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National Advisory Committee for Aeronautics, Research Memorandum - Acceleration Characteristics of R-3350 Engine Equipped with NACA Injection Impeller

Qualitative investigations have shown that use of the NAGA injec-
tion impeller with.the 3-3350 engine increases the inertia of the
fuel-injection system and, when the fuel—metering system is
used, this increase in inertia results in poor engine acceleration
characteristics. This inrestigation.was therefore undertaken to
determine whether satisfactory acceleration characteristics of the
engine equipped_ with the injection hapeller could be obtained by
simple modifications to the fuel—metering system. The engine was
operated with two types of carburetor; namely, a hydraulic-me' vering
carburetor incorporating a vacuum-operated accelerating pump and a
direct-metering carburetor having a throttle-actuated accelerating.

The vacuumroperated accelerating pump of the hydraulic-metering
carburetor was modified to produce satisfactory accelerations by
supplementing the standard air chamber with.an additional 75—cubic-
centimeter air chadber and by decreasing the strength of the primary
spring.

The throttle-actuated accelerating pump of the direct-metering
carburetor was modified to produce satisfactory accelerations by
replacing the standard 0.028-inch~diameter bleed in the load-
compensator balance line with a smaller bleed of 0.0225-inch diameter.

The results of this investigation.indioated that both carburetors
can be easily modified to produce satisfactory acceleration character-
istics of the engine and no definite choice between.the types of
carburetor and accelerating pump can be made. Use of the direct—
metering carburetor, however, probably resulted in better fuel
distribution to the cylinders during the acceleration period and
reduced the backfire hazard because all the fuel is introduced through
the injection impeller.

At the request of the Air materiel Command, Army Air Forces, a
general investigation of methods for improving the cooling character-
istics of the 3-5550 engine has been conducted at the NAGA Cleveland
laboratory. Use of the NAGA injection_ impeller improves the mixture
distribution and consequently the temperature distribution of the
engine (reference 1).