MINIMUM SPECIFIC FUEL CONSUMPTION OF A LIQUID-COOLED MULTICYLINDER AIRCRAFT ENGINE AS LFFECTED BY COMPRESSION RATIO AND ENGINE OPERATING CONDITIONS

An investigation was conducted on a 12-cylinder V-type liquid-cooled aircraft engine of 1710-cubic-inch displacement to determine the minimumspecific fuel consumption at constant cruising engine speed and compression ratios of 6.65, 7.93, and 9.68. At each com-pression ratio, the effect.of the following variables was investigated at manifold-pressures, l of 28, 34, 40, and 50 inches of mercury absolute:. temperature of the inlet-air to the auxiliary-stage supercharger, fuel-air ratio, and spark advance. Standard sea-level atmospheric pressure was maintained at the auxiliary-stage super-charger inlet and the exhaust pressure was atmospheric. Advancing the spark timing from 340 and 280 B.T.C. (exhaust and intake, respectively) to 42 0 and 360.B.T.C. at a compression ratio of 6.65 resulted in a decrease of approximately 3 percent in brake, specific fuel consumption.. Further decreases in brake specific fuel consumption of 10.5 to 14.1 percent (depend'_ng on power level) were observed as the compression ratio was increased from 6.65 to 9.68, maintaining at each compression ratio the spark advance required for maximum torque at a fuel-air ratio of 0.06. This increase in com-pression ratio with a power output of 0.585 horsepower per cubic inch required a change from. a fuel- blend of 6-percent triptane with 94-percent '68--R fuel at a compression ratio of 6.65 to a fuel blend of 58-percent, triptane with 42-percent 28-R fuel at a compression ratio of 9.68 to provide for knock-free engine operation. As an aid in the evaluation of engine mechanical endurance, peak cylinder pressures were measured on a single-cylinder engine at several operating conditions. Peak cylinder pressures of 1900 pounds OU S- S IF19#10. 0 A h " L _ LLU'' 2 NACA RM No. E6L31 per square inch can be expected at a compression ratio of 9.68 and an indicated mean effective pressure of 320 pounds per square inch. The engine durability was considerably reduced at these conditions