WHERE THE ASPIN ENGINE PRINCIPLE IS DIFFERENT
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IN a recent issue I described the 'mechanics of the Aspin engine, making reference to the methods of oiling this unusual form of rotary head and reviewing the cycle in general terms. It is now proposed to analyse comparatively with other rotary valve engines the functioning of this novel mechanical conception in which a single rotary member assumes the combined duties of combustion space, valves, anti-detonative flame directioning, charge stratification, and scavenging.
That its composite properties are unusual in the aggregate is evident from the tests carried out on a singlecylindered air-cooled unit of 249 c.c. in which quite phenomenal performances are obtained in respect of speed, power, consumption, and resistance to detonation, even with fuels of low octane values.
Before proceeding to discuss these, I will give certain relevant data of the unit under consideration. The bore and stroke are 67 ram. and 70.5 mm., respectively; a maximum output of 32 b.h.p. is developed at 11,500 r.p.m., and the compression ratio is 14 to 1. The consumption rate at the torque peak on full-power setting, giving 21 b.h.p. at 6,000 r.p.m., is 0.32 lb. per b.h.p. /hr., which is equivalent to 47.6 per cent, thermal efficiency.
• At an economic setting, giving 13.5 b.h.p. at 5,000 r.p.m., the figure is 0.25 lb. per b.h.p. /hr., that is, 59.6 per cent, thermal efficiency.
It does not require an expert eye to realize that these figures are abnormal, barring only the b.h.p., viz., 32 at 11,500 r.p.m., which, although extremely high as judged by touring engine standards (12.8 b.h.p. per litre), are by no means impossible in modern supercharged engines. Remember, however, that the Aspin engine is unblown, which makes a great difference.
Whence the Efficiency?
The power curve of this engine is much straighter and longer, the zonsumption curve much lower and the thermal-efficiency figures much higher than those of any other engine known to the writer either in or out of its class, and it is now proposed to consider the reasons why.
Take first the exhaust-valve function of the Aspin rotary head. The main objectives in seeking alternatives to the present poppet system in the order of their importance are probably three in number. First and most important comes removing the limit imposed by poppet exhausts on all super-high output designs by the difficulty of keeping them cool.
Secondly, I place inviting greater A28 functional constancy by the employment of rotary movement only. This, by eliminating high-periodicity surface impacts and directional changes will cut out the elements of rapid seating, tappet-guide, cam, and valvestem wear which beset the poppet engines during sustained outputs.
Thirdly, may be named the permitting of much, higher operating speeds which are essential to the improvement of power-weight ratio in view of the so-called " constant torque" classification of the internal-combustion principle.
The effect of supercharging, in this connection, is fully recognized, but the employment of a blower is in the nature of an additional power-weight boost that can be applied equally to both principles and, therefore, does not enter into the comparison.
In all non-poppet systems known to the writer, including three sleeve engines and several more with valves taking the form of pistons, one slidevalve unit and various engines having valves of the simple rotary order, the designs were conceived with one or more of these objectives in view; and it is fair to say that at least two of them—a resuscitation of the single sleeve, and a rotary valve—have achieved quite a measure of success, particularly the former, which has now gained fame in aircraft engine design. Moreover, when the mechanical obstacles, especially operating difficulties in the sleeve, and oiling and sealing troubles with the rotary valve, can be overcome, marked functional improvements are assured on a basis of local heat reduction and functional constancy alone.
In none of these engines, however, is the remarkable fuel economy of the Aspin obtained, nor are they independent of the octane factor, which is one of its outstanding qualities. This engine being quite immune from detonation, the calorific value of the fuel is apparently all that matters here, knock rating being of no account.
In addition to the advantages obtained in common with other nonpoppet designs in regard to suppression of excessive exhaust-valve heat, the Aspin goes one farther, in that the combustion cell, with its contained charge, is removed in its entirety from the hot precincts of the exhaust port, and during inflammation is at the opposite side and coolest part.
One of the accompanying illustrations shows the sectional elevation of the rotary head with the cell opposite the valve port, whilst from another, which is a plan view, it is easy to see that this off-set depression, which is the combustion head de facto, has now rotated (anti-clockwise) from the ex haust port to the inlet port where it receives its charge and, thence moves. on to the plug which, incidentally, is so placed that immediately its spark passes, the trailing edge of the cell masks it so that it suffers no harm from the eventual combustion flame, nor, on the other hand, can it play any pre-ignition tricks on the live charge, which plugs are apt to do in highoutput engines.
There would, in fact, appear to be no question—on comparing the Aspin with a simple rotary valve unit—that whatever benefits the latter might receive from the substitution for the poppet of a sleeve or rotary valve (and they are no doubt considerable) the valve and its surrounds, although at a much lower temperature than the original poppet, are still exposed to the live charge during all phales of combustion. This is not the case with the Aspin.
