In many engines the air valve is noisy in action. This, ordinarily, is due to its having too great a lift. The remedy is to cut off part of the valve guard, thereby reducing the valve lift. In those engines where the air valve is a brass poppet with a rubber ring on the valve seat and a spring to cause the valve to seat quickly, the noise is usually not great. In cases where it is objectionable, it is better to increase the thickness of the rubber ring, thus decreasing the valve lift, rather than to change the spring tension. If the springs are weakened, the time interval of closing is increased, thus reducing the compression efficiency by allowing the air column to reverse and flow back out the suc tion. Muncie Oil Engine Co. makes use of a valve of this design.
Some of the inclosed engines are three-ported instead of two ported. This 'design eliminates the air suction valve, and the piston, by uncovering a port, allows the air to flow into the crank case. The one objection that has been advanced against the three-ported engine is the lower volumetric efficiency of the com pressor. The piston must move a considerable distance on the return stroke before the suction air port is closed, lowering the volumetric efficiency by the percentage of the total stroke the piston moves before covering the port. Figure 278 is a cross-section of the Mietz and Weiss engine which uses the three-ported design.
It is well to remember that it is impossible to make the air suction noiseless as long as there is no muffler on the air suction line. Some engine designs include a muffling effect by drawing the air through the hollow engine sub-base. In most of these cases the air passes across the top of the foundation and below the oil pan. There is a tendency in such a construction for the current of air to pick up any particles of cement that are on the surface of the concrete foundation. To safeguard this, on erec tion of an engine a couple of gallons of linseed oil should be poured on the foundation surface beneath the engine. This makes a dust-proof seal that will eliminate all cement, dust or grit.
Some engines make use of a piston air valve, while others have adopted a rotating valve of the so-called " Corliss" type. These valves usually last for a number of years before replacement. The objectionable feature is the wear on the valve cage or cavity. When the valve is replaced, it is imperative that the valve cavity be rebored if it is not of the renewable type. These two designs of valves require lubrication, or they cut quickly. They elimi
nate all breathing noise, and this is an attractive feature in an engine that is located in a congested district.
Figure 255 shows the location of the Corliss air valve on the Bes semer Oil Engine under the cylinder casting; the vale is driven by an eccentric from the crankshaft. Figure 279 shows the details of the same valve. This is made of a valve bushing; this reduces replacement cost in case of wear. In order to secure the proper amount of air the eccentric must not be allowed to slip. In one plant the engine refused to run, despite the effort of the engineer and an engine expert. By accident, after both had given up in disgust, it was discovered that the eccentric had shifted, prevent ing the air valve from opening until late in the air compression stroke. Various adjustments of the timing of this air valve have been tried out. The builders recommend that the valve be set to open when the crank is 40 degrees past the front, or outstroke, dead-center, and to close when the crank is 30 degrees past rear, or the cylinder head, dead-center. This gives ample time for the complete filling of the air cavity. There is no reason why the valve should not open when the crank is 30 degrees instead of 40 degrees past dead-center. The column of air rushing into the cyl inder has sufficient inertia to prevent any backward flow when the suction valve is opened while the discharge port is still un covered. This will allow the valve to close earlier, preventing air loss_through the suction valve. This will increase the volu metric efficiency.
Adjustments.—Air valves, of all types, require Teplacement when the wear allows the air to leak through. This air leakage reduces the amount of scavenging air and its pressure, which is seldom over 4 pounds gage. ' If the pressure is lowered but slightly below .its normal value, the air will be insufficient to completely scavenge the cylinder of exhaust gases. This air valve leakage invariably causes loss of power, increased fuel con sumption and a smoky exhaust.
Crankshaft Air Seal.—Any inclosed crankcase engine must have some means of preventing the air from blowing out along the shaft. Some builders make use of a stuffing-box arrange ment at the outer end of the main bearings, Fig. 280. With this design the bearing cap must make an air-tight joint with the lower part of the bearing or housing. It is better, in case of re newing the shims, to use either a paper or a thin rubber gasket. It is well-nigh impossible to secure air-tightness when using copper or tin shims.