ASPIN ENGINE REPRESEN1
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)VANCED 9 THOUGHTL. Jatellr,
Novel Design of Head al Rotary V abbe Fermi Departures from Comm( Principles of Controllt Combustion
onditions Closely Conformg with the Theoretic Ideal .?sult in a loci per cent. am n in Respect of Thermal
Efficiency
ALTHOUGH there has been marked progress in the development of the basic principles of controlled combustion during the past two years, it is, perhaps, fair to describe the advances made rather as progressive steps in the improvement of orthodox design than as fundamental departures from either past or current concepts. Where poppet valves are employed, limits are unfortunately imposed, on the one hand, by considerations of size and lift, and, on the other, by exhaustvalve heat, which circumscribe greatly the extent of really basic departures from accepted principles.
Great ingenuity has been shown in taking advantage of the heat of one valve and the relative coolness of the other to locate, in the combustion head, corresponding hot and cold areas, as desired, by suitable emplacements and by the astute shaping of their surrounds in relation to plug position, etc. Moreover, a review of past developments teaches us the inadvisability of even toying in unspoken imagination with such an indiscreet word. as " finality."
Nevertheless, there can be little doubt in the minds of anyone, with a sound knowledge of the subject and a well-developed sense of " prospective," that the poppet must nearly have run its course where outstanding improvements are in contemplation.
Metallurgy's Contribution.
It is an open secret, for example, that the ,resuscitation of certain old systems, such as the Burt-McCulIum sleeve, which remained tacitly dormant for years, is now being made very profitable by the aid of modern metallurgical improvements which were denied to the original post-war inventors.
Rotary valves, also, when they are designed with an eye to the mechanical and oiling difficulties which beset such devices, are meeting with a fair measure of, at least, functional success.
For both, the reason is the same— suppression of the intense local hot spot which is relentlessly present in all combustion heads fitted with poppets— for, however cleverly one may moderate the thermo-dynamic iniquities of the type by turning local temperature differences to account, they still loom behind the camouflage as a potential " thus far."
The arrival of the Aspin engine, therefore, is well timed and, after being thoroughly tried out as a single-cylindered unit, the details of which will presently be described, its appearance in various practical multi-cylindered forms for widely different purposes is now imminent.
The desirable attributes in a combustion head viewed thermo-dynamically are perhaps, broadly, three in number: (1) the provision of a valve £28 system of constant and uni-directional motion in operation; (2) the elimination of all local hot areas such as are ordinarily exposed to the charge during combustion, and (S) the inclusion of such pre-spark turbulent factors as will prevent, if possible, the development of a defined flame front moving in a specific direction, because this is the precursor of all detonative troubles.
In the Aspin engine these conditions appear to be fulfilled to a considerable degree and in such a simple manner that doubts as to its mechanical practicability are excusable in those having no personal knowledge of the engine. Here the duties of both valves, the scavenging, the hot-spot shielding, and the prevention of flame frontal formation 'are discharged by a simple onepiece member of uni-directional rotation at uniform speed and requiring no appreciable effort to drive.
Combustion-head Design.
Accompanying drawings show the Aspin combustion head sectionally in elevation and in plan. It will be seen that the space ordinarily described as the head is Of tall conical formation and acts as a housing for the solid rotating member. This is sharply coned externally to act as a bearing and bluntly coned underneath to an angle almost agreeing with that of the piston crown, which fits fairly closely into it at top dead centre.
Note also that the apex is extended vertically into a stout spindle on which two large roller bearings are mounted and that above them is keyed a halftime gear meshing with a smaller wheel which caps a vertical shaft driven by bevels from the crankshaft—not shown in the illustration.
Oiling is effected by the upwardinclined duct, seen to the left of the cylinder head, which forlis at the top, one branch supplying the bearings and the other a distributor groove on the face of the coned housing.
Consider now the functional aspect and in this connection observe the recess in the side of the lower cone face, described as the ",cell." This is the combustion head itself, but, instead of being a fixture as in the ordinary way. it follows the unique practice of rotating. Having noted its general shape from the elevation drawing, tarn to the plan, from which its cyclic action will be easy to follow.
The cell is here seen opposite the plug and is, therefore, at the firing position. Take the cycles, however, in order and picture an anti-clockwise rotation until the cell is opposite the inlet port. A charge having been inspired, the valve rotates until the recess aperture clears the inlet port. Then compression takes place until nearing the peak, when the sparking plug is momentarily exposed and inflammation follows.
Good Sealing and Shielding.
Note here two important and useful facts; first, the distance of the cell from the inlet port on one side and the exhaust port on the other, which insures a maximum compression seal by the length of the intervening wall, and, secondly, that compression and combustion take place while the aperture is remote from the exhausting section and at the coolest part of the heath Even the sparking plug is covered after igniting the charge. Combustion having now started, the driving stroke goes on, while the cell advances until the exhaust port is uncovered and the cycle is practically complete.
