In the two-stroke type of petrol engine the cycle of operations is completed in two working strokes of the piston instead of the four required by the “Otto” cycle; there is thus one explosion or power stroke in every revolution of the crankshaft. Theoretically this represents a great advance over the “Otto” cycle, but difficulties and complications arise in the practical carrying out of the cycle. The cycle on which it is desired to operate the engine is: 1st stroke—Compression; 2nd stroke—Explosion. The charge would be introduced on the compression stroke and exhausted towards the end of the explosion stroke.

Now the charge of gas required by the engine consists of a mixture of petrol vapour and air, and it must either be sucked in or pushed in under pressure. In the “Otto” cycle the charge is sucked in, and in the two-stroke cycle it is delivered to the cylinder under pressure; hence in the two-stroke cycle some form of pump is required which will suck in the charge of air and gas, compress it a small amount, and deliver it to the working cylinder at a pressure of 5 or 6 lb. per square inch above atmospheric pressure. This is where the complications commence; if we fit a separate pump for each cylinder, which is what would generally be done, or if we made one pump serve for two cylinders, we have to provide pump cylinders, pistons, rods and valves, and therefore there is practically no gain over the four-stroke engine. Hence it is that inventors all try to avoid the use of a separate charging pump and turn their attention to the production of an engine in which one or more of the existing portions is made to serve as a pump for charging the working cylinder or cylinders with gas. A favourite and fairly successful device is to make the crankchamber gas-tight and use it as the cylinder of the pump, the underside of the engine piston then forming the pump piston which draws the charge from the carburettor into the crankchamber on its upstroke and compresses it on its downstroke, delivering it to the working cylinder through the inlet port as soon as the piston has uncovered it by its downward movement.

There is no exhaust valve, as the piston uncovers the exhaust ports a little before the inlet ports are opened. To prevent the new charge escaping directly across the top of the piston from the inlet ports to the exhaust ports, a deflector is fitted on the top of the piston equal in height to the height of the exhaust opening and situated immediately in front of and facing the inlet ports.

A two-stroke engine of the type referred to is shown diagrammatically in Fig. 63. E is the gas-tight crankchamber, upon which the water-cooled cylinder A is mounted in the usual manner and fixed by studs or bolts. The piston P carries the deflector H, which is equal in height to the height of the exhaust opening G. The piston rings are prevented from turning by pins so arranged that the joint of the rings does not pass across the ports. The connecting rod D is of usual form, and also the crankshaft C. The carburettor, or induction pipe leading from the carburettor, would be attached to the flange L, and the automatic valve F controls the admission of gaseous mixture from the carburettor to the crankchamber. The inlet ports N are often only half the height of the exhaust ports. On the upstroke of the piston a partial vacuum will be formed in the air-tight crankchamber, which will allow the atmospheric pressure to force open the valve F against the pressure of the spring and enable the air to flow into the crankchamber through the carburettor and induction pipe, carrying the charge of petrol vapour with it. We must note, however, that no vacuum can be formed until the port N has been covered up by the piston, so that a portion of the stroke is idle. On the downward stroke of the piston the charge in the crankchamber is compressed, and as soon as the piston uncovers the ports N the charge from the crankchamber flows up into the working cylinder, displacing the burnt gases as it comes into the cylinder. Exactly what happens next it is difficult to say; the probability is that this new charge rises in the cylinder a short distance (but not a sufficient amount to displace all the dead gases from the top end of the cylinder) and that some of it gets squeezed out of the exhaust opening as the piston rises and before it has had time to cover the exhaust ports. Thus, owing to the idle portion of the stroke during admission to the crankchamber and to the low compression pressure adopted in the crankchamber, the pumping portion of the engine has what is termed a very low volumetric efficiency.

It can be proved that this type of engine which endeavours to draw sufficient gas to fill its working cylinder into the crankchamber by means of a piston having only the same diameter as the diameter of the working cylinder itself, and which cannot avoid some idle movement during the operation together with further loss from the exhaust opening, is incapable of retaining more than a little over one-half a cylinder full of fresh combustible gas at the instant when compression commences; the remainder of the contents must be dead exhaust gas. Thus, even allowing for the double number of power impulses resulting from the use of the two-stroke cycle, it is difficult to see how this form of engine could ever give more than about one and a quarter times the power of a four-stroke engine having the same bore and stroke even when the many difficulties experienced in the practical working of two-stroke engines have been overcome. To use a high compression pressure in the crankchamber would increase the volumetric efficiency, but would result in lost work during the pumping process, besides being undesirable at the delivery stage of the process; it is much better for the transfer of the gases to take place as gently as possible. If too high a delivery pressure is used the fresh gas will enter in a sharp gust and get badly contaminated by mixture with the foul exhaust products instead of gently displacing them in bulk. The use of an automatic valve is very desirable for the gas inlet to the crankchamber, but unfortunately it limits the speed of the engine and also its flexibility or ability to pull well at all speeds. An engine with an automatic valve runs best at that speed for which the tension of the spring is most suitable. If the spring is weak the speed will be low. Tightening the tension on the spring will allow the engine to speed up, but will prevent it running well at low speeds. At high speeds and with correspondingly high tension the valve does not give enough opening, and therefore limits the power of the engine. It will, therefore, readily be seen that when a two-stroke engine with automatic inlet valves is pitted against a four-stroke engine with mechanically-operated inlet valves, the comparison is unfair to the two-stroke cycle. With the position and arrangement of ports shown in the drawings, one must have a deflector on the piston head to prevent excessive loss of fresh gas through the exhaust opening. After the engine has been running for some time at a high speed this deflector becomes very hot, and as a general rule the cooling effect of the incoming gases is not sufficient to prevent it attaining a red heat on the compression stroke, thus igniting the charge before the piston reaches the top of the stroke. This defect, which is called pre-ignition, causes the engine to knock, and results in a loss of power; it may be partly overcome by admitting lubricating oil with the charge, the oil then serving to cool the deflector as the charge enters the cylinder. At high engine speeds there is great risk of the hot exhaust gases in the working cylinder setting fire to the incoming charge in the inlet ports, thus causing backfiring into the crankchamber. To avoid all possibility of backfire, some form of air scavenging must be adopted, but the general arrangement of this form of two-stroke engine does not lend itself to such an addition—it would merely reduce still further the quantity of gas reaching the cylinder.

