Internal Combustion Engine

INTERNAL COMBUSTION ENGINE.) Aero Engines - Elements of the problem.

The great problem with which the designer of aero engines is faced is reduction of the ratio weight to horse-power, or as it is customarily expressed the "pounds per horse-power," to the smallest possible value consistent with reliability. But pounds per horse-power in the air cannot be taken simply as a figure obtained on the test-bed; an aeroplane must carry its own fuel, so that economy of fuel consumption is also of primary importance. If we take, in order to fix our ideas, the example of a 450-h.p. engine which itself weighs, say 9oolb., then the weight of fuel used per hour would be of the order of 2251b., depending, of course, at what height the flight was made.

If we consider, not the weight of the engine alone, which is 1.451b. per h.p., but the weight per h.p. of the "power unit (en gine and fuel) for a four hours' flight," then the value of this when the aircraft leaves the ground will be not 2 but and it becomes at once clear that the effective weight to power ratio of an engine in flight will be materially affected by its fuel economy. In 1915 there was no water-cooled engine which weighed, com plete with water and radiator, less than about 41b. per h.p., and no air-cooled engine of less than 31b. per horse-power. Since that time these figures have been reduced to less than 2 and 1.5 re spectively.

Tendency of Design.

For an engine of given size (bore and stroke) both the power and economy are increased by an increase of the compression ratio. The tendency of design has therefore been toward the use of higher and higher ratios of compression : from the figure 5.5 of the average motor-car the compression in aero engines has been pushed up to 6 to 1, and even much higher in special cases. The limit to which it can be raised depends largely on the physicochemical properties of the fuel available. With some fuels, of high octane rating, an engine of 7 to 1 com pression ratio will run quite smoothly. With others which differ only in chemical analysis and not at all in appearance, volatility or specific gravity, it would be impossible to run the engine at all. Owing to the high degree of compression, the combustible gas when ignited would explode with such violence as to cause "de tonation" or "knocking" in the cylinders. In order that the com pression ratio may be pressed to the highest possible point, there fore, the supply of suitable fuels for aero engines is a matter of great importance.

The Factor of Head Resistance.

The aero engine designer must bear constantly in mind the factor of "head resistance." When an aeroplane is flying level, the whole available power of the engine is used up in overcoming the "drag" on body and wings as the machine tears forward through the air. In practically all modern machines the engines are at the front of the body or streamlined into the wings, and if the head resistance is to be kept down, the area presented by the engine and accessories. when viewed from the front, must be as small as possible. A most important accessory, in this connection, is the radiator necessary with liquid-cooled engines.

Bearing in mind the four considerations of power, weight, fuel economy and head resistance, their effect upon aero-engine design will now be traced. The designer must be a master of compromise. Compactness, rigidity, revolutions, inertia forces, piston speed and area, bearing loads, valve area, effective cooling and the rest . . . each member of this complex company must be harmonized with all the others and at the same time consideration of each must be pressed to the farthest limit in the service of the two great aims of power and lightness.

Two Main Types.

It is appropriate in the first place to point out the main subdivision into liquid-cooled and air-cooled types. With our primary concern for weight to power ratio the advan tages of the air-cooled type are not far to seek; there is the clear gain in weight of all the circulating water, besides the elimination of the weight and drag of the radiator. But the air-cooled engine designer cannot have it all his own way. For minimum head resistance of the engine itself a design of the liquid-cooled "cyl inders-in-line" type with the crank-shaft fore-and-aft will clearly have the advantage; but if for cooling we rely wholly upon a high velocity air-stream it is imperative that no cylinder should be seriously more screened from this air-stream than any other.

Hence, although some air-cooled engines of the cylinders-in-line type have been built, the two types which have been widely suc cessful have been those in which the cylinders are arranged radi ally in one plane (see Pl. 11.–i), or sometimes in two parallel planes (see Pl. 11.-2), perpendicular to the axis of the machine; so that all cylinders share equally in the cooling air. Such engines, unless effectively cowled, whether of the "rotary" or "static radial" types, present a large frontal area and offer very serious head resistance, in fact, far in excess of the radiator. The difficulty of air cooling is that the specific heat as compared with water is only 1/4.2, lb. for lb., or 1/3,500, volume for volume.

Water-cooled Engines.

The power developed by an engine depends upon the mean piston speed, the total piston area and the mean-effective pressure in the cylinders. As to the first two the question at once arises : "Shall the requisite piston speed and area be achieved with a few large or many small cylinders, and what in each case are the limiting factors in regard to speed?" Cylinder Design.—The answer given by designers has been "many and small"; largely on account of the resulting compact ness and of the rigidity obtainable with light construction. A long stroke means a heavy crank-shaft and a cylinder reaching out a long way from the shaft centre-line and upsetting the slip stream from the propeller. For getting adequate piston speed, therefore, the tendency is toward a short stroke and high revolu tions. Large-bore cylinders in line, again, would lead to a long crank-shaft, involving both weight in itself and in the resulting crank-case, besides the danger of non-rigidity in a big engine of light crank-case construction. A further advantage of small cyl inders is the reduced danger of distortion due to uneven heating while in service.

