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Car Engine for Heavy Commercials ?
Page 65
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Rolls-Royce would develop all-aluminium V8 petrol engine for lorries and buses if demand were sufficient HAS the high-speed, lightweight petrol engine in the 200-250 b.b.p. category a potential for long-distance goods and passenger vehides? Technicians of Rolls-Royce Ltd., motorcar division, Crewe, consider that a unit of this type could possibly be employed economically in selected applications, and the company would be prepared to develop a suitably modified version of the 1841 allaluminium V-8 engine, fitted to RollsRoyce and Rentley cars, given that a favourable climate of operator opinion created a demand of sufficient quantity for the unit.
Having a dry weight of 561 lb. (244 kg) with auxiliaries, the engine develops a maximum gross output of 245 b.h.p. at 4,000 r.p,m, when operating with a compression ratio of 9 to 1 on 400 octane petrol. If the engine were modified to -run on commercial fuel with an octane rating of 90, it would be necessary to drop the compression ratio to about 7.6 to 1, but although this would necessitate derating the unit, its power output would be in excess of 200 b.h,p. Maximum gross torque of the high-rated engine is 372 lb. ft. at 2,250 r.p.m.
Cubic capacity of the unit is 6-23 litres (380-2 Cu. in.), the bore and stroke being 10-41 cm. and 9.14 cm. respectively (4-1 in. and 3.6 in). Overall length from the front of the fan to the crankshaft flange is 37-125 in, (94.3 cm.), whilst the unit has a width of 23-25 in. (59l cm.) across the induction system and a height of 31.5 in. (80 cm.).
It is emphasized by a leading technician of the company that the maximum weight-saving advantage of employing the unit would be attainable only if it were matched to appropriate transmission components, further gains being possible in some applications by using lighter frame sections, front springs and so on. If optimum advantage were taken of the
unit's low weight and high operating speed in these ways, it should provide an overall weight saving of at least one ton. compared with the latest types of fourstroke diesel in the same horse-power category. The cost per horse-power would be about the same.
Although no specific claim is made by the company for the probable life of the unit between major overhauls in typical commercial-vehicle applications, the characteristics of the engine and its proven durability give promise that the cost of maintenance would be no higher than that of a diesel and that vehicle serviceability. would be comparable. Reducing the compression ratio and operating on commercial fuel would reduce fuel costs.
In assessing the fuel consumption cf a maximum-load, multi-wheeled vehicle equipped with the unit—compared with a diesel-engined 'counterpart—due regard lutist be given to operating conditions, notably to the average load factor. The difference between the consumption of the two types would diminish as the load was increased because the effective com
pression ratio — and therefore the efficiency of a petrol engine—progressively increases as the throttle is opened, an eventual decrease at speeds in excess of around 60 per cent of the maximum r.p.m, being a function of breathing difficulties. In practice, therefore, sustained high-load running at, say, 50-70 per cent maximum output favours the petrol engine, whereas the advantage of the diesel is increased if the load factor is small and if ,the engine idles for long periods.
As a basis of comparison, the probable petrol consumption of an L841-engined vehicle of 6 m.p.g. may be , matched against a consumption of an equivalent diesel-engined lorry of 10 m.p.g. which would probably apply on typical trunking runs, with a relatively small proportion of motorway running and if the vehicle were loaded in one direction only, If, however, the vehicle were engaged on a motorways' shuttle service, fully laden in both directions, the consumption of the petrol-engine vehicle might well be improved to something better than 6.5-7 m.p.g. Moreover, petrol injection might enable the engine to burn commercial fuel without an appreciable reduction of power.
The L841 was designed as an aft aluminium engine and is based on an extremely rigid crankcase. Wedge-shaped combustion Chambers are employed and the valves are operated by short pushrods and a centrally located camshaft, the tappets being of • the self-adjusting hydraulic type. Each bank of cylinders is fed by an S.U. diaphragm-type car buretter with automatic control of the choke. Full-skirted aluminium pistons with three compression rings and one oil.; control ring are carried by balanced connecting rods comprising H-section forgings, whilst the big-end bearing shells (as well as the main-bearing shells) are lined with copper-lead or copper-leadindium. The crankshaft is a dynamically balanced, chrome molybdenum steel forging with five journals and four crank pins.
A special turbine type of vane impeller is used to circulate the coolant, and it is notable that the rate of flow round the head passages is many times higher than the flow round the bores and that coolant jacket pressures are maintained at a comparatively high figure to reduce the possibility of water-side attack on the wet cylinder liners. • Uniform cooling of th: cylinders and cylinder .heads is promoted by this system: