only on engines installed on small boats. Engineslike the GM-6-71 do not have indicator cocksinstalled.When taking compression readings on a 6-71engine, you will perform the following steps:1. Check the manufacturer’s technical manualfor the minimum compression pressure requiredfor the engine.2. Start the engine and run it at approximatelyone-half the rated load until normal operatingtemperatures are reached.3. Stop the engine and remove the fuel pipesfrom the injector and the fuel connectors on thecylinder to be tested.4. Remove the injector and install theindicator adapter, with pressure gage attached,and use the crab nut to hold the adapter in place.5. Use a space fuel pipe to fabricate a jumperconnection between the fuel inlet and the returnmanifold connectors to by-pass fuel to and fromthe injector.6. Start the engine again and run it at approx-imately 600 rpm.7. Observe and record the compressionpressure as indicated on the gage.Another type of trapped pressure indicator isthe Kiene indicator (figure 3-3). This indicator isbasically a Bourdon gage connected to a cylin-drical pressure chamber. The pressure chambercontains a check valve which allows the gas toENGINEMAN 1 & Cflow from the engine into the chamber until thepressures are equalized. This gage is attached tothe chamber and the pressure is read directly. Thecheck valve is an inverted piston seating on a seatpiece. The valve moves up and down in a guide.A stop nut is used to adjust the travel of the checkvalve.Most of you should become familiar with thisindicator since it is widely used to check both thecompression and firing pressures on main dieselengines and emergency generator diesel engines.Review figure 3-4A and B. It is a PMS situationrequirement to be performed when the engineoperating conditions indicate problems.EXHAUST AND CYLINDERTEMPERATURESOne of the most useful tools that the engineoperator has for monitoring an engine’s perform-ance is the thermocouple pyrometer. The prin-cipal use of this device is in the exhaust system(but it can also be used for other purposes) whereit is used to measure the exhaust gas temperaturesat each cylinder or the common temperature inthe exhaust manifold. By comparing the exhaustgas temperatures of each cylinder, the operatorcan determine if the load is balanced throughoutthe engine.The two types of pyrometers in use are thefixed installation and the portable hand-heldinstrument (figure 3-5). Both types use a ther-mocouple unit, such as the one shown in figure3-5, installed in the exhaust manifold.In its simplest form, a thermocouple consistsof two dissimilar metal wires, usually iron andconstantan (55% copper and 45% nickel) that arejoined at both ends to form a continuous circuit.When the temperatures at the junctions are dif-ferent an electrical current is produced and flowsin the circuit. The greater the temperature dif-ference, the greater the voltage produced.Courtesy of Bacharach, Inc., USA75.238XFigure 3-3.—Trapped Pressure Indicator.One junction, known as the hot junction, iscontained in a closed-end tube, installed in the ex-haust manifold of each cylinder. The other junc-tion called the cold junction, is exposed to roomtemperature, and is located at the pyrometer wire3-4
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