Introduction
Mainly historical
Fifty years of experimental work
Cylinders
Port design
Mechanically timed admission
Applications of rotary valves
Crankcase compression
Piston design
Composite pistons
Deflectors
The "Zephyr" 2-1/2 cc engine
Piston head transfer valve
Crankshaft design
Bearings
Connecting rod bearings
Connecting rods
Flywheels
The Atom V engine
Phoenix 15 cc engine
The Busy Bee 50 cc engine
Carburation and combustion
Combustion chamber design
Twin two-stroke engines
Introduction
In a series of articles published in M.E. over a year ago, I reviewed the various aspects of the miniture four-stroke engine and its progress over half a century of development. As a sequel to this series, it will, I trust, be appropriate to deal in a similar way with the two-stroke, which, in the size and form used by model engineers, is at least as popular as its more sophisticated rival. One of my friends, referring to the above articles, said "Very nice for those who like to play with valves and cams and things, but why have you neglected the two-stroke for so many years?"
True enough, I have not produced a new design for a two-stroke engine for a long time. This is not due to lack of interest, or even ideas for development of design; but the fact that miniature two-strokes have been produced commercially in great numbers and variety, and are readily available to those who want them, makes it more difficult to persuade model engineers to undertake their construction. Let me make it clear that I have nothing against the ready-made engines or those who use them for propulsion of various kinds of models; most of them work very well, and provide a source of power far more efficient than anything previously available. They also enable impatient enthusiasts to buy time, to say nothing of the elimination of hard work and skill involved in construction.
I have on many occasions given technical assistance to the manufacturers of engines, and many of the features of design which are now common practice were first applied by amateur designers, among whom I venture to include myself. But I have also tried to encourage individual constructors (when you come to think of it, this is really what model engineering is all about), and this is sometimes interpreted as a private vendetta against commercially made engines. Such an idea is clearly absurd, as the number of engines likely to be made in home workshops, even at the most optimistic estimate, could never be a serious menace to manufacturers. In same respects, any individual enterprise may even be helpful to them, by demonstrating the application and possibilities of miniature engines to the uninitiated more convincingly than the most expensive publicity campaign.
In this review, therefore, I propose to concentrate on the home-produced two-stroke, and trust that I shall not offend any persons or parties by doing so. Of necessity, I propose to indulge freely in reminiscence (or as some may consider, ancient history) about two-strokes in general and miniatures in particular, but I shall also do my best to consider modem problems and to answer some of the innumerable questions which have been put to me by readers.
Mainly historical
have always found it helpful, in trying to explain the development of an engine or other mechanical device, to give at least a brief review of how it originated, and its early development. There are many who consider this unnecessary and, in support of their opinions, are fond of quoting the famous aphorism "History is bunk!" ascribed to the great Henry Ford. But nobody can deny that everything in the present and future has its roots in the past, sometimes a long way back. One of my objections to many modem text-books is that they often assume that the reader knows all about the foundations of the subject beforehand--or ought to --before starting to read them.
One of the questions I am very frequently taked is "Who invented the two-stroke?" This is a very simple question, but, like many such, not really so simple to answer. The early "atmospheric" internal combustion engines, such as the Hugon and Lenoir, were two-scokes, in the sense that combustion took place once par complete revolution of the shaft, i.e., two strokes of the piston. But these engines were completely superseded by the Otto engine, in which the mixture charge was compressed in the cylinder prior to ignition and nearly all later i.c. engines follow this principle, though not necessarily the identical working cycle. This includes two-strokes, in which it became necessary to incorporate some form of air pump, compressor or blower to force the charge into the cylinder as distinct from sucking it in during one complete piston stroke, as in the Otto cycle engine.
Most authorities credit Dugald Clerk with the invention of the precompression type of two-stroke, and it is probable that his engine was the first of such to be employed to my great extent. But Clerk's patent of 1881 was predated by that of James Robson in 1879. Besides individual features of design, the major difference between the two engines was that Clerk employed a separate pump cylinder for charging, while Robson utilised the reverse side of the piston, in a cylinder like that of a double acting steam engine, for the purpose.
Fielding's patent of 1881, engine construction was simplified by using an enclosed crankcase as a charging pump. The engine patented by Day in 1891 carried this idea still further, besides eliminating mechanically operated valves and their operating gear. In Fig. 1, based on Day's parent specification, it will be seen that the charge of air/fuel mix (carburetion and ignition details are not shown) is taken in through a large disc valve in the end plate and, after precompression in the crankcase, is transferred to the cylinder through an automatic valve in the piston crown, while the exhaust is released through a port uncovered by the piston. Both these events take place dining the later part of the down stroke and the beginning of the up stroke.