Before proceeding to analyse the functioning, let us consider the mechanical aspect of the Aspin head, especially as regards the differentiation of loading, and the lubrication.
The first doubt that assails one is the practicability of these arrangements to permit adequate oiling and to carry the heavy thrusts that must be anticipated from the fairly considerable area of the lower face of the rotary member which carries the full peak pressure. • In ihe single-cylindered model on which all the original developments were made, the compression ratio was 14:1, the speed from 8,000 r.p.m. to 11,500 r.p.m., arid the pressure at the torque peak, 1,900 lb, per sq. in.
When first assembled, the cone member is in actual bearing contact with its housing, and although provided above with double tapered-roller bearings the contacting cone surfaces carry the bulk of the load.
Contact, however, is " only just," and an oil-film cles:rance is soon established by running it in gently, after which these two load carriers—the roller bearings and the cone faces—are stated to act differentially, each functioning as a locating and stabilizing member for the other.
Immediately above the roller bearings is a stout coil spring. This permits the rather-heavy depression of die induction stroke to draw the cone fractionally downwards, thus taking the load momentarily off the bearing surfaces and allowing space for the entry of a little oil each time and, therefore, the maintenance of the film. Pressures taken at various points of the cone bearing, by tapping the housing locally, would appear to confirm that this actually takes place as intended, and further proofs of good lubrication are provided by the very slight driving effort necessary to rotate the cone.
Free Working of Valve.
It has been found, also, that quite small-diameter vertical shafts can tie used experimentally, provided that sudden accelerations and decelerations be avoided, because, otherwise, twisting or fracture can result from the inertia of the rotary head, which behaves like a flywheel. This would not be possible unless the valve worked with tolerable freedom, the presence and extent of which is thus determinable.
As regards the reciprocating members, these are of normal design but extremely robust, especially in all those dimensions which have a bearing upon resistance to whip, twist, or other distortional forces in evidence at high revs.
That the Aspin engine is sound from the mechanical angle appears to be borne out by a run to destruction at speeds generally, in excess of 8,000 r.p.m,, and with a b.m.e.p. well up in three figures, when the engine broke down after no less, than 2,261 hours, and then only through the collapse of the main-shaft ball bearings. Examination of the head showed little wear, and nothing to indicate that the above speeds and loads could not have been sustained for a much longer period.
When radical departures from existing principles are made, especially where the substitution of a rotary exhaust valve for a poppet is concerned, as in the Aspin engine, two questions of prime importance arise, where normally there is only one:—
First, whether it will work in the mechanical sense, which in this case always intimately involves the subject of oiling; secondly, what will be its functional effects in the thermodynamic sense and why, Regarding the former, it has now been established that, given correct materials, workmanship and assembly, this head will outlast, between major overhauls, any poppet system, and with a considerable margin to spare. The functional aspect, therefore, remains to be discussed, especially in comparison with other non-poppet designs. A second article on this subject will be devoted thereto and it should make interesting matter for discussion in view of the unique performance that characterizes the Aspin engine.
The single-cylindered engine, referred to above, measures 67 rem, by 70.5 mm. (249 c.c.). Its b.h.p. curve goes almost in a straight line from 1,000 r.p.m. to 8,000 r.p.m. where it indicates 27 b.h.p., and finally peaks at 11,500 r.p.m. and 32 b.h.p.
Whilst an orthodox engine of this size and general design would produce a consumption of about 0.6 lb. per b.h.p./hr., the Aspin unit, with a full. power setting, as above, consumes only 0.32 lb. per b.h.p./hr., and with an economy setting, which produces 18 b.h.p. at 6,200 r.p.m. as against 25 b.h.p. with the maximum-power setting, it will consume the phenomenal figure of only 0.25 lb. per b.h.p./hr.
Remarkable Thermal Efficiency.
The last-named, it may be mentioned, represents a thermal efficienzy of about 60 per cent., or about double that of the most efficient petrol engines of orthodox design known at present. It will run without detonation on a variety of fuels, including paraffin, and at compression ratios up to over 20 to 1, the highest yet tried.
Its power output is a measure of the calorific value of the fuel used and octane numbers have no bearing thereon.
Whereas an average engine loses about 50 per cent, of its fuel efficiency via the exhaust, the Aspin exhaust is describable in engineering parlance es " cold," that is, it is possible to pass the hand in front of and three or four inches from the open exhaust port at full load and speed without suffering any undue discomfort, moreover when the engine is run in a dark room, there is no luminosity at the port.
To sum up, the Aspin is quite out of line with existing practice both in design and performance and to reassure those who are excusably sceptical, and inclined to regard the figures I have quoted as bordering on the fantastic, I can state tIlat they have been observed and checked by several responsible engineers.