In England, the first vertical aero-engine of note was that designed by Green, the cylinder dimensions being 4.15 inch diameter by 4.75 stroke - a fairly complete idea of this engine can be obtained from the accompanying diagrams. At a speed of 1,160 revolutions per minute it developed 35 brake horse-power, and by accelerating up to 1,220 revolutions per minute a maximum of 40 brake horse-power could be obtained - the first-mentioned was the rated working speed of the engine for continuous runs. A flywheel, weighing 23.5 lbs., was fitted to the engine, and this, together with the ignition system, brought the weight up to 188 lbs., giving 5.4 lbs. per horse-power. In comparison with the engine fitted to the Wrights' aeroplane a greater power was obtained from approximately the same cylinder volume, and an appreciable saving in weight had also been effected. The illustration shows the arrangement of the vertical valves at the top of the cylinder and the overhead cam shaft, while the position of the carburettor and inlet pipes can be also seen. The water jackets were formed by thin copper casings, each cylinder being separate and having its independent jacket rigidly fastened to the cylinder at the top only, thus allowing for free expansion of the casing; the joint at the bottom end was formed by sliding the jacket over a rubber ring. Each cylinder was bolted to the crank-case and set out of line with the crankshaft, so that the crank has passed over the upper dead centre by the time that the piston is at the top of its stroke when receiving the full force of fuel explosion. The advantage of this desaxe setting is that the pressure in the cylinder acts on the crank-pin with a more effective leverage during that part of the stroke when that pressure is highest, and in addition the side pressure of the piston on the cylinder wall, due to the thrust of the connecting rod, is reduced. Possibly the charging of the cylinder is also more complete by this arrangement, owing to the slower movement of the piston at the bottom of its stroke allowing time for an increased charge of mixture to enter the cylinder.

A 60 horse-power engine was also made, having four vertical cylinders, each with a diameter of 5.5 inches and stroke of 5.75 inches, developing its rated power at 1,100 revolutions per minute. By accelerating up to 1,200 revolutions per minute 70 brake horsepower could be obtained, and a maximum of 80 brake horse-power was actually attained with the type. The flywheel, fitted as with the original 35 horse-power engine, weighed 37 lbs.; with this and with the ignition system the total weight of the engine was only 250 lbs., or 4.2 lbs. per horse-power at the normal rating. In this design, however, low weight in relation to power was not the ruling factor, for Green gave more attention to reliability and economy of fuel consumption, which latter was approximately 0.6 pint of petrol per brake horse-power per hour. Both the oil for lubricating the bearings and the water for cooling the cylinders were circulated by pumps, and all parts of the valve gear, etc., were completely enclosed for protection from dust.

A later development of the Green engine was a six-cylindered vertical, cylinder dimensions being 5.5 inch diameter by 6 inch stroke, developing 120 brake horsepower when running at 1,250 revolutions per minute. The total weight of the engine with ignition system 398 was 440 lbs., or 3.66 lbs. per horse-power. One of these engines was used on the machine which, in 1909, won the prize of L1,000 for the first circular mile flight, and it may be noted, too, that S. F. Cody, making the circuit of England in 1911, used a four-cylinder Green engine. Again, it was a Green engine that in 1914 won the L5,000 prize offered for the best aero engine in the Naval and Military aeroplane engine competition.

Manufacture of the Green engines, in the period of the War, had standardised to the production of three types. Two of these were six-cylinder models, giving respectively 100 and 150 brake horse-power, and the third was a twelve-cylindered model rated at 275 brake horse-power.

In 1910 J. S. Critchley compiled a list showing the types of engine then being manufactured; twenty-two out of a total of seventy-six were of the four-cylindered vertical type, and in addition to these there were two six-cylindered verticals. The sizes of the four-cylinder types ranged from 26 up to 118 brake horse-power; fourteen of them developed less than 50 horse-power, and only two developed over 100 horse-power.

It became apparent, even in the early stages of heavier-than-air flying, that four-cylinder engines did not produce the even torque that was required for the rotation of the power shaft, even though a flywheel was fitted to the engine. With this type of engine the breakage of air-screws was of frequent occurrence, and an engine having a more regular rotation was sought, both for this and to avoid the excessive vibration often experienced with the four-cylinder type. Another, point that forced itself on engine builders was that the increased power which was becoming necessary for the propulsion of aircraft made an increase in the number of cylinders essential, in order to obtain a light engine. An instance of the weight reduction obtainable in using six cylinders instead of four is shown in Critchley's list, for one of the four-cylinder engines developed 118.5 brake horse-power and weighed 1,100 lbs., whereas a six-cylinder engine by the same manufacturer developed 117.5 brake horse-power with a weight of 880 lbs., the respective cylinder dimensions being 7.48 diameter by 9.06 stroke for the four-cylinder engine, and 6.1 diameter by 7.28 stroke for the six-cylinder type.