The Petrol Engine / A Text-book dealing with the Principles of Design and Construction, with a Special Chapter on the Two-stroke Engine

by Francis John Kean

Fig. 65.—General arrangement of the “Kean” Four-cylinder High Speed High Compression Duplex Two-stroke Engine employing Air Scavenging. In this Engine there is no Crankchamber Compression.

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.