Chapter 2ADMINISTRATION, 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
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
1. Cylinder compression pressures.
2. Cylinder firing pressures.
3. Fuel pump rack or governor power piston
4. Cylinder exhaust temperature.
5. Crankcase vacuum.
6. Lubricating oil pressure at engine inlet or
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
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