A difficulty that is peculiar to multi-cylinder engines of the two-stroke type arises from the use of open exhaust ports. The several cylinders generally discharge their exhaust gases into a common exhaust pipe or box, so that if one cylinder happens to be missing fire the exhaust from another cylinder may set fire to the wasted charge—this is usually referred to as flashing-back from the exhaust and results in irregular and spasmodic knocking. It will be clear from the foregoing that this cycle of operations, which is so attractive from the theoretical point of view, is not by any means so encouraging from the practical standpoint, as many inventors have discovered. The difficulties and failures of the early inventors which were so discouraging for them have only encouraged their successors and spurred them on to further efforts. After a time the attempt to produce a simple two-stroke engine was abandoned generally, and inventors turned their attention to improved forms of two-stroke engines, some of which were very costly and complicated, and none of which have survived for motor-car purposes.
The writer of this volume became interested in the problem of the two-stroke in connexion with one of these inventions for an improved engine, and at a later stage patented and designed an improved engine of the two-stroke air scavenging variety, which by that time had become a recognized type of two-stroke engine. This engine was constructed and exhibited at one of the motor shows held in London some years ago. A vast amount of experimental and research work was carried out on it by the writer, but the work had to be abandoned when incomplete owing to the Syndicate which financed the venture having exhausted its resources. The promoters of the Syndicate were anxious to produce an engine that would give double the power of a four-stroke engine, but their early attempts were not at all successful. One of their four-cylinder engines, which would have been rated at 35 horse-power on the four-stroke cycle, only gave 12 brake horse-power when tested by the writer. The engine designed by the writer, which we may call the Kean two-stroke engine, would have been rated at 25 horse-power on the four-stroke cycle, and gave approximately 35 brake horse-power. Although this result was excellent, so much advance had been made in the four-stroke engine that it did not quite come up to the best results obtained on that system, and hence we were unable to show any marked advantage to be gained from its adoption. My experiments clearly pointed out the road to further success, but owing to the partial failure of my attempt to beat the four-stroke engine we could not influence sufficient capital to reorganize and reconstruct the Syndicate. My engine had not been designed to secure a high speed of rotation but rather for strength and durability, but it exceeded my expectations by turning up to 1,500 revolutions per minute. The four-stroke had, however, got well ahead of me by that time, and 2,000 was becoming common for it, hence I was heavily handicapped in the race for horse-power.
Fig. 64.—Diagrammatic Sketch showing how the Duplex Type of Two-stroke Engine operates with Air Scavenging.
A description of my engine will probably prove of interest. To understand the principle of the engine we must turn to the diagrammatic sectional view of Fig. [64]. Instead of using the crankchamber of the engine as a gas pump, this type of engine has a duplex piston, and the pump chamber is formed by an annular extension of the main engine cylinder. At first sight one would say this resulted in a very high engine, but as a matter of fact the increase in height is not more than about 25 per cent. in the cylinders, and there is no difference in the crankchamber height to that of a four-stroke engine. The outstanding feature of the invention is the provision of a pump piston of larger effective diameter than the main piston and the arrangement of transfer pipes by which one pump feeds its neighbour’s power cylinder, and vice versa. These are the basis of the invention, and were being used a long time before the writer had even heard of this type of engine, but it was left for him to seize upon their capabilities and correctly proportion the area of the annulus with respect to the main engine piston. A careful study of the two-stroke problem revealed the inherent defect of the low volumetric efficiency and the tremendous possibilities of having an unlimited volume for the pump chamber by simply increasing the area of the lower or annular piston. Then followed the writer’s attempt to tackle the outstanding practical difficulties enumerated above. The engines already employed air scavenging, but could not really use it effectively until proper proportions had been fixed upon for the respective pipes, valves, and ports. The cycle of operations is as follows:—On the downstroke of No. 1 piston the annular portion draws a charge of gas from the carburettor into the annular chamber D1 (Fig. [64]) through the inlet valve B1 and at the same time pure air is drawn into the transfer pipe by the valve A2. On the upstroke the charges of air and gas are compressed into the transfer pipe, and as soon as the piston P2 uncovers the inlet ports the air and gas enter the working cylinder. In my engine I used a relatively high compression pressure for the transfer of the charge and curved the inlet ports up towards the head of the cylinder as shown. The head of cylinder I made curved, and the exhaust ports were carefully rounded and curved also. The deflector on the head of the piston I inclined, to curl the gases back against the wall of the cylinder, and I reduced the height of the deflector to that of the inlet port (instead of the exhaust port). My ultimate aim was to abolish the deflector entirely by suitably shaping the inlet ports, and I estimated that the path of the gases would be in the direction of the arrows. The object of raising the compression pressure in the lower cylinder was twofold. First of all I aimed at an increase of volumetric efficiency there, and secondly I hoped to propel the scavenging air and the new charge right up to the head of the cylinder and so clear out all the dead gases. Then by suitably curving the head of the cylinder I expected to compel the scavenging air to keep going ahead of the gaseous mixture and curl round and down, then following the exhaust gases out of the exhaust port.
