RECIPROCATING ENGINE The chief advantage of the reciprocating engine at the present time is in low specific fuel consumption, especially at partial load. This makes it particularly applicable to long-range flight at present cruising speeds. Typical reciprocating engines now in use have specific fuel consumptions which are in the neighborhood of 0.7 lb/bhp-hr at full-rated rpm, decreasing to about 0.45 lb/bhp-hr at cruising conditions.
The principle characteristics of several current and proposed reciprocating engines of higher horsepower of interest for comparison with gas turbines, are given in Table I.
The largest reciprocating engine now under development is the 5000-hp Lycoming R-7755. One of the chief aims of this engine is fuel economy for long-range bomb
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pr application. The estimated minimum specific fuel consumption is approximately 0. 37 lb/bhp-hr, the improvement being associated largely with increased compression ratio. However, it is noted that there is no improvement in either specific weight or power per unit frontal area of the R- 7 75 5 over the R-4 360. Theratio of dry weight to take-off power is 1. 21 lb/hp for the R-7 7 55 as compared with 1. 14 lb/hp for the R4360 and the ratio of take-off power to frontal area is 178 for the R-7755 as compared with 198 for the R-4360.
Further increase of reciprocating-engine power involves further increase in cylinder volume, which is already at a critical stage as regards number and arrangement of cylinders, complexity of the drive, excessive frontal area, and difficulty of cooling. There are development projects at present aimed at increasing the output of the R-3350 to 3200 hp and the R-4360 to 4300 hp.
Some increase in fuel economy can reasonably be expected by utilizing higher pressure ratios, improved fuels, improved metallurgy, etc., but engine manufacturers consider that it would be difficult to attain a specific fuel consumption essentially lower than 0.35 lb/bhp-hr.pdf net