AT periodic intervals throughout the history of the motor cycle the large single-cylinder two-stroke has made its appearance, but, in most cases, it is heard of no more. Each new engine, however, has shown some advance upon its predecessor, and, in view of the new discoveries concerning air-cooling which war experience afforded many engineers, it is not unreasonable to expect that all previous obstacles can be now overcome.
We know of at least three two-strokes with cylinders of 500 c.c. or over which are emerging from the experimental stages with every promise of success. One of these may be said to have passed this stage, and will be offered at Olympia as a serious proposition, intended for solo or sidecar work, at a popular price. This machine is the Dunelt, the production of Messrs. Dunsford and Elliott, Ltd., large Sheffield steel manufacturers, who have an experimental department in Birmingham.
For Cheap Production.
Often the view has been advanced that, with a moderately efficient two-stroke in the neighbourhood of 500 c.c, the main essential of the Ford type of motor cycle is secured; that with such a power unit it would be possible to produce a machine capable of taking the same place in the motor cycle world as that enjoyed by the famous American vehicle in the field of four-wheelers.
Messrs. Dunford and Elliott, recognising the enormous scope for a motor cycle suitable for solo or sidecar work, and which could be sold at a figure approximating to that of the lightweight, decided that only a two-stroke engine could make this possible; but, realising that the efficiency of the small conventional three-port engine would be difficult to obtain in a much larger size, it was decided to "borrow" one of the features of large two-stroke engines which are known to be satisfactory for marine and stationary purposes. This is the truncated piston - a type which is not exactly new to motor cycle practice, though this is the first time we have seen it embodied in a large single-cylinder engine which has been developed beyond the paper stage.
By the use Of the two-diameter piston, it is possible to send through the transfer passage into the cylinder a charge greater than the capacity of the cylinder, while with the conventional two-stroke engine it is impossible to replace the dead gas by a volume equal to it. This will be appreciated readily when it is remembered that the displacement of a piston is only about 80% of the capacity of the cylinder, and dead gases must always be present in both two and four-stroke engmes. For this reason, many engineers contend that the six-stroke engine promises greater efficiency. This, however, is by the way, and illustrates the fact that scavenging is desirable with a four-stroke engine, and almost essential in a two-stroke if anything but moderate efficiency is to be obtained.
A Double Diameter Piston.
In the Dunelt engine the scavenging is done by making the charge of new gas greater than the actual volume used, the displacement of the trunk piston being 770 c.c, while the working piston only displaces 500 c.c.
It may be argued that this will result in an extravagant waste of gas, but probably the overall efficiency will be quite proportionate to the more simple type of two-stroke, as, owing to the large quantity of dead gases always mixing with the fresh charge, an over-strong mixture is necessary to make the whole combustible.
In the Dunelt engine a better and "cleaner" mixture is retained in the cylinder, and approximates to the proportion of air and petrol vapour used in the four-stroke. We understand that 70 m.p.g. has been obtained with the Dunelt on the road, so its consumption cannot be said to be unduly heavy.
The use of a truncated piston necessitates a larger cylinder than usual, hence the Dunelt engine is quite an imposing unit. The bore and stroke of the working cylinder are 85 mm. and 88 mm. respectively, while the diameter of the trunk is 105 mm. With its large outside fly-wheel and massive cylinder the engine looks heavy, but, placed on the scales at our request, it was found to weigh but 65 lb., the average weight of a 3 1/2 h.p. four-stroke engine.
From the diagrammatic view on page 509 it will be seen that, in addition to the crank case, there is an annular space above the piston trunk. This increases the compression in the crank case. The "cycle is quite easy to follow with the aid of the diagram. On the up stroke of the piston a partial vacuum is created in the crank case, which is filled with new gas from the carburetter when the trunk exposes the intake. In the mean: time, a small charge is being compressed in the annular space above the trunk, and immediately after the main charge has entered the crank case from the intake the up-going piston exposes a transfer port communicating with the annular space, thereby transferring this extra charge into the main volume of new gas. On descending, the trunk of the piston compresses the charge in the crank case while the annular space above the trunk is taking in its charge. Incidentally, it may be mentioned that this upper chamber is not required, but is one of those minor things which have to be endured for the sake of other advantages, but as the ports are shallow, in all probability the crank case space is not completely filled at high speedsj and the upper side of the pump is utilised to make good any partial vacuum at high speeds. It is the opinion of several internal combustion engine experts that the most important thing in engine design is to ensure that the gas reached the cylinder, which is an acknowledgment that this is a far more difficult undertaking than getting rid of the exhaust.
Roller Bearing Big End.
In actual construction the engine is good, very substantial ball and phosphor-bronze bearings being used throughout, the gudgeon pin being fin. diameter and the big end of the roller type. The drive is taken off the side opposite to the flywheel. Tlie piston has two rings at the top and one on the trunk.
It will be seen from the illustration of the complete machine that, while unconventional, the design of the Dunelt is extremely clean. The engine is inclined towards the head and drives a neat two-speed gear of the expanding ring type by means of chains, the final drive being by belt. Embodied in the gear box is a kick-starter with enclosed mechanism.
The wheels are interchangeable, a brake rim being used on the front wheel exactly the same as that on the rear wheel. The tyres are 26in. x 2iin. Domed mudguards Tin. across the beads, a two and a half gallon tank, and aluminium foot plates are included in the equipment, and altogether at the price at which the makers hope to market it, ?i.e., about £70, the Dunelt will undoubtedly create a great deal of interest when it is exhibited at Olympia.
The Motor Cycle, November 6th 1919. p510
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