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148 
.ocalized overcooling a bar to ligher outputs
Page 97
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(FIE extent to which an engine is overcooled for a period or continuously, is :ctly related to the temperature of the
• lant. But whilst overcooling can be largetvoided by improved temperature control, rheating may be localized and in the nt it cannot be cured by modifying the iator system. In every case, it results m an engine design fault, the rectification which could be extremely costly in terms -e-tooling and so on.
When outlining the problems of cater, for high-output engines a few weeks ), Mr. D. A. Law, chief engineer of Wellrthy Ltd., and a director of the company, ntioned in particular that the hottest rt of a vehicle engine was undercooled a typical instance, and he pointed out it the makers of heavier industrial and wine engines had tackled the problem ich more realistically than had vehiclegine manufacturers.
Mr. Law emphasized that large industrial gines were normally turbocharged and crated at much higher b.m.e.p. than hide engines. For many years the de;ners of such engines had been fully aware the necessity for adequately cooling the :a round the upper end of the cylinder er and the base deck of the head, and had cen measures to provide good heat dissinon in the area by detailed attention to olant-passage design and to obtaining itimum flow rates.
As mentioned by Mr. Law, inadequate oling in this area relative to the b.m.e.p. 'hich may prevail despite a comparatively w radiator temperature) results in the rmation of hard brittle carbon which is tremely abrasive and is correspondingly :structive when it flakes off. Cyclic loadg from idling to full load increases the te at which hard-baked carbon is formed in practice, destructive flaking can :cur after a vehicle has covered a mile;e of 5,000 to 20,000.
avitation effects
Dealing with one of the most vexed probms facing engine designers today— irritation erosion of wet cylinder liners— Er. Law described chrome plating of the ler (externally) as an expensive palliative tat could transfer the area of attrition from re hard chrome surface to the rougher irfaces of the crankcase, which the bubbles attacked after bouncing or sliding off the chrome.
Mr. Law observed that the use of dry liners did not necessarily obviate the erosion problem because, in the same way, the area of attack was transferred to the crankcase. Adding a soluble oil to the coolant could assist in the prevention of erosion by virtue of the resultant increase in the surface tension of the water. Localized crevice attack in small corners of the system could be prevented by the application of chrome.
Although a lesser problem, the anodic corrosion of liners could reduce their useful life and it was pertinent that experimental work in the Wellworthy laboratories included the development of a surface coating that afforded galvanic protection for the life of the liner, the thickness of the coating being more than adeT quate to cater for the dissolution rate.
After he had mentioned that increasing gudgeon-pin diameter to a maximum of 0.6 of piston diameter represented a limiting design factor, Mr. Law cited the use of multi-piece or composite pistons as a probable future necessity in the development of high-output engines, which would enable a high-conductivity material to be employed for the upper section of the piston and a high-strength material for the lower section, the two attributes being impossible to combine in one material.
The materials involved could include malleable irons, high-conductivity copper alloys and low-expansion irons having a high nickel content, in addition to aluminium alloys. Thermal cycling in the form of regular heating up and cooling down could represent a particular problem in the case of vehicle engines and unless the material had good elasticity, cycling created residual tensile stresses.
Cracking of the edges of combustionchamber bowls could be caused in this way, and although the cracks were sometimes of the relaxation type that did not spread, propagating cracks were the more common and both were undesirable.
Pistons in current Wellworthy ranges featured iron inserts for the top rings which, it was claimed, greatly improved the wearing and corrosion-resistant properties of the assembly. The inserts were of austenitic iron.
Looking further into the future, Mr. Law mentioned fibre reinforcement of alloys as an interesting prospect. Whilst impairing conductivity to some extent, "whisker technique" improved the strength of the material and had been the subject of a considerable amount of research, notably in America. Fibre reinforcement had been successfully applied to rocket mechanisms and for the time being was very costly.
On the subject of lubrication, Mr. Law suggested that the oil companies had "made mistakes" in the choice of additives and that this could explain the high incidence of scuffing in recent times on a world-wide scale. Oil-cooled pistons would probably be required for high-output engines and the oil would be circulated through an annular channel in the crown, which would also enable the weight of the piston to be reduced.
Mr. Law forecast that the variable-compression-ratio piston eventually would be fitted to the majority of larger engines to facilitate operation at b.m.e.p. in excess of 300 p.s.i. The v.c.r. piston would favour part-load running and would limit the peak firing pressure.
Other basic problems of designing highoutput engines reviewed by Mr. Law included the difficulty of injecting the extra fuel without extending the period of injection which was necessary in the interests of efficiency. Conventional types of pump could not accommodate the higher mechanical loads involved, but the difficulty could be overcome by utilizing a hydraulic-servo system of the BIC ERA type.
Whilst Mr. Law did not consider that piston inertia would create any problem of note, he stressed that a low piston weight was desirable as it promoted mechanical smoothness. With increases in firing pressure, engine scantlings had to be improved and this, together with the adequacy of bigend bolts, would accommodate anticipated inertia stresses.