Oil Engines Supreme: Small Units Most Economical?
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A Decade of Development in Chassis Design Shows Marked Progress in Oilengine Construction OF 511 British vehicles with payload ratings ranging from 10 cwt. to the legal maximum, 74 per cent, have oil engines, 4 per cent. are offered with either an oil or a petrol unit, whilst the remainder are available with petrol engines. These figures were given by Mr. W. C. Wilson, A.Inst.T., M.I.R.T.E., general manager of Stockton-on-Tees Transport Department, in a paper which he read to the Institute of Road Transport Engineers in London yesterday.
The direct-injection oil engine with its comparatively high thermal efficiency, easy starting and remarkable freedom from maintenance difficulties, was, he said, easily the most popular.
New engines had been designed or existing ones modified to give "square" characteristics. The speaker quoted the Commer Superpoise engine as an example. In this unit the bore had been increased by 12 per cent, with no change in the stroke. This had raised the maximum torque by 31.5 per cent. with practically no increase in external dimensions.
A 10-year Survey
Mr. Wilson quoted some figures • gained from a survey of the performance of engines fitted to American heavy vehicles in the period 1938-1948. During the 10 years covered by the survey, engine size, measured in terms of displacement, had increased by 5 per cent., and r.p.m. by 12 per cern. at maximum b.h.p. Compression ratios had gone up by 27 per cent. By cubic in. of displacement, b.h.p. had increased by 32 per cent, and maximum b.h.p. by 37 per cent. Some credit must be given to improved oil fuel, an advantage we could not share.
The extension of engine life had been a notable achievement by British makers. The Leyland concern had doubled the life of its big oil engines by eliminating weaknesses which had shown themselves during operation. This company had pioneered the prefinished slip-fit cylinder liner which had proved so successful that the need for a separate cylinder block was obviated and a cast-iron monobloc crankcase had been adopted.
Gudgeon pins and crankshaft journals had been increased in diameter in keeping with loads of an order associated with oil engines. Nitrided forged-steel crankshafts had been found to have far greater fatigue strength than those made of other metals, whilst they could be lapped to provide an excellent wearing surface. Shafts of this kind carried in steelbacked copper-lead bearings having an electrical-deposit flash of indium, had given mileages of up to 200,000 and had then needed little attention.
Referring to the use of white metal for ell-engine bearings, Mr. Wilson said that larger bearings were called for and when the unit had to be as compact B14 as possible, the designer might find it impossible to accommodate them. Reduction of the thickness of a whitemetal bearing from, say, 0.025 in. to 0.014 in. would raise the load capacity by 60 per cent. Many big-end bearings were not now provided with grooves, because it had been found that they had served to break down the oil film. Consumption tests carried out by the Leyland company had provided some interesting figures. Four combinations of engine and axle ratio were incorporated in a double-deck bus, having a gross weight of 115 tons. The combinations were: (a) 100 b.h.p., 5.4 to 1 axle ratio, (b) 125 b.h.p., 5.41 axle, (c) 125 b.h.p., 4.81 axle and (d) 125 b.h.p., 6.51 axle.
When tested non-stop on trunk routes, the best performance was put up by a, whilst d proved to be the worst, although there was only a m.p.g. between them. In hilly country at five stops to the mile, b came out on top, with a, a close second In the other two cases the big engines showed up badly with high and low ratios, and between the best and the worst there was a difference of nearly 2 m.p.g.
Economy with Big Engine
Many experts had stated that a big engine running well within its capacity offered the best results in terms of both performance and fuel economy. An operator who was noted for efficient working had found the Meadows 10.6litre oil engine cheaper to run in m.p.g. than a standard unit of smaller capacity.
A petrol-injection unit, even under ideal conditions, did not compare favourably with a modern oil engine in general use. Injection-pump phasing was less critical than in oil-engine practice, where an error of 5-degree would affect performance. Two degrees plus or minus on the petrol-injection pump miiht be permissible, so that no provision was made for fine adjustment.
In the case of a petrol-injection pump the mating of the plunger and barrel called for a degree of accuracy down to 0.00004 in. per inch of diameter, and special measuring instruments were necessary to reveal minute irregularities. The Nuffield Organization, said Mr. Wilson, had pointed out that the cost of the petrol-injection equipment might equal or exceed the cost of an engine. Progress in the design of pistons had continued by way of small but important modifications, the emphasis being laid on the rapid transfer of temperature from the piston crown to the cylinders. Compression rings had been reduced to three, allowing a deep top-land to be used, which meant that the top ring operated in a comparatively cool zone.
Whilst the advantages in maintenance of pre-finished slip-fit dry liners were fully recognized, there were theoretical disadvantages. One was the poor metal-to-metal contact, which could lead to reduced head conduction. Another was that, because of the prefinish, the bores could not be honed to correct possible bore irregularities. Nevertheless, the tendency to employ this type of liner, which was sponsored by the Leyland company, was growing and troubles consequent upon its use had been remarkably few.
Constant Mesh Popular
Crash-type gearboxes were installed in 32 per cent. of goods vehicles at present produced. Boxes having constant-mesh gears claimed 53 per cent. The remaining 15 per cent. were three, four and five-speed boxes incorporating synchromesh mechanism.
In passenger chassis, pre-selector units came first with 32 per cent., followed by crash-type boxes with 27 per cent., constant-mesh, 23.5 per cent., and synchromesh, 17.5 per cent.
In some quarters, said Mr. Wilson, the two-speed axle was cOnsidered to be overrated, it being pointed out that an auxiliary gearbox enabled the ratios of the main unit to be duplicated. Additional ratios offered by a twospeed axle were said to be restricted in range and it was more difficult to select suitable ratios than it was with an auxiliary gearbox.
Whilst plate clutch assemblies remained substantially unchanged, longer life and improved accessibility had been achieved by detail improvements. The Leyland company had found that the life of clutch plates was shortened by extreme temperature build-up on the frictional surfaces.
Present braking systems for the most severe operating conditions might not prove satisfactory and an answer might be found in the electrical transmission brake or retarder, as employed on some Continental makes of vehicle.
In the Michelin Metallic tyre, steel cord was used to overcome the limitations of cotton and rayon, and the plies could be reduced to four or even two to give cool running with a higher load for a given size than was possible with a fabric tyre.