Under pressure

A gas turbine engine with the potential to be up to 20 per cent more efficient than comparable systems has been designed by UK company Numerics Technology.

The rotary internal combustion turbine or RICT is a constant volume (CV) pulse turbine that uses much higher pressures and instantaneous temperatures than a normal constant pressure turbine. It will be able to run on a range of liquid and gaseous fuels and uses a different principle of gas expansion to that of normal turbines.

As well as the automotive market, it could be used in the aerospace, marine and rail markets as well as for domestic combined heat and power (CHP) energy generation.

Being a turbine, the company says the time between maintenance and between failures should be longer than for piston engines, meaning the unit will have a much longer lifespan. Numerics Technology estimates the device will last about 7,000 hours between failures. The design also has fewer individual parts than piston engines and gas turbines, so the manufacturing costs should also be much lower than conventional turbines, equalling those of mass-produced piston units, perhaps being even lower.

A one-litre capacity prototype has been constructed and was tested in March. Work to construct a second version is due to start soon. The RICT is claimed to have up to 10 times the power density of a normal turbine — a one-litre unit would be designed to produce 1,900 bhp at 30,000 rpm.

The company also aims to build a true compression ignition diesel unit and a detonation ignition unit comprising a diesel that runs on petroleum or gaseous fuels. These would have potential efficiencies of about 50 to 60 per cent, compared with up to 25 per cent achieved by petrol-powered piston engines, with the potential to reduce fuel use and carbon emissions.

The RICT achieves low-speed output by using two rotors, one smaller unit driving the compressor at high speed and a larger slow-speed rotor for power output. The device consists of a two-stage centripetal compressor, next to a toroidal intercooler unit. The core of the engine consists of a rotor, plus two stator units. Finally, a large, shaft-mounted turbine assistance unit uses the waste exhaust gases to provide more power.

Unlike a normal turbine the gases pass radially around the turbine axis, undergoing multiple chamber based expansions in the process, so producing shaft power. The prototype has 14 chambers in both halves of the rotor and stators, producing 196 individual expansions. This allows the design to be both powerful and compact. As the chamber sets are horizontally opposed, the engine is totally balanced.

In the combustion chamber, gases at high pressure expand progressively into increasingly larger chambers of the rotor which, in turn, expand into the stator chambers as the rotor rotates.

The shape can be constructed with normal workshop techniques and materials, meaning that the manufacturing costs should be kept low.

‘The design will not just be limited to using diesel or petrol,’ said Roland Heap, Numerics Technology’s managing director. ‘You could also use gas without the danger of causing damage through fuel detonation as unlike with pistons, this design has no parts that can be knocked out.’

In the case of the compression and detonation ignition variant of the engine, the engine rotor-stator system has no moving links, such as with a piston engine.

As the units are fully balanced, both can take the forces involved with detonation ignition, meaning they are unable to do any damage.

Numerics Technology is now applying for EPSRC funding in a partnership with London’s Queen Mary University, Cranfield University and power systems specialist Cummins Generator Technologies, which will allow a three-year in-depth analysis of the engine to take place.

‘The next step is to improve performance at high speeds,’ said Heap. ‘At present the prototype engine is only running at speeds of 10,000 rpm but with further work we hope to reach the full 30,000 rpm achieved by commercial engines.’