Two types of inspections and tests that can be used to spot impending trouble in an internal combustion engine before it can seriously affect its operation are called trend and spectrographic analyses. We will now discuss and explain their importance and use in detecting problems in internal combustion engines.

ENGINE TREND ANALYSIS.Preventive maintenance receives a great deal of attention from everyone in the field of diesel engine operation, since the idea of letting an engine run as long as it will run and fixing it only after a breakdown occurs is not only foolish, but extremely costly. On the other hand, it would be just as foolish to be constantly tearing down an engine for inspection. It is a known fact that vital parts of an engine last longer and operate better if they are not tampered with unnecessarily. Therefore an attempt must be made to find a happy medium between these two forms of maintenance. One way is to determine the condition of an engine is by monitoring its operation. This is done by regularly obtaining certain engine operating data and by studying, and analyzing, and com-paring it with previous data. This information is then reduced to a form which all engineering 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 obtained 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 important that all data be obtained under similar conditions. 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.