Delta-Cosworth CatGen automotive gas turbine generator
A team of engineers at a UK company called Delta-Cosworth (formally Delta Motorsport) have developed a miniature gas turbine generator for use as a range extender fitted to electric vehicles. Based at Silverstone, Scott Heiring and his team have perfected a powerplant sometimes referred to as a “microturbine generator” or “turbo generator”. This prototype machine has so far been trialled in a medium size van. Range anxiety is an emerging problem for EV drivers wishing to travel long distances, and it’s a problem for bigger vehicles too such as trucks and vans with an appreciable payload and a busy working schedule. It’s also a clean burning unit meeting high emissions standards without after-treatment.
Small Gas Turbines
A miniature gas turbine may be similar to a small jet engine. A simple jet engine consists of a gas turbine expelling it’s own exhaust through a propelling nozzle. A gas turbine is a prime mover that consumes fuel releasing heat and can be made to do mechanical work such as driving a generator, operating itself as an air compressor or expelling it’s exhaust creating thrust. A simple gas turbine consists of three basic components: A rotating compressor wheel, a combustor or combustion chamber and a turbine wheel. The compressor and turbine are both mounted on a common shaft and rotate at high speed. Air is drawn in by the compressor where it is compressed to several atmospheres, the air is then heated by burning fuel in the combustion chamber and it is then expanded though the turbine. The turbine extracts energy from the hot gases and in turn drives the compressor. The process is self-supporting and additional useful power may be extracted from the same shaft to drive a load such as a generator. The engine has to be started of course, usually by means of an electric motor able to rotate the shaft fast enough to obtain a self-sustaining speed and stable combustion. The working principle is continuous unlike a piston engine that operates in repeating cycles.
In small gas turbines the compressors and turbines often take the form of impellor type wheels and are similar in construction to those commonly found in automotive turbochargers. The compressor wheel is surrounded by a stationary ring of blades that form diverging ducts or passages known as a diffuser. The turbine is also surrounded by a ring of nozzles that direct the hot combustion gases on to the turbine in the direction of rotation. Very hot gases pass through the turbine and nozzles so consequently these components have to be made of high temperature resistant steel alloys.
In a conventional gas turbine fuel is burnt in the combustion chamber in the form of a fine atomised mist or from a vaporiser. This process is initiated once only and ignited usually by an electric spark. Once lit the combustion is continuous and the igniter is switched off. It is the heat released from the combustion that drives the operating process and so many types of fuel may be burnt. Fuel flexibility is an advantage of the gas turbine prime mover. There are no piston-engine pre-ignition or detonation issues with a gas turbine but there are a few others!
The first gas turbines appeared before WW2 but they were very large stationary units. The genius of the jet engine envisaged by Frank Whittle was to utilise the exhaust gas flow as a reactive jet. Gas turbines are commonplace in commercial aircraft and helicopters, they may also be found in power generation and the oil and gas industries. Small gas turbines found in any other purposes are quite rare.
Small model size engines have evolved over the past decades to propel realistic scale model aircraft. Could it be also possible to use these small units in other applications such as portable power? Unfortunately, they are inefficient, noisy and require frequent overhauls and re-builds, they are also expensive compared to piston-engine solutions of the same scale. Another issue with RC size engines is exhaust gas emissions due to poor combustion having to occur in a very small size.
Gas turbines in cars
Gas turbines as fitted to road vehicles have had mixed success over the past decades. Small gas turbines were first developed in the 1950s and 60s and a few car makers tried them out in road vehicles such as cars, trucks and buses. In the USA Ford and Chrysler built prototypes, and in the UK Rover, Austin and Leyland built and tested models too. The famous tilting train, the Advanced Passenger Train designated Experimental (APTe) was also fitted with Leyland truck gas turbine generators. In more recent times through the 1990s, the Volvo car company installed a gas turbine generator in various saloons and utilised an electric power train that included a battery. And around the year 2000 a grand tourer sports car in the USA known as the CMT-380 was fitted with a Capstone microturbine generator and battery powertrain. In the UK a company acting on behalf of Ford also installed the same microturbine in an SUV.
