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 sire of a single clyinder 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 cylinders may be so arranged that the impulses will occur twice for each revolution instead of once as in a single cylinder. This gives a more even turning effect on the shaft and consequently steadier running, and also requires a less heavy fly-wheel. The vibration also is much less, as one set of working parts may be made to travel upwards while the other is travelling downwards, thus neutralizing the throw of each and lessening the vibration.

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Fig. 59.

In the case also 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 flywheel 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.

The original method of constructing a multiple engine, and one which is still used by some builders, is to simply use two or more single cylinder engines copuled 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. 59. It consists of two single cylinders mounted on a common base of special design bringing the cylin-

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Fig. 60.

ders 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 vaporisers V-V being used. The gasoline enters at G and branches to each vaporiser. 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.

A common form of two cylinder engine is shown in Fig. 60. The two cylinders are cast together and bolted to the base. This particular motor is of the three port type with the inlet at I. The exhaust port J is E. The cooling water inlet is at O. the outlet being on the other end of the engine. S-S are spark plugs for jump spark ignition. This engine is of the high speed type, carbruettor and timer not being shown.

In Fig. 61 a four cycle engine of common type is shown. The cylinders are independent, and are bolted to a common base. The carburettor is shown at C

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Fig. 61 with the inlet piper I leading up and branching to each cylinder. It is quite important that distance from the inlet pipe to each cylinder should be the same. In this way only can it be certain that each cylinder draws the same amount of mixture.

The cooling water pump is at P discharging water directly into the jackets. This water comes out at the top of the cylinders and discharges into the exhaust pipe E into which the two cylinders also exhaust.

A cast iron base B supports the engine and also carries the reverse gear R. This base holds everything in line and has flanges for bolting to the bed.

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Fig. 62 Fig. 63

Three or four cylinder engines are arranged in much the same manner. A four cylinder engine of the type of Fig. 60 has two pairs of cylinders as shown, bolted to a common base. In the three or four cylinder four cycle type such as is illustrated in Fig. 61 additional cylinders are added with the proper base.

The pistons and cranks may be arranged in a variety of ways. Fig. 62 shows the best arrangement for a two cylinder two cycle engine. The cranks are opposite, one piston ascending while the other is descending. Each thus balances the throw of the other. This arrangement also gives an impulse in each cylinder for each revolution.

Two cylinder four cycle engines may have the cranks arranged either as shown in Figs. 62 or 63, in the

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Fig. 64 former the pistons are balanced, but the impulses occur irregularly two occurring on adjacent strokes, with an interval of one revolution before the next two. In the latter, Fig. 63. both pistons move together: this arrangement so distributes the impulses that they

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Fig. 65 occur regularly, one during each revolution. This arrangement, however, requires heavy counterbalances on the opposite ends of the crank, to belance the weight of the pistons; and it is likely, under certain

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Fig. 66 conditions to cause considerable vibration. Both arrangements are used, however, in representative engines.

The arrangement shown in Fig. 64. or "opposed*' motor has some advantages. It is almost absolutely balanced as the parts are moving either towards or away from each other and thus absorb the throw.

In the two cycle motor of this type the impulses occur together and on each revolution. In the four cycle they occur regularly and one for each revolution. This arrangement has some advantages, as it lies very low in the boat and may even be placed under a transverse seat. For auxiliary work it may be placed under the standing room floor.

In the cylinder engines of either two or four cycle types the cranks are almost always arranged as in Fig. 65-or 120° apart. This arrangement gives a good mechanical balance. The two cycle engine thus has three impulses for each turn and the four cycle has three for each two turns, they occur regularly, and are so timed that one cylinder is receiving its impulse while another is compressing, which is conducive to steady running.

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Fig. 67

Although for automobile work the three cylinder- four cycle engine has been discarded it is still popular and satisfactory for marine work. Four cyclinder engines of the two cycle type are very commonly arranged as in Fig. 66. The cranks are arranged in pairs, the two cranks of each pair being opposite each other, and the planes of the two pairs being at right angles. This makes the engine well balanced, each pair being balanced in itself. It also gives an even turning effort with four impulses per revolution, occurring regularly.

Four cycle four cylinder motors are probably best arranged as in Fig. 67, the cranks being all in one plane. The positions of Nos. 3 and 4 may, however, be reversed, bringing 1 and 3 up and 2 and 4 down. The former is, however, considered to be the better. Either arrangement may be made to give regularly occupying impulses, four during each two revolutions. For high powers a larger number of cylinders are used-six or eight. A six cylinder engine would consist of two units like Fig. 65 coupled together and an eight cylinder, two like either Figs. 66 or 67 coupled together.

The advantages of multiple cylinder engines are becoming better appreciated and their use is increasing, even in small sizes; the exception being in working and fishing boats and other cases where cost is a prime consideration.