AA Scott could fairly be called the father of the two-stroke motor cycle. Here’s a treat: the great man explains how he came to design his legendary two-stroke.
“Mr Alfred A Scott needs no introduction to our readers. He may be described as the ‘godfather’ of two-stroke motor bicycles, for it is undoubtedly due to his skill and untiring perseverance that the two-stroke motor cycle has gained such popularity. In the accompanying article Mr Scott relates his early efforts with two-stroke engines, and why he favoured this type of motor. It is interesting to recall that the first description of the Scott was published in this journal on September 2nd, 1908, after its successes at the Sutton Bank and Newnham hill-climbs.”
Why I favoured the two-stroke engine
By Alfred A Scott
“MY INCLINATION TOWARDS THE DEVELOPMENT of a practical two-stroke engine is no doubt due to early training and association with engineers manufacturing marine and high-speed steam engines, so that in turning my attention to the gas engine and consequently to the petrol engine, I was naturally attracted by the possibilities of the two-stroke cycle of operations by which the regular impulse, the simplicity of design, and the sound mechanical motions employed in steam engineering practice could be retained; in preference to the four-stroke with its irregular impulses and complication of valves, gearing, and cams. I do not mean to infer that in my opinion the four-stroke system is in any way defective or unmechanical; I am simply attempting to respond to
the Editor’s invitation to explain my early preference for the two-stroke system, and consequent neglect of the type universally adopted in the early days of motor cycles. My initial preference for the two-stroke was further strengthened by the reliable and excellent service obtained from a two-stroke gas engine designed by my brother (Mr AF Scott, MIME) and employed for many years in driving the machinery in my experimental workshop. The two-stroke petrol engine first made a place for itself in marine work, being almost universally used for small sizes in America, and was considered eminently suitable for the purpose of driving a boat, where variation in load was not recognised and where absolute simplicity was desirable, and so long as it was confined to this field no serious attention was made to design a two-stroke which would give the elasticity of control required for road work. I wish to avoid giving the impression that we are particularly indebted to American genius for the invention and development of the two-stroke. Priority in its invention and development can undoubtedly be attributed to Mr Day, and in recognition of this I have frequently referred in previous articles to the Day cycle in contradistinction to the Otto cycle. My first two-stroke engines were designed for marine work, and I found a decided advantage in experimenting with an engine on a boat where it was possible to make tests and obtain diagrams under working conditions. My first bicycle motor was fitted to a Premier bicycle in 1901. The twin cylinders…were made of steel tube with aluminium radiator flanges shrunk over the outside. Plain phosphor bronze bearings were first of all fitted, and later metallic bearings with floating gland joint. The engine drove the front wheel by friction contact with the tyre, a successful drive in dry weather, but useless in wet. I was delighted with my first ride on this machine, but found that the steel cylinders were quite impracticable, for, in spite of liberal lubrication, they scored badly and showed no signs of taking a polish. My next engine was a development of the same idea, but with cast iron cylinders…This engine eventually drove by belt to clutch counter-shaft and thence by chain to back wheel. One trembler coil was used, and the spark distributed to either cylinder by a simple device provided by fixing a projecting electrode on the piston head. Later on a plain coil was used. A rotary contact-breaker was driven by link work from a pin placed mid-way on the connecting rod, and by deriving its rotary movement from the
elliptical motion of the rod, a variable motion with quick and slow periods was devised, useful for the sudden interruption required in a mechanical contact-breaker. At the same time I extended the development by experimental work on a two-cycle marine engine 4x4in, and by keeping this engine on the test for the greater part of a year derived useful information as to what could not be done. The engine was fitted with a large water-cooled brake wheel, so that it could be run at full load for any length of time without heating up. On the average this engine developed 10hp at 800rpm and showed 85% mechanical efficiency. The present Scott type of piston with the now generally adopted curved top was developed during these tests, and many developments as to port proportions made and the effect observed by the brake and indicator readings. The satisfactory behaviour and proved efficiency of both