In later development by Day and some other engineers, the automatic valves were eliminated, and all there events controlled by the piston through ports in the cylinder, as shown in Fig. 2. A deflector is fitted to the piston head with the object (only partially achieved) of directing the fresh mixture upwards and preventing it from escaping or mixing with the exhaust gases. In the "three-port" engine, as it is called, the mechanical system is reduced to the minimum of three working parts, and this has been the pattern for the great majority of two-stroke engines ever since. The diagrams are explanatory only, and not complete or exact in detail, but they are approximately correct in preportions and design of the actual engines.
Before the end of the nineteenth century, many individual inventors filed patents for "improvements in or relating to" two-stroke engines, but whatever their merits might been, manufacturers have not been enthusiastic about any departure which involves complication of design or construction. Much the same applies to the innumerable attempts to improve thes engines in more recent years and, apart from engines of quite large size, the simple three-port engine is still predominant, though it has bean considerably improved in detail, and its performance has been enhanced by long and patient development by many individual experimenters.
As a rival to the four-stroke engine, however, the two-stroke has many limitations and disadvantages. Although it is sometimes thought that an engine which fires twice as frequently as a four-stroke (other things being equal) ought to produce twice as much power, this is rarely, if ever, achieved in practice, and it is even difficult to attain parity of performance. Two-stroke engines have to answer for many sins; they usually more noisy, less economical, and more difficult to control than font-strokes; in some cams they are liable to overheating, tricky to start add generally temperamental.
Sophisticated engineers, for the most part, have tended to regard two-strokes as unworthy of serious consideration, and have neglected them in the general scheme of engine research. I have even encountered a rooted opposition to them, and have been told "We wouldn't have anything to do with two-strokes at any price!" This may partly account for the fact that in the development of full-size aircraft engines, few attempts have been made to use a two-stroke engine, though some promising designs have been evolved at various times. The late Professor A. M. Low once facetiously remarked "These wretched little (two-stroke) engines have no right to existence; they ought not to work at all, but, surprisingly, they do!" At the time when I became interested in motor cycling, two-stroke machines were considered definitely inferior to four-strokes, and, in fact, their riders were regarded as declasse. Things are very different now, and over 90 per cent of two and three wheeled vehicles have two-stroke engines.
I have always considered that the two-stroke should be given due consideration in technical literature, if only because most mechanically minded youths obtain their first introduction to i.c. engines by way of two-strokes of some kind or other. But many of those who use them have no more than a vague idea of how they work, to judge by some of the very naive questions they ask about them.
Fifty years of experimental work
I have been personally interested in the design of both four-stroke and two-stroke engines in small sizes almost as long as I can remember, but the first that I actually designed were of the latter types. They included an air-cooled motor-cycle engine, a three-cylinder engine for light aircraft, and a watercooled stationary (or marine) engine, all of which worked fairly well, but failed to achieve commercial success for reasons unconnected with technical merit. Most of the engines designed since that date have been "models" in the sense that they were intended to drive or propel models of some kind, though it is often contended that such engines are not models in the true sense of the term, as they do not emulate full-size engines in scale proportion. But this applies to nearly all miniature i.c. engines up to the present day; though it is not absolutely impossible to build them more or less accurately to resemble full-size engines of certain types, it is not generally expedient to do so, if they are required to work as efficiently as possible.
In common with four-stroke engines, miniature two-strokes in the early days were not only few and far between, but also of dubious design, and no information on their construction was available. The first of my engines to be described in M.E. was Atom 1, built in 1925 with the intention of installing it in a half-scale model of the Comper CLA3 light aeroplane constructed by Cranwell RAF Apprentices. The engine ran quite well, except for sparking plug trouble (the miniature plugs then available were not at all satisfactory), but the 'plane never became airborne. As a "guinea pig" for further experiments, however, Atom 1 served a very useful purposes though it was too large to suit the popular sizes of model aircraft. I tried to stimulate interest in the development of suitable engines, but found it extremely difficult to persuade aeromodellers that anything better than rubber motors could be produced for this kind of work. It was not until the early '30's that I succeeded in obtaining the co-operation of (then) Capt. C. E. Bowden, and was able to prove that engine-powered model aircraft were practicable.
