A New Aero Engine
Five-cylinder Radial Two-stroke of Unique but Simple Design.
ALTHOUGH the two-cycle petrol engine is nearly as old in conception as the four-stroke type, it was not until comparatively recent times that it advanced in development and favour to any appreciable extent. Not that it has failed to claim the attention:of a large number of inventors and designers for Many years ; attracted by its salient features of simplicity and smooth running, engineers the
world over have sought in the past to perfect the two-stroke by all manner of means, and with varying degrees of success. But while this work was being carried on, the four-stroke had already become the pre-eminent type of petrol engine, and in addition to grappling with the difficult problems of scavenging, cooling, fuel consumption, and flexibility, two-stroke exponents found themselves also having to contend with a certain prejudice, which had grown up against this form of power unit. In spite of this, however, its development continued, and ultimately in one sphere, at any rate—that of motor-cycling—it h a s now established itself as an engine possessing to a remarkable degree, stamina, speed . :.and economy, so that in the smaller capacities it is a
formidable rival to the four-stroke. Moreover, in reaching this satisfactory state, its simplicity has not been sacrificed, the vast improvement in efficiency being due, in practically every case, to the increase in knowledge of metallurgy and the involved technicalities peculiar to the two-stroke itself. The fact that we now have (and have had for some years) tiny engines of egg-cup capacity with but “three which
are capable of producing really respectable b.h.p., which can withstand the strains imposed by prolonged full-throttle runs, and which, at the same time, are economical in the consumption of oil and petrol, has led many people to ask why the two-stroke is still taboo as a power unit for cars and aeroplanes.
It has been argued that especially for aero work, the two-stroke has much to conunend itself, and it is therefore interesting to find that a new engine of this type has now appeared. Known as the Caunter, this unit is of the radial type and has five air-cooled cylinders arranged around an alnminium crankcase in the usual way. In preparing the engine, the designer kept in mind two points, i.e., that it was to be simple (in order to keep down the cost of production and
to make for reliability), and that it was to have good volumetric efficiency. And in order to secure these qualities he has embodied various patented features in his layout. The cylinder charging is effected by means of truncated or stepped pistons, the fresh gas being drawn into the underside of the latter (which has a volume 50% greater than that of the upper working bore) from an induction ring, through valves located in the crankcase. By this means 180 degrees of inlet suction, in the pump cylinders, which are sealed by simple ball slides through which the connecting rods pass, is obtained, thus ensuring a full charge being passed, via orthodox transfer ports, to the combustion chambers. a The stepped pistons are also utilised to aid in the exhaust scavenging. This is effected by using the vacuum generated in the annular space, by the descent of the piston. The main exhaust port opens first, and when atmospheric pressure has been reached in the working bore, auxiliary exhaust ports are uncovered by the piston skirt so that the residue gases are sucked into the annular space, where they are compressed on the following up-stroke, and forced out through a small exhaust
transfer passage and the main exhaust port.
In other respects the Caunter engine follows normal practice. The crankcase is in one piece of Alpax, with detachable front and rear covers, the latter carrying the two magnetos, the oil pump and revolution counter drive. The induction ring and inlet valve pockets are included in the main casting. The carburettor, a type R.R.C.H. Claudel-Hobson, is bolted to the crankcase between the two lower cylinders and the magnetos are B.T.H., type G.14.5. The cylinders have cast iron barrels, with detachable aluminium heads, the whole being liberally finned. The auxiliary exhaust transfers are also detachable, and are of aluminium, so shaped as to permit air to pass between them and the cylinder walls to avoid hot spots. The pistons are made from aluminium alloy and have cast iron inserts to take the rings, with a view to giving long wear and preventing distortion. The deflector, in conjunction with the cylinder head formation, is calculated to produce a good gas turbulence and flow. The gudgeon pins are of the floating type, and the connecting rods are tubular with slipper type big-ends. They are secured to the crankshaft by two substantial retaining rings. The former is a solid single
throw type and runs on a big ” no-gap ” ball bearing (which takes both journal and thrust loads) in front, and a plain phosphor bronze bearing at the rear.
The principal data, including estimated h.p. of the Caunter motor is as follows :—bore and stroke dimensions-85 ram x 88 m.m. ; cubic capacity, 2 litres, weight 160 lbs. ; power output at 2,000 r.p.in., 55-60 h.p.
At the present time the engine is undergoing its preliminary running tests and its actual performance figures are not yet available. But initial trials and an inspection of the engine indicate that it is a promising proposition, which may lead ultimately to an aero engine which will be cheap to make (according to its designer, Mr. C. F. Caunter it could be marketed at about 275), simple and thoroughly reliable, and admirably suitable for the really low-priced light ‘plane which has yet to come.