Aero Engines - Future Developments

AERO ENGINES - FUTURE DEVELOPMENTS Increase of Size v. Increase of Gas Pressures.—Apart from piston area and piston speed the only way of increasing the power of an engine is to increase the mean effective pressure. This can be done either by raising the pressure or lowering the tempera ture of the combustible mixture drawn in. The latter alternative has obviously only limited possibilities, but within these limits the use of fuels with high latent heat of evaporation, which therefore give more cooling of the charge, has been successfully applied in racing motor-car practice and may be also in aircraft engines. • The Two-stroke Cycle.—To design for a working stroke every revolution is not the easy way toward reducing weight per h.p. which at first sight it appears. Getting the charge into the cylinder with only the limited area available for inlet valves is, at high speeds, one of the limiting conditions even when the time of half a revolution is available. If it has to be accomplished in one-third of the time, or less, the problem is proportionately greater. Any lowering of engine speed, if combined with all the extra weight of the necessary compressors for scavenging and delivering the fresh charge, may very soon bring down the extra horse-power per lb. from a two-stroke cycle to a very narrow margin. It is unlikely, moreover, that the same thermal efficiencies will be achieved as with the four-stroke cycle. Nevertheless the two-stroke cycle is a possible line of advance, more especially with the greater perfection of compressors for supercharging and in combination with the Diesel type of fuel injection.

Sleeve Valves.

One of the most difficult aero engine prob lems is the satisfactory operation of poppet valves at high speeds and high temperatures. The drawback to the use of sleeve valves has been, in the past, a tendency toward gas leakage, excessive oil consumption and large mechanical loss due to friction; but with the single-sleeve valve it is possible, by skilled design, to mini mize these troubles, and this type of valve notably in the case of British Perseus has achieved substantial success. The absence of all springs in the first place and of the excessive accelerations which are so detrimental to cams, at once eliminates a constant source of trouble.

The Compound Engine.

Forty per cent or more of the heat energy of the fuel is at present thrown away in the exhaust of a high-duty engine ; where weight is no consideration some of this can be used for raising steam, but such methods of improving efficiency are probably out of the question for aircraft. The question remains, "Can some of this waste heat be saved by expanding the hot gases much farther in a low-pressure cylinder?" The difficulty lies in the transfer of the hot gases at i,000° C or more from the high- to the low-pressure cylinder. So long as this has to be controlled by poppet valves the difficulties are probably insuperable; no valves could be expected to stand up to such treatment, when it is remembered that they would need to be operated from outside, and that therefore there would be con tinuous leakage of hot gas down the valve stems where they slide in the guides. The only hope for the compound engine seems to be through the development of the sleeve valve.

Jet Propulsion.

More than one proposal has been made that the force for propelling an aircraft forward should be obtained by shooting out a jet of air behind, on the principle of the rocket. To obtain efficiency, however, when using this principle, it would be necessary that the forward speed of the aircraft should be of the same order as that of the jet backwards, while at the same time the drag due to air resistance has to be overcome by the reaction between the aircraft and the jet. The two conditions are incompatible at present airplane speeds. An alternative of greater possibilities is to have an air-screw which is rotated by jets at the blade tips and eliminates other driving mechanism, but this could never hope to achieve more than a very poor fuel economy when compared with a present-day heat engine. This has been proposed many times, both in water and air craft, but all tests have been so extremely inefficient as to border on the absurd.

(D. R. P.; X.) The Diesel Engine.—Attempts made to solve the problems of the aircraft Diesel engine by refinements in design were at first disappointing, but in the recent years have been meeting with success, particularly in Europe. The two-cycle water cooled en gine is somewhat better due to recent advances in super-charge design, this showing 2•08 pounds to the h.p., and weight of the air-cooled Diesel engine is lower by reason of a radical crank case and cylinder construction, and changes in air intake pas sages to increase the air charge and greatly augment the turbu lence. Both water and air-cooled Diesels have passed the rigid Government tests, and are in commercial production. One engine, developing 500 h.p. normally has been reported attaining Soo h.p. with super-charging.

