Mechanical Blowing and Turbl larging Weighed Up
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Although Turbo-chargers have been Subject to Development, Mechanical Blowers Probably More Suitable for Small Power and Vehicle Engines
THE possibility that mechanically driven superchargers might be more suitable for the smaller sizes of vehicle engine than turbo-chargers, was one of the points brought out in a symposium on superchargers and supercharging, presented at a general meeting of the Institution of Mechanical Engineers in London on Tuesday. The symposium consisted of four papers:—
" An Approach to the Problem of Pressure Charging the Compression ignition Engine," by Mr. D. W.
Tryhorn.
"The Supercharging of High-speed Diesel Engines by Mechanically
Driven Compressors," by Mr.
B. W. Millington.
"The Turbo-charging of High-speed Diesel Engines, Present Position and Future Prospects," by Mr.
C. H. Bradbury.
" The Design and Development of Small Radial-flow Turbo-chargers," by Mr. C. A. Judson and Mr. E. Kellett.
ing range of a Roots blower. To meet this requirement, B.I.C.E.R.A. had developed the Bicera compressor. This machine weighed 189 lb. and would be suitable for pressure-chaiging engines in the 150-200 h.p. class when naturally aspirated. A compressor, as distinct from a displacing machine, was essential if a satisfactory efficiency was to be obtained at, delivery pressures of over 7 p.s.i.
The biggest problem in designing a pressure charger with internal compression was not making the working space within the machine decrease in volume and so compress the charge, but making the delivery port open quickly enough, and to sufficient area, to get rid of the charge without over-compression.
This was done in the fficera compressor by making the whole of each rotor into the rotating part of a rotary valve, one rotor controlling the inlet of the charge and the other the delivery. The rotors ran over slotted tubes attached to the casing, which actedas the stationary members of the rotary valves and served as ducts to feed the air into and out of the machine. An interesting feature resulting from the use of tubular valves was that by simple modification it was possible to vary the port opening periods at will.
Something for Nothing?
It might well be asked whether there was any future for the positive-displacement charger in these days of the turbocharger. A consideration of the character of the turbo-charger suggested that there was. At first, hirbo-charging looked as though it would give something for nothing by making -use of energy which would otherwise be wasted. On closer examination, however, it was found that Nature was not quite so generous.
Basically, the turbo-charger was a pressure charger driven by means of a hot-air turbine. The hot air was supplied by a pressure pulse and a piston'pump (the exhaust stroke of the engine). This form of drive was basically inefficient, and it was only when there was an abundant 'supply of surplus energy available that it became effective.
The fact was not to be neglected that the turbo-charger was still a more expensive unit than a mechanically driven charger, and so in any application it should give an equivalently higher maximum engine output to make it economic.
To meet the requirement for increased output for compression-igni
tion engines, pressure charging should he used. This could be done without
losing the oil engine's inherent advantage of cool running when compared with the petrol engine.
Mr. Tryhorn thought that the Roots blower, operated alone, did not give • a good enough all-round performance to make its use attractive, but manufacturers were no longer limited to this type. The Bicera compressor offered much higher efficiency and was capable of working to higher delivery pressures.
MechanicalBlowing Under 10 Litres
THE most usual reason for adopting supercharging was to increase the specific power output of an engine. thereby making possible a lighter. smaller, cheaper or more economic power unit, said Mr. Millington. The alternatives to supercharging for raising specific power output were an increase of rotational speed or an increase of combustion efficiency. Both these factors would have already been raised as high as practical expediency allowed.
The gain in economy in using a turboblower on an engine of capacity less than 10 litres became problematical, but the greatest difficulty arose where a wide speed range was encountered and where a high torque was required at tlie lower end of the speed scale. It was in these circumstances that an efficient mechanically driven supercharger became a real asset.
Mr. Millington believed it was possible to raise the output of a welldesigned normally aspirated oil engine by about 50-60 per cent., at the lower speeds, but by only about 20 per cent. at full speed without departing materially from the basic engine design. If this was achieved, a good step had been made to the constant power engine.
Sir Harry R. Ricardo, in his 1950 Thomas Hawksley lecture, had spoken of the advantages of supercharging four-stroke oil engines for use in vehicles "if and when a suitable blower can be developed for this service." It was possible that he had already settled in his mind the layout of the wobbleplate compressor for this.
