EXHAUST-VALVES AND GUIDES FOR AIRCRAFT ENGINES 240033
Trouble with the exhaust-valves of the Type-J air-cooled cylinder caused an investigation to be made of valve-cooling and of valve and guide wear. A temperature of 1300 deg. fahr. invariably caused fractures of the exhaust-valve stem at the junction of the stem and the neck. A file-hard tungsten-steel valve with a shallow hole and no filling eliminated breakage but scaling was apparent. The same valve, using a hard tungsten-steel guide, when tested with salt filling, gave improved cooling; the area of the hot zone was reduced in size and the stem remained dead-black. Scaling was reduced and the wear of the valve-stem and guide that appeared was overcome by substituting a roller tappet for the solid tappet previously used. Tests showed that extreme hardness is of advantage even for inlet-valves.
Experiments with a Type-K air-cooled cylinder gave excellent results with a salt-cooled valve in spite of a very high head-temperature; with an unfilled valve the results were not so good. The tests indicated that cylinder temperature has far less influence on valve-cooling than is commonly supposed and that the major factor is the design of the valve, valve-port, valve-seat, guide and guide-boss.
When the fins had been removed from the head and the barrel of the cylinder and a water-jacket had been welded into place, no difference in performance was observed but the valve-cooling was improved. In the opinion of observers, the cast-iron Type-K cylinder, in air-cooled form with a salt-cooled valve, or in water-cooled form with either type of valve, seemed to be the most effective in cooling the valve.
Inasmuch as the mechanical efficiency of a single-cylinder is lower than that of a multi-cylinder engine, the horsepower and mean-effective-pressure figures used were obtained from those observed by adding the difference between the friction loss of the single-cylinder test-engine and that of a modern first-class aircraft engine.
The considerations that enter into the internal cooling of valves are discussed, including the requirements of a medium for filling the valves that would prove satisfactory. The filling most used is said to be the eutectic mixture of sodium and potassium nitrates, containing 45.5 per cent of sodium nitrate and 54.5 per cent of potassium nitrate by weight.
Although practically all work done with salt-cooled valves up to the present time has been confined to the valve-stem, experiments are now in progress with valves having both the head and the stem filled with salt. If hollow-head valves are to be used, it will be necessary to secure the absolute reliability of any welds that are exposed to explosion pressure. Trouble might also arise from excessive fluid-pressure set up within the head when the engine is started because of the melting and expanding of the filling in the head while that in the stem is still frozen.
Up to the present time, high-tungsten steel, containing from 14 to 18 per cent of tungsten, has been found to be most satisfactory for internally cooled valves, for internal cooling eliminates the scaling and burning that have constituted the chief disadvantages of the use of tungsten steel for exhaust-valves; its principal advantages are hardness and resistance to wear.
Tulip-shaped valves are said to be superior to those of the flat-head or the mushroom type as regards cooling, gas-passing ability, and resistance to warping and stretching.
In service engines, normally about 10 per cent of the stems and guides are found to be scored by the time the first overhauling becomes necessary. Increasing their hardness has served to overcome this difficulty, the use of file-hard tungsten steel guides being probably the best general solution of the problem. The principal arguments against the use of this material are its cost and the difficulty of machining. Case-hardened guides give excellent service when conditions do not cause loss of hardness at the valve-tip end. For air-cooled cylinders, the Engineering Division successfully uses valves of relatively smaller diameter but with considerably higher lift than are current practice in this country.