There are problems encountered when fitting gas turbines to cars. Early designs utilized twin shaft units. A twin shaft gas turbine operates with a mechanically separate free power turbine driven by the engine exhaust that propels the vehicle though a reduction gearbox. The turbine drive may be similar to a “torque” converter in a conventional automatic car. The main problems with this arrangement were poor part-load fuel consumption, throttle lag and a lack of engine breaking.
An alternative to using the free turbine transmission is to utilise an electric transmission. Here the gas turbine simply drives a generator and can run at constant speed. The generator is a special high-speed device that is directly coupled to the compressor shaft and may rotate at speeds as high as 100,000 rpm. An advantage of this arrangement is the generator is physically small and light weight compared to a conventional low speed AC generator and, it doesn’t require a reduction gearbox. With an electric drive system, a battery may also be utilised to work with the generator to suit varying road conditions.
All small gas turbines suffer from a lack of efficiency when compared to piston engines and particularly at part load. Nearly all automotive gas turbines intended for vehicle use, are fitted with a heat exchanger device to improve thermal efficiency and reduce fuel consumption. A device known as a recuperator recycles the heat from the exhaust gases and pre-heats the air before it is passed to the combustion chamber. Consequently, less fuel is burnt for a given power setting, and an additional advantage is the final exhaust exit temperature leaving the vehicle is also reduced.
The recuperator is the Achilles heel of the automotive gas turbine. The recuperator design may consist of a simple heat exchanger matrix manufactured from high temperature corrosion resistant steel. There are two gas paths through it that must be kept separate, and heat is passed from the exhaust gases to the compressed unburnt air. A problem with the exchanger is size and weight, for best efficiency it must be large in order to create a sufficient interface surface area for best heat transfer. It may also have significant thermal inertia that can lead to problems with engine load transients and throttle response. Pressure losses may also be a problem as it must not significantly impede the flow of gases through it leading to reduced efficiency. The Capstone C30 powered CM-380 car used this type of recuperator. Life and deterioration of the recuperator can be a problem.
An alternative type of recuperator used in some vehicle gas turbines consists of a slowly rotating glass-ceramic matrix that collects heat from the exhaust and carries it to the compressor outlet. Sealing the gases on to the moving matrix is a challenge but the overall assembly can be made lighter and smaller than a conventional static heat exchanger matrix. It also has low thermal inertia. This type of heat exchanger was used by Rover, Chrysler, Leyland and Volvo. There are significant challenges with this type of recuperator design including life, durability, manufacturing difficulties and of course cost.
Piston engines may also be used to purely drive generators in EVs and this has been done commercially, examples being the BMW i3 and the Vauxhall Ampera. Both these vehicles use only electric motors to drive the wheels and are known as a “series hybrid” type as the generator purely charges the battery whilst on the move. A more common hybrid such as the Toyota Prius uses an engine and electric motor that can drive the wheels simultaneously, this type is known as a “parallel hybrid” vehicle. In both cases the engine may be turned on and off as required dictated by driving conditions. A disadvantage of a piston engine in this application is noise and vibration, as when running it’s working relatively hard, and it spoils the otherwise smooth quiet ambiance of a typical EV.
The Delta micro gas turbine
The unit built by Delta-Cosworth is known as a Catalytic Generator or Cat-Gen for short. It consists of a small single shaft gas turbine fitted with a static recuperator matrix and a high-speed permanent magnet generator. The power plant also features a unique combustion system specially developed by Delta. A special “flameless” catalytic combustor heats the air, without hot spots that lead to poor exhaust emissions (Nox – oxides of nitrogen). This single-shaft recuperated machine develops up to 35Kw to charge a vehicle battery and is sufficient to prevent range anxiety in a variety of vehicle types.
