Chapter  2—ADMINISTRATION,  SUPERVISION,  AND  TRAINING personnel can interpret and, based on the findings, decide  whether  or  not  the  engine  needs  to  be overhauled in order to ward off serious and costly damage or just be temporarily shut down for some simple  maintenance. The key to utilizing engine performance data as a tool is to make graphs from the data which show at a glance the signs of impending distress. Analysis of this graphical display is commonly called  trend  analysis. In order to get a good indication of the engine condition,  the  following  specific  items  are recorded. 1. Cylinder compression pressures. 2. Cylinder firing pressures. 3. Fuel pump rack or governor power piston position. 4. Cylinder exhaust temperature. 5. Crankcase vacuum. 6. Lubricating oil pressure at engine inlet or upper header. 7. Manifold air or scavenging air pressure. To produce meaningful graphs, all data must be plotted under the same conditions, and be ob- tained  at  some  readily  duplicated  condition.  It  is not important that the engine be under full load at 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  at 80% load and 100% speed. 2.   Always   obtain   data   from   propulsion engines; for example, standard or full. Data need not be plotted daily. In most cases, a set of readings should be plotted every 200 hours of operation. In some cases it may be prudent to repeat a set of readings when a large change in operating characteristics has apparently occurred. The first step in preparing the graphs for trend analysis is to collect the data. This is done by observing and recording the above items with the engine operated at a selected type of condition for a sufficient time, prior to taking data, to allow pressures and temperatures to stabilize. (It can be assumed that conditions have stabilized when lube oil and freshwater temperatures are within ± 5° of the normal operating temperatures. These data are then plotted on 10 × 10 lines per inch graph paper as shown on the examples (figures  2-8  through  2-15).  For  convenience,  the first points are located at zero time for an engine that has just been overhauled or at the number of hours on the engine since the last overhaul (0, 400, 1000, 1600 hours, etc.). The first point for lube oil consumption occurs at 200 engine hours. This is done because it is easier to start with a full engine  sump  and  monitor  the  amount  of  oil added each 200 hours to obtain the consumption rate. Once the initial points have been plotted, all that is required is to record and plot the same information each 200 hours and observe the trends that develop. (NOTE: remember to always take data under the same controlled conditions!) A close look at the sample graphs will reveal how they can be used to determine engine condi- tion.  For  purposes  of  illustration,  the  ideal  trend of each graphed value is shown for a hypothetical engine. Unfortunately, the Navy does not have too  many  ideal  engines  so  some  samples  of problem indications that may be expected are also included. On figures 2-8 and 2-9, a high, average, and low value is plotted for both firing and compres- sion pressures. Under normal conditions these curves  will  remain  flat  until  the  engine  is approaching the time of overhaul, then the curves will  start  to  fall  off.  The  high  and  low  firing pressures will remain at about ± 50 psi (100 psi spread) from the average firing pressure for a well- balanced engine. If you look at figure 2-8 you can see that a decided drop in firing pressure has oc- curred at 1600 hours (point A). This failure in the compressing pressures indicates that the rings are either sticking, broken, or beginning to wear; that the valves are not functioning properly; or that the liner is beginning to score or possibly that a piston has cracked. Remember that any change in a curve (beyond normal limits) indicates that immediate  attention  is  required.  At  this  point,  it should  be  pointed  out  that  more  than  one indicator  will  usually  reveal  the  same  distress signal. Therefore, before any corrective action is taken, it is best to make a study of other curves to deny or confirm the problem. In this case check the lube oil consumption, crankcase vacuum, and exhaust temperature curves. In figures 2-9, 2-10, and 2-15 the typical indications for this problem 2-13