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Continent Ahead of Britain : Turbo charging Brings Big Increase in Power Output
OF 36 papers on oil-engine and gas. turbine design, which were read during the fifth conference of the Congres Internationale des Machines a Combustion at Wiesbaden this week, seven were concerned with pressure charging of relatively small high-speed oil engines. The three papers summarized below were those which had most bearing on commercial-vehicle 'developments and appear to demonstrate that the Continent is well ahead of this country in this field, with experience of proved high-output turbocharged oil engines operating in vehicles.
90% Power Increase in 9.6-litre ,Engine
"VOLVO have obtained 285 b.h.p. at V 2,200 r.p.m. from a 9.6-litre sixcyl indered direct-injection oil engine through high-pressure supercharging by a turbocharger and intake-air cooling. The engine when normally aspirated gave 150 b.h.p. at the same speed, or 215 b.h.p. with turbocharging only. The 285 b.h.p. version weighed only 105 lb. more than the normally aspirated unit. This information whs disclosed by Mr. Ake Larborn, chief engineer of the Volvo development laboratory, and Mr. John Stalblad, chief engineer of the engine design department, during their paper on "High-pressure Turbocharging of Small Engines." The increased outputs had been obtained without redesigning the engine, although the Schwitzer turbocharger used gave a pressure ratio of 2.1 to 1 at 1,500 r.p.m. and 2.6 to 1 at 2,200 r.p.m. For successful operation it was, however, nece'ssary to start with a robust engine which provided liberal cooling for the valves and injectors in particular, whilst the injection pump must be able to withstand increased injection pressures. For the turbocharged 9.6-litre unit valve overlap had been increased to 78 crankshaft degrees from the standard 28-degree setting. This had necessitated cutting recesses in the piston crowns to accommodate the valves, which had tended to offset improvements in scavenging by causing combustion deterioration.
The intake-air cooler, which had been most successful, took the form of an additional air-to-air cooling block bolted on to the front of the normal water radiator. For the Volvo installation it had been fitted to a normal truck without
• modification to the engine compartment or detriment to normal engine cooling. As with most simple turbochargers, insufficient pressure at low speeds led to over-fuelling and exhaust smoking.This was a drawback which could be over-. come by producing a turbocharger with a wider operating range at the expense of overall efficiency. It seemed easier to limit the useful power range to lie between 1,200 and 2,200 r.p.m., which could he suitable for commercial vehicles with-closely spaced transmission gearing. Although piston temperatures were increased by the use of turbocharging, this characteristic had not caused serious difficulty. A rise of 25° C. was measured at the top piston ring, which was quite acceptable, but could, if necessary, be reduced by oil-jet cooling inside the piston crown. Cooling of this kind had been found most efficient.
Volvo had also investigated the possibilities of the mechanically driven supercharger and had developed a type which was driven by the engine outnut shaft through a differential gear. The gearing adjusted the speed of the Lysholm screw compressor used to provide a pressure ratio roughly proportional to engine-torque output. Fitted to a six-cylindered 6.12-litre oil engine, this supercharger arrangement proved capable of approximately doubling the normally aspirated engine's output over the entire speed range with an equal or improved specific fuel consumption. The shape of the torque curve could be adjusted with this layout merely by altering the rate of fuel delivery. This type of drive had many advantages over the directly driven supercharger, although on the grounds of cost and complication it could not compete with the turbocharger,
Turbocharger Economy Uppermost in U.K.
WHEN used with an engine frequently VII subjected to part-load operation, turbocharging could give valuable improvements in fuel economy, coupled with relatively small increases in power, said Mr. E. Kellet, of the Birmingham Small Arms Co., Ltd. He was reading a paper on "Problems in the Application of Radial-flow Turbochargers."
Fuel savings with turbocharged engines in public service vehicles had, he claimed, been found on test to be up to 12 per cent. This aspect of turbocharging was particularly valuable in areas where fuel was expensive and power requirements were fixed by specific service needs. He thought that in the future engine design would be affected by turbocharging, but that present engines were capable of withstanding much-increased power outputs. Trouble had been experienced with leaking cylinder-head gaskets on high-speed engines because of the increased cylinder pressures involved, but this could he easily cured. Crankshafts did not give trouble.
For the large slower-speed engine it was most important to keep the maximum pressures to a low level. This could be done by increasing valve overlap. but it was generally better to retain a narrow overlap and rely on variable infection timing to keep cylinder pressures within reasonable limits.
Alterations to reduce peak pressures could jeopardise easy starting and it was probably wiser to opt for robust engine design rather than to resort to complex devices giving variable pressUre ratio. If a turbocharger was properly matched to the engine with which it was used, exhaust smoking was no more than that experienced with the naturally aspirated unit and was of much shorter duration. Using a light radial-type turbocharger, lag in response -to a change in engine speed should be negligible. Even this could be improved further as lighter materials became available for the manufacture of the rotor.
Turbocharging May Change Engine Design
X JARIABLE-COMPRESSION pistons V were a possible development for the high-pressure turbocharged oil engine as a means for reducing peak firing pressures. said Mr. C. H. Bradbury, of Simms Motor Units, Ltd., in his paper on "High-pressure Turboeharging of Small Engines." Although their use greatly raised costs, the only other satisfactory method of overcoming the problem was to increase the strength, and, therefore, almost inevitably, the weight of the engine structure. Alteration to the valve timing to give greater overlap was not the way to tackle the problem, as this necessitated deep gashes in the piston crowns, which could lower the compression ratio and bring starting difficulties. Intake-air coolers, which were now used in negligible quantities because they were not warranted by the low-pressure turbochargers commonly available, would assume greater importance and become a necessity as turbocharging pressure ratios rose.
Pressure ratios of up to 6 to 1 could probably be achieved with turbocharging, but it was unlikely that ratios above 3 to 1 would be employed, because above this level, piston and injector cooling became a serious problem. On test, pressure ratios up to 2.65 to I had been used with surprisingly few complications, although the engine on which the tests were carried out was economically proportioned both mechanically and thermally.
However, the high-pressure turbocharged oil engine would undoubtedly require attention to a number of detail features. Cylinder heads should have hardened valve seats, whilst exhaustvalve seats should be Stellited. Piston cooling was not essential if the design was capable-of withstanding upper-ringgroove temperatures of 200° C., although oil-spray cooling from the end of the connecting rod might be necessary for the highest ratings.
Fuel-pump drives would have to be strengthened, whilst accurate metering of small quantities of injected fuel at idling speed would be essential.