Page:EB1922 - Volume 30.djvu/70

40 FIG. 22.

adapted for water-cooling, engines of this type have been built and operated successfully. Among these is the recent 300 H.P. 9- cylinder Fiat, weighing 1-7 Ib. per H.P. The difficulty of arranging the water circulation so as to avoid all danger of air locks in the inverted cylinders is, however, appreciable, and the head resistance of the completed engine is large. For these reasons there is not likely to be any great future for the water-cooled radial engine on aeroplanes of present types.

Installations of Air-Cooled Engines. Some form of cowling is needed to distribute the air evenly over the various cylinders, and the success of a Vee engine depends largely on the cowling, whereas even air-cooling is more easily obtained on a " radial."

With rotary engines the cooling is not as good as might be expected from the high peripheral velocity, and the windage losses, even with a cowling, amount to some 10% of the total power developed.

In these engines the air-petrol mixture is led through the hollow crank-shaft to the crank-case. In the original Gnome engine auto- matic inlet valves fixed in the piston heads and opened by the suction on the inlet stroke admitted the charge. These valves were light, often broke, and were inaccessible.

In the Monosoupape Gnome the valve in the piston is eliminated and a mechanically operated valve in the cylinder head is used. This serves as an exhaust valve, but, instead of closing at the end of the exhaust stroke, it remains open for a part of the inlet stroke and then admits air to the cylinder. When it closes, the further motion of the piston produces a partial vacuum in the cylinder, until, near the end of its stroke, the piston uncovers a ring of openings in the cylinder walls communicating with the crank-case. The fuel jet is adjusted to give a mixture too rich to be explosive, and this mixture enters into the cylinders and mixes with the air admitted through the inlet valve to form an explosive charge.

Other modern rotary engines have mechanically operated inlet and exhaust vajves, with which efficient valve timing becomes possible. The mixture in the crank-case then passes into a circular box fixed to the rear of the crank-case and rotating with it, whence it is led by inlet pipes to the cylinders in the ordinary way.

These methods of mixture supply, though crude, gave the rotary engine the advantage of having a fuel supply adjustable by hand to suit the air density when flight at great heights first became impor- tant. On the other hand, the non-rotary engines, fitted with normal carburetters, received a mixture too rich for efficient operation at considerable heights. To obviate this, automatic carburetter con- trols had to be devised, but pending this the rotary engine had a distinct advantage for high Hying.

The lubrication of the rotary engine is peculiar to the type. All oil in the crank-case is thrown centrifugally into the cylinders, and once there cannot be drained out, cooled, and circulated again as in fixed-cylinder engines, but must be discharged through the exhaust valves. Consequently the oil consumption is high. Moreover the lubricating oil must be insoluble in petrol, so that castor oil is necessary.

The power of the rotary engine falls off more rapidly with height than that of the fixed-cylinder engine if the latter has a suitably controlled carburetter, and at a height of 15,000 ft. the difference in horse-power is about 10 per cent.

The Differential Engine. For large powers, each of the two types of radial engine has its own peculiar limitations. In the fixed radial the fly-wheel effect is small, while it becomes difficult to design an engine exceeding about 400 H.P. on a single crank because of the excessive load on the big-end of the connecting-rod. In the rotary radial this difficulty is less, but windage losses, centrifugal stresses, gyroscopic effects and valve-gear difficulties are encountered.

The "differential" engine has been proposed to combine some of the advantages of each type. Here the cylinder ring rotates in one direction and the crank-shaft in the opposite direction at the same speed. In this way the big-end loading may be kept within reasonable limits; the gyroscopic effect is negligible ; centrifugal forces and wind- age losses are comparatively small ; and the speed of rotation is low enough to permit an efficient airscrew to be fitted.

If the big-end loading be taken as the criterion, the power of the differential engine is about 30% greater than that of the fixed radial engine, or, deducting the windage loss, about 26 per cent. Whether this advantage outweighs the complication in design, remains to be proved.

Cycles of Operation. All aero engines are of the single-acting type in which driving impulses are received on one side only of the piston, and in the majority of engines the four-stroke cycle is adopted. The two-stroke cycle has not hitherto been adapted successfully to the aero engine, owing to its comparative inefficiency in a high-speed engine which requires to operate over a wide range of speeds.

A six-stroke cycle is in the experimental stage. It consists of the four-stroke cycle with the addition of a suction and compression stroke. The first suction stroke draws in a charge which is com- pressed into an auxiliary reservoir on the succeeding stroke. The next stroke is also a suction stroke which draws in another fresh charge. At the end of this stroke a valve opens and admits to the cylinder the charge compressed during the preceding stroke, and during the succeeding stroke both charges are compressed into the clearance space and fired. In this way a charge of double weight is obtained and the mean effective pressure during the expansion stroke is twice as great as in the four-stroke cycle. The mean effective pressure over the whole six strokes of the cycle is thus 33 % greater than the mean effective pressure over the whole four strokes of the ordinary cycle. Since the explosion pressures are approximately twice as great as in the four-stroke cycle the cylinder construction is heavier.

For evenness of turning moment, the two-stroke is better than the four-stroke, and this than the six-stroke cycle.

In each of these cycles the mixture is drawn into the cylinder, compressed, burnt at constant volume, and expanded to the same volume as before compression. The theoretical efficiency of this cycle

is given by the expression I ( Jy l where r is the ratio of the

volumes before and after compression and y is the ratio of the specific heats of the working fluid at constant pressure and constant volume. This is known as the air standard efficiency. It assumes that the specific heat is constant at all temperatures, and that there is no loss of heat to the walls of the cylinder, in which case the value of y is 1-408.

Taking into account the variation of specific heat with tempera- ture, the appropriate value of y in this expression becomes 1-295 and except for losses of heat to the cylinder walls and piston, the efficiency of an aero engine should attain the values corresponding to its compression ratio, which are:

Compression ratio

4-0

4-5

5-o

5-5

6-0

6-5

7-0

7-5

8-0

,-,-(!),-.

336

359

378

396

411

424

437

449

460

These figures indicate the importance of a high compression ratio. This is particularly important in the case of an aero engine, since the drop in power with height diminishes as the compression ratio is increased.

A limit is, however, set to the compression ratio in practice by the tendency of a petrol-air mixture to detonate when compressed to a high pressure and temperature. Such a mixture has a " spontaneous ignition " temperature corresponding to any definite pressure, at which it will detonate, and should this combination of temperature and pressure be attained in operation it is apt to cause overheating of
 * he sparking plugs and to lead to general overheating of the cylinder

and ultimately to pre-ignition.

The tendency to detonation depends largely on the design of the combustion chamber. It is less where this is compact and symmetri- cal than where it contains pockets as in a cylinder of the L-headed
 * ype. It also depends appreciably on the position of the sparking

Dlugs, and on the composition of the fuel. The addition of benzol or Denzene to petrol enables a higher compression ratio to be used, but owing to the comparatively high freezing-point of benzol, not more
 * han about 25 % can be used in admixture with petrol, for use at

reat heights.

By attention to design it is now found possible to use c6mpression ratios as high as 5-5 when using petrol as a fuel, and as high as 6-5 when using petrol-benzol mixture. With such compression ratios,