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.

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.

Much trouble was caused at one time in the new engine by knocking of various kinds, and many hours were spent in locating these troubles and curing them. The first kind of knocking was most violent and almost made one hold one’s breath in anticipation of seeing parts of the engine go skywards. This turned out to be partial seizure of a piston owing to a hard spot in the cylinder. After curing this, general knocking from all cylinders began, and was found to result from worn gudgeon pins. These had been mild steel and case-hardened; they were discarded for Ubas steel of slightly larger diameter, and this trouble disappeared. Then pre-ignition was discovered. When the magneto was switched off the engine slowed down and nearly stopped, then began to run on again, knocking and hammering in a most diabolical manner. All cylinders were taken off again, all parts ground up, and corners well rounded off, but still it continued. At first it seemed to be due to the deflectors, but on several very careful examinations (which of course meant dismantling the whole engine every time and removing the cylinders) no trace of overheating or burning could be found on these or anywhere else in the interior of the cylinder. Then the trouble was traced to the electrodes of the sparking plugs. This was followed by two or three weeks’ continuous experiments on fitting different types of plugs, and the same type of plug was tried in four different positions inside the cylinder. Then the device of fitting the plugs to an adapter and so keeping them at the top of a small hole instead of projecting into the cylinders was tried. They still showed signs of overheating, and strange to say no loss of power or flexibility was noticeable. Finally, I fitted a water tank on the dashboard and allowed the engine to suck water into the induction pipe while it drew its mixture from the carburettor in the usual manner. I had previously fitted separate drip-feed of water to the air scavenging valves with a view to effecting cooling of the engine, but abandoned it owing to lack of results. Very soon I discovered that for every gallon of petrol the engine consumed I could let it take nearly half a gallon of water into the induction pipe. The engine ran much quieter and very smoothly, and for a time I thought I had succeeded, although the water gave me trouble in restarting if I happened to stop the engine while it was in use. It meant that the water had to be shut off some minutes before the engine was going to be stopped. The day after I thought I had effected a cure for the pre-ignition intermittent knocking began, and there was also general knocking always for a second or two when accelerating quickly under load. After much loss of time and the expenditure of a large sum of money on experiments, I persuaded the Syndicate to let me take some diagrams from the engine with an optical indicator, and eventually after nine months they consented, but they would not agree to my taking the engine out of the chassis and putting it on the bench for a proper power test. Therefore my diagrams were taken while the engine was in the garage in its chassis, and the load was applied by the propellor shaft brake, the shaft itself being withdrawn. Anyone who has attempted even in a well-equipped laboratory and with the aid of a proper brake to take diagrams from a petrol engine when the indicator is driven by a flexible shaft, will understand and appreciate my work in securing thirty photographic records under as many conditions of load and speed. After carefully analyzing my diagrams, I came to the conclusion that the intermittent knocking was undoubtedly flashing back from the exhaust, and the acceleration knocking was due to a cushion of hot gas which accumulated in the head end of the cylinder at times when the engine speed was low and the load on the engine was heavy.

Having explained these things to the Syndicate and pointed out the need for still larger valves, they set about attempting to raise fresh capital for the final attempt at success. They were not successful, and up to the present nothing more has been done. The Syndicate was wound up, the members drifted apart, and the patents were allowed to lapse.

The engine and chassis were eventually sold, and are still doing good service somewhere in the North of England. Meantime the writer has not rested, but has steadily formulated his ideas for the improvement of the engine, which have resulted in the securing of a fresh patent early this year. In the new engine the charge enters at the head end of the cylinder, there is a special transverse combustion chamber, and many improvements are introduced in the scavenging and flow of gases; also there is no deflector at all on the piston head. Funds have not yet been secured to enable an experimental engine to be constructed, but it is to be hoped they will be forthcoming, for the benefit of the motor-car industry generally, as the future undoubtedly lies with the two-stroke.

