A 250 b.h.p.
Page 47
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Two-stroke Engine
ADESIGN of engine, from which there seems good reason to expect a remarkable performance, has been evolved by Mr. A. Cuddon Fletcher, of Fletcher and Winterbottom, Ltd., Somerton Hall, near Great Yarmouth, a new company that has been formed to develop it.
The features of the design have been incorporated in a converted privatecar four-cylindered engine, the perfOrmance of which proved most encouraging, in spite of the fact that "the new engine operates "on the twostroke principle and the experimental • unit had a ISO-degree crankshaft. .
• High Power Output 'Anticipated.
A complete set of drawings for a ,four-cylindered 70 mm. by 70 mm. engine of 1.079-litre capacity has been prepared. Some of these we have examined, and, as a result, accept with more credulity the inventor's statement that he confidently hopes for an output of 250 b.h.p. at 7,000 r.p.m. The constructional work is being carried out for the inventor by M. A. McEvoy, Ltd., Derby, the maker of the Zoller supercharger used.
The engine has an overhead rotary exhaust valve, inlet ports around the lower parts of the cylinders, and, as mentioned above, is supercharged. A compression ratio Of 6.5 to 1. is used and the" mixture is supplied at 28 lb. per sq. in. Thus theactual compression pressure is equiValent to that of an unsupereharged.,unit 'with a cornpressitin 'ratio of 11 :to 1. Neverthe
leSs, the unit... is designed . f Or " straight " petrol. Composed of three steel tubes, fitted one within the other, the rotary valve runs in an aluminium housing along the top of the block, which is also made of the latter metal. The bores are unlined, and the aluminium pistons have east-iron protruding rings which constitute a bearing surface. The cranks are, of course, set at 90 degrees, and the shaft has five main bearings.
Reverting to the exhaust valve, this has, in addition to the ports (which are the length of the cylinder diameter) a number of longitudinal slots formed in its three components. These slots are arranged so that they do not coincide with each other, and provide for changes of temperature, giving the complete laminated valve a degree of springiness. Regarding the valve in cross-section between any two ports, it resembles three piston rings, each fitting closely within that outside it and the largest forming a seal within the housing. Inside the valve is a fixed water-jacketed exhaust pipe, ported above each cylinder. This discharges from both ends. The valve is driven by chain, the sprocket , running on its awn bearings and driving through dogs. It is stated that the effect of pressure On the bottom of the valve is to give it a tendency to expand.
Efficient Induction-pipe Layout.
From the supercharger, gas is fed to two induction manifolds, one on each
side. Each supplies three ports per cylinder. It is noteworthy that the intake end of one manifold is at the rear of the engine and of the other at the front. Thus any leanness of mixture entering the cylinder most remote from the blower from One manifold is compensated by a proportional richness from the other. All induction passages are designed to maintain .constant gas
• velocity. 'and it is 'proposed to provide a control by which one manifold .can be closed off, so that the turbulence of the gases in the cylinders shall not materially be reduced at half throttle.
There are 2.4 sq. ins, of inlet-port area per cylinder and the period of opening is of 110 degrees duration. • It is estimated that a volumetric: efficiency of 80 per cent, will be afforded.
Highly efficient cooling arrangements
have been evolVed. In' the cylinderbarrel jackets, water circulates by thermo-syphon, but supply pipes running along both sides Of the head, discharge jets of water around. the. sparking-plug bosses. (There are two plugs each side of each cylinder, supplied by two magnetos.) This water then flows upwards around the valve, the' two streams converging above it and escaping from a central discharge pipe at the top. A separate supply feeds the exhaust-pipe jacket from the rear.
Lubrication of the rotary valve is assured by taking pipes from the pressure feed to points between each cylinder. The oil enters a chamber, one side of which is formed by the. valve (rotating in a clockwise direction) .between about 30 degrees to •90 degrees after bottom centre. In a similar manner on the other side, the lubricant carried around by the valve is extracted, Thisprevents the cylinder from being flooded with oil.
It is calculated that even when running by compression-ignition with an explosion pressure of 1,200 lb. per sq. in. the pressure on the top of the valve would not exceed 84 lb. persq. in.
As deSigned the overall length of this engine,, with blower and flywheel, is 33 ins., its height 22 ins, and its frontal area only 140 sq. ins. • One definitely attractive feature of the design is that the hot gases are not exhausted around the tops of the pistons.
Applicable to the Commercial Vehicle.
The Cuddon-Fletcher engine is suitable for use in many fields, among which are included commercial vehicles, private cars and aeroplanes; in all of these the importance of high output per unit of weight is considerable, In view of the lack of support the two-stroke type has received by commercial-vehicle makers, it is noteworthy that this principle has been chosen by the inventor of the power unit under review in his. efforts to achieve a big horse power from an engine of small dimensions. Some engineers will consider his claims to be fantastic, and, whilst we have heard the designer's convincing reasoning in justification of his estimates, we admit that we ourselves shall not be wholly without scepticism until they have been substantiated by actual Performance.