Chapter 3—ENGINE MAINTENANCEcutout mechanism, or a loss of compression. If,after checking the air cleaner, the filter, and thefuel cutout mechanism, you determine that theproblem is due to loss of compression, then youmust perform a compression check with a cylinderpressure indicator.There are several different types of indicatorsthat may be used. Most indicators used withdiesel-cylinder engines are either of the springbalanced type or the trapped pressure type. Theyare manufactured by various companies such asKiene, Bacharach, and Kent-Moore. Some ofthese indicators measure only compressionpressure, others measure both compression andfiring pressures.Spring Balanced IndicatorA spring balanced indicator, such as the onemanufactured by Bacharach (figure 3-1), employsa spherical ball piston, which is held on its seatby the force of a helical spring actuated by thecylinder pressure which acts against the bottomof the ball piston to oppose the spring tension.Before the indicator is attached to the engine, thevulcanized handle must be rotated clockwiseuntil the reading on the counter is greater thanthe maximum cylinder pressure expected. Theamount of this pressure is listed in the enginemanufacturer’s technical manual. When theindicator is installed, the operator must make surethat it is placed as near the cylinder as possibleand position it so that it can be read easily. Afterthe indicator is installed the engine is operated atthe specified rpm, then the fuel to the cylinderCourtesy of Bacharach, Inc., USA75.238XFigure 3-1.-Spring balanced Pressure Indicator.being tested is cut out, the cylinder test cock isopened, and the spring tension on the indicatoris adjusted. The tension of the spring is reducedb y r o t a t i n g t h e v u l c a n i z e d h a n d l ecounterclockwise until the maximum cylinderpressure barely offsets the spring pressure. At thispoint, the latch mechanism of the indicator tripsand locks the handle firmly in position, giving adirect and exact reading of the pressure in poundsper square inch (psi). To reset the lock mechanismfor a new reading, the handle must be rotatedcounterclockwise one-fourth turn. When this in-dicator is stowed for future use, the indicatorspring must be unloaded by rotating the handlecounterclockwise until a zero pressure reading isobtained.Trapped Pressure IndicatorsIn this type of indicator, the cylinder gasesenter past a valve into a chamber which leads toa gage. When the pressure above the valve equalsthat of the cylinder, the valve seats and traps thegas above the valve at its highest pressure, thenthis pressure is read on the gage. There are severalother types of indicators. The one pictured infigure 3-2 is used to take compression readingsCourtesy of Bacharach, Inc., USA75.238XFigure 3-2.—Trapped Pressure Indicator (small boat).3-3
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