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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).
Analysis of combustion roughness, flame propagation, and spark advance in engines.

Identifier  ExFiles\Box 133\1\  scan0255
Date  21th July 1937
  
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to get the flame away to a good start, and to obtain a maximum flame travel by placing the plug over the valves.
We thus have a quantitative measure of roughness, viz:-

Combustion roughness (P2-P1)/(t2-t1)

Analysis of a curve by Ricardo showing optimum spark advance against C.R. shows that:-

Optimum spark advance 1/ (r-1)
clearance volume.

This leads to the conclusion that for say an 8-cylinder engine as compared with a 6-cyl. of the same capacity since the clearance volume per cylinder will be less on the eight, then the time of burning will be less. As, however, maximum pressures will be about the same, then clearly the "natural roughness" of the eight will be greater, though due to the smaller bore this may not be felt.

Again, increased C.R. gives rise to roughness due to increased rise in pressure, coupled with shorter time of burning.

The essential knowledge of flame propogation is

(1) Mechanism and speed of flame propogation.

(2) Relation between pressure developed and volume of combustion products.

(3) Distribution of volume around the sparking plugs.

(1) MECHANISM OF FLAME PROPOGATION.

Fig.2 shows how flame speed varies with distance of travel, whilst Fig.3 gives flame speed as a function of time. This S-shaped curve is characteristic of all flame movements in closed vessels, with the possible exception of the sphere.
  
  


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