turbotech engineers the confidence that greatly accelerated the design process. The microtubes were also designed with the help of ANSYS Mechanical simulations to verify structural integrity and ANSYS CFD for analyzing the fluid flow of gases and performing thermal analysis.

The gas turbine in the center is a specially designed regenerative turbine capable of handling the injection of recirculated gases efficiently. The turbine is coupled directly to a high-power-density electric generator module on the left, which provides electricity to recharge the batteries.

Adding 40 kg of Jet-A1 to this 55 kg turbogenerator yields 130 kW-hr from a 95 kg package — the highest power-to-weight ratio of any electrical storage technology. Li-ion batteries with the same amount of power would weigh about one metric ton. If you take a car and add a ton it will still run, if only slowly. An aircraft with an extra ton just won't take off, so the higher energy density of the turborgenerator is needed to make these vehicles viable and flyable.

Another 40 kg of fuel doubles the power yield to 260 kW-hr in a 135-kg package, while the equivalent power in Li-ion batteries would weigh two metric tons. Clearly, if you want to be airborne, the extreme weight advantage of the hybrid-electric turbogenerator system is the only way to go.

Keep in mind that this power output is continuous, not intermittent like the variable power output of a car’s engine. Continuous power puts the turbogenerator under a lot more stress, which requires extra diligence in the design phase. ANSYS Mechanical simulations were run to ensure that the components of the turbogenerator could withstand this extra stress over its expected time-between-overhauls.

The TG-R55, designed for VTOLs carrying 4-6 passengers, has already run several thousand hours in testing protocols. Turbotech engineers are now working on the next generation 90 kW turbogenerator, which will weigh only 5 kg more than the 55 kW version. Scalability is built into this technology.

This article was written by Bill Kulp, Lead Product Manager, Fluids Business Unit, ANSYS (Canonsburg, PA). For more information, visit here .

Aerospace & Defense Technology Magazine

This article first appeared in the April, 2020 issue of Aerospace & Defense Technology Magazine.

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