Since the combustion chamber is cooled by the same water that flows around the cylinder walls; there is no way of varying the cooling effect on the head in accordance with the load carried. In a few installations engineers have made the combustion cham ber cooling system separate from the jacket, with very desirable results.
The casting is rather complicated, and, as a result, there are severe shrinkage stresses existing in the head; these strains, augmented by the explosion stresses, frequently cause fractures. Some cases of fractured heads are directly traceable to the engi neer's negligence; he starts his engine without first having the cooling water running. Afterward, when this neglect is noticed, the sudden impinging of the cold water on the hot sides of the head breaks it.
Figure 254 outlines an ignition bowl employed on the Buckeye Barrett engine that is in principle the same as Fig. 250. In this case the interior of the bowl is spherical shaped, with the upper part cut away, affording an entrance to the cylinder. This casting is cylindrical in outer form and fits into the cylinder head, which is entirely water-cooled as is also the cylinder head cap. The bowl is heated by a starting torch, blowing through the passage A. Since it has no starting tube, more time is re quired to start as the bowl heats slowly. The design contem plates the isolation of the vaporized fuel charge in the bowl until dead-center has been reached, when the air charge entering the bowl furnishes the necessary oxygen for combustion.
Operation.—In starting ap engine using any of these devices, the operator should not pump too much oil in by hand before starting. There are two objections to this all too common prac tice. First, the amount of oil vaporized will be so great as to fill part of the cylinder volume, and the extra quantity of gas to be compressed will raise the compression pressure high enough to explode the charge early in the compression stroke. The engine will then run backward until stopped by the explosion on the next stroke. This is not advisable, especially when pulling a direct-current dynamo. The second objection lies in the heavy carbon deposits that this excess fuel charge causes. Experiment will soon reveal just how much of an initial oil charge must be injected.
Figure 255, cross-section_of the Bessemer oil engine, illustrates an ignition bowl that operates on the same general principles as does the above. In this case, however, a starting tube, which is cast on the bowl, is used.
Engine Ignition few builders have adopted the four-stroke-cycle in their low-pressure engine.
The only prominent American firm doing so has adopted the ignition device illustrated in Fig. 256. In this design the cylinder head is water-cooled and has embodied in it the engine combus tion chamber. As the drawing shows, the admission and exhaust valves are in the top of this combustion chamber. The lower part of the chamber is formed by a separate uncooled cup. This cup or cover is heated by a torch, and the vaporization and igni tion of the fuel proceed in a manner quite like that in the design of the two-cycle engine, Fig. 250.
The four-cycle design insures a cylinder charge of pure air as the exhaust stroke of the piston dispels all the burnt gases. The scavenging is usually more than is desired. In many cases the inertia of the moving gases tends to clear the combustion cham ber of the burnt charge. This fills with fresh air on the suction stroke, and the oil, then, is injected into a mass of pure air. The proneness toward preignition is present even on loads below engine rating. The intense heat radiated from the hot cup is partially absorbed by the overhead valves. The engineer should examine the valves periodically and regrind as often as necessary. In case of loss of power it is usually safe to figure that the valves are leaking. If the water contains much mineral matter, a solu tion of muriatic acid should be used occasionally. Due to the form of the cooling space, the sediment is liable to deposit around the port into the cylinder. This is a dangerous condition, for fire cracks will develop, resulting in leakage.
General.—From the.foregoing it is apparent that there are a number of different cylinder head designs, although all these igni tion devices are based on the process of ifaporization and ignition, by temperature, of the fuel charge.
The first and all-important factor in successful operation of any of these hot-igniter engines is to keep the ignition device clean. The operator should have an extra part on hand and, when cleaning one, should use the spare part. Soaking the carbonized igniter in lye for a few days and ending up with a thorough washing in kerosene will clean it very satisfactorily. If the engine tends to preignite on ordinary loads, inserting copper gaskets between the head and bulb, or the head and cylinder casting, thereby increasing the clearance volume, will probably relieve this. If the engine, on starting, blows through the joint between the head and bulb, too much pressure should not be used in tightening up the studs. The engineer should bear in mind that, as soon as the engine warms up, this leakage will cease.