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From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
The measurement and comparison of engine breathing capacity.

Identifier  ExFiles\Box 134\2\  scan0036
Date  30th May 1935
  
E/PSN.2/MN.30.5.35.

Engine Breathing Capacity.
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The maximum power that can be obtained from an engine naturally aspirated or the boost necessary to give a required m.e.p. on a supercharged engine is controlled primarily by the breathing capacity of the cylinders. A measure of the potential breathing capacity of an engine cylinder is the ratio of maximum inlet valve or port area to piston area, and on such a basis cylinders of different sizes can be compared. A comparison of the actual breathing capacities is given by the i.m.e.ps. developed at similar piston speeds and C.R's. A comparison of potential and actual breathing capacities will show the efficiency with which the available inlet area is being used, or more accurately the orifice coefficient of the valving.

Attached is a table showing the breathing capacities of various cylinders compared on the above lines. Comparisons are all between single cylinders as the breathing of the cylinder itself on a main engine is obscured by the breathing of the engine as a whole, including induction system, supercharger and chokes. The m.e.ps have been corrected to a C.R. of 6.5:1 and a boost temperature of 70°C. It will be seen that m.e.ps are given at piston speeds of 2500, 2750 and 3000 ft. per minute. The lowest of these figures is about the current piston speed, the middle one is what we are aiming at with our new designs, while the highest represents about the maximum piston speed at which we are likely to operate for some time.

It is proposed to draw a number of comparisons between various engines in the table and suggest reasons for the differences in performance.

(1) Pegasus and Kestrel Ramp Head.

It will be seen that the Pegasus is slightly better valved than the Kestrel, about 4%, but this is insufficient to account for its superior breathing capacity, which is very marked. At a piston speed of 2500 ft./min. the superiority of the Pegasus is only 4%, but at 3000 ft/min. it is 7% naturally aspirated and 15.5% or 22 m.e.p. better

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