My efforts in this direction were very unfortunately frustrated to a large extent by the fact that the cylinders of my engine had already been cast before I fully realized the tremendous importance of curving the cylinder head and giving a very steep inclination to the inlet ports. We did our best to rectify matters in the machining and finishing stages, but any engineer will understand the limitations now imposed upon us. It was impossible to get new cylinders cast owing to lack of time and funds, as we were intending to exhibit the completed engine. Thus I cannot say that my ideas were ever given a really satisfactory test; the inlet ports were curved and inclined and the cylinder head was rounded off, but not to such an extent that I can feel certain no further improvement could ever be made in those directions. Other improvements which I introduced were an improved automatic inlet valve for the gases, which was fitted inside the induction pipe and whose spring tension could be adjusted while the engine was running without letting any air leak into the induction pipe; also an improved air scavenging valve, which could be set to give the full amount of air to the engine and yet be controlled from the dashboard of the car to give any desired quantity of scavenging air from no air up to full air. Very large inlet valves were fitted, but when indicator diagrams were eventually obtained from the engine they showed that they were not nearly large enough and that the carburettor opening was too restricted, thus cutting down the power (and very likely the speed) of the engine by probably over 25 per cent. High tension magneto ignition was fitted and thermo-syphon cooling. Arrangements were made to carry 80 lb. of water in the system, so that the engine never showed any tendency to boil even when the car had been running for long periods on the low gear. A pump was afterwards fitted, but it did not effect the cooling of the water any better than the natural circulation, which was quite satisfactory. The range of speed was from 150 revolutions per minute up to 1,500 revolutions per minute; the lower figure is very good indeed, and can be attributed to the large number of impulses obtained due to the two-stroke cycle. At the highest speed the crankshaft received 6,000 impulses per minute, or equivalent to a four-stroke engine running at 3,000 revolutions per minute. The effective pressure in the cylinder was, however, only just over 40 pounds per square inch, due to the throttling at inlet already explained. In a four-stroke engine we would expect just double that figure. The extraordinary thing about this was that, under heavy load, when the speed was brought down to about 300 revolutions per minute, the effective pressure had risen to nearly 200 lb. per square inch, but this appears to be due to imperfect scavenging (or cleansing) of the cylinder under these conditions.
The question of silencing the exhaust from the engine had caused me some difficulty in the earlier experiments, so that I now tackled this problem and designed a special form of silencer in which the gases were first expanded to remove their pressure and then afterwards their velocity was taken up without shock. This answered so well that a cut-out made no difference whatever, and on taking diagrams with the optical indicator I discovered that the exhausting process was divided into equal periods of slight pressure and slight vacuum with an average of zero pressure (just atmospheric). We have seen in the earlier part of this chapter how the fitting of automatic inlet valves is liable to hamper the engine and reduce its flexibility, and this impressed me very much with the earlier engines so that at one time I adopted dual springs for the inlet valves. These springs were mounted one above the other, the lower one being much stiffer than the upper one. The idea of the invention was that the weak springs would serve for slow running and all loads up to say half the lift of the valve, and then the stiffer springs would secure correct action at high speeds. Further than this, I had them all coupled on a bar which was controlled from the driver’s seat, and by means of which I could cut out the weaker springs or reduce their effect at will. It certainly answered well in the older engines, but my new engine, shown in Fig. [65], was so satisfactory that I abandoned the idea. About five different systems of lubrication were experimented with and many lubricating oils. Finally, forced lubrication was employed for all the bearings and a drip sight-feed for the pistons.
