IN previous articles on unorthodox engines, I referred to attempts which have been made to dispense with a crankshaft.
The substitute for the normal crank has usually been a swashplate or equivalent device, which produces an axial or arcuate motion of the pistons. Alternatively, there have been a few examples of engines in which the pistons, or extensions thereof, bear directly or indirectly on formed cam tracks which control their order of movement.
This is sometimes done to vary the travel or duration of individual strokes to obtain, theoretically at least, an increase of efficiency. In other cases, multi-lobed cam tracks are used to increase the number of cycles per revolution of the shaft. One such engine employed a wave track on a cylindrical cam which produced eight inward and outward strokes of each piston per revolution--equivalent to using a reduction gear of 8 : 1 ratio.
Generally, such engines have failed to show any improvement over those with conventional cranks and connecting rods-rather the reverse, in fact, because of the lower mechanical efficiency in transmitting motion through cams.
Even when ban or roller races are used as the thrust members, both the axial and side thrusts are difficult to cope with: and so far as i.c. engines are concerned, they have never attained practical success.
At one time, cam-track engines operating on air pressure were popular for portable tools, where compactness was more important than economy, but they have now been superseded by rotary motors of the vane type, Which have no reciprocating parts and no valve gear.
Axial piston motion, though usually favoured in cam-track engines, is not an essential feature; some engines have had peripheral cams producing a radial movement of one or' more pistons. They are less compact than the axial type, and if the cams are made to control piston movement in both directions, their construction is also more complicated. Complication is often necessary for Lc. engines, though not for single-acting steam or air motors.
Both axial and radial cam-track engines have been made in "static" and "rotary" types; the latter term, as explained in previous articles, is somewhat ambiguous, though in engines with reciprocating pistons, it means that the engine structure rotates, while the crank or camshaft remains stationary. As Einstein told us, it is all a matter of relativity. But it introduces centrifugal force, which can be usefully employed in certain cases, though it is not always an unqualified advantage and may have undesirable effects.
An interesting steam engine produced some years ago employed four pistons tangentially located in a rotating member, the centre of which formed a bearing for a hollow stationary shaft. This served as a steam supply pipe, and was ported to form a rotary admission valve. The rotating member was eccentrically mounted inside a circular casing, the inner surface of which formed a track on which articulated slippers attached to the pistons took their bearing, under the influence of centrifugal force.
Rotation of the member caused the pistons to move inward and outward once per revolution, their stroke being determined by the eccentricity of the casing. Exhaust could be controlled either by ports and passages in the rotary valve, or by uniflow ports uncovered by the pistons. It is probable that the mechanical efficiency of this "tangento-rotary" engine was low, because of the friction of the piston slippers. Undoubtedly rollers would have been better in this case.
One of the most promising cam-track engines-potentially at least was that produced in the Twenties by the Fairchild Aircraft Corporation of America. This had four radially disposed cylinders (or banks of four), the pistons of which were provided with rollers making contact with a large "figure-eight" cam, carefully designed to produce a true harmonic motion of two complete inward and outward strokes of each piston per revolution.
Connecting rods between the roller pivots of the pistons tied them together, on the principle of a rhombic parallelogram, so that one opposed pair, in moving outwards, caused the other pair to move inwards, and vice versa.
Side thrust eliminatedThis mechanical system imposed no side thrust on the pistons, and loading on the cam and the rollers was relatively light if the contour of the former was properly designed. The engine could be made either static or rotary, with no basic alteration in design. Presumably it did not work out as expected, though it is difficult to understand why; at any rate, it never went into production, so far as I know.
A new cam-track engine brought to my notice a few weeks ago embodies some of the features already described, including the outward-mouthed cylinders and rotary valve of the "tangento-rotary" steam engine, and the "figure-eight" cam of the Fairchild-Caminez engine; though the latter surrounds the rotating system, and is internally contoured.
The new engine, for want of a better term, described as an " inside-out rotary," is the invention of Mr A. C. Mercer, of Bradford, who has built a six-cylinder working model of 1-1/8 in. bore and stroke (111 c.c. total displacement).
It works on the normal two-stroke principle, but as it cannot obtain crankcase compression, it is fed by a rotary blower, which is belt driven. In the centre of the assembly is a stationary hollow shaft. which forms the transfer passage, and has two ports diametrically opposite to control admission to the cylinder. The rotating assembly comprises a hexagonal hub, around which are equally spaced the six open-mouthed cylinders.
Each piston is fitted with a roller bearing which engages with the cam track, and as the system rotates, makes two complete inward and outward strokes per revolution. Contact is maintained by centrifugal force which pushes the pistons outwards so that it is not necessary to provide for positive operation both ways.
At the outward end of the stroke, exhaust ports, similar to those of an ordinary two-stroke, are uncovered by the piston. A sparking plug is fitted in the side of each cylinder, near the inner end, and the six plugs serve as their own h.t. distributor by coming in turn into close proximity to stationary conductors, to which current is supplied by two coils and contactbreakers.
The internal cam track may be defined as an undulating plane or "switchback", wrapped round the inside of a cylinder. When the rollers of the pistons are on the "downhill" parts of the track the pistons move outwards; on the "uphill" parts they move inwards.
As each comes to the inner end of its stroke, or the "top dead centre", the cylinder charge is ignited by the sparking plug, and the resulting combustion pressure forces the piston outwards. This produces a torque reaction on the assembly, which in turn acts as the propelling force. As the piston nears its outward position, the exhaust port is uncovered, allowing the products of combustion to escape, and very shortly afterwards, the ports of the rotary valve are opened to admit fresh mixture from the blower.
At 90 deg. from t.d.c., the piston is at the outer end of its stroke, and the next 90 deg. are occupied in pushing it back into the cylinder, thereby compressing the charge, which is then ignited, and the sequence of operations repeated.
Each cylinder, therefore, completes two working cycles per revolution, and the six-cylinder engine has an impulse frequency equivalent to that of ordinary two-strokes with 12 cylinders, or a four-stroke with 24 cylinders.
As the reciprocating parts of this engine are always working exactly in opposite directions at equal speeds, the balance can be made to all intents and purposes perfect. The working parts are few in number, and present no special machining or metallurgical problems.
Cooling and lubrication are not difficult, but centrifugal force may be the limiting factor in the permissible speed of the engine, which has been restricted to 1,500 r.p.m. in the model illustrated.
It is said that plans are in hand for a further experimental version of the Mercer engine, a four-cylinder type for installation in a motorcycle. The principle is applicable to any number of cylinders, though with the cam form shown, the minimum number which will give good dynamic balance is two, and in this respect even numbers are desirable in any case.
In a road vehicle of any kind, there is much to be said in favour of an engine which produces the maximum number of firing impulses for the minimum r.p.m. This offers possibilities of simplifying the transmission gear, and the old idea of building the engine into one of the wheels, first exploited by Singer at the beginning of the century, may be feasible with this kind of engine.
In comparing the relative virtues of cranks and cams for converting reciprocating to rotary motion, the major issues are mechanical efficiency and durability, but there are many other factors which influence the prospect of practical success, including convenience of application, ease of manufacture, and also of maintenance. Up to the present, most orthodox, and many unorthodox, engines still use cranks in some form or other, but one never knows what the future may bring.
There is no denying the ingenuity of the many engines which have forsaken the beaten tracks of standard practice. In common with many other model engineers, I always find them extremely interesting. They deserve attention not merely as mechanical curiosities, but also as examples of design which prove that there are still many regions in the world of engineering which remain to be explored by pioneers.
Edgar T Westbury, April 1962
Ref: ME 3172