Limitations of the Two-stroke.

SUPERCHARGING AND ,( ITS POSSIBILITIES.

By H. HAGENS.

SUPERCHARGING is opening up possibilities in engine design which hitherto have remained dormant to a more or less degree.

It must be remembered that supercharging in itself is:not new, but has been made use of on various occasions to achieve some object or overcome some difficulty which may have arisen. One marked instance of this was the supercharging of aeroplane engines, in order that an engine at a high altitude might still develop its maximum power, in spite of the rarefied state of the atmosphere. More recently supercharging has been used on racing motor cars, in order that an engine of a certain cubical capacity might propel a car at speeds which could not even be hoped for except by installing a considerably larger engine.

To-day a new field for the designer is being opened up by making use of supercharging.

Limitations of the Two-stroke.

Hitherto, two-stroke engines have never been considered much for motor vehicles, except lightweight motor cycles, as there have always been certain disadvantages which apparently more than counter-balance any of the advantages of this type of engine.

Let us consider some of these points, as they have a direct bearing on the engine I am going to describe subsequently.

Firstly, the small two-stroke engine as fitted to the motor cycle is delightfully simple, there being no valve gear, the crankcase serving as pump chamber for the purpose of charging the cylinder. Without going into details, it may be said that this construction leaves a great deal to be desired from an engine function point of view, but the moment one attempts to rectify the weak points on such a design of two-stroke engine, it loses the wonderful simplicity of construction (which simplicity naturally makes it a This is Mr. Hagen’s. second article on this very interesting subject and we should like to remind readers that the author is responsible for the design of many successful racing engines, among them being two of the fastest machines in the world,

Mr. C. F. Temple’s Anzani Motor Cycle and Mr. Eldridge’s Car. We are sure all OUY readers will eagerly await the appearance of his 150 m.p.h. Motor Cycle.

It is interesting to note how so many of Mr. Hagen’s predictions in the April, 1925, number of the Brooklands Gazette have subsequently been borne out.

cheap engine to manufacture), and then the four-stroke engine holds its own easily. In other words, the fourstroke engine becomes the simpler engine of the two.

Supercharging, however, serves a dual purpose where the two-stroke engine is concerned, the supercharger or blower acting as the pump for charging the cylinder of the two-stroke engine, apart from supercharging the cylinder.

Having these points before us, we may now consider how best to make use of them in designing an engine.

To begin with, I will enumerate the points I consider of importance in designing a modern engine.

The engine must be light, compact, capable of developing a large amount of power for its size and weight ; it must be cheap to manufacture and capable of hard work, with only reasonable wear and tear.

To obtain a great power out of a small engine it is necessary that the engine should run at a high speed. This necessarily imposes severe stresses on many of the working parts due to inertia forces ; the forces increase in magnitude very rapidly as the engine speed goes up, and over 3,000 r.p.m. become quite formidable. Therefore, one has to make the reciprocating masses as light as possible, in order to keep down the inertia forces, and to bring these weights down to a minimum we must make the reciprocating parts as small as possible ; hence we will choose a multiple cylinder engine ; in other words, make an engine with a large number of small cylinders. The engine I have in mind is a 12-cylinder engine, 1,500 c.c. capacity, running at 6,000 r.p.m. and supercharged, each cylinder giving a pbwer stroke every revolution of the crankshaft. Such a twostroke engine cannot be made valveless if we are to obtain the best results, but the valve gear can be very simple if rotary inlet valves are used, this type of valve having the great advantage that it opens and closes equally well at all engine speeds, whereas the operation of the poppet valve at high speeds becomes quite a problem due to valve bounce.

Valve Details.

Rotary inlet valves run cool, clean and silent, and are therefore quite permissible, but I would consider their

use as an exhaust valve as courting failure, as in the case of the exhaust they would be running under adverse conditions due to high temperature and by-products of combustion, which are not as clean as the incoming charge. I therefore intend to design the engine with exhaust ports cast in the cylinder skirt, so that the piston uncovers the exhaust port when it reaches the bottom of its stroke, the same as in ordinary two-stroke practice.

In order to keep the peripheral velocity of the rotary valves low, I intend using two instead of one. This procedure enables me to make their diameter small and still maintain an effective port area, as both the rotary valves in one cylinder will open and close together and the port area (and width) in each will be half the total area (and width). These valves will be perfectly lubricated, as the fuel will be mixed with lubricating oil, in order to lubricate the supercharger and the valves.

The ignition system presents some difficulty if it is desired to use a magneto, but this is easily overcome by using a battery and coil ignition which system is rapidly gaining favour, as it surely deserves to, it has great advantages, and is quite reliable. Nevertheless, a certain amount of prejudice exists against this system over here. Its advantages are easy starting, good operation at low speeds, and elimination of the magneto drive. Whatever this happens to be, chains or gears, and where one has already an electrical installation for lighting and starting, it seems out of place to instal another electrical machine, such as a magneto, for the sole purpose of ignition.

In order to make the engine of compact design, I intend to arrange the twelve cylinders in three rows of four cylinders, each row being at an angle of 6o degrees to its neighbouring row. Figure i shows an end view of the arrangement, and I suggest driving the rotary valves by means of gears and chains.

A convenient bore and stroke for an engine of this type to come within the 1,500 c.c. class is 54 by 54.

It may be queried why an equal bore and stroke have been chosen instead of a smaller bore and longer stroke. A short stroke keeps down the piston length, which is necessarily longer than in standard practice, on account of the necessity of not uncovering the exhaust port, when the piston is at the top of its stroke ; and since the engine is supercharged, there is not much to be gained, as it is possible by means of supercharging to have quite a high compression and yet retain quite a reasonable combustion space without increasing the stroke. By a reasonable combustion space I mean one where the wall area to volume ratio is small, which is a point of considerable importance in high efficiency engines, as the jacket losses due to a poor combustion space can be so great that it would on occasions pay to reduce the compression ratio in order to improve the compression space.

Probable Performance.

A carefully designed engine of the type under discussion would be capable of developing approximately 150 h.p. at 6,000 r.p.m., and the overall dimensions of such an engine would be I ft. 8 ins, long by i ft. 6 ins, high by

ft. 4 ins. wide approximately. I will now briefly explain the functioning of the engine.

The supercharger is placed in between the carburetter and the engine.

When the engine is rotated the supercharger draws from the carburetter and forces a charge into those cylinders which are ready to receive their charge, namely, those that have started exhausting, the incoming charge which enters the top of the bore through the rotary valves assisting to scavenge the cylinders. The charging of the cylinders continues after the rising piston has covered the exhaust ports, and the rotary inlet valves close at a point when the charge in the cylinder, which is being compressed by the rising piston, equals in pressure the supercharging pressure. The piston then continues to rise and compresses the charge in the ordinary way till the end of the stroke, when ignition takes place and the power stroke occurs. The firing order of the cylinders will be as follows, the numbering of the cylinders is shown in the accompanying diagram :— I & 4 6 & 7 2 & 3 8 & 5 9 & 12 IO&II 2 3 4 8 7 6 5 9 ro II 12

Regarding an engine of this type from a manufacturer’s point of view, the proposition is delightful. When we consider that the engine is built up of small parts such as small bores, small pistons, etc., it must be obvious that they can be machined easily and rapidly and at a minimum of cost which is what is required for a good production job.

I think I may venture to say that before this season is over we will be acquainted with a new speed motor cycle fitted with a multi-cylinder supercharged engine capable of attaining a speed of somewhere in the region of 150 m.p.h.