The Story Behind the
Page 51
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Modern Oil Engine— Told by Mr. S. Markland and Mr. N. Tattersall, of Leyland Motors, Ltd., in a Paper to the Institution of Mechanical Engineers APAPER presenting a wide survey of the development and production of high-speed compressionignition engines was read before the Automobile Division at the Institution of Mechanical Engineers in London on Tuesday. Mr. S. Markland, chief engineer, Leyland Motors, Ltd., and Mr. N. Tattersall, the Leyland research engineer, who contributed the paper, stressed that the oil engine had long superseded the petrol engine for heavy goods and public-service vehicles.
In the earlier stages of the high-speed oil engine, said the authors, it was generally recognized that the pre-combustion chamber type of engine was preferable to the direct-injection unit, despite the high thermal efficiency of the latter, which was, however, restricted to a low maximum speed. For this reason, earlier developments were mostly confined to designing new combustion chambers, and at one period there were dozens of designs from which to choose.
Efficiency of Direct Injection
In a small number of experiments, the direct-injection engine was used, and now, 16 years later, it had become the more popular for its high efficiency and easy starting from cold.
A further school of thought had advanced the claims of the supercharged two-stroke-cycle engine, which, however, had fundamental limitations which had for the present placed it in the background.
The experimental stages of the directinjection engine proved that the reduced surface area of this type of unit, in comparison with the pre-combustion engine, produced a higher thermal efficiency. Swirl, which was a natural factor of the pre-combustion engine, had to be promoted in the direct-injection engine by means of screened valves and contoured inlet ports.
Previously, the single-hole sprayer had been popular as a means for distributing the fuel throughout the combustion space, but with the direct-injection engine it was found preferable to use a multi-hole sprayer, the four-hole pattern being found to produce the optimum results.
Maximum Turbulence
In addition to orderly rotational air swirl, the " squish " produced by the piston as it reached top dead centre had an important bearing on the mixing of air and fuel. A compromise had to he drawn between combustion space in the piston crown and the clearance between the cylinder head and the top of the piston at top dead centre, to promote maximum squish and swirl.
The relationship between bore and stroke of the engine, over which much controversy had raged among technicians, was essentially one of mechanical convenience, depending on a number of factors. A main consideration was the length available for the engine, which would decide the width of bearings, and the valve and port diameters, assuming that the valves were in line.
With a conventional six-cylindered direct-injection unit employing a counter-weighted crankshaft to permit use of a shorter centre main bearing, a compromise might be reached by employing a bore-to-stroke ratio of 0.85 or 0.9 to 1.
The relatively short stroke permitted the use of a short connecting rod, which reduced the height and weight of the engine. In addition, the path of the connecting-rod journal was kept to a minimum, and as the contour of the crankcase closely followed this path, a narrow engine could be built. It was preferable, said the authors, to keep the stroke as short as possible.
In designing a cast-iron cylinder block, adequate spacing betwOen cylinder barrels was necessary if foundry scrap was to be kept at a low percentage, a tninimum of in. having been found most practicable. Spacing between the centre cylinders must be adjusted to allow for a wide centre bearing to carry the crankshaft; this width could be minimized by utilizing a counterweighted crankshaft.
Crankshaft Specifications
In earlier days, crankshafts were made from identical materials and with heat treatments similar to those of petrolengine crankshafts. The result was that the crankshafts were too slender, and, with incorrect materials and heat treatments, premature failures in service were a common occurrence. It was apparent that loading failures predominated, regardless of a factor of safety of 6 or 7 to 1, when considering maximum bending stress in relation to ultimate tensile strength.
Thus the fallacy of basing safety factors on -the physical properties of conventional materials was exploded .at an early stage. The shaft diniensiOns could possibly be deduced -by using higher mathematics, but for experimental purposes a Wohler testing machine was used Results from this Machine compared closely with those obtained under. normal service conditions. Fatigue-testing specimens ',Over the past 12 years showed the nitrided 'forged-steel crankshaft to be' -vastly "superior: in fatigue strength to all others -tested. "
In nitriding the shaft, it was found that the surface layer of the steel expanded, leaving the core unaffected, thereby "restraining the surface expansion and leaving the nitrided layer under a cornpression'stress. The nitrided "surface was extremely hard, had .excellent wear-resisting properties, and was not easily softened by heat. Nitrided bearing "surfaces subjected to overheating by seizure lost little of their surface hardness.
By virtue of natural frequency periods, it was difficult to design a crankshaft for a six-cylindered-in-line engine without setting up a torsional vibration, but, by adding a frictional, harmonic or pendulum damper to the system, the amplitude of the peaks could be reduced, the authors pointed out.
Provided that -sufficient length was available in the engine, a babbit-lined bearing might be used, but in most cases this space was not available, and an alternative harder material had to be used. Aluminium alloy had been found to have several shortcomings which restricted its use, and copper-lead bearings on steel backings gave far better results.
A further improvement to the copperlead bearing was found when the surface was treated with an electrically deposited flash of indium thermally infused into the lead. The rate of wear with this type of bearing was low and the indium layer provided a safeguard against scoring or seizure, also acting as an anti-corrosive against fatty acids which might be present in the lubricating oil.
Catering for Excessive Wear
The engine-lubrication pump must be deSigned to cater for engine requirements when excessive wear had taken place in the engine. Allowances for operation under tropical conditions, when the oil viscosity was low, must also be considered. There had been much controversy as to whether the " by-pass" or " full-flow " oil filter should be used. Both had their respective merits, and a compromise between the two was, perhaps, preferable, the authors suggested. • Detergent oils of good quality reduced sludging in the engine and tended to keep the piston rings free in their grooves. The importance of choosing the correct grade and quality of oil was, however, stressed in the paper.
' Of the many types of fuel equipment 'tested; the remote seating nozzle tip g,aye good ,results; as the guided portion Of the nozzle needlewas protected from the heat of ;combustion. A special phbadgraphieapparatus assembled to take micrp-flash. photographs . of the spmy.in.free: air .disclosed Spray propagation. From these „ photographs the form of atomizer, -size, of plunger, and effective ' pumping Stroke could be. 'determined.
Desirable' Testing Conditions It was not considered necessary or desirable, on test, to run the engine in at a very-low Speed but it was advisable to start' at half the maximum engine speed with light load for the first 30 minutes, increasing the speed; but not the• load; over a period of two hours. The *maximum speed should then be maintained an-I the load increased over a further -period of two hours to fullload conditions.
Power and consumption curves should then be made and compared with a known performance for the particular class of engine.