To achieve compactness of design. which was most important, a high piston speed was necessary. This would lead to high piston friction and the need for lubrication of the cylinders in a normal assembly, but, by the adoption of pistons carried on guided piston rods and running clear of the bores, both lubrication and friction troubles were largely eliminated.
Grouping of the cylinders around a central rotary valve with wobble-plate operation gave compactness, whilst the adoption of a double-ended layout with a bank of cylinders on each side of the wobble plate not only made better use of the one driving member, but also gave the piston rods support from widely spaced bearings.
The whole question of the economy of the supercharged engine hinged on the ability to employ the engine so that it operated in the region of good efficiency and avoided the region of bad efficiency. As the engine must operate within the torque range in which the supercharger was really necessary, it must pull hard. '1 he way to do so was to give it the job of a larger engine by fitting it to a larger chassis.
An alternative method was to use a higher axle ratio than that associated with a normally aspirated engine. By this means the average operating point moved to a region of higher torque and lower speed which had already been shown to be towards the region of highest efficiency.
More Interest in Turbo-chargers
'THERE was now an upsurge of A interest in turbo-chargers for automotive and high-speed engines. The. reason why it had been so long delayed was two-fold, said Mr. Bradbury.
First, the fact that a component was satisfactory on mediumand high-speed engines was no guarantee that it would be equally satisfactory on smaller engines where servicing techniques were completely different from those of power stations and ships. Secondly, for a given form of construction, cost was not reduced in proportion to size and turbo-chargers built on the design principles of those for larger engines could not be sold at an economic price.
With normal turbo-charging up to about 140 p.s.i.. brake mean effective pressure, dynamic and thermal engine loadings were not materially increased. Such was not correet, however, when higher b.m.e.p. were considered.
Steps had to be taken to ensure when an engine was to operate in the region of 200 p.s.i. brake mean effective pressure, it was designed from the beginning to withstand higher pressures, or to avoid exceptionally high pressures.
Some means for maintaining the compression ratio at light load and reducing it at full load, in order that the maximum cylinder pressures might be correspondingly reduced, was needed.
Such a device, a variable-compression piston, had been tested in both petrol and oil engines from 31 in. to 81 in. bore. It had been found to operate perfectly satisfactorily over wide load ranges, in the oil engine up to 250 p.s.i. brake mean effective pressure, and to control cylinder pressures to within a small percentage of the allowable maximum.
A second problem associated with highly turbo-charged engines was that of the fuel-iniectiop system, When b.m.e.p. ol 200 or more were reached the injection period became exceptionally long with conventional fuel pumps, and any attempt to fit larger pumping elements or more massive equipment resulted in much overloading of the drive mechanism.
An appreciable increase in the rate of injection, without an increase in pump drive loading, was needed. This has been achieved in several instances by servo operation.
With a compressor pressure ratio of 4 to 1, a BSI. rating of 360 p.s.i. brake mean effective pressure was possible with a brake thermal efficiency of about 45 per cent., provided that adequate after-cooling was used. At this pressure ratio it would be necessary to use a maximum cylinder pressure of about 1,500 p.s.i. even with special variableratio pistons. The reason was that, to maintain lower pressure, the full-load engine compression ratio would be so low that the engine would tend to misfire.
Pistons would not present undue difficulty so far as temperature was concerned, but present ring layout would require modification.
The fuel-injection system would be either servo-operated or would be of the common-rail type.
When pressure ratios of 4 to I had been reached, the engine was unlikely to remain in its present form. It might becomea gas generator, still crankshaft controlled, with a free-running turbine as a power unit after the turbo-charger.
Automotive Engine Influence
TURBO-CHARGERS were already I well established for oil engines of
moderate and high outputs, and there was no reason why this should not also apply to oil engines of 100 h.p. and upwards, said Mr. Judson and Mr, Kellett. In the former case, axial-flow turbines were used, but these suffered from serious disadvantages in the smaller sizes, so that radial-flow turbochargers were more likely.
The range of applications on the small oil engine was extensive compared with its larger counterpart. Automotive engines would have a serious influence on the design'and development of the small turbo-charger. Because of the rapidly and widely varying load conditions, the component efficiency had to be high and the rotating assembly as light as possible to give rapid response.
This involved extensive development of turbine and compressor, and the use of shaft speeds of 40,000 to 50,000 r.p.m., which until recently would have been regarded as impracticable for service conditions where a lite of several thousand hours had to be accepted.
The automotive field was also highly competitive, and production quantities were large. In consequence, the design of the turbo-charger had to be such that it could be made economically and in large numbers.