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Aids to piston power
Page 55
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THE BEST THING that can happen to the reciprocating piston engine," a well-known engine designer recently observed, "is that its apparently impregnable position should be challenged by the gas turbine or some other form of rotary unit. Only then will designers be spurred on to apply those aids to power that would undoubtedly raise the tempo of progress to a peak that has not been achieved in the history of the internal-combustion engine.
-Many authorities consider that the petrol engine had too easy a victory over the Steam engine and that it would not have succumbed to the diesel in such a wholesale way for application to heavier commercial vehicles if its protagonists had been given sufficient financial encouragement," he said.
"It is to be hoped that the gas turbine is here to stay—it has its place in the transport industry—so long as manufacturers are not encouraged to go over to the turbine at a later date on a wave of publicityinspired enthusiasm for a new concept.
"Given that the turbine and piston engine had reached a comparable relative development stage, this would have been more likely 40 years ago when a power unit was acceptable if it didn't give much trouble in 20/30,000 miles running. Today heavy lorries and long-distance p.s.v. often cover upwards of 400,000 miles before an engine overhaul is required and this has created such an enviable standard of durability that ousting of the diesel by the turbine in say 10 to 20 years is unthinkable.
Over-confidence
"Such durability, combined with an economy that is unlikely to be surpassed, could however create over-confidence in the diesel. On the one hand its durability stabilizes the situation; on the other it could retard progress by promoting conservatism.
-The petrol engine is challenged by the Wankel and in this case applications are dominated by the private-car market and extending durability beyond a mileage of say 70,000 represents over-engineering in the case of a popular car. The Wankel may earn a place in the car and light-deliveryvehicle field but it will not replace the piston engine unless development of the latter is neglected. It would be fortunate for the conventional petrol engine if it came back into its own, at least to a limited extent, in applications to heavier commercial vehicles. And this could happen."
All the aids to power that were later mentioned by this designer have been employed in some form or another over the years either in practical applications or ex
perirnentally, but then the road-vehicle gas turbine is by no means novel and its development was inspired by the success of the turbo-prop aircraft engine. The Wankel was derived from one of literally hundreds of rotary-engine designs.
A list of the means by which pistonengine performance may be improved in one way or another is a short one and includes increasing the volumetric, mechanical and thermal efficiencies of the unit, and raising its combustion pressure and peak r.p.m.
Good combustion is more easily obtained in a slow-speed long-stroke engine partly because of its better surface/volume ratio. Mechanical efficiency is favoured by low piston speed, weight and surface area; optimum filling depends on correct proportioning and positioning of intake passages, ports and valves and can normally be obtained over a relatively narrow speed range.
Turbocharging offers the only practical means of pressure-charging a four-stroke as it does not absorb power and tends to improve combustion efficiency and fuel consumption. Above a critical speed volumetric efficiency falls off sharply, good combustion is more difficult to obtain and in a typical case a limit to r.p.m. increase is imposed by radiated mechanical noise if the unit is to be marketed in a country where strict noise regulation is enforced.
The problems here summarized—together with weight reduction and providing efficient cooling—have dominated the design scene over the years. The exercise of "matching the variables" to the best advantage still offers big rewards, but unless a practical form of gasifier /power-turbine compound engine is produced, accelerated progress probably depends on: (a) producing a stratified-charge petrol engine that will operate on full throttle at part loads, and (b) the development of a simple lightweight variable-compression-ratio piston or of a VCR system in which the effective stroke of the pistons can be varied.
In the background is the threat that the superfluity of lighter-fraction fuels will one day necessitate imposition of a differential tax on dery to encourage the use of petrol. It is pertinent that fuels in this category are a by-product of the manufacturer of plastics materials.
Effective stratification depends on the separation of the stratum of combustible mixture from the remaining strata of pure air and on having a form of combustion chamber that enables the mixture to burn without part of it being weakened by the trapped air before burning is complete.
Although the German MAN company has developed a stratified-charge system that is claimed to give satisfactory operation over the full load-speed range, it is doubtful whether a road-vehicle unit could be produced that would in practice provide acceptable part-load efficiency over more than a very limited speed range.
Strong inducements
The promise of a saving in weight comparable to that afforded by a gas turbine and the ability to burn a low-grade fuel are strong inducements, however, to engine makers to develop stratified units, whether or not the use of petrol engines is encouraged by differential taxation.
A variable-compression-ratio piston or system would enable the compression ratio to be matched to cylinder pressure, which would reduce fuel consumption of a petrol engine at part loads and logically it would be employed in conjunction with a fuelinjection system.
But as shown by applications of the BIC ERA VCR piston, the diesel would probably be the main beneficiary of its development in a commercially acceptable form. All engine designers, with whom ratio control has been discussed, agree on this point. It would provide for a reduction of compression ratio to say 11/12 to 1 for full-load-operation in the case of a medium-sized unit and ratio of 15/17 to 1 for light-load running and starting.
Increases in power output of up to 50 per cent should be obtainable without impairing fuel consumption or increasing peak mechanical stresses. Although there is a small theoretical gain in raising the compression ratio above 12 to 1, in practice this is offset by a reduction in mechanical efficiency.
The rocker-beam arrangement of the Rootes-type opposed-piston two-stroke engine could readily be adapted to provide ratio variation by mounting the rockets on mechanically controlled eccentrics, and this would be a relatively simple method of control compared with employing a twopart piston, the relative movement of which displaces hydraulic fluid.