Langley Memoir on Mechanical Flight, Parts I and II / Smithsonian Contributions to Knowledge, Volume 27 Number 3, Publication 1948, 1911

by S. P. Langley; Charles M. Manly

The reason for building the engine with five cylinders instead of some other number, and for arranging them radially on a central drum using only one crank pin may not appear quite obvious. The advantages gained by such a construction, however, are very great, and may be briefly summed up as follows:

First, since in a gas engine of the four-cycle type there is only one explosion in each cylinder every two revolutions, and the crank shaft and crank pin therefore are loaded only one-quarter of the time for each cylinder, it is obvious that by having four cylinders arranged radially around a central drum the load on the bearings of a single crank shaft and crank pin may be kept very uniform. However, with four cylinders thus arranged it is impossible to have the cylinders explode and exert their effort on the crank at uniform intervals in the cycle, it being necessary to have the cylinders explode in the order of 1, 3, 4, 2, 1, etc., thus giving intervals between explosions of 180 degrees, 90 degrees, 180 degrees, 270 degrees, etc., or to have them explode in the order of 1, 3, 2, 4, 1, etc., thus giving intervals of 180 degrees, 270 degrees, 180 degrees, 90 degrees, etc. On the other hand, with any odd number of cylinders the explosions will occur at equal intervals in the cycle. With three cylinders they will explode in the order of 1, 3, 2, 1, etc., or at equal intervals of 240 degrees, while with five cylinders they will explode in the order of 1, 3, 5, 2, 4, 1, etc., or at equal intervals of 144 degrees. It is therefore seen that there is a great advantage in smoothness of operation and uniformity of torque of the engine through having an odd number of cylinders instead of an even number.

Second, it is readily apparent that the greater the number of cylinders, provided the number is an odd one, the more uniform the torque will be, and it would seem at first that seven cylinders would therefore be better than five, since the uniform intervals between explosions with seven cylinders would be only 103 degrees (approximately). The advantage gained, however, through seven cylinders instead of five is largely, if not completely, counterbalanced by the added number of parts and the difficulty of providing sufficient circumferencial width for the connecting-rod shoes on the crank-pin bearing, even with the improved construction of this bearing already described. There is considerable fluctuation of the torque in each revolution of the engine with five cylinders, but this fluctuation of torque is more easily smoothed out by the use of very light fly wheels than by increasing the number of cylinders, and thus adding to the complication of the engine.

Third, the strongest point in favor of the radially arranged cylinders is the reduction in weight and complication which it permits. The crank shaft is reduced to the very minimum, there being only one crank pin with two main bearings which can, without any difficulty whatever, be kept absolutely in line with each other and thus prevent binding and loss of power. Again, the use of a single-throw crank not only reduces the cost and weight of the crank [p246] itself, but makes it very much less liable to damage; long crank shafts with several crank pins being frequently twisted by improper explosions in the cylinders. The supporting drum or crank chamber is likewise reduced to the very minimum, both in weight and simplicity, the drums being perfectly symmetrical with no lost space either inside of them or on their exteriors. The cam mechanism for operating the valves is reduced to a simple ring carrying (for a five-cylinder engine) a double-pointed cam and journaled on the exterior of the hub of one of the drums, the cam being driven by a train of gears journaled on studs mounted on the drum, and co-acting with a gear fastened to the crank shaft against the crank arm.

The radial arrangement of the cylinders is thus seen to give not only an engine with the smallest number of parts, each of which is as far as possible worked to a uniform amount during each complete revolution of the crank shaft, but it also gives a very compact and readily accessible mechanism with its center of gravity coincident with its center of figure, and with the liability of damage to it, in case of a smash of the vehicle on which it is used, reduced to the minimum from the fact that the greatest weight is located at the strongest part.

Fourth, and of almost as great importance as the reduction in weight which the five-cylinder radial arrangement permits, is its unusual qualities as regards vibration. Since these five-cylinder engines were built by the writer a very thorough treatment of their properties as regards balancing has been given in a treatise on the balancing of engines,[44] so no discussion of the mathematical formulæ involved in a study of the question of the inherent balancing properties of these engines will be here given. It is sufficient to call attention to the fact that in an engine having five cylinders arranged radially, all of the reciprocating parts are balanced for all forces of the first, second and third orders. As it is only the reciprocating parts which give any trouble in balancing any engine, the unbalanced rotating parts being readily balanced by placing an equal weight at an equal distance from the center of rotation, and on the opposite side thereof, it is readily seen that the properties of balancing which are inherent in this type of engine are unusual. A six-cylinder engine having a six-throw crank shaft is not nearly so thoroughly balanced as this type having its five cylinders radially arranged, for in the latter case all the moving parts are in one plane, while in the former case the moving parts are in six separate and parallel planes, and there is consequently considerable longitudinal vibration which can never be overcome. While this is true as regards the vibration due to moving masses, it is still more impressively true as regards vibration due to reaction arising from the force of the explosions in the engine cylinders, especially when the engine is running slowly and having heavy explosions.

