Among the first internal combustion engines to be taken into use with aircraft were those of the horizontally-opposed four-stroke cycle type, and, in every case in which these engines were used, their excellent balance and extremely even torque rendered them ideal-until the tremendous increase in power requirements rendered the type too long and bulky for placing in the fuselage of an aeroplane. As power increased, there came a tendency toward placing cylinders radially round a central crankshaft, and, as in the case of the early Anzani, it may be said that the radial engine grew out of the horizontal opposed piston type. There were, in 1910 - that is, in the early days of small power units, ten different sizes of the horizontally opposed engine listed for manufacture, but increase in power requirements practically ruled out the type for air work.

The Darracq firm were the leading makers of these engines in 1910; their smallest size was a 24 horsepower engine, with two cylinders each of 5.1 inches bore by 4.7 inches stroke. This engine developed its rated power at 1,500 revolutions per minute, and worked out at a weight of 5 lbs. per horse-power. With these engines the cranks are so placed that two regular impulses are given to the crankshaft for each cycle of working, an arrangement which permits of very even balancing of the inertia forces of the engine. The Darracq firm also made a four-cylindered horizontal opposed piston engine, in which two revolutions were given to the crankshaft per revolution, at equal angular intervals.

The Dutheil-Chambers was another engine of this type, and had the distinction of being the second largest constructed. At 1,000 revolutions per minute it developed 97 horse-power; its four cylinders were each of 4.93 inches bore by 11.8 inches stroke - an abnormally long stroke in comparison with the bore. The weight - which owing to the build of the engine and its length of stroke was bound to be rather high, actually amounted to 8.2 lbs. per horse-power. Water cooling was adopted, and the engine was, like the Darracq four-cylinder type, so arranged as to give two impulses per revolution at equal angular intervals of crankshaft rotation.

One of the first engines of this type to be constructed in England was the Alvaston, a water-cooled model which was made in 20, 30, and 50 brake horse-power sizes, the largest being a four-cylinder engine. All three sizes were constructed to run at 1,200 revolutions per minute. In this make the cylinders were secured to the crank case by means of four long tie bolts passing through bridge pieces arranged across the cylinder heads, thus relieving the cylinder walls of all longitudinal explosion stresses. These bridge pieces were formed from chrome vanadium steel and milled to an 'H' section, and the bearings for the valve-tappet were forged solid with them. Special attention was given to the machining of the interiors of the cylinders and the combustion heads, with the result that the exceptionally high compression of 95 lbs. per square inch was obtained, giving a very flexible engine. The cylinder heads were completely water-jacketed, and copper water-jackets were also fitted round the cylinders. The mechanically operated valves were actuated by specially shaped cams, and were so arranged that only two cams were required for the set of eight valves. The inlet valves at both ends of the engine were connected by a single feed-pipe to which the carburettor was attached, the induction piping being arranged above the engine in an easily accessible position. Auxiliary air ports were provided in the cylinder walls so that the pistons overran them at the end of their stroke. A single vertical shaft running in ball-bearings operated the valves and water circulating pump, being driven by spiral gearing from the crankshaft at half speed. In addition to the excellent balance obtained with this engine, the makers claimed with justice that the number of working parts was reduced to an absolute minimum.

In the two-cylinder Darracq, the steel cylinders were machined from solid, and auxiliary exhaust ports, overrun by the piston at the inner end of its stroke, were provided in the cylinder walls, consisting of a circular row of drilled holes - this arrangement was subsequently adopted on some of the Darracq racing car engines. The water jackets were of copper, soldered to the cylinder walls; both the inlet and exhaust valves were located in the cylinder heads, being operated by rockers and push-rods actuated by cams on the halftime shaft driven from one end of the crankshaft. Ignition was by means of a high-tension magneto, and long induction pipes connected the-ends of the cylinders to the carburettor, the latter being placed underneath the engine. Lubrication was effected by spraying oil into the crank case by means of a pump, and a second pump circulated the cooling water.

