Revolutionary Development for Diesels?
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U.S. Experimental Works Promises Greatly Increased Outputs
By P. A. C. Brockington
A.M.1.Mech.E.
IN two recent papers of the Society of Automotive Engineers, mention is made of experimental work in America on a modified B.I.C.E.R.A. variable-compressionratio (VCR) piston, and on a highly supercharged two-stroke diesel engine, and the results obtained are of significant interest as possible pointers to future developments. Advances in diesel engine design over the years have been progressive rather than spectacular, but forecasts by leading technicians in the past 18 months give promise that the diesel is on the threshold of revolutionary development in terms of output per litre. The test information in the papers gives substance to this promise, and in the case of the paper on the VCR piston, the potential value of ratio control to the designers of spark-ignition engines is also mentioned.
The authors of "A Variable Compression Ratio Engine Development ", Mr. W. A. Wallace, of the Continental Aviation and Engineering Corporation, Washington, and Mr. F. B. Lux, of the U.S. At !fly Tank Automotive Centre, state that a VCR piston can increase the output of a diesel engine by 50 per cent without a corresponding increase in maximum combustion pressure, and in "Highly Supercharging a Two-Cycle Compression Ignition Engine" Mr. Irmin 0. Kamm, of the Davidson Laboratory, Stevens Institute of Technology, and co-author Mr. Michael R. Dragon, of the U.S. Army Tank Automotive Centre, describe tests of a singlecylinder laboratory engine which has been operated at b.m.e.p. up to 360 p.s.i. A projected 12-cylinder turbocharged V engine of 312 cu. in displacement, will, it is anticipated, develop a maximum b.h.p. of 830 at 3,000 r.p.m. and a peak torque of 1,450 lb.ft. This would correspond to a b.m.e.p. of 350 p.s.i., and an output per litre of approximately 160 b.h.p.
B.I.C.E.R.A. Piston
It will be recalled that the B.I.C.E.R.A. piston essentially comprises a gudgeonpin carrier and an outer shell or ring carrier, and that an upper and lower chamber are supplied with fluid under pressure from the lubricating system through non-return valves. The inertia of the shell on the upper end of the exhaust and intake strokes tends to move the shell upwards, which increases the effective volume of the upper chamber, additional oil being supplied through the appropriate valve. At the same time, a corresponding volume of oil in the lower chamber is forced through a fixed orifice and flows back to the sump, the diameter of the orifice being such that movement of the shell in each cycle is relatively small. In effect, the lower chamber acts as a dashpot Opposing upward movement of the shell, the force of combustion creates a hydraulic pressure in the upper chamber and above a critical limit this opens a spring-loaded discharge valve, which allows the shell to move downwards, total movement depending on the excess pressure relative to the valve setting and the duration of the excess-pressure phase. This provides automatic control of the compression ratio between wide limits, adjustment of the shell to a
position of equilibrium taking place over a number of cycles.
Detailed modification to the B.1.C.E.R.A. piston incorporated in the Continental VCR piston have included the replacement of the aluminium shell with a malleable iron shell in the interests of durability, redesigning and " capsulating " the valves, re-shaping the combustion chamber and matching nozzle characteristics to the form of the chamber, with due regard to a reduction of intake swirl to a. minimum and providing a special annulus for piston-groove cooling. A modified type of upperchamber sealing ring has operated satisfactorily and development is continuing.
The authors point out that the use of a VCR piston in a naturally aspirated engine is only justified if improved startability is required, notably in arctic conditions, or if the unit is intended to run on a variety of fuels (a very high ratio is required for starting a diesel on petrol at low temperatures), both of which attributes are of great value in military applications. In highly turbocharged compression-ignition engines, peak cylinder-pressure control is considered "mandatory" if a significant improvement in specific weight and bulk is to be obtained, which it is emphasized, must not incur a material increase in cost or reduction in durability. Tests have shown that the existing design of Continental VCR pistons enables output to be increased by 50 per cent without materially increasing peak loading on the big-end bearings, and it is notable that evaluations are being made of its performance at substantially greater outputs. These indicate that the limits of increase with regard to mechanical and hydraulic operation have not, so far, been attained.
A VCR piston in a naturally aspirated engine is particularly useful in cold climates because the automatic provision of a high ratio at reduced pressures aids starting and idling by increasing the heat created by compression, whereas a high ratio is undesirable at full load because the peak pressure and, therefore, the cost and weight of the unit, are disproportionately high relative to any gain in output.
The same reasoning applies to a more marked extent to a pressure-charged engine, the difference between the optimum ratios for starting and full-load running limiting the peak boost pressure that can he economically employed in terms of engine weight, cost and reliability unless VCR pistons are fitted; part load running also benefiting by their use,
It is notable that the marginal increase in maximum big-end loading of the VCR high-output engine is accounted for wholly by the inclusion of an oil groove in the wearing surface. A particularly significant merit of the piston is that the rate of combustion-pressure increase is relatively low. Cylinder pressure diagrams of a single-cylinder test engine and a 12-cylinder test unit confirm that the VCR piston gives highly accurate pressure control.
Evaluation of VCR piston performance in small-bore, high-speed, spark-ignition engines has confirmed that ratio control affords a substantial improvement in part-throttle consumption, the compression ratio being automatically raised from 6.5 to 1 to 16.5 to 1 with a reduction in loading. According to the authors it would appear logical to predict an overall improvement in vehicle m.p.g. of 20-40 per cent as the result of using a VCR piston.
The experimental two-stroke engine employed by the U.S. Army Tank Automotive Centre is a single-cylinder loopscavenged air-cooled unit of 26-cu.-in. capacity, the combustion chamber being located in the cylinder head. Oil is delivered to the cylinder wall on the thrust side for part of the stroke by a passage communicating with the gudgeon pin_ oil groove.
Oil and Fuel Separation A basic principle of all the combustion chambers tested is separation of the oil and fuel, apart from a small volume of air which mixes with the fuel at a con
trolled rate. This is performed by physical separation into a series of small compartments, by dynamic distribution in the chamber or by combining the two. Control of the rate of pressure rise is provided by passages connecting the cylinder to the chamber. The injector is placed centrally above the chamber.
In view of the claims for the VCR piston, it is particularly significant that combustion pressures have been controlled to give a ratio of peak pressure to compression pressure of 1.2 to 1 up to b.m.e.p. of 300 p.s.i. Air and fuel are mixed in a tangential pattern, which is essential to pressure control. Specific fuel consumption varies between 0-3 and 04 lb./i.h.p./hr. up to 250 b.m.e.p., the consumption at the peak output of 450 i.m.e.p. being 0.5 Ibli.h.p.Thr.
In the case of the projected 12-cylinder engine, additional gas energy will be supplied to the turbocharger by an external by-pass combustion chamber between the compressor and turbine if the exhaust pressure is inadequate.