The hot-plate design is good in many respects. It is better than the hot tube when using heavy fuel oil since it provides more vaporizing surface. It is not liable to become overheated or burned since it loses part of its heat to the exhaust. Further more, as the air enters the ports it blows across the plate, cooling it to a considerable extent. This is an advantage at full loads, but on lighter loads this cooling effect quite often results in the plate not igniting the fuel charge. It is doubtful if the hot plate helps the lubrication of the cylinder; certainly, many engineers are in constant fear of faulty lubrication due to the high tempera ture of the plate burning the lubricating oil off the cylinder walls on the compression stroke. There is another little trouble that must be guarded against. The plate may fracture, and the broken part's, becoming wedged in the exhaust' ports, will ruin the piston. This probably happens but seldom; nevertheless, it is worth the attention of the careful engineer. If a battering sound is heard in the cylinder, it is advisable to stop the engine and see if the plate is intact.
Figure 252 shows a cross-section of the Chicago Pnuematic Tool Co.'s "Little Giant, " an engine making use of a modification of this principle.
Separate Combustion attain fair stratification or separation of the oil vapor and air charges, the ignition device shown in Fig. 250 has been brought out. This combustion chamber, in some respects, is a reversion to the original Hornsby Akroyd design. With this construction a cavity is cast in the head with the open end outward, while a small opening acts as a port of communication between the cavity and the engine cylin der. To complete the combustion chamber a semi-spherical casting is held against the combustion chamber by means of a retaining collar, not shown. The cylinder head is completely water-cooled, consequently the half of the combustion chamber formed in the head is also water-cooled; the combustion chamber cap is partially water-cooled by means of a cored passage around its juncture with the cylinder head. The oil is sprayed in at the side of the chamber.
To start the engine, the starting tube A is first heated until it is red-hot. The fuel is then injected by means of a hand lever operating the fuel pump. It is usually necessary to give the pump several strokes. Then the engine is pulled back against the compression until the piston almost reaches dead-center, whereupon the fuel explodes and the engine turns over. In cases of the larger size engines an air starter is used; it is neces sary to turn the engine over two or three times with the air starter before the fuel will ignite. After the engine has run a few
minutes, the vaporizer or combustion chamber absorbs enough heat to fire the fuel without the assistance of the torch.
The scavenging air frees the cylinder of at least 75 per cent. of the exhaust gases. Owing to the restricted opening into the combustion chamber, it remains filled with, inert gases during the compression stroke. The fuel is injected directly into the combustion' chamber, when the piston is from 20 to 40 degrees from dead-center and the temperature of the uncooled chamber cap is sufficient to vaporize it. When the piston approaches the head, the air is forced into the chamber and, uniting with the oil vapor, causes combustion. The piston leaves but a slight clear ance in the cylinder, and all the combustion occurs in the cham ber; the hot gases, passing through the narrow opening, act on the piston face. This design is one of the best used. The fuel being burnt outside the cylinder, there is but little danger of unburnt carbon deposits cutting the cylinder walls. As a result, the amount of lubrication necessary is considerably less than in the case of a hot-tube or hot-plate engine. Since the combustion chamber is filled with burnt gases when the fresh oil charge is injected, there can be no ignition until the piston forces the pure air charge into the combustion chamber. Having the advantage of being partly water-cooled, the combustion chamber seldom becomes overheated. However, on full loads, the amount of heat absorbed by the walls is so considerable as to cause the cap to get cherry red. To reduce this temperature the cap, as already mentioned, has a small cooling space, and the operator must open the cooling-water connection. This sudden chilling quite often causes the cap to break; in fact, this is the one serious drawback to this design of combustion chamber. Where fuel oil of 30° Baume or lower is used, it is absolutely neces sary that this cap be allowed to bcome cherry red in order that all this heavy oil be vaporized. With the light distillates the oil will preignite if the cap gets too hot. If it is cherry red, the oil vapors will absorb so much heat that their volume will exceed that of the chamber, and some of the vapor will enter the cylinder and, uniting with the air, explode. Each engine has certain characteristics of its own; the engineer must experiment and ascertain under what temperature condition his engine best handles the fuel used.