The Cat-Gen is neatly installed under the bonnet (hood) of the medium size van. A sophisticated monitoring and control system manages the engine start up and shut down. Precise control is needed to manage the catalytic combustor, as might be expected, the power plant runs with a distinctive turbine-whine sound rather like a small jet. This high frequency sound is relatively easy to silence although some driver’s may even like the sound as it mimics a jet aircraft!
The future for micro gas turbines
The history books show gas turbines in cars have had a tough time. The problem with developing such a unit is there is no state-of-the-art in small gas turbine design and manufacture. And piston engines are a hard act to compete with. Piston engines have enjoyed steadily evolution over decades, starting as crude side-vale units, then overhead valve (OHV), then overhead cam (OHC), then aluminium, multivalve (16V), fuel injection, variable valve timing, turbo charging, direct injection and even variable valve lift throttle control. The latest units may soon be available with variable compression stroke too!
The micro gas turbine is a difficult beast to manufacture. Interestingly no commercial turbo charger manufacturer has yet taken up the challenge, it might be imagined that they would have good turbo-machinery expertise that would lend it self to micro gas turbine production. Honeywell (Garrett) produced a machine (Parallon 75) in the early 2000s but it was a CHP unit and not destined for automotive use. It's long since been discontinued. Although seemingly simple, a micro gas turbine consists of delicate precisely manufactured components that require careful assembly. New ways to mass produce and minimise the cost of these components need to be found. The heat exchanger in the form needed to operate as a gas turbine recuperator also represents a manufacturing challenge.
There are virtually no commercially successful small gas turbines in existence suitable for automotive use. Two companies Capstone and Ansaldo Energia offer stationary units that were originally derived from models develop for automotive use. These products operate as combined heat and power systems for installation in buildings. They are both gas-fired recuperated units and fulfil a niche market role. Around the world various development and start-ups have been reported developing small GTs but crucially none of them have demonstrated any significant sales and are not in cost-cutting volume production. The company Bladon Jets partnering with Jaguar promised the world a turbine-extended sports super-car, but the reality was just a mock-up at the Geneva motor show. They now offer a disappointingly bulky 12Kw machine intended to power mobile telecommunications. It’s not clear how many have been sold and the estimated development cost over the past ten years is over £100 million!
An interesting company to follow is Bowman Power Systems. They make a piston engine exhaust energy recovery device consisting of a turbine impellor married to a high speed generator. Bowman once produced a microturbine in partnership with Elliott but after low sales they've shifted their business model to energy recover systems for retro-fitting to existing installations.
The Delta machine may also have a tough time winning market acceptance when pitted against its piston engine rivals. What’s different today is the proliferation of EVs, the automotive world is changing fast, and outright range and range anxiety must be addressed particularly with commercial vehicles. The automotive gas turbine faces an opportunity to provide an innovative and different solution here. In the aviation world there is a certain “snobbery” and kudos associated with flying turbine powered aircraft. Every pilot wants to fly a turbine and the same could become true for premium and luxury cars. Hi-end vehicles could offer turbine power but it’s probably the commercial sector where the appetite may be greatest for turbine extenders. Commercial drivers don’t have the time to sit and wait at charging stations, time is money. Even if not on the move, a plumber or builder could have their van sit outside a property recharging from the turbine if convenient power was not available.
Hydrogen fuels are another possibility for the Cat-Gen. Although tried before in piston engines and fuel cells, hydrogen may re-emerge as a viable fuel source for larger vehicles at least.
Lets see what happens Scot and the Delta team could be re-writing those history books!
Gas turbines were tried in cars back in the 1960s famously by both Chrysler in the USA and Rover in the UK. In more recent times circa 1990s Volvo installed a recuperated gas turbine (HSG40) in an electric vehicle. Several porotypes were built including this amazing Volvo 850 but also a version of the V70 was built. So not an entirely new idea, but the automotive landscape is changing now as the world transitions to lower or even zero carbon propulsion solutions. The automotive gas turbine may make a comeback! More on Volvo and gas turbines.
