A New Four-cylinder Engine.
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A Novel Form of Monobloc Engine in which the Four Pistons Work Indirectly on to a Double-throw Crankshaft.
The illustrations which we reproduce on this page will convey a general idea of the form of construction of a new engine, known as the Banner engine, the invention of Messrs. W. J. Robb and W. Ii, Welsh, of Banner Motors, Ltd., Redcliff Backs, Bristol. In some respects, the method of connecting the pistons and connecting-rods to the crankshaft somewhat resembles that adopted in the early Arrol-Johnston horizontal engines. In the Banner engine, however, the four cylinders are disposed vertically, and, in the plan view, they occupy the four corners of a square. The connecting-rod from each piston is coupled to a rocking lover, which is pivoted on a short shaft that takes its hearing in bosses in the crankcase casting; this rocking lever is coupled to the crankshaft through a short connecting-rod. Two pistons, in this way, work on to one crank throw. It will he seen that the ordinary reciprocating motion of the pistons is not directly converted into rotary motion, but that the pistons are caused to rock the lever whence the motion is transmitted to the crankshaft. By this arrangement, very little angular thrust is transmitted on to the piston walls, and another point of importance is that a high piston-speed does not necessarily mean a high rate of revolution for the crankshaft. The rocking lever has the effect of gearing down the piston's movement; a stroke of 5 in. of the piston only corresponds to a throw of 11 in. on the crankshaft.. The four cylinders of this engine are 101 mm. in diameter, with a stroke oi 127 mm. (5 in.), and, according to R.A.C. rating, the engine should develop 25 h.p.
The valve gear is a combination of the rotary and the mushroom systems. The actual distribution of the charge is effected by means of a single rotary valve, whilst the mushroom valves separate the rotary valve from the burning gases during the periods of compression and firing; thus, one of the early objections to the use of rotary valves is removed. The rotary valve is housed in the centre of the cylinder casting, and is rotated through skew gearing and a vertical shaft from the crankshaft. The mushroom valves are all operated by means of a single cam, which is mounted on the vertical shaft, the order of firing (1, 2, 4) making this arrangement quite permissible. As the distribution valve is rotated, a single port in the valve is alternately brought into register with a corresponding port. in each of the mushroom-valve pockets, and the charge is drawn into the cylinder through the opening thus formed.
Let us consider the cycle of one cylinder only, in describing the method of operation: as the distributing valve rotates, during the upward stroke of the piston, the valve port is brought into register with a similar port in the cylinder head, and the exhaust gases pass out of the cylinder through the ports and down the central passage around the rotary-valve spindle, the mushroom valves meanwhile being held wide open by means of the single cam to which we have already referred. The distributing valve continues to rotate—the poppet valve remaining open, and the exhaust port is gradually closed, and another port opened for the admission of a fresh charge of gas from the inlet pipe; the mushroom valve then closes, and remains closed for the remainder of the cycle. Obviously, this valve-cycle takes place for all cylinders, each of which has its own mushroom valve, although, of course, it depends on the common central rotary valve. .Az the two crank-pins are set at 180 degrees to each other, the pistons which are diagonally opposite to each other will move in the same direction.
Among the advantages claimed for this form of construction is that, for a given power, a four-cylinder engine need be no longer than an engine having only two cylinders of the same diameter. Its width, of course, is slightly increased, but that only tends towards increased stability. The engine's balance should be very good, and its running, at heel' fast and slow speeds, extremely smooth. A member of the Editorial staff of " The Motor " has been afforded opportunities to examine and witness tests with one of these engines, and his reports are highly favourable. That the engine is no mere experimental toy appears quite evident from the fact that Rarvev Frost and Co., Ltd. of Great Eastern Street, RC., has acquired considerable interest in it, as well as the full rights for licensing its manufacture. Bench tests are to he conducted shortly, and we hope then again to refer to the engine, and to give a fuller description, together with the neceesary photographs and drawings, when such tests ale completed.