During the 1920's there was a great upsurge of interest in the development of model speed boats, and although flash steam was then regarded as the most efficient means of their propulsion, petrol engines were beginning to challenge their supremacy. From over the channel came that wizard of two-strokes, M. Gems Suzor, to astonish the natives with the performance of Canard and, later, Nickie 11. The M.P.B.A [rc: Model Power Boating Association] and the Modele Yacht Club de Paris got together to set up an International class of petrol-driven speed boats with engines limited to 30 c.c. capacity, and this was undoubtedly a milestone in the history of model power boats.
This application of small petrol engines offered, at the time, a more promising field for their development, and by taking advantages of lessons learnt by experiments with Atom I, I designed an engine of 30 c c. capacity, having features which I considered suitable for efficient speed boat work. It was named Atom II, and gave excellent results on the test bench, but, unfortunately, ended its career in a spectacular breakdown under load, due to a faulty crankcase casting. Its successor Atom III, of more robust construction, though similar in its general design, withstood the most stringent tests, and produced speed and power beyond my expctations; I still regard it as one of the most successful of my two-stroke designs.
Most model exponents of petrol-driven speed boats, however, preferred four-stroke engines, and regarded two-strokes with suspicion. I can remember only a very few of them who had any faith in the possibility of achieving high boat performance with a two-stroke engine. Notable among them was that great individualist from the North, Mr A. D. Rankine, whose Oigh Alisa dynasty of boats put up spectacular performances; another was the late Bill Rowe, of the Victoria M.S.C., whose boats were mostly noted for high speed, long distance endurance runs. My own attempts to produce results in this sphere were handicapped by lack of knowledge of hull design, and usually ended in crashes or capsizes.
Following up the success of Capt. Bowden's engine-powered aircraft, in demolishing records in their class which had been held for many years, I designed a two-stroke engine of 15 c.c. capacity in 1932, which I called the Atom Minor. In power and reliability, demonstrated in Bowden's Blue Dragon and other aircraft, proved beyond all doubt that all existing records in endurance, speed and altitude of model aircraft would become things of the past.
I shall refer later to the particular features of these early engine designs, which, although now becoming antique, are still capable of producing good results and for this reason cannot be dismissed as completely obsolete. While I have never claimed brilliance of design or super-efficiency in any of my engines, I think I can justly claim that many of their features, in structural methods and functional working parts, were innovations which have influenced the progress of design even up to the present date, including that of commercially built engines. I have always sought to exploit as fully as possible the potential advantages of two-strokes, including low frictional or other mechanical losses, and low weight, while reducing their inherent disadvantages such as imperfect charging and scavenging.
Up to and during the last war, I made many experiments in design of two-stroke engines from 50 c.c. down to 2-1/2 c.c., most of which, but by no means all, have been described in M.E., and nine of these designs are still available from the M.E. Plans Dept.; but others, and mutations (or mutilations) of them, have never been recorded, beyond references to them in a series of articles in Improving the Two-stroke published during the war years.
When the decision was made to produce an aircraft engine smaller than the Atom Minor 15 cc, the opinions of interested aeromodaillers were consulted, to ensure that the engine would be well suited to their requirements. Not only the capacity of the new engine, but also relevant features of in design and construction, and the mode of installation in the airfrme, were discussed in detail. The outcome of all this was a specification for an engine of 6 cc, which was described as the New Atom Minor, though I preferred to class it a "Mark II," became the term "New" has a short-term significance, and often comes all too soon outdated or unfashionable.
Though this engine fulfilled its puprpose, and gave good results in model aircraft of managable size, it was soon brought home to me that there was vary little interest in the construction of engines for this particular purpose. Ready-made engines were becoming available, and have since become almost universal, not only for the propulsion of aircraft but also for other kinds of models.
During the war, I was invited by a well-known firm of motor-cycle engine manufacturers to submit a simple engine design, suitable for production in moderate quantities, for a specific purpose not entirely unconnected with the war effort. The engine I produced was an improved version of the 6 cc Atom Minor. It gave results which satisfied requirements, and a number of "tool room" prototypes were made, but I was never fully informed what they were used for! (Official secrets and all that, you know.) The Atom Minor Mark III has never been described in M.E., but a handbook on its comstruction was published after the war by Percival Manhall & Co, and was fairly popular, though 6 cc has since proved to be an unfashionable size. An external view of this engine is shown in Fig. 3
But in the chronological order of things, the Mark III is out of its proper place, because in 1937 I produced the 5 cc Kestrel engine, which was not only successful from the functional aspect, but also appealed to many constructors who had not previously attempted to build a petrol engine. It featured in several early attempts by model engineers whose names later became well known in connection with model cars and boats.
