The Case for Gas Turbines in Buses
Page 35
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R. H. H. Barr, B.Sc., Maintains that the Turbine is Practicable for Road Vehicles, and Claims Economy, Smoothness and Ease of Control
ACOMPARISON between the performance and construction, of a reciprocating-engined and a turbinepropelled bus was made by Mr. R. H. H. Barr, B.Sc., in a lecture given last week at the School of Gas Turbine Technology, Lutterworth. Mr. Barr was of the opinion that should a 100 b.h.p. oil-engined bus averaging 8-10 m.p.g. or a similar-sized petrol-engined bus operating at 5 ntp.g. be fitted with a gas turbine, a fuel consumption of 7 m.p.g. could be reasonably expected. The turbine would be the 160 b.h.p,. model which is at present under consideration for road transport.
Acceleration from rest would be smooth, because, during all normal running, no gear-changing would be necessary with the gas-turbine vehicle, although an emergency low gear might be required for ascending a hill with a gradient exceeding 1 in 8. Absence of gear-changing would save time, because of the rapid acceleration from rest, whilst the driver would be relieved from fatigue.
in estimating the performance of the gas turbine for a bus, allowance would have to be made, for its low weight. the present 160 b.h.p. seven-chambered unit weighing only 250 lb. In addition, there would be no radiator drag on the turbine vehicle, as water cooling is not required.
In the discussion which followed the lecture, a question was asked whether it would be dangerous to stand close to the gas-turbine vehicle, having regard to the possible high velocity of the air intake and exhaust gases. Mr. Barr pointed out that the air intake to the engine would pass through a large-capacity filter, and that the suction would be only four to five times that of the reciprocating engine. Exhaust gases would be conducted away through twin 4-in. diameter pipes, the velocity of gas in them being approximately 250 ft, per sec. As the thermal efficiency of the turbine was equal to that of the oil engine, the exhaust gases would be somewhere near the same temperature. approximately 3-50-400 degrees C.
When asked to state the size of the fitter required for the air intake, Mr Barr replied that the filter dimensions must be adequate for the purpose and that no more than Cal lb. per set. in. depression could be permitted in the system. For the 160 b.h.p. unit it would be necessary to have a filter approximately 200 sq.. ins, in area, using a fine-mesh gauze.
Easy to Drive
On the question of control, he said that the turbine bus would be simple to drive. It would be equipped with
accelerator and brake pedals, and a gear lever for changing the direction of the drive. It would not be necessary to use a gearbox, because the power turbine would take the place of the clutch and gearbox or alternative fluid transmission_ Mr. Barr considered that the ultimate cost of the turbine, on a quantity-production basis, would be the same as, if not less than, the cost of the retiprocating engine, By comparison, there would be only one-fifth of the weight of material used, the quantity of high-grade material employed being comparatively slight. In fact, be elaborated, the cost of the fabricated unit might eventually, be very close to that of the basic tight alloy.
In his lecture he outlined the construction of the small turbine for road use and the transmission system uhkh would be employed to reduce the shaft speed to that of the normal propeller shaft. Basically, he said, the gas turbine consisted of an air compressor, a turbine which drove the compressor, and a second turbine supplying the shaft power. In aircraft practice it was common to have both turbines attached to a single shaft, but for road purposes it was preferable to employ independent shafts. With this simple form of unit, air entering the compressor was compressed and passed into the combustion chambers, where fuel was burned continuously in an excess of air. The products of combustion then expanded through the two turbines and uere finally passed to exhaust. To economize in fuel, a refinement might be added in the form of a heat exchanger, consisting of a heat-transfer apparatus, which enabled the exhaust gases from the turbine to be used in preheating the compressed air before it entered the combustion chamber.
Large turbines were known to have a high efficiency when compared with a piston engine. In scaling down the turbine to a size suitable for use on the road, certain factors had to be considered to avoid serious loss of efficiency. The process did not necessarily involve working to closer limits or machining all parts with a high-polish finish, which would tend to raise the cost of the turbine out of all proportion to its purpose. On the contrary, the value of the highly polished finish of turbine blades had been found to be grossly overrated, and tests on turbines fitted with unpolished blades had shown no appreciable loss of efficiency.
A gas-turbine engine, of 160-200 b.h.p. required to run at between 35,000 ap.m. and 40,000 up.m. Gears and bearings running at this speed were not in common use, Mr. Barr continued, and the question arose as to whether the long life of the road-transport engine could he attained with such high-speed machinery. For normal purposes, the bus or goods-vehicle engine operated for only 1 per cent. of its life at maximum speed, most of its life being run at speeds between 20 and 60 per cent, of maximum r.p.m.
Rigid Gear Mountings
Bearings could be made to operate satisfactorily at these speeds, but gearing, to reduce the turbine speed to the speed of a normal propeller shaft, presented a more difficult problem. Great attention would have to be paid to the rigidity of their mounting, stiffness of the casing and the provision of a copious supply of lubricating oil. It was necessary that the gears should he accurately machined, and unless care was taken, added Mr. Barr, an objectionable noise would be set up in the drive.
Gear noise, he explained, could be reduced to a minimum by using helical gears running within stiff casings. Little noise should arise from the exhaust because of the relatively low gas velocity, and the diffuser noise could also be kept at a reasonable level by having a thick blower casing and carefully designing the diffuser tips. Air-intake noise might easily be screened; in fact, the air filter would probably be sufficient. The whole engine could, if necessary, be well insulated against the transmission of sound to the rest of the vehicle, and it was not expected that the general intensity of noise would be any greater than that of a normal reciprocating engine.
Starting the turbine might present a difficulty should the
normal electrical starter fail. It would not be possible to tow the vehicle to start it, because the compressor was not linked directly by the shafting to the road wheels. Further, it was doubtful whether the compressor rotor could be raised to "lighting-up" speed by hand, even if step-up gearing were employed.
There would be no water system, radiator or electrical system, other than the starter, to cause trouble. Oil consumption would be negligible, because there would be merely the bearings and gears to lubricate, and there would be no carbonization of the oil. Of the auxiliaries, the fuel and oil pumps would' be most likely to cause failure. Mr. Barr mentioned that the high-duty fuel purrips required for aircraft were sometimes the most unreliable components on this type of engine_ A questioner asked whether the turbin,e could be maintained by. the usual vehicle repair staffs, or whether it would be necessary to employ specially trained personnel. Mr. Barr said he considered that the turbine should receive specialized treatment, but ultimately the operation of gasturbine units would enable maintenance staffs to be much reduced. This had already been proved in the use of turbine-engined aeraplanes.