These are the points that should be observed each time the engine is stopped for inspection or for a shut-down. If the shut down is only for a few hours and is a matter of frequent occur rence, this inspection of the bearings is not necessary. No matter how short is the period of shut-down, the engine should be thor oughly wiped with waste or rags. Nothing is so conducive to a long-lived engine as thorough cleanliness. All the small moving parts, such as make up the fuel pump and governor assembly, must be kept clean. • Low-pressure Indicator Cards.—The use of the indicator is not general, even where the units are Diesels. The operator of small low-pressure engines usually does not feel justified in investing in an indicator; where the engine is above 50 h p., this accessory is almost indispensable if economy in operation is to be achieved. By the intelligent use of the indicator an engineer is in the position to know when his engine is not working efficiently.
a preignition sound is mistaken for a pound pro duced by a worn bearing. An indicator card would reveal at once if the fuel was exploding prematurely. Again, the engine may labor and the engineer think the load is excessive, whereas the trouble may be due to late injection or delayed combustion.
The different fuels have marked differences in behavior, as revealed on the card. Figure 342 is a card from a De La Vergne . Type D.H. engine using 24° &time crude oil. The ignition is a trifle late since the vertical line slopes to the right, showing that the piston had passed dead-center before the fuel ignited. The horizontal hook "a" indicates that, after most of the fuel exploded, there was a percentage of heavier particles that con tinued the combustion. The compression pressure at the point "1 " reached 200 lbs. per sq. inch while the maximum explosive pressure ran above 360 pounds. The fact- that the fuel did not ignite just before dead-center can probably be attributed to a cold combustion chamber. With this engine the fuel injection occurs considerably ahead of dead-center, and evidently then the trouble was not delayed injection.
Figure 343 is a card from the same engine while using 32° Baume fuel oil. The charge exploded about 6 degrees ahead of dead-center, where the compression was around 175 lbs. per sq. inch. The maximum explosive pressure reached 380 pounds. This card is well-nigh ideal for a low-pressure engine. The explosion had better occur a trifle before dead-center than after the piston commences its return stroke.
Figure 344 is a card from a 50 h.p. Fairbanks-Morse Type Y engine running at 257 r.p.m. The compression at "a" reaches 160 pounds while the maximum cylinder pressure exceeds 375 pounds.
The fuel used was 27° fuel oil, which resulted in a flattening of the explosion peak. The waves in the expansion line are not due to delayed explosion but are caused by the inertia of the indicator piston and other parts. The combustion was a trifle late, which produced a slight pound in the piston-pin bearing. On investiga tion the piston-pin bearing was found to be somewhat worn. For an engine operating at a speed of 257 r.p.m. the card is exceed ingly satisfactory.
Figure 345 is an example of the freak cards occasionally taken with the indicator. The engine was the same from which Fig. 344 was secured but at a later date. The fuel used was kerosene. The compression reached approximately 100 pounds when the kero sene charge in the combustion chamber fired. Due to the inertia of the indicator piston and levers the pencil jumped to "b," far beyond the actual cylinder pressure at this position of the piston. The pencil then moved downward toward its true position, but the inertia of the spring forced the pencil to "d," whereupon, being stable now, the point moved up approximately along its true course to the maximum point This erratic behavior of the indicator mechanism completely distorted the card, and the events as outlined on this card are by no means rep resentative of the actual occurrences in the cylinder. The dotted line from "a" to probably is the path the pencil should have taken. Regardless of the action of the indicator, this card shows that if kerosene is to be used in an engine without water injec tion, either the fuel must be injected practically at dead-center or the compression must be reduced. Due to the design of connecting-rod on this engine, no great alteration in the compres sion is possible, making it necessary to delay the injection pump action.
Figure 346 is a card from a Muncie engine which was taken after the engine had been operating on low load. The bulb had not become thoroughly hot on the resumption of full load. As a result of the fairly cold bulb, the dead-center was passed before ignition actually took place. The compression line at dead center reached the point " a. " Because of the cooling water absorbing part of the heat of compression, as well as because of leaky piston rings, the compression dropped to "c," practically at dead-center or a few degrees beyond center. The explosion occurred at this point reaching to the point "b" where the piston was fairly well advanced in the power stroke. This con dition is very likely to exist in any engine that has been operating at a low load and has a heavy load suddenly thrown onto it.