During the whole of this time the writer was engaged as Chief Assistant in the Engineering Department of Leeds University, being in charge of the experimental work of the students in the laboratories there. Many of the drawings were made by students in their vacation, and the writer is greatly indebted to his friend, Professor John Goodman, for so kindly allowing him the necessary freedom during vacation times when there is often much miscellaneous work that requires attention.

Before closing this chapter one may add a few words on carburation and ignition for two-stroke engines. A four-cylinder two-stroke engine should have cranks at right angles to secure the maximum torque on the shaft. Looked at in end view the cranks form the four arms of a cross and thus four impulses are given every revolution, but as the ordinary magneto only gives two sparks in every revolution it must be driven at twice the crankshaft speed. This puts a great strain on the machine at top speed, and also on the insulation of the windings and the plugs, so that the plugs require constant attention. Magneto troubles were found to be eliminated by the use of the special racing pattern magneto supplied by some manufacturers and the choice of high grade sparking plugs.

Carburation troubles were not so easily dealt with. A multi-cylinder two-stroke engine should undoubtedly have a multiple jet carburettor and some form of hand-controlled extra-air inlet valve on the induction pipe; also the mixing chamber of the carburettor should be water-jacketed by hot water. It was also found necessary to fit a hot water-jacket round a portion of the induction pipe, as the demand for petrol vapour was so great and the rate of evaporation so high that frost readily formed on the induction pipe unless the weather was very warm. The two-stroke engine requires its petrol much faster than the four-stroke, so that the float of the carburettor should be delicately balanced and the height of the petrol in the jet should be quite level with the top of the orifice, although this often leads to flooding.

Reviewing the description of what we have designated the Kean two-stroke engine, we may sum up the results of these experiments by saying that the engine could have developed considerably more power than it did had diagrams been taken from it in the first instance and the severe throttling in the carburettor and automatic inlet valves been discovered; moreover, the flashing back from the exhaust would have been located much sooner and probably cured by a re-arrangement of the exhaust manifold. If the exhaust manifold had been arranged so that there was a separate branch for at least each pair of cylinders, it would very likely have been stopped, or at any rate greatly reduced. But what could not have been altered was the acceleration knocking. It must not be imagined because I have been very frank in the criticism of my own work that the engine was a failure; it was a great success, but not sufficiently successful to represent an improvement on the best four-stroke practice. The car ran well, was very reliable and efficient in petrol consumption; the engine was quiet and extremely flexible; but it had one very objectionable feature in that every time you pressed the accelerator pedal down sharply, either to put on a spurt for the purpose of passing slower traffic or in rushing a short gradient, a peculiar knocking or hammering arose from the engine cylinders—this is what I describe as acceleration knocking and must not be confused with the knocking or hammering of a four-stroke engine when labouring on a gradient. My engine would be full of life all the time it was knocking like this, and gradually as the speed increased the noise would ease-off, even though no change of gear had been made.

The diagrams proved to me that this knocking was due to pre-ignition caused by a cushion of hot gases remaining in the top of the working cylinder, and in my opinion no alteration of the ports or cylinder head would have influenced this defect to any marked extent; therefore I should never attempt again to feed the new charge in at the bottom end of the cylinder of a two-stroke engine if I wished to obtain the maximum amount of power from it. It seems to me that other people must also have been impressed with similar misgivings, for in one or two types of engine using crankchamber compression we see a special attempt made to overcome it, although the method adopted leads to a rather undesirable arrangement of the engine mechanism. In the type of engine I refer to the charge may be drawn into the crankchamber in the usual manner, if desired, but the working cylinder is a casting with two bores having two separate pistons and a common combustion chamber. The charge enters above one piston while the crank is on its bottom dead-centre and is exhausted from the space above the other piston simultaneously, and the path of the gases is from the inlet port up to the top of No. (1) bore, then down to No. (2) bore, and out of the exhaust. This ensures that there shall be no cushion of hot exhaust gases left in the combustion chamber (or top end of the cylinder).

These engines have given quite good results, and would be much more extensively used but for the fact that there is double compression to overcome in starting, and their running torque, due to the number of impulses given to the crankshaft, is no better than a four-stroke engine. Fig. 66 shows the arrangement of the cylinders and the path of the gases. A₁ and A₂ are the twin pistons working in the water-jacketed cylinder casting B, and having the common combustion chamber C. The connecting rods may drive separate cranks in opposite directions or both be coupled together and work a single crank. It will be seen that in this type of engine the piston does not require any deflector.

The simple two-stroke engine described at the beginning of this chapter is often constructed in such a form that no automatic inlet valve is required on the crankchamber.

In this case the induction pipe is connected to a third set of ports just below and a little to one side of the inlet ports to the working cylinder, and these are uncovered by the piston towards the completion of its upstroke, thus allowing the carburetted air to enter the crankchamber. Such an arrangement constitutes a three-port two-stroke engine, which is of course less efficient than a two-port engine with automatic valve, but has the great merit that it is entirely valveless, and therefore extremely simple and cheap to manufacture. It is much used for motor boat work, both in this country and in America, on account of its relatively low speed of rotation.