Ignition Devices. 247 shows across section of the ignition device used on one engine long on the market. In this the cylinder head is a plain water-cooled casting of a design free from danger of heat fracture. To this head is bolted the bulb A projecting outward, and against this bulb is placed a "spoon," or lip, B, which passes through an opening in the head and projects into the cylinder cavity (in a somewhat similar design the spoon is integral with the bulb). There is also a heavy block or combustion chamber C fitting into the cylinder space. In starting, the bulb is first heated to a high temperature (not red-hot) by a torch. As soon as the engine is started, either by cranking or by using an air starter, the fuel pump, actuated by a cam, injects a charge of oil through the nozzle D immediately above the hot spoon. This oil, on striking the spoon, is "cracked" and vaporized and, mixing with the air charge, is ignited by the temperature of the compressed air charge.
Since this engine operates on the two-stroke-cycle, using crankcase compression, the air charge, as it blows through the air ports, is unable to completely scavenge the cylinder of all the burnt gases. This is traceable to two defects that are in herent in all two-stroke-cycle engines. One of these defects is the volumetric efficiency of the crankcase air compressor. Due to the volume of the enclosed crankcase in respect to the displacement of the piston, the air actually passing into the cylinder through the air ports is not equal to the cylinder volume by any means; 60 per cent. is as much as can be expected. The second defect lies in the inability of the air to force the exhaust gases out through the exhaust ports without mingling with these gases and partially escaping into the exhaust pipe. Complete scavenging of the cylinder is highly desirable in order that suffi cient oxygen be supplied to unite with the oil. On the other hand, scavenging of the hot bulb is far from being desirable; in fact, the entire scheme of operation is based on the bulb being charged with burnt and inert gases at the moment the fuel charge is injected onto the "spoon." Consequently, when the fuel is vaporized by the hot spoon, the movement of the air charge in front of the advancing piston pushes this vapor into the bulb where it mixes with the inert gases. As there is no free oxygen present in the bulb, there can be no explosion, although the temperature, due to the hot bulb and to the compression pressure, is much above the ingition point.
As the piston advances toward the head, the air is forced into the bulb, where it mixes with the oil vapors and burns. This stratification of the oil vapors, the burnt gases and air is fairly successful at loads up to about three-quarters of the engine's rating. At values approaching full load, the beginning of the injection of oil is much advanced, and the amount of fuel in jected is increased to such a value that the bulb does not accom modate all the vapors. As a result part of the fuel charge mixes with the pure air in the cylinder. As the temperature existing in the cylinder, due to the hot bulb and to the compres sion, is above the ignition point of the "cracked" light oils, preignition of the charge frequently takes place, resulting in piston pounding and loss of power. In order to avoid the pre ignition, this engine, as do most others, makes use of water in jection, whereby at each cycle a small quantity of water is injected into the cylinder, reducing the compression pressure and the temperature existing in the cylinder. Because of the
successful stratification of the gases on loads below three-quarters rating, it is not necessary to use water at lower load values.
The conbustion block C merely serves as a reservoir of heat to assist in the vaporizing of the fuel. When using kerosene or the lighter distillates, this block gives off enough heat to vap orize the oil before it strikes the spoon; with heavy fuels, below 30° Baume test, most of the charge actually strikes the spoon before "cracking" and vaporizing.
Oils of different gravities will ignite at different temperatures and at different pressures. In order to make the time of igni tion constant, it is necessary to vary the compression pressure according to the character of fuel oil used. To do this, the Muncie Engine Co. has adopted the plan of inserting steel com pression plates behind the combustion block. By varying the number and thickness of the plates used, it is possible to vary the compression from 90 to 140 pounds; the former suitable for kerosene, the latter for the heavy fuel oils.
The hot bulb has a number of advantages. Among these is the fact that, being made with fairly thin walls, the bulb will break in case of any dangerous increase of pressure in the cylin der, thereby protecting the more expensive parts, such as the cylinder and the cylinder head. It is inexpensive in replacement cost, quite unlike several of the more complicated devices. It has another advantage that is of material assistance in operation. The bulb is of sufficient size to contain in its walls a great amount of heat. The heavier the load the greater is the amount of heat absorbed by the bulb. This increased heat serves to raise the compression temperature to a higher degree than usual. The oil burns earlier in the cycle, and by suitable adjustment of the water injection the engine can be made to fire just before dead-center, giving the maximum of power. The bulb is open to objection because of its tendency to fill with unburnt carbon in cases of leaky fuel injection nozzles or heavy overloads. The same de posits will occur where leaky piston rings allow the compression to escape or where the bulb is too low in temperature. This latter may be due to cold air currents striking the bulb or to the cooling water in the cylinder head having too low a discharge temperature. Carbon deposits in the bulb are invariably accom panied by decrease of power. The carbon frequently is the cause of preignitions. Carbon is not as good a conductor of heat as are the iron walls of the bulb; consequently the carbon will become red-hot. This, of course, raises the cylinder temperature suffi ciently to ignite the oil practically at the instant, of its injection. In cases where the engine preignites even with the normal supply of injection water, the engineer is practically safe in assuming that the bulb is full of carbon. When the bulb is badly car bonized, the easiest way to cleanse it is by soaking it for several days in a bucket of lye, following up with a thorough washing in kerosene. It pays to keep at least two extra bulbs on hand.