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The challenges of micro turbine hybrids

It has been an exciting couple of weeks for Bladon Jets thanks to the Paris motor show launch of the magnificent Jaguar C-X75, which featured its micro gas turbines as part of a hybrid powertrain. However, it is now back to the drawing board for the Isle of Man-based company as far as this advanced development is concerned. Some may ask: Have we not been here before? Is this not a case of déjà vu?

It is some 60 years since the noise of gas turbines on test first broke the silence in sleepy Solihull, UK, as Rover Gas Turbines ploughed a lonely furrow in the development of UK automotive gas turbines. What was then the ‘engine of the future’ proved to be just a cul-de-sac.

Given their best efforts, Chrysler (the first), Fiat, Ford, General Motors, Mercedes-Benz, MAN and latterly, Volvo, also came away empty handed. Now, two generations later, a new breed of engineers is battling the gas turbine’s corner. Can they do it and how long will it take?

The gas turbine is a compact, lightweight but noisy power unit. Instead of the previous beefy 400hp (535kW) road-going turbine engines, Bladon Jets has opted for micro turbines. The two, small diameter single-shaft 95hp (127kW) gas turbines, without any heat exchanger, form part of a complex hybrid system. Jaguar claims zero emissions on electric drive and 28g/km of CO2 with turbines running. and whereas 60 years ago a 90hp (120kW) turbine engine weighed 62kg, Bladon Jets now talks of just 3kg – a twenty fold gain for an engine that could fit in a shoebox. Overall weight is 30kg.

In the C-X75, each micro turbine is direct-coupled to a ground-breaking high-speed generator from SD Drives of Harrogate. These charge the batteries in range extender mode but can power directly the four 145kW (108bhp) wheel motors. The entire length of the rotating package is 500mm. According to Jaguar, the multi-fuel ultra-lightweight range extender (ULRE) offers a range of 560 miles on a single tank and reaches a top speed of 205 mile/h.

whereas 60 years ago a 90hp (120kW) turbine engine weighed 62kg, Bladon Jets now talks of just 3kg

However, as any engineer who has cut his or her teeth on automotive gas turbines knows only too well, micro turbines with their high rotational speeds bring their own challenges, challenges requiring a team of high-flying, intellectual engineers. The challenges are as they were six decades ago, but now they are more intense as the engines are that much smaller. Even turbocharger manufacturers do not face such challenges.

For example, high efficiencies for the multi-stage axial flow compressor and turbine, while not paramount, are nevertheless essential for low specific fuel consumption, even given short duration running. These small diameter components require a clear understanding of fluid flow, very close tolerances and mastery of manufacture to minimise leakage.  Computational fluid dynamics will play a key role and the minute blade profiles and passages will require the best that electrochemical machining (ECM) and/or electrical discharge machining (EDM), can give.

Compounding these issues are: air filtration, thermal differential, multi-zone combustion and fuel injection, sealing, erosion, possible lubrication, packaging and noise, vibration and harshness (NVH). Then add the complexity and stability of low-friction air bearings for a shaft running at around 90,000 rev/min. Finally, overarch these with a need for materials technology and the simple gas turbine is not quite so simple.

High temperatures and high rotational speeds characterise any gas turbine. The third element is combustion efficiency. These three alone determine not only how well the engine performs, but also its durability, reliability and overall fuel economy. The combustion chamber is an amalgam of many technologies that may not only defy simulation but which also find their solution only through trial and error.

Put together as a complex jigsaw, all of these elements pose an awesome challenge, a challenge that in the 21st century is made that much more arduous by constraints of manpower, finance and time

Put together as a complex jigsaw, all of these elements pose an awesome challenge, a challenge that in the 21st century is made that much more arduous by constraints of manpower, finance and time. Of these, time is the great imponderable.

Bladon Jets has a tight time schedule if it is to meet its deadline. To bolster its staff the company requires highly qualified engineers familiar with an understanding of high temperatures, fluid flow, materials engineering, machining, high thermal differentials, noise containment, and what can loosely be called ‘systems engineering’, namely pulling together the entire complex system of air, gas, and fuel flow for these shoebox turbines. The compressor for example is only 100mm in diameter. The number of engineers with hands-on micro turbine experience is miniscule.

One parameter in the gas turbine world has not changed with the passage of time. Sixty years ago, everything always took much longer to bring to fruition than anyone in authority planned or contemplated. And so it is today, except that today there are new issues in the engine which may not be simulated or anticipated.

With just over 12 months before it must deliver operable demonstrator engines, Bladon Jets has its hands well and truly full. That TATA Motors of India, with its European Engineering Centre close to Jaguar, has taken a 20% financial interest in Bladon Jets is hardly surprising. For it will require not only access to substantial finance, but the best engineering brains and management expertise to steer this new system through R&D to production and cost/benefit viability.

The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.