Turbines are aiming at a moving target
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kn interview with Scania's chief engines and transmissions engineer govides a cool assessment of the prospects for truck gas turbines
ly Brian Cottee HE truck gas turbine will not be a real )mpetitor for the diesel until longer%red high-temperature materials can be rovided in production quantity.
That opinion, expressed to me :cently in Sweden by Mr Folke arlsson, Saab-Scania's chief engineer, igines and transmissions, would prob)1y have been almost equally true at ly time in the past eight years; but, as pointed out, the competitive situ:ion has really changed quite consider)1y because the diesel engine has made .g strides in that period, and some of ie turbine materials problems which rice looked near to solution are proving ;ry intractable.
Diesel engineers have not been idle, id the turbine designer is still aiming at moving target —"moving" so fast that 1r Karlsson believes the heavy-truck irbine would not now be competitive ith the diesel at less than about 370 kW 00 bhp), whereas a few years back the t. was being thought of as an .ternative engine at powers from round 220/ 260 kW (300/ 350 bhp) pwards.
The increase in diesel outputs through xbo-charging, and potentially through )mbining this with intercooling for the nnbustion air, as some manufacturers ive already done, has shown that )wer can be raised by perhaps 20 per nt without increasing Ptak cylinder -essures and thus imperilling liability. A big factor in diesel/ gas turbine :onomics, says Mr Karlsson, is the imparative manufacturing costs; and I be successful, the turbine would prob)1y need more careful servicing perips less frequently), while the exotic gh-temperature materials used for .rbine blades, nozzles, guide Vanes and !rhaps heat exchanger are costly and ould need to be treated as rare scrap id returned for recycling when engines components ended their service life. Whether 370kW (500 bhp) engines ill be in demand is a big question in ;elf, especially with so much resistance raising gvw limits in Europe, and American operators cutting power to save fuel. But at least some American engineers can see this output as a possibility, and Folke Karlsson had just returned from a US visit which included a ride in a 335 kW (450 bhp) turbinepowered commercial vehicle developed at General Motors' research centre at Indianapolis.
Whereas some manufacturers have dropped their truck turbine plans, and companies like British Leyland have had to admit that a successful truck g.t, is a lot farther away than they announced in the late Sixties, GM apparently remains very optimistic. Certainly its 230kW (310 bhp) Allison GT-404-3 turbine engine performed impressively in the Greyhound coach which Martin Hayes described in our April 12 issue. General Motors has developed a new gas turbine combustion chamber which enables the exhaust emissions to meet the 1976 Californian regulation limits.
So it looks as though one major goal has been achieved, but the bigger hurdle of durable high-temperature materials remains. This really takes two forms — the need for a heat exchanger material that will not degenerate too rapidly, and will continue over a long life to transfer heat from the burnt gases to the incoming combustion air with high efficiency; and the need for durable materials for the very hot parts of a g.t. engine, capable of running at the 2000 degrees F which engineers like Mr Karlsson consider essential to give the turbine engine an edge of efficiency over the diesel. The critical parts here are the nozzles which direct the hot, combusted gases on to the compressor-driving turbine, and the blades of that turbine.
While weight, size, cost, durability and serviceability are very important factors in the success of any engine, and particularly so in the case of a new power unit, the long-term prospects for the automotive gas turbine depend mainly on its overall efficiency compared with competitors — for heavy trucks this means the diesel. But the efficiency race could prove very close indeed.
As is well known, the diesel is the most efficient internal combustion engine — a really well-designed truck diesel can convert 42 per cent of the fuel's heat energy into work. With exotic high-termperature materials, the gas turbine is theoretically capable of reaching about 45 per cent efficiency — but if (as Folke Karlsson thinks possible) some of those high-temperature materials were used on the piston crown, cylinder head and other very hot parts of a diesel, its efficiency too might be pushed to 45 per cent. The use of heat-resistant ceramics, for instance, could reduce the diesel's heat losses through the cooling system. At present about one-third of a diesel's heat energy produces power, a third is lost through the exhaust and a third dissipated through the cooling system.
The fact that the high-powered gas turbine is thus potentially at least as efficient as the big diesel suggests that it will not necessarily remain at a disadvantage on fuel consumption, and Mr Karlsson confirmed this — indeed he sees it as capable of being more economical than the diesel, though it is inherently poorer when running at part throttle.
