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Laurels the Road
by P. A. C. Brockington,
With the Power Increased Nearly 30 Per Cent. by an Exhaust-driven Blower, the Thornycroft KRN6 Oil Engine Designed for Supercharging has an Outstanding
THE success of industrial oil engines fitted with exhaustdriven superchargers has for many years been an irritant to vehicle-engine designers who have appreciated the potential advantages of such a system, but have hesitated to adopt it for road use on account of slow engine response to throttle opening. High first cost and bulk were added deterrents.
The development of the KRN6 11,3-litre oil engine by Transport Equipment (Thornycroft), Ltd., liasingstoke, had the specific aim of providing a power unit suitable for turbo-charging, and the subsequent fitting of an Eberspacher blower has shown that former objections are now invalid. Acceleration lag has been virtually eliminated, the unit is quiet in operation and exhaust pulsations are appreciably reduced.
Torque and Consumption
Moreover, the torque curve compares with that of . a normally apirated engine of the same output and the. specific fuel consumption closely approaches that of the basic engine in its unblown form.
As announced at last year's Commercial Motor Show, the power output of the KRN6 has been increased from 155 b.h.p. to 200 b.h.p. at 1,900 r.p.m. by turbo-charging and the maximum. torque has been raised from 508 lb.-ft. to 625 lb.-ft. at 1,000 r.p.m., with corresponding b.m.e.p. figures of 111 p.s.i. and 135 p.s.i. The engine was built for outputs in excess of 250 b.h.p., and the importance of this design policy is strongly emphasized by Thornycroft technicians. /
Cylinder Pressure Raised
Although the percentage increase in maximum cylinder pressure is less than the gain in power output, it was considered essential that the crankshaft assembly and so on should be built to accommodate the greater power if a high standard of reliability and a long life between overhauls were to be maintained. The peak cylinder pressure has been raised from 950 p.s.i. to 1,200 p.s.i.
The turbo-charger is of German origin and weighs 37i lb., which gives a total increase of installation weight of under 40 lb. The dry weight of the unblown engine. including accessories, is about 2,860 lb. and assuming that a normally aspirated 200 b.h.p. unit would have the same power-to-weight ratio, the compressor provides a saving of over 7 cwt.
The engine is remarkably free from combustion noise and, having `a robust crank assembly, is generally quiet. Supercharging raises the density of the air charge and shortens the delay period, thus giving more controllable combustion. It is also relevant that a high intake pressure reduces the engine's sensitivity to such factors as the fuel volatility and cetane number, and efficient combustion is maintained over a larger range of fuels.
There are also many design advantages of raising the output without increasing the bore. Piston friction is not increased and torsional-vibration problems are not introduced by lengthening the crankshaft. The inertia of the moving parts remains unchanged and it is unnecessary to reduce the peak r.p.m. This would lower the b.h.p. per litre obtainable from an unblown engine.
Operationally, the otter of a basic engine with variable ratings dependent upon the boost pressure has the great merit that standardization is simplified and the spares service can be improved at a lower cost. This particularly applies to areas where the numbers in use are small.
Thornycroft's decision to design the KRN6 for turbo-charging was taken after consideration had been given to the possible use of mechanically driven superchargers of the Roots and vane typed Encouraging performance results with exhaust-driven blowers were achieved by Swiss engineers from 1910-1920, but poor reliability created a bias against the unit in favour of the positively driven supercharger operating at a relatively low speed. In the 1930s, valuable development work was undertaken in the U.S.A., which eventually enabled the petrol-driven power units of the Flying Fortress bombers to be equipped with lightweight superchargers, which gave the aircraft unprecedented altitude capabilities.
At the same time, progress in metallurgical research associated with the production of jet engines and gas turbines in this country and similar work in Germany gave promise that various problems connected with weight, cost, efficiency and bulk could be satisfactorily solved. The production of the Eberspacher unit represented the fulfilment of this promise with regard to road-vehicle engines.