In the United States, a Packard Diesel engine powered a Bel lanca high-wing monoplane that established a non-refueling en durance record, but development of this engine was curtailed after the death of Captain Woolson, generally credited with being the moving figure behind it. In England, however, successful Diesels have been built by such well-known firms as Beardmore, Rolls Royce and Bristol, and, of course, Napier. Beardmore built the Tornado, an eight in-line rated at 585 h.p. in 1929, and it was successfully tested in the British airship, "R-1 1." On the Con tinent, Diesels have been developed by the Italian Fiat and Isotta-Fraschini, Germany has been prominently successful with compression-ignition designs, and at least six French manufac turers have been engaged with aircraft Diesel development. In France, M. Clerget has been occupied with a two-row fourteen cylinder design which is said to deliver Soo h.p. normally and Boo h.p. when fitted with a Gnome-Rhone supercharged. Interest has also been centred on the Salmson-Szydlowski engine, a two-cycle design with U-type cylinders, there being nine pairs of water cooled cylinders in radial arrangement. The engine is said to de liver 600 h.p. at 1600 r.p.m. for a specific weight of 2.08 pounds per h.p.

The lower cylinder temperatures of the Diesel as compared with the petrol engine allow of the single-sleeve valve being suc cessfully employed, which, combined with the high supercharging pressure, introduces another very important factor—turbulence. This turbulence can be regulated and brought to an extremely high velocity, which is probably the most significant of all the gains. Not only is this velocity high at the moment of fuel injec tion at the end of the compression stroke, but it is found to per sist throughout the remaining three strokes of the compound cycle. This has a three-fold significance. First, it allows the fuel to commingle with and finally reach a greater percentage of oxygen than has ever before been possible ; secondly, it allows this process to go forward at very much higher speeds; thirdly, it ensures completeness of combustion, especially that fraction known as after-burning, which here takes place under conditions of extreme turbulence.

It is found that such an engine as this turns out to be + two cycle with very much higher net mean effectives to the crank for all power pistons than in two-cycle practice. The of the engine which remains four-cycle is performing an extremely useful function in producing the high velocity turbulence above referred to. In such an engine it is found that the negative work is only I : 42, accounting for the very high mechanical efficiency observed in this cycle.

The great advantages of the Diesel cycle in addition to those already referred to are : first, there is no difficulty in the distribu tion of fueling between the cylinders; secondly, the low fuel demand is extremely important in increasing the radius of flight or the pay load in commercial aircraft ; thirdly, the complete elimination of all fire risk inasmuch as the fuel oil is not volatile. It is practically impossible to ignite it by any ordinary method and it will actually extinguish a small bonfire when dashed upon it. Besides this, it costs between and 5 less and develops as much as 35% more B.Th.U.'s than "aviation gas," volume for volume. Fourthly, it does away with all of the electrical ignition complications and troubles, including spark plugs, together with the serious interference with wireless and radio reception.

Engines, How and Where Used.

In the more spectacular accomplishments in aviation, the high powered radial and in-line engines naturally are in prominence, and although there are many types of motors of moderate and small horsepower serving a wide variety of tasks the world over, some of the foremost applications of the higher powered types will indicate how and where many aero engines are used.

The air-cooled radial engine leads in production in the United States where it has found great favour. It is in use on practically every commercial craft, in the majority of service planes and has been used by almost all of the recent American record holders. The U. S. transcontinental speed record has consistently been broken by radial powered craft; Wiley Post had a radial in the plane in which he twice circumnavigated the world ; Lincoln Ells worth flew across Antarctica with a radial engine. The Curtiss Conqueror V-in-line type has been used effectively in the United States, although chiefly in service craft, the other American in line models being of much lower horsepower and used only on lighter planes.

In Great Britain, particularly with the army and navy, the V-in line liquid-cooled engine is in very wide use, the Rolls Royce, for instance,. being under considerable restrictions for government use. There are many air-cooled in-line power plants in British commer cial and private craft, although the application of this type seems to be equalled by the number of radial models in use. British air cooled in-line engines drove Scott and Black to victory in the famous MacRobertson London-Melbourne race in radials were used by Lord Clydesdale and Flight Lieutenant McIntyre in the first flight over Mount Everest in 1933.

The world's maximum speed record has consistently gone to planes almost of necessity powered with in-line engines because of the need for tremendous horsepower with a minimum of frontal area or resistance. The plane used by Francesco Agello of Italy in setting the speed record at 44o m.p.h. was almost a flying mo tor, long lines of cylinders banked as a "V" extending aft of a small pointed propeller hub.

Maurice Rossi and Paul Codos, French pilots, used an in-line engine to set the world's non-stop distance record at 5,657 miles, although two previous American holders of this record had used a radial. Commander Renato Donati of Italy, setting the altitude mark at 47,352 feet, used an Italian plane powered with a British radial.