Aircraft and space programmes have forced the development of hightemperature materials which could solve the truck turbine's problems, but the automotive industry has to compete with the aerospace industry for them; they are scarce and costly, based on nickel, chrome, cobalt and the like.
But it is good to know that Mr Karlsson thinks the British are ahead in this materials field. He mentioned materials evolved by Lucas, who have licensed the Norton company in the USA to do ,research based on a British development; and the silicon nitride, for example, produced by Advanced Materials Engineering Ltd, of Team Valley, Co. Durham. The part required is cast in pure silicon and then nitrided in a furnace. The result is a material capable of withstanding very high temperatures, but it is brittle so the parts have to be designed specifically to allow for this and to derive strength from bulk or shape.
One of the attractions of a breakthrough in materials, allowing much higher temperatures to be used and therefore achieving higher efficiency, is the possibility of making turbine engines competitive down to perhaps 75 kW (100 bhp) and so broadening the market to include cars. (But they would not be suitable for small trucks, which need to be cheap to buy and economical at low speeds.) As well as nozzle, turbine and combustion chamber heat problems, automotive gas turbine designers are still looking for a better way to build heat exchangers, units which are essential for acceptable fuel consumption.
There are two main types of g.t. heat exchanger — the stationary recuperator which works rather like a vehicle's radiator, with adjacent tubes carrying the hot exhaust gases in one direction and the cool intake air in the other, and the regenerator in which some form of disc or wheel is rotated continuously through the hot gas and the cool incoming air.
In both cases the intention is to re-use some of the waste heat in the exhaust and so make more efficient use of the energy in fuel; without this heat recovery the gas turbine would not have a chance of competing in the automotive world — it would be hopelessly uneconomical. With pre-heated intake air, less fuel needs to be burnt to reach the designed operating temperature.
The recuperative heat exchanger is comparatively simple, can be made durable and is capable of operating at pressure ratios of perhaps 6 or 7 to I; so it can be compact, which is important. The top limit on temperature is about 400 degF, using stainless steel.
By contrast the rotary regenerator is limited to pressure ratios of 4 or 5 to 1; so it has to be bigger for a given amount of transfer: General Motors and British Leyland, among others, achieved the required size by having two rotating discs, but this reportedly proved a major drawback for the Leyland design (CM April 12 1974) because it blocked servicing access, so a single disc is now used. And the rotary heat exchanger, since it moves between hot and cold gas flows at different pressures, needs efficient high-temperature seals. Mr Karlsson told me that the maximum life for the heat seals was around 2000 hours and the seals were important not only to stop pressure and heat losses but to prevent damage to the expensive recuperator disc.
He said that GM had gone to stainless steel for the recuperator discs, having found ceramics limited to 700 or 800 hours life. Also the current ceramics used for the honeycomb-type discs are prone to degeneration and eventual failure through the chemical effects of the sulphur in the fuel. Several new substances, one of them magnesium based, are being worked on to overcome this.
Saab-Scania has neither the particular expertise, the facilities nor the staff to develop truck turbines, though it is constantly monitoring g.t. progres: around the world and is doing sorm studies in conjunction with an America! company. But the Swedish company ha! been doing its own research into a nove form of automotive turbine, based on at intermeshing screw-type compressor — rather like two huge woodscrew meshed in parallel, invented by Al: Lysholm. Mr Lysholm, who died las year, had his patent taken up in man3 countries as a compressor, but Scank envisaged using his compressor in vehicle engine, driven by a reverse( version of it which was, in effect, a loni "screw" turbine. There would be a corn bustion system in between.
Scania did a lot of work on the screv elements, especially on lobe design, bu the project at present looks rat he doubtful. One of its basic attraction was the ability to use normal materials avoiding the exotic, but this assumed internal cooling. They tried liquid cool ing for the screw turbine but thi lowered the overall efficiency too much They could go to high temperatur materials, but that would be a risky step since the conventional "mainstream g.t. development would benefit pro portionately more than the screw-typ from the development of metals o ceramics with greater heat tolerance.
All in all, the automotive turbine is a a difficult stage, and the prospects o producing a truly competitive true] engine still depend on the developmen of longer-lived sulphur-resistan materials for rotary heat exchangers (o more efficient recuperators) and mor practical and "hotter" materials for th critical parts of the engine which Ii downstream from the combustiol process. And even that assumes that diesels di not make further strides in quietness operating efficiency and productioi cost-saving.
Next week: Higher torque or mor gears? Mr Karlsson discusses likel. truck trends and reveals Scania's think ing on automatic transmissions.