Another good example of this type of engine was the Eole, which had eight opposed pistons, each pair of which was actuated by a common combustion chamber at the centre of the engine, two crankshafts being placed at the outer ends of the engine. This reversal of the ordinary arrangement had two advantages; it simplified induction, and further obviated the need for cylinder heads, since the explosion drove at two piston heads instead of at one piston head and the top of the cylinder; against this, however, the engine had to be constructed strongly enough to withstand the longitudinal stresses due to the explosions, as the cranks are placed on the outer ends and the cylinders and crank-cases take the full force of each explosion. Each crankshaft drove a separate air-screw.

This pattern of engine was taken up by the Dutheil-Chambers firm in the pioneer days of aircraft, when the firm in question produced seven different sizes of horizontal engines. The Demoiselle monoplane used by Santos-Dumont in 1909 was fitted with a two-cylinder, horizontally-opposed Dutheil-Chambers engine, which developed 25 brake horse-power at a speed of 1,100 revolutions per minute, the cylinders being of 5 inches bore by 5.1 inches stroke, and the total weight of the engine being some 120 lbs. The crankshafts of these engines were usually fitted with steel flywheels in order to give a very even torque, the wheels being specially constructed with wire spokes. In all the Dutheil-Chambers engines water cooling was adopted, and the cylinders were attached to the crank cases by means of long bolts passing through the combustion heads.

For their earliest machines, the Clement-Bayard firm constructed horizontal engines of the opposed piston type. The best known of these was the 30 horse-power size, which had cylinders of 4.7 inches diameter by 5.1 inches stroke, and gave its rated power at 1,200 revolutions per minute. In this engine the steel cylinders were secured to the crank case by flanges, and radiating ribs were formed around the barrel to assist the air-cooling. Inlet and exhaust valves were actuated by push-rods and rockers actuated from the second motion shaft mounted above the crank case; this shaft also drove the high-tension magneto with which the engine was fitted. A ring of holes drilled round each cylinder constituted auxiliary ports which the piston uncovered at the inner end of its stroke, and these were of considerable assistance not only in expelling exhaust gases, but also in moderating the temperature of the cylinder and of the main exhaust valve fitted in the cylinder head. A water-cooled Clement-Bayard horizontal engine was also made, and in this the auxiliary exhaust ports were not embodied; except in this particular, the engine was very similar to the water-cooled Darracq.

The American Ashmusen horizontal engine, developing 100 horse-power, is probably the largest example of this type constructed. It was made with six cylinders arranged on each side of a common crank case, with long bolts passing through the cylinder heads to assist in holding them down. The induction piping and valve-operating gear were arranged below the engine, and the half-speed shaft carried the air-screw.

Messrs Palons and Beuse, Germans, constructed a light-weight, air-cooled, horizontally-opposed engine, two-cylindered. In this the cast-iron cylinders were made very thin, and were secured to the crank case by bolts passing through lugs cast on the outer ends of the cylinders; the crankshaft was made hollow, and holes were drilled through the webs of the connecting-rods in order to reduce the weight. The valves were fitted to the cylinder heads, the inlet valves being of the automatic type, while the exhaust valves were mechanically operated from the cam-shaft by means of rockers and push-rods. Two carburettors were fitted, to reduce the induction piping to a minimum; one was attached to each combustion chamber, and ignition was by the normal high-tension magneto driven from the halftime shaft.

There was also a Nieuport two-cylinder air-cooled horizontal engine, developing 35 horse-power when running at 1,300 revolutions per minute, and being built at a weight of 5.1 lbs. per horse-power. The cylinders were of 5.3 inches diameter by 5.9 inches stroke; the engine followed the lines of the Darracq and Dutheil-Chambers pretty closely, and thus calls for no special description.

The French Kolb-Danvin engine of the horizontal type, first constructed in 1905, was probably the first two-stroke cycle engine designed to be applied to the propulsion of aircraft; it never got beyond the experimental stage, although its trials gave very good results. Stepped pistons were adopted, and the charging pump at one end was used to scavenge the power cylinder at the other ends of the engine, the transfer ports being formed in the main casting. The openings of these ports were controlled at both ends by the pistons, and the location of the ports appears to have made it necessary to take the exhaust from the bottom of one cylinder and from the top of the other. The carburetted mixture was drawn into the scavenging cylinders, and the usual deflectors were cast on the piston heads to assist in the scavenging and to prevent the fresh gas from passing out of the exhaust ports.