The hot bulb or other ignition device may be too cold, thus failing to vaporize all the oil, which then exhausts while in a liquid Or at least a saturated condition. The cold bulb may be due to two causes—the bulb may be too large, thereby exposing too great an exterior surface to the cooling action of the air; or carbon deposits may cause the interior to be closed, shutting off the absorption of heat. If the device be water-cooled, the jacket may be absorbing too much of the heat from the combustion chamber. In those engines using a hot bolt, the bolt may be burned or oxidized to such an extent that there is only a small amount of metal left in the bolt to absorb the heat of combus tion. The proper adjustments to relieve these various defects are apparent.
Frequently an over-abundant supply • of injection water lowers the temperature of the entire cylinder and ignition device, preventing proper vaporization. The heavier oil particles, even though partially gasified, will then blow out, giving a decidedly dark hue to the exhaust, unless an unusually good exhaust pit is used to trap the free oil particles. This condition is always accompanied by the loss of power in the engine. The water should always be reduced in quantity until the engine begins to preignite; the water supply should then be increased a slight amount—sufficient to destroy the preignition sound.
On the other hand, too little injection water may produce a smoky exhaust. This applies where a heavy fuel is used. The water injected in the cylinder seems to assist the heavier particles in burning, even though these heavier parts do not vaporize. If the water supply is insufficient, these heavier particles of oil blow out in a liquid state.
If a dirty fuel oil be used, the injection nozzle check valve will probably cut, thereby imperfectly sealing the nozzle. The oil which drips into the cylinder during the latter part of the power stroke does not burn. This, of course, will cause a dark exhaust.
Another very frequent cause of a smoky exhaust lies in the use of entirely too much lubricating oil in the cylinder. If more of this oil is supplied to the cylinder walls than the gas flame will completely burn, the unburned part will settle in the exhaust. ports and pipe. The exhaust gases, as they pass through the exhaust line, pick up this oil and blow it out the discharge.
Low-compression pressure is largely responsible for an objec tionable color to the exhaust. If the compression leaks past the piston, the pressure and temperature in the cylinder and bulb will not be sufficient to vaporize and ignite the fuel. It follows that at least the heavier ftiel particles will blow out the exhaust in an unconsumed state.
Preignitions.—If the ignition device is at too high a tempera ture, the vaporized fuel will mix with the air charge early in the compression stroke. The natural result of the mixing, already discussed in a previous chapter, is premature combustion before the piston reaches dead-center. Much depends on the character of the oil. If it vaporizes rapidly, the preignitions will continue. If water injection is not used, or if the bulb is not equipped with some form of a cooling jacket, which may be. provided with a valve to control the bulb temperature, it is necessary to alter the size of the hot ball. If the bulb be made larger, it will radiate heat faster, thus maintaining itself at a lower temperature. It is a wise precaution for the engineer to keep two or three bulbs or tubes of different sizes on hand so that this change can be made without any delay. Another remedy is the adjustment of the fuel injection timing, making it occur later in the compression stroke.
If the load be around the engine's full rating, and a light fuel is used, the vaporized charge has too large a volume to be con tained in the combustion chamber. Expanding into the cylinder proper, it mixes with the air and ignites. If the fuel is of light gravity, of course it will ignite earlier in the compression stroke. This tendency to preignite at full load is more evident in "dry" engines than in those making use of water injection.
Where the injection nozzle leaks, this oil drip, which does not blow out the exhaust, is trapped in the cylinder. Even though the cylinder temperature is fairly low, the interval of time, during which the piston is compressing the air charge, is ample to allow this fuel to vaporize and ignite very early in the stroke.
If the fuel used possesses a low flash point, as in case of kero sene, the tendency of the oil to ignite prematurely is always present. A more liberal supply of injection water will cool the hot device and cylinder, thereby reducing the preignition.
Strange as it may seem, a cold hot bulb or other ignition device will occasion preignitions as will also a bulb when too hot. If the bulb is cold, the fuel charge will not burn completely. The unconsumed portion blows out the exhaust. Due to imperfect scavenging, some of the fuel remains in the cylinder where it gradually vaporizes and ignites on the succeeding compression stroke. -This oil more frequently collects in the exhaust passages where a blast of hot exhaust gases ignites it, producing what is mistakenly termed preignitions, although it is more in the nature of a back-fire.