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National Advisory Committee for Aeronautics, Report - Performance of B.M.W. 185-Horsepower Airline Engine

This report, which was prepared for the Engineering Division of the Army Air Service and
submitted to the National Advisory Committee for Aeronautics for publication, deals with a
test made in the altitude chamber of the Bureau of Standards upon a B. M. W. (Bavarian
Motor Works) engine. The engine was submitted by the Engineering Division of the Air
Service for test according to their standard program of altitude tests. In preparing this pro-
gram it was the purpose of the Engineering Division to subject engines to conditions sufliciently
typical of those encountered in service to reveal any feature of superiority which would warrant
further investigation.
The engine has six vertical water-cooled cylinders with a bore of 5. 90 inches, a stroke of
7 09 inches, and a compression ratio of 6. 7. Report No. 1350 of the Engineering Division
describes the engine. Descriptions have appeared also 1n some of the automotive journals. 1
The engine is not designed forfull-throttle operation at ground-level densities. In fact, because
of the high-compression ratio, such operation with ordinary aviation gasoline is likely to result
in preignition. Throughout the ground-level tests made at the bureau the throttle was so
adjusted as to give an absolute manifold pressure of 24 inches of mercury. McCook Field
experiments had shown that no preignition would result under these conditions The method
of air—fuel ratio control 18 such that changes 1n charge quality are accompanied by changes in
charge quantity. Both the quantity of charge received and its quality influence the amount
of power developed However, at all densities lower than 0.064 pounds per cubic foot, and
even at this density at all speeds above 1,000 r. p. m., maximum power was obtained with the
throttle wide open, the position which gave the leanest mixture the carburetor permitted.
Hence, at these densities, the necessary compensation was secured only 1n so far as the carbu-
retor gave inherent compensation for the changes 1n air fuel ratio which usually result from
changes 1n air density.
Tests were made m the altitude chamber of the Bureau of Standards, where conditions
of temperature and pressure can be so controlled as to simulate those of the desired altitude.
Both the test chamber and the auxiliary apparatus have been described in detail in Report No.
44 of the National Advisory Committee for Aeronautics.