Effect of Exhaust Pressure on the Cooling Characteristics of a Liquid-Cooled Engine Page: 4 of 21

                                
NACA PM No, E7A20

The compression ratio of the engine was 6.65, reduction gear ratio
2:1, and spark advance 280 B.T.C. on the inlet side and 340 B,T.C.
on the exhaust side.
A general view of the test setup is shown in figure 1. The
engine was connected by an extension shaft and two flexible couplings
to a 2000-horsepower eddy-current dynamometer.
The exhaust-gas collector was the type used on the P-38 air-
plane modified to incorporate bellows-type expansion joints to
insure a gas-tight system. The collector was attached to the lab-
oratory altitude exhaust system through a length of 8-inch pipe,
A piezometer ring located approximately 1 foot downstream of the
junction of the two collector halves was used in measuring the engine
exhaust pressure.
The coolant used was a mixture of approximately 70 percent (by
volume) ethylene glycol and 30 percent water. The actual composi-
tion of the coolant was regularly checked by determining its boil-
ing point, Coolant-flow rates were measured with a thin-plate
orifice installed according to A.S.M.E. specifications. Coolant
temperatures were measured with single copper-constantan thermo-
couples mounted in the inlet and the outlet of each cylinder bank,
The coolant temperature was taken as the average of the inlet and
the outlet temperatures.
Cylinder-head temperatures were measured with 12 iron-constantan
thermocouples, one embedded between each pair of exhaust valves, The
cylinder-head temperature  Th was taken as the average of the 12
thermocouple readings. The location of a thermocouple in the cylin-
der head is shown in figure 2,
The carburetor-air temperature was measured with four iron-
constantan thermocouples connected in parallel and mounted just
above the carburetor top deck. Combustion-air flow was measured
with a thin-plate orifice installed according to A,S.M.E. specifi-
cations and the fuel flow (AN-F-28). was measured with a rotameter.
The investigation covered a range of exhaust pressures from
approximately 7 to 62 inches of mercury absolute, engine speeds from
1600 to 3000 rpm, inlet-manifold pressures from 30 to 50 inches of
mercury absolute, and fuel-air ratios from 0.063 to 0.100.
The procedure followed in obtaining the data was to maintain
a specified engine speed, inlet-manifold pressure, and fuel-air