With many small cylinders in line we get less piston area to the same length of crank-shaft, but it is possible to make two or more cylinders operate on one crank and so the balance is redressed. Considerations of this kind have led to the develop ment of the "Vee," "Broad Arrow" and "X" types of engine with two, three and four banks of cylinders. Fig. 2 on Plate I. shows one of the successful modern liquid-cooled engines of the Vee type, the Curtiss Conqueror V-157oF, an unsupercharged type de veloping 65o h.p. at 2,400 r.p.m. It has a bare weight of goo pounds, height of 39.25 inches and width of 26.562 inches. Two other Conquerors, one of which is supercharged, develop respec tively 675 and 705 h.p.

The Rolls-Royce Kestrel, one of the best known Vee types, has two banks of six cylinders each, like the Conqueror. The Kestrel has a crank throw of 51 inches and a bore of 5 inches. The un supercharged type gives 575 b.h.p. at 2,500 r.p.m. and with mod erate supercharging it delivers 63o b.h.p. at 2,500 r.p.m. at 3,000 feet, and fully supercharged delivers 600 b.h.p. at 2,500 r.p.m. at 1 i,000 feet.

Valve Problems.

The combination of high piston speed with good volumetric efficiency involves ample valve area and the freest possible form of induction system. This, combined with the need for a compact and symmetrical combustion chamber to diminish detonation, has resulted in the universal adoption of four valves in the cylinder head, two inlet and two exhaust, or two larger valves. The effective area of one or two inlet valves is from 15 to 20%. With this provision mean piston speeds up to well above 2,000ft. per minute can be employed with good volumetric efficiency.

Next to the big-end bearing loads, valve operation is most critically affected by high revolutions. To get sufficiently rapid opening and closing of a poppet valve for an engine speed of 2,500 r.p.m. the accelerations involve very heavy springs and ter ribly severe stresses in the cams and in the valve spindles. This difficulty of high-speed poppet valve operation makes the sleeve valve so attractive.

Fuel Distribution.

With a number of cylinders in line the equal distribution of the fuel between them is a difficulty. Two or more carburetters must be used and the cylinders drawing from each carburetter must be arranged, so far as possible, to do so at equal time intervals. No one arrangement has been standardized as the best: on most 12-cyl. Vee engines one car buretter serves one whole bank of cylinders, but another arrange ment divides the engine into two groups of the front six and rear six cylinders, and each group is served by one carburetter. In at least one design some weight is sacrificed for the sake of good distribution in the provision of a carburetter to every two cylinders.

Air-cooled Engines.

Air cooling, while using an extremely inferior cooling medium, is found possible by the discovery that engines will operate at very high cylinder and valve stem tem peratures. The two main classes of air-cooled engines are the Rotary and the Static Radial types. In each the cylinders are ar ranged radially in one or two planes round the circumference of a circular crank-case. Thus all cylinders are satisfactorily ex posed to the cooling air-stream, but at the cost of presenting a large frontal area. The rotary engine, in which the cylinders and crank-case rotate with the air-screw, besides the fore and aft air-stream, gets the extra cooling due to the rotational speed of the cylinders, but at the expense of considerable waste of power in air friction.

The speed, and therefore the power, of the rotary engine is limited to about 1,200 r.p.m. by the enormous centrifugal forces on the rotating cylinders, and in consequence this type has now become obsolete.

One of the chief attractions of the static radial type is the pos sibility of a very compact and rigid crankcase construction (see Pl. 1.-4). This takes the form, very roughly, of a 'drum, with cylinders fixed radially round its circumference. Only one crank is employed for all the cylinders which lie in one plane; one piston having a connecting rod with a "big-end" as ordinarily understood, while all the other pistons have "articulated" con necting rods, which are attached by pin joints around the cir cumference of the big-end of the one master-rod, as shown in Pl. 111.-3, where the master-rod points vertically upwards. The loading on the one crank-pin is very severe, and special designs of big-end bearings are necessary to stand up to it.

Valve Design.

Valve opening is accomplished by push-rods and rocker-arms which are carried on the cylinder heads; the rods being operated from a cam-ring which is driven at the speed appropriate to the number of cylinders by an epicyclic gear on the main shaft. Since the cylinders of a large, air-cooled, radial engine may undergo an expansion while running of as much as 7o/1 000ths of an inch, which is not shared by the push-rods, very careful design and proportioning of the rocker-arms are necessary if valve operation is not to be upset.

Several of the successful air-cooled radial engines are shown in the illustrations. Rated horse-power of the 22 Wright Cyclone models ranges from 635 to 77o; diameter of all of them is 53ii inches, and dry weight ranges from 947 to 1,050 lbs. The Pratt & Whitney Twin Wasp Junior has an overall diameter of 43i inches; its 14 cylinders have a rated horse-power of 700; bore and stroke are each 56 inches, and the bare weight is 990 pounds with 3:2 gear or 1,009 pounds with 4 :3 gear. The 9-cylinder 600 h.p. Wasp and Boo h.p. Hornet engines have a frontal area of inches and inches, respectively.

Among the more successful radial engines used in England and on the Continent are the Bristol Perseus and Pegasus, the Arm strong Siddeley Tiger, and the Gnome-Rhone. The Perseus and Pegasus motors are both of nine cylinders, the first being rated at 64o h.p. and 77o maximum, and the second rated at 690 and 750 maximum. The Armstrong Siddeley Tiger has 700 h.p. rated and 744 h.p. maximum. Dry weight (bare) is 1,14o lbs.