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Aviation History
1918
1918 - 0401.PDF
APML II, 1918. Valves.—The two sketches shown below show the form of two average valves. It will be seen that dotted lines have been drawn, cutting off a part of the valve, these lines show the part of the valve to be cut away, leaving the seating- only us in. in width. The corners (shown at " B ") should be removed in the lathe, leaving a rounded B B Fig. 2. edge, which eases the ingress or egress of the gases. The reduction in the width of the seating lessens the effective " life " of the seating, but improves its quality; the attendant lightening of the valve is all to the good, reducing the load on the cam- shaft. We will not go into the question of the inertia of the gases in the inlet pipe and valve ports or passages, important though it be, except to remark that these gases have to make their entry into the cylinder at a very high velocity, and on this account, it is of the highest importance that its passage should be as free as possible, which is ensured by making the inside surfaces of the inlet manifold and ports as smooth as possible. All sharp edges must be rounded off and the corners radiused, as they would other- wise tend to set up eddies in the gas flow. As regards the combustion chamber, this should be made as smooth as its accessibility will allow, the smooth surface then being wiped with oil which has a big percentage of defloculated graphite mixed into it. The excellent properties of this form of graphite are not nearly so widely known as its qualities warrant, and a few words regarding it will not be misplaced. Graphite, when defloculated, while not becoming soluble in oil, is capable of being held in suspension in that medium, being split up into particles of in- finitesimally small size. These particles form a practically homogeneous "face" on the cylinder walls, or any other parts which slide or are subject to frictional contact. If the reader should happen to have an engine which is inclined to be heavy on oil consumption, an admixture of this graphite will speedily improve it. To revert to the matter in hand, the reader will have gathered, ere this, that the smallest points are not to be overlooked in getting every ounce of power, which can be got without going to extremes, and one of the " small points " is the fit of the pistons in their respective cylinders. Few people realise what an extremely delicate thing the average piston of an aero engine may be. A very slight tap may distort a piston to an extraordinary degree. Should the reader doubt this, let him obtain the piston of an Austro-Daimler engine (or Beard- more) and carefully take the diameter of it. Then let him administer what he thinks is " a slight tap," and then check the diameter again ! A series of " taps " at different points will show that the piston can be " tapped " into and out of shape in a really astonishing manner. To return to the fit of the pistons, let the cylinder and piston, be thoroughly cleaned and wiped dry, then apply some " rubbing " to the cylinder walls, taking care to only apply it very lightly; lamp black (not red lead) should be used, the latter being too coarse, and hence not readily applicable when fine results are needed. The piston will then show if there are any " high places." These can be removed by scraping, or should the engine be one employing pistons similar to the " Beard- more," the aforementioned light tapping will often correct any slight distortion. Lapping the piston and rings into the cylinder with pumice powder may give the final finish. The question of compression is approached with certain misgivings, and yet it cannot be omitted. Needless to say, "good" com- pression is a sine qua non, but the writer cannot advocate increasing the compression beyond that point, which can be gained by avoiding the loss of the compression which the designers and makers between them 'may have settled upon. In one instance, the writer deliberately increased the com- pression of an old engine, which had seen many hundreds of hours in the air on a school machine, and when this long-suffering engine had been sub- jected to many of the " hints and tips " here retailed, it gave surprisingly good results, although it then ran at about 400 to 450 r.p.m. more than it had been allowed to by the makers. While on this subject, it may be well to mention that an " Indica- tor " should be used to test the compression. There are several of these on the market, which, if care- fully used, will give accurate results. It is of great importance that the compression in each cylinder should be equal, to ensure even and " sweet " run- ning, lack of vibration and so forth. AH exhaust and inlet springs should be tested to ensure against loss of elasticity. A certain weight should depress the spring a certain known distance. A small device may easily be constructed for gauging the tension of the spring, as shown in the sketch (Fig. 3). One cannot lay down any directions for fitting bearings, as this depends entirely on the skill of the re. I a x a a, lomprjisco. I\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\N Fig. 3. fitter who does the job, and there is no doubt that some of the finest engine fitters are to be found in the ranks of the Flying Services. It is only necessary to point out that the crankshaft bearings must be perfectly fitted, ensuring both absence of vibration and correct lubrication. It is almost superfluous to say that big ends and small ends must also be perfect. Carburaiion.—As these notes are not directly applicable to any one make, type or power of engine, no very definite instructions can be laid down, but the following notes may not be out of place. In 397
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