These engines when properly arranged are made to do service as marine motors, and are then installed either horizontally or vertically. A vertical engine has been shown on previous pages, but perhaps a little further explanation may not be amiss. Engines for boats are made with one cylinder or with more, and there are many considerations which make an engine of two or more cylinders particularly desirable. It is a self-evident fact that when the limit of size of a single-cylinder is reached, it is necessary to add other cylinders if greater power is desired. Even for moderate or small powers, there are many advantages. Among these may be noted the fact that with the proper arrangement of cylinders the impulses may be made to occur at shorter intervals than with a single-cylinder engine. Thus with a two-cylinder engine, the cylinder may be so arranged that the impulses will occur twice for every revolution instead of once, as in a single-cylinder. This gives a more even turning effect to the shaft, and consequently steadier running, and it also requires a less heavy fly-wheel. The vibration is much less, as one set of working parts may be made to travel upward while the other is travelling downward, thus neutralizing the throw of each and lessening the vibration.
In case of the disablement of one cylinder, there is the chance of getting home on the remaining ones. The weight, power for power, of the multiple-cylinder engine is less than that of the single-cylinder engine, as the weight of the fly-wheel and other working parts is less.
While for marine work, single-cylinder engines have been built as large as eight or ten horse-power, they are so large as to be rather cumbersome and the practice now is to build engines of more than six horse-power with two or more cylinders. There are several firms who are making double-cylinder engines as small as four horse-power, which both as to weight and reliability are much superior to those of a single-cylinder.
Fig. 224. Two-cylinder engine
The original method of constructing a multiple engine, and one which is still used by some builders, is simply to use two or more single-cylinder engines coupled together. This is a cumbersome method and takes up a great amount of space. The simplest method which can be recommended is that shown in [Fig. 224]. It consists of two single-cylinders mounted on a common base of special design, bringing the cylinders much nearer together than when a coupling is fitted to connect two separate engines—as the shaft can be made in one piece. This particular engine is of the two port type, two vaporizers V-V being used. The gasolene enters at G and branches to each vaporizer. The pump is shown at P with the discharge at W, piped with a branch to each cylinder. The cooling water outlet is at O. The exhausts are connected to a common pipe with the outlet at E. The igniting gear for each cylinder is independent and on opposite ends. By means of the lever L, which is connected to both igniting gears, the time of ignition is regulated and kept the same on both cylinders. This allows multiple-cylinder engines to be built with very few extra parts, as the cylinders, ignition gear, etc., are the same as in the single-cylinder engine.
Fig. 225. Single-cylinder engine