AERO ENGINES - OTHER PROBLEMS Fuel Economy.—As mentioned earlier, the economical use of all fuel carried in the air is of vital importance. This means the uniform distribution to all cylinders of the minimum fuel required under all conditions of operation. This uniform distribution is a problem of induction manifold design very difficult to solve. A workable method of metering out to each cylinder its proper quantity without adding excessive weight to the engine would con stitute one of the most important advances which at present re main to be achieved.
While the engine designer has been improving the power and efficiency of his magneto and ignition system to suit high-compression engines, great strides have been made by the wireless branch in short wave-length instruments. The magneto and high-tension leads produce waves which upset the sensitive wireless sets, and methods must be devised of so screening off the effects of the ignition system as to allow ad vantage to be taken of the improvements in signalling apparatus. The high-tension leads are therefore enclosed throughout their whole length in a woven metal sheath which is "grounded" to the engine body, and at the same time the plug-heads and contact breaker are enclosed in small metal cowls which are likewise grounded. But the hard-won improvement in ignition efficiency is cut down by the added electrical capacity introduced by the new screening arrangements, the effect of which, and the additional weight involved, are a dead loss to the engine designer.
The functioning of a system of forced lubrication, in which oil is delivered by a pump to all parts of the engine, is very greatly affected by the viscosity of the lubricant. Since viscosity varies widely with temperature some control of oil temperature is necessary. During the cold period at starting, excessive pressures will be developed by the pump; and although these can be dealt with by relief valves, the flow to the bearings will be very small, and a long period of warming up is necessary until the flow is sufficient to allow full loading to be imposed on the bearings. When flying at low altitudes and in warm climates, and more especially with airship engines which are usually not open to any cooling air-stream, it is necessary to provide for cooling of the oil after it has left the engine.
The problem is a peculiar one, for although the amount of heat to be abstracted is comparatively small, the rapid rise of viscosity as the oil is cooled has the effect of producing a thick coating of viscous oil on any surface where cooling is effective. The conductivity of this viscous layer being very bad, the re mainder of the hot oil passes on out of the cooler with a very inadequate reduction of its temperature. The problem is to pre vent the oil, as soon as it is cooled, from remaining in contact with the cooling surfaces and so preventing the cooling of that which comes after. Good results have been obtained with coolers which are light and compact, and which are provided with relief valves to prevent damage from high oil pressures when the engine is started cold.
A problem peculiar to airships is that of compensating for the weight of fuel used during a long flight. In flying to Egypt a large ship would burn some 25 tons of fuel oil, and unless this weight can be made up in some way an equivalent quantity of hydrogen will have to be allowed to escape. An alter native is, while drawing liquid fuel from the tanks, to use less of this and to make up the balance of fuel required by drawing from the gas bags the corresponding weight of hydrogen necessary to maintain balance in regard to buoyancy. Research upon the use as fuel in an engine of a mixture of oil and hydrogen has yielded most promising results and this may prove the most workable solution of the problem.
The alternative of maintaining the dead load on the airship by condensing and storing the water in the exhaust gas from the engines is a matter of considerable difficulty, but it should not prove insuperable. With a water-recovery apparatus capable of cooling the exhaust gas to within i o° C of whatever may be the surrounding atmospheric temperature, and of collecting all the water formed, the possible recovery should be from 0.9 to i•I times the weight of fuel burnt. This apparatus has for some time been working very successfully on big U.S. Naval airships. This is more important when helium is used, owing to the increased cost of that gas over hydrogen.
