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National Advisory Committee for Aeronautics, Technical Notes - Supplementary Report of Oil Scraper Piston Rings

Due to the unfortunate accident noted in paragraph 15 of
Technical Note No. 88, whereby aluminum particles were distrib~
uted throughout the lubricating system of the oil-scraper ring
engine, the results of the seventy-five (75) hours and fifty-
five (55) minutes of operation were inconclusive, so far as ac-
curately estimating bearing life is concerned. To supply this
deficiency, this report, based upon the operating record of this
engine subsequent to the overhaul necessitated by this accident,
is herewith submitted.

This engine, after the overhaul — consisting of the clean;
ing of the entire lubricating system, the removal of aluminum
particles from the surface or the babbitt of the main bearings,
and the fitting and installation of new crank—pin bearings —
was installed on a torque stand and used for incidental tests.
The majority of these tests were to determine the characteristics
of the engine when fitted with various types of manifolds; thus
imposing operating conditions no more severe than those encoun-
tered in normal service. For a total of approximately five (5)
hours, however, the engine was used on runs for the development
of glass spark ;iugs, which pre-ignited badly even in this low
compression engine; thus imposing the most unduly severe condi-
tions of operation possible.

During the entire life of the engine (when equipped with oil
scraper rings) of two hundred and sixty—two (268) hours and
fortyueight (48) minutes, to date, the carbon deposits have been
removed four (4) times; or an average interval between carbon
remoVals of sixty-five (65) hours and forty—two (42) minutes.
Except on the occasion of the last overhaul, the engine would
have functioned entirely satisfactorily if it had been re—
assembled without the removal of the carbon - the value of the
above average interval between carbon removal is, therefore,
doubtful and unquestionably too low; 'On the last overhaul, it
was found that an accumulation of carbon had built up in the
piston and ring oil grooves and in the piston ring gaps and
grooves. These deposits had been unnoticeable on previous over—
,hauls but would have caused trouble on this occasion if they-had
not been removed.