the marine and the smaller motor bicycle engine convinced me of the superiority of the two-stroke for both purposes, and I decided to improve upon previous efforts and build a complete two-stroke water-cooled motor bicycle. An accident on an early motor bicycle enforced some considerable leisure time, in which I was able to think the matter out and forecast what was wanted, and devise in its main outlines the machine associated with my name. The first complete edition of this machine (1908) had to depend upon coil ignition, but the substitution of a magneto greatly improved the rapidity of ignition and overcame the most annoying of two-stroke troubles. It will be remembered that a twin cylinder two-stroke firing twice every revolution consumed four times as much current as the single-cylinder engine of its time. This meant a big drain on accumulators, excessive wear on the platinum points of the mechanical break, and constant failure of the electrical equipment. The development of the magneto into the perfect machine of to-day has had a great influence in making the reliability of the two-stroke motor bicycle, where certainty and rapidity of ignition are particularly required. The absence of working valves on the two-stroke presented another difficulty. It was obviously necessary in producing a chain-driven two-stroke motor bicycle to provide some substitute for the usual valve lifter, since at that time all the early motor bicycles were controlled more or less by the valve lifter. This was met by providing a secondary exhaust port placed further up in the cylinder wall, which could be controlled by a lever on the, handle-bar, so that the compression the cylinder could be reduced at will. This half-compression lever was combined with a magneto cut-out switch, so that the engine could be completely controlled to the same extent as was possible with the ordinary valve lifter on the standard four-stroke motor bicycle. At the outset the Scott motor bicycle was regarded as an elaborate freak by the motor bicycle world of that time, which could scarcely be expected to swallow all at once this combination of open frame, water-cooling, twin-cylinder, two-stroke, two-speed gear, kick starter, chain drive, etc, and in view of this natural conservatism, I was strongly advised to limit my ambitions and develop the two-stroke on the accepted lines of the ordinary belt-driven machine. As far as the development of the two-stroke engine
is concerned, there was no difficulty in proving immediately its efficiency in competition. I began the list of Scott wins at the Bradford club’s hill-climb at Wass Bank, and then gaining three firsts on formula at the Coventry club’s hill-climb at Daventry evoked a storm of protest, with the unfortunate result that the ACU was induced to impose a stiff handicap on two-strokes, which, although it did not completely stop the record of success, checked any further inducement to compete in hill-climbs, and thus materially delayed the general appreciation and recognition of the peculiar pulling power of the two-stroke engine. The question of exhaust naturally arises from comparison with the four-stroke, where a full stroke of the piston is spent in expelling the exhaust gases, and in addition a full stroke of the piston is utilised to draw in the fresh charge. In the two-stroke, where exhaust takes place towards the end of the stroke, the exhaust gases move under the influence of considerable pressure; in fact, a very, persuasive influence to depart is exerted compared with the mild invitation extended to the contents of the carburetter. Taking into consideration the relative conditions of pressure, area and time, I feel that in some two-stroke engines at high speed this invitation to come inside must seem so casual and offhand that I can quite imagine the reluctance of the charge to enter the engine at all, and can sympathise with the natural attitude of the greater part of the mixture in remaining sullenly in the inlet pipe with a feeling of ‘The welcome on the mat ain’t meant for me’. In attempting to predict the tendency of future design, I would expect the ultimate success of the simplest means for attaining increased economy, control and power, and do not think that real improvement will be secured by the complication presented by double diameter pistons, additional displacer pumps, etc. I do not, for instance, see that any real practical advantage is gained by adopting a complicated and heavier design of engine in order to avoid the use of the crank case for compression of the charge. The crank case compression two-stroke engine, in spite of its defects, is irresistible on account of its simplicity, and I think that in future development this characteristic simplicity will be conserved. I anticipate the successful application of the Diesel system in combination with water cooling to increase economy and efficiency of the two-stroke, and no doubt we shall depend in future upon some form of independent pneumatic ignition.”