Two-Cycle v. Four-Cycle Oil Engines.

Progress, Volume II, Issue 6, 1 April 1907, Page 226

Two-Cycle v. Four-Cycle Oil Engines.

By H. C. Christian, Christchurch

A two-cycle engine (sometimes called a singlecycle engine) is an engine which gives its charge in one cylinder at every revolution, and which usually makes charge in the crank chamber. I say usually, as these are engines made with separate compression chambers, although this is not the rule. This class of engine is often recommended by its makers on account of its even motion and also on its capability of being built lighter per h.p. than a four-cycle engine. It is a very simple engine, being practically valveless. Its arrangement is practically this • a charge of oil and air is admitted into the crank chamber of the engine by the suction of the upstroke of the plunger being drawn through the vaporiser en route, which mixes to a gas. On the down stroke the charge is compound, and at the bottom of the down stroke a port is opened into the cylinder by the passing of the plunger, and the charge is admitted to the cylinder, where on the return stroke it is compressed and fixed in the usual way, either by flame, hot tube or electric spark. The charge exploding expands and drives the plunger down and it again uncovers a port on the opposite side when the exhaust gas escapes. With this arrangement it will be noticed that at one position of the plunger both the inlet and exhaust ports are open at the same time, the new charge being prevented from shooting into the exhaust by a vertical shelf on the piston head which directs the charge into the top of the cylinder. This arrangement, although very simple, has notwithstanding, some very bad points. For instance, the crank chamber must be practically pressure tight before a satisfactory charge of gas can be made, which is very difficult as the bearings begin to wear. Then again, when a plunger is passing open ports they must assuredly cut away to some extent, and form uneven surfaces in the cylinder, with the inevitable result of losing compression m which is confined the power of the engine. Then engines, if well made, will sometimes last from five to six years ; but beyond that are worn out. It is, however, only the best of them that have a life as long as this. Numbers of the cheaper class have never lasted out twelve months ordinary use. It is, therefore, a risky matter to buy a cheap oil engine, especially a two-cycle. We now turn to the two-cycle engine with separate compression chamber. This is an infinitely superior type of engine, but is naturally more complicated than the ordinary two-cycle engine . With this engine a much higher compression can be obtained, which means greater power. The movement of these engines can be made much more rapid, as the suction stroke is practically done away with, and the charge of gas is admitted direct to the cylinder head. They have, however, so far not been accepted for general use, being confined almost entirely to motor cycles, cars, etc. It must be clearly understood that the first thing required in an oil engine of any description is a proper quantity and quality of gas to explode, and to have it admitted quickly at the correct time ; it is also very important that the charge shall be fired at the light time, which varies according to the speed of the engine. For instance, an engine travelling at say 200 r. p. m. should fix its charge iust as the plunger reaches the top of the stroke, but in an engine travelling at 500 r. p.m. a considerable amount of lead should be allowed for. This is to counteract the speed of the plunger, and is regulated so that the explosion comes on to the plunger just as it is beginning to descend. The greatest fault, however, of the two-cycle engine is its oil consumption. It is practically impossible to obtain the same amount of economy out of a two-cycle engine, power for power and type for type as out of a four-cycle one, owing largely to the fact that the surplus amount of power over and above the necessary power required to drive the engine m a two-cycle is very small. This is due largely to the small compression and the limited amount of time possible for the charge of gas to sef into the cvhnder, and thereby leaving a narrow margin of efficiency. Now we will go on to the four-cycle, or Otto type, engine. This is an engine which, firstly, on the down stroke of the engine draws in a charge of gas through Rie intake or compression valve, and, secondly, on the return stroke compresses it, and at the end of the compression stroke fires it, which thirdly drives down the plunger as the gas expands,

and on the fourth and return stroke discharges the products of the explosion into the exhaust. By this movement it can be easily seen that there is ample time for everything to be done, drawing in, compressing, fixing and exhausting. This is undoubtedly a superior class of machine, and has many good qualities, although it is a little more complicated than the two-cycle, owing to the necessity of having exhaust and intake valves to operate which do not exist in the two-cycle motor. There is also a two to one gear in this class of engine, on account of having to reduce the lay or cam shaft down so as to produce one movement of the exhaust valve to two revolutions of the fly wheel. The fixing gear is usually operated from this shaft for the same reason. It can be easily seen, therefore, that if an explosion is required at every half revolution there must be four cylinders. With this type of engine the economy can be worked to a nicety, owing to the abundance of time allowed for each movement of the engine to do its required duty. This engine, unless built in two cylinders, takes a good deal of holding down and usually (unless running very rapidly) takes a heavy fly-wheel on account of its somewhat uneven motion. It is, however, generally acknowledged to be the most reliable, economical and durable machine, is much easier kept in order, and although the parts are more numerous, it is easier to locate a trouble with the engine than with a two-cycle one. The four-cycle engine is of course more expensive than a two-cycle, but this brings us back to an old proposition, t.e., if you want a good serviceable article you must pay for it.