The usual practice in balancing the rotating parts of an engine is to attach [p247] balance weights to the crank arms which are prolonged beyond the center of the crank shaft and on the opposite side from the crank pin; the radius of rotation of these balance weights being made approximately equal to the radius of the crank pin. But aside from the constructional difficulties which would be introduced, it was seen that if this plan was followed in this engine it would require a very large additional weight. Since the amount of this weight could be diminished in exact proportion to the increase of the radius of rotation of the balance weights, it was at first decided to attach the weights to the rims of the fly wheels, the relative amount of weight attached to each wheel being inversely proportional to its actual longitudinal distance from the crank-pin center. It was very soon found that the attachment of these balance weights to the fly wheel caused excessive strains on the rims of the wheels, thereby causing them to go out of line. In order, therefore, to keep the amount of balance weight small by carrying it at a considerable distance from the center of the shaft, the weights were finally arranged as clearly shown in the drawings, Plates [78] to [80]. There it is seen that the main portion of each of the balance weights consists of a flat arm bolted between the flanges which couple the transmission shafts to the engine shafts. The flat arm terminates in a lozenge-shaped lug, additional weight being provided by a plate fastened to one end of a tube, the other end of which terminates in a collar fastened around the transmission shaft. The tube is inclined at an angle of about thirty degrees with the flat balance arm, thus acting as a brace to prevent the balance arm from wobbling, the plate on the bracing tube being fastened to the lozenge-shaped lug by means of small bolts.

The tabulated statement of the weight of this large engine is given below. From this it will be readily seen that the net weight of the engine proper is 124.17 pounds. The fly wheels were in no way necessary to the engine itself, but were used solely for the purpose of smoothing out the torque of the engine so that the transmission shafts and propeller shafts might be kept down to the very minimum in weight. Including the two fly wheels, the weight is 140 pounds.

Including the 20 pounds of cooling water the total weight of the power plant is 207.47 pounds. Without flywheels the total weight is 191.64 pounds.

The construction of this large engine was completed in December, 1901, and the first tests of it were made in January, 1902. As already stated, these first tests were made with the engine mounted on a special testing frame and delivering its power to two water-absorption dynamometers, no fly wheels being used, as none were required. Later, when it became necessary either to use fly wheels or to greatly increase the weight of the transmission and propeller shafts, in order to overcome the reverse torque, the two light fly wheels were added, and another series of tests was made of the engine on its testing frame. The arrangement of the engine, dynamometers, and accessory [p248] apparatus is clearly shown in Plates [82], [83] and [84]. The engine ran in a clockwise direction, as viewed in Plate [82]. AA are the fly wheels; BB the balance weights; CC the dynamometer shafts, on which are fastened the rotor plates which revolve inside of the dynamometer drums, DD, between stator plates fastened therein. The drums have a hub on either side, by which they are supported, these hubs being journaled on ball-bearings in the pedestals resting on the wooden framework. The rotor plates do not touch the stator plates in the dynamometers, but drag on the water with which the drums are partially filled, and thus tend to cause the drums to revolve around with them. The torque on each drum is measured by means of a rope, not shown, fastened into the hook at the top of the drum, the rope being given a partial coil around the drum and passing off tangent thereto at the horizontal diameter is fastened to a pair of spring scales hung from the ceiling vertically above the point of tangency. The scales and ropes were unfortunately not in position when these photographs were taken, but the arrangement of them should be readily understood. As the friction of the rotor plates on the water heats it in exact proportion to the amount of power absorbed, the small amount of water in the drums would be soon converted into steam unless continually renewed or cooled. When the rotor plates are revolving the centrifugal force keeps the water pressed toward the circumference of the drum, and the friction at any speed is dependent on the area of the rotor plates in contact with the water. The horse-power required to revolve the plates at any definite speed can therefore be controlled by having an outlet for the water at the proper radial distance from the shaft. The water from the water mains is led through the upper vertical pipe and allowed to flow into the funnel, and thence into the drum near the center where the centrifugal force throws it to the circumference of the drum. The lower vertical pipe is connected to the drum at a suitable radial distance from the center, and the heated water thus passes through this pipe and into the lower funnel connected to the sewer. By the use of the funnels the drums are allowed to rock sufficiently to exert their pull on the spring scales without being affected by the supply and exhaust of water.