The fundamental characteristic of a turbo-charger is that it is driven by waste heat which would not otherwise be used. Any increase in back pressure is self-compensated, in that the energy absorbed is given back in the form of increased air charging, and there are no power losses as with a mechanical drive. this is a particularly valuable tue if the vehicle is running under ht load, when the percentage loss a mechanically driven superarger would be relatively high. e power required for such a unit substantially constant and if, for ample, the figure were 21 per cent. a peak engine output of 200 b.h.p., would increase to 10 per cent, as output was reduced to 50 b.h.p. The Eberspacher turbo-charger is out 11 in. in diameter and r in. long, the small dimensions ing made possible by the high ational speed of 38,000 r.p.m. The mpressor casing is constructed of st aluminium and cast steel is iployed for the casing of the turie. The rotors of the two sections ! mounted on a single aft which is carried in arings between them. Both the turbine and mpressor operate on radial-flow principle, turbine having an kard flow and the
)wer an outward flow similar to at of the conventional form of ntrifugal unit. Despite the accepwe of axial flow by leading signers as the most efficient prinsle for larger sizes of turbine and mpressor, it would appear that the rmer type of flow offers optimum rformance when applied to small 1,11-speed units. The maximum perssible gas temperature of the ierspacher turbine is 1,256° F.
A cc ordin g to the makers, !ratebau Eberspacher, Q.H.G., of slinger am Neckar, a basic blower the dimensions given can be uipped with various types of izzle ring and other components r fitting to engines of 4-litre capay and upwards, with normally pirated ratings of 75 b.h.p. to 180
h.!). They point out that the haust manifold must be adapted to commodate the turbine. For some ;tallations, duplication of the Dwers is advised, each unit being ed to charge a bank of cylinders. Rotating parts weigh only 2.2 lb., th a correspondingly low moment
of inertia, which explains the rapid response of the turbine to load changes. On the Thornycroft KRN6 engine, the response is such that no boost-control capsule is required, which is attributed in part to the low inertia of the rotor and in part to careful matching of the turboblower characteristics to those of the power unit.
For most vehicle applications an • air-cooled unit is completely satisfactory, a location in the air stream of the engine fan (as exemplified by the KRN6) being favoured. If necessary, a semi-water-cooled unit can be supplied having a water-jacketed bearing. A fully water-cooled unit is also available in which both the bearing and turbine are jacketed.
Maintaining a fairly low air-intake temperature is important in the interest of combustion efficiency, and because the cooling of the bearing is enhanced by an air bleed from the intake, it is necessary also for mechanical reasons. The flow of air from the fan assists in the external cooling of the bearings and of the intake pipes.
Although in the majority of applications straightforward modifications of the exhaust manifold and intake connections suffice to accommodate the blower, the makers state that engine manufacturers must be prepared to " match " the blower and engine characteristics. This involves development over a long period.
The only routine maintenance required is to top-up the lubricant in the blower casing with engine oil at regular intervals, the bearings being fed by splash from a selfcontained reservoir.
A technical representative in this country states that overhauls should not be attempted by engine makers or operators, and a scheme is planned by which a replacement unit can be supplied for £15, the time required for the change being about half an hour.
The mileage interval at which renewals normally become necessary is given by the makers as 30,000, but the tests conducted by the Thornycroft research department indicate that a mileage "considerably in excess of 30,000" should be possible.
Tests equivalent to over 100,000 miles have been completed on turboblown KRN6 engines, as fitted in Thornycroft Big Ben chassis. The gross laden weight of the Big Ben for this application is 31+ tons. The tests have indicated that blower per formance is not sensitive to a limited deposit of carbon on the blades.
An increase in output of about 30 per cent, is stated by the Eberspacher company to be the advisable limit for road-vehicle engines requiring a good low-speed torque. An increase in b.h.p. of 20 per cent. at 30 per cent. maximum r.p.m., rising to an increase of 30 per cent. at 50 per cent.. of the rated speed, is then obtainable.
It is claimed that the blower provides an average saving in specific fuel consumption of nearly 9 per cent. Its maximum isentropic efficiency is approximately 70 per cent., and in a typical application a boost pressure of 1.42 atmospheres is produced at 35,000 r.p.m. The' makers emphasize that at high speeds the blower appreciably reduces the thermal load on the engine.