The first model racing car built by Jim Crankshank was, I believe, powered by a Kestrel, and the design formed a basis for many modified and improved engines by other individual constructors. (Fig. 4.)
In post-war years, a 5 cc engine specially intended for model racing cars, known as the Cadet, was designed, together with another of 10 cc known as the Ensign, for either cars or boats. The early International class hydroplane engines of 30 cc Atom II and Atom III were followed up by Atom V, which had several unorthodox features of design. A great interest arose in the use of small engines for attachment to pedal cycles, and I designed the Busy Bee, 50 cc engine which was described in M.E. in the early 1950s and has been built and used by many readers. Some of these Bees, I understand, are still buzzing! A modified version, the Bumble Bee, was designed for light stationary work. Other post-war designs include the Zephyr 2-1/2 cc engine, the Ladybird 2-1/2 cc twin (C.I.) [rc: Compression Ignition, aka "diesel"] and the Cherub 10 cc twin.
It is not my intention to bore readers with a catalogue of engine specifications, but the above engines are, I think, worthy of mention as examples of experimental design. To illustrate their salient features of design, drawings of some of them have been reproduced. In a few details, these differ from the drawings originally published, to incorporate minor modifications introduced in the course of development. The word "experiment" so frequently used in these descriptions is often loosely applied, and may mean anything or nothing in particular instances, but generally it indicates some attempt to improve design in respect of functional efficiency, power-weight ratio, or materials and methods of construction. There is no advantage in altering design simply for the sake of being "different," but it is equally futile to adhere slavishly to conventions which, for all one knows, may be based on ancient fallacies.
The dominant component in an enclosed engine such as a two-stroke is (in most cases) the crankcase, which may incorporate other items in order to reduce the number of parts, reduce all-up weight, or simplify machining operations. In the early days of motor-cycles, a form of crankcase was developed which was well suited to the enclosure of internal flywheels; it was split on the vertical transverse centre-line, with main bearings in the two halves, and machined flat on the top to form a cylinder mounting platform.
This design has persisted, even when its original purpose no longer holds good, but I have rarely used it in two-stroke engines, as I consider that it involves unnecessary problems in machining the concentric register of the two halves, and the cylinder seating and register after bolting the halves together.
The blind-ended or "cup-form" crankcase is suitable for overhung-crank engines which do not need a power take-off on the end remote from the flywheel. But, the "barrel-form" crankcase, open on both ends, may be better suited or even necessary for assembly in certain cases. The former type was employed in the Atom I engine, in which the crankcase was extended upwards above the level of the exhaust and transfer ports; this was done mainly to reduce engine weight as much as possible. A cylinder casting (in iron) with ports and passages in it would inevitably have been heavier and might also involve molding difficulties. By forming the ports, with flanges, and also the transfer passage, in the light alloy crankcase castings, weight was reduced, and machining simplified. The cylinder was machined all over from fine-grained cast-iron, with fins on the upper part, and an external fine thread below the port level to screw, into the crankcase.
This method of construction, though mechanically sound, involves the problem of keeping close clearance in the port region to avoid leakage from one port to the other, and also in the orientation of the ports when the cylinder is screwed fully home. It is not generally convenient to fit a ring nut to lock the cylinder in the correct position, though this been done in some cases. The use of four (or more) long screws or bolts to secure both the cylinder and its head, as in the Kestrel and Atom Minor Mark II and III, is more common, but is open to the objection that differential expansion may cause the bolts to become loose (this has not happened in my engines).
If the crankcase or body casting is carried still higher, and provided either with fins, or an annular space to form a water jacket, it must necessarily be open at the main bearing end, or it would not the possible to assemble the connecting rod and piston. In the Atom Minor, engines, both ends of the crankcase were open, and apart from convenience in machining and assembly, this made it possible to reverse the engine from left to right, if this should be necessary, such as for paired engines running in opposite directions. In other engines, such as Atom II and Atom III, separate cylinders were employed, and the crankcase incorporated the main bearing housing. The Kestrel engine also had an integral crankcase and bearing housing, but the casting was carried upwards to a point below the ports, which were formed by a narrow belt pressed on to the cylinder skirt; a unique feature which undoubtedly simplified construction and contributed to the success of the engines.
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