Chapter 2—ADMINISTRATION, SUPERVISION, AND TRAININGpersonnel can interpret and, based on the findings,decide whether or not the engine needs to beoverhauled in order to ward off serious and costlydamage or just be temporarily shut down for somesimple maintenance.The key to utilizing engine performance dataas a tool is to make graphs from the data whichshow at a glance the signs of impending distress.Analysis of this graphical display is commonlycalled trend analysis.In order to get a good indication of the enginecondition, the following specific items arerecorded.1. Cylinder compression pressures.2. Cylinder firing pressures.3. Fuel pump rack or governor power pistonposition.4. Cylinder exhaust temperature.5. Crankcase vacuum.6. Lubricating oil pressure at engine inlet orupper header.7. Manifold air or scavenging air pressure.To produce meaningful graphs, all data mustbe plotted under the same conditions, and be ob-tained at some readily duplicated condition. It isnot important that the engine be under full loadat full speed when taking data, but it is impor-tant that all data be obtained under similar con-ditions. For example:1. Always obtain data from generator sets at80% load and 100% speed.2. Always obtain data from propulsionengines; for example, standard or full.Data need not be plotted daily. In most cases,a set of readings should be plotted every 200 hoursof operation. In some cases it may be prudent torepeat a set of readings when a large change inoperating characteristics has apparently occurred.The first step in preparing the graphs for trendanalysis is to collect the data. This is done byobserving and recording the above items with theengine operated at a selected type of condition fora sufficient time, prior to taking data, to allowpressures and temperatures to stabilize. (It can beassumed that conditions have stabilized when lubeoil and freshwater temperatures are within ± 5°of the normal operating temperatures.These data are then plotted on 10 × 10 linesper inch graph paper as shown on the examples(figures 2-8 through 2-15). For convenience, thefirst points are located at zero time for an enginethat has just been overhauled or at the numberof hours on the engine since the last overhaul (0,400, 1000, 1600 hours, etc.). The first point forlube oil consumption occurs at 200 engine hours.This is done because it is easier to start with a fullengine sump and monitor the amount of oiladded each 200 hours to obtain the consumptionrate. Once the initial points have been plotted, allthat is required is to record and plot the sameinformation each 200 hours and observe the trendsthat develop. (NOTE: remember to always takedata under the same controlled conditions!)A close look at the sample graphs will revealhow they can be used to determine engine condi-tion. For purposes of illustration, the ideal trendof each graphed value is shown for a hypotheticalengine. Unfortunately, the Navy does not havetoo many ideal engines so some samples ofproblem indications that may be expected are alsoincluded.On figures 2-8 and 2-9, a high, average, andlow value is plotted for both firing and compres-sion pressures. Under normal conditions thesecurves will remain flat until the engine isapproaching the time of overhaul, then the curveswill start to fall off. The high and low firingpressures will remain at about ± 50 psi (100 psispread) from the average firing pressure for a well-balanced engine. If you look at figure 2-8 you cansee that a decided drop in firing pressure has oc-curred at 1600 hours (point A). This failure in thecompressing pressures indicates that the rings areeither sticking, broken, or beginning to wear; thatthe valves are not functioning properly; or thatthe liner is beginning to score or possibly that apiston has cracked. Remember that any changein a curve (beyond normal limits) indicates thatimmediate attention is required. At this point, itshould be pointed out that more than oneindicator will usually reveal the same distresssignal. Therefore, before any corrective action istaken, it is best to make a study of other curvesto deny or confirm the problem. In this case checkthe lube oil consumption, crankcase vacuum, andexhaust temperature curves. In figures 2-9, 2-10,and 2-15 the typical indications for this problem2-13
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