Additional benefits, therefore, must definitely accrue, if the charge be completely insulated from all hot areas, including the plug and its housing during its combustion, as in this case, also from the mechanical fact that the peak pressure rise takes place when the cell is at an approximately maximum distance from the nearest source of leakage. Observe the length of the arcs which represent the extent of the sealing areas between the cell and either port.
So far, fairly orthodox comparisons have been made, but reasons for ultraefficient and complete combustion must now be sought.
Reference has already been made, in the preceding article, to the relatively cold exhaust gases, even at peak outputs. As one feels obliged to associate this property with the remarkable fuel efficiency, an analytical consideration of the probable charge and flame movements during combustion is called for.
Specialists in combustion are fairly well agreed as to the generalities ot detonative incentives, and whatever differences of opinion there may be as regards their detailed development it is probable that all are in accord that they are a product of definitely directional flame-front movement.
Turn now to the diagrams and envisage a slow-up of the progress of combustion immediately following the passage of the spark and formation of a flame globule. Picture first the descent of the piston at the beginning of the induction stroke as it is receding from the lower face of the rotary head. The cell, which contains the whole of the charge when the piston is at T.D.C., gradually becomes now a swept part of the inlet port, and a cursory glance is sufficient to reveal the notably unrestricted nature of the entrances and exits for live and spent charges, respectively, with this layout.
After assuring ourselves of the excellence of the filling, which, incidentally, is reflected in the power curve (it rises in a practically straight line to 8,000 r.p.m. and then climbs to a peak at 11,500 r.p.m.) we turn to a consideration of the flame-front direction. The cause for the singular immunity from detonation will be fairly apparent, for there cannot evidently he a flame front, describable as such.
It was found by Sir Dugald Clark last century, confirmed by Prof. Hopkinson early this century, and experimentally established by Whatrough some years ago, that a moving ffiarie front accelerates proportionately, but in the opposite direction to arty live charge projected upon it.
To appreciate how this applies here, picture the piston ascending on its compression stroke, but not yet quite arrived at top dead centre. As this point is approached, endeavour to visualize the charge and the resulting flame movements.
As the whole of the combustible charge will eventually be crowded into the cell, it must be ejected into the cell more or less radially in relation to the cell centre, and, on that account, with an immense number of different directions and, consequently, speeds, because the live-charge velocity, at the moment of entrance, must be a fair measure of the length of the radial path finally traversed and of these there must he a great variety.
Here, therefore, lies, very probably. the main reason for the high efficiency of the Aspin head and its freedom from detonation, for it will be noted that as there is no defined flame front, there can be no detonation, for the latter, so far as present knowledge goes, is dependent on the former.
At the same time, however, the flame rate is so high that alt combustion, both primary, secondary and that mysterious type, called, by the Americans, the " after-glow," is completed early in the stroke, permitting thermo-dynamic conversion of the flame into mechanical energy so quickly and completely that, not only is there no luminosity at the exhaust port during maximum output, but, also, it is even possible to pass one's hand slowly before the port and at a distance of 3 ins, from it.
All kinds of fuel, paraffin, vaporizing oil, alcohol, etc., will burn, and will put up a performance according to the calorific value of the fuel and not to its knock rating, which, up to now, has been the dominant factor in fuel classification.
One other functional picture remains and it is difficult to estimate its value, namely, charge stratification. Some years ago much money was spent in trying to effect stratification by employing double-step pistons to inject a cushion-like layer of inert gas behind the ordinary charge. It was finally abandoned as impracticable, because any definition of strata was impossible owing to the general turbulence which took place during compression.
There is, however, no such uncertainty in the Aspin, and the recent researches into the potentialities of this property of centrifugal separation have disclosed possibilities which were previously considered to be unattainable.
All that now remains is to repeat that any lubricating uncertainties of the head have been removed by the use of a special and newly discovered alloy, which combines the necessary toughness and surface resistance to wear with a high adhesion for lubricants.
Although the employment of lower compression ratios reduces—as it theoretically should—the thermal efliciency, according to the gamma curve in the air cycle, the efficiency of this form of head, compared with orthodox designs, even with the same compression ratios, shows a marked ascendancy, both in power and economy. Moreover, whilst in the latter types the thermal efficiency appears generally as a progressive falling off, after the torque peak, the Aspin head. on the contrary. improves with increasing r.p.m. up co a very high speed.
Engines of various types, cylinder multiplication, and arrangement, both two-stroke and four-stroke, and with spark and compression ignition, have now been made, and the results show no reason to think that the principle is not universally applicable to all kinas of internal-combustion engine, especially those of ultra high output.