The clearance volume of a cylinder (often presented as a percentage term) is that part or proportion of the cylinder volume that is not swept by the piston. It is therefore the volume (or proportion of total volume) taken up by the steam passages and cylinder-head cavities - i.e. the volume contained between the piston head and valve port when the piston is at dead-centre.
Ideally, the clearance volume should be as small as possible because that part of the steam that fills the clearance volume at the end of each stroke is wasted in that it has done no "work" on the piston. However reducing the clearance volume also necessitates reducing the area of the steam passages which has the effect of restricting steam flow in and out of the cylinder. Furthermore, low clearance volume can result in very high compression that can cause cylinder cover pressure release valves to blow, which is reported to have been a tendency on GW King Class 4-6-0s that clearance volumes of around 6%.
A compromise has therefore to be adopted and the clearance volume of most latter-day steam locomotives has been in the order of 10%. In the case of the 5AT with its double-valve arrangement, Wardale adopted a clearance volume of 10.6%.
In his "Compounding" paper published in Camden's book "Advanced Steam Locomotive Development - Three Technical Papers", Porta makes the following observations relating to Clearance Volume:
"Fig. 19 (below) shows the influence of the clearance volume: for high boiler pressures it requires prohibitive compressions (thus spoiling the (alpha coefficient) much as happens in compressors. The convenience of a small clearance volume was appreciated early on by Churchward, but his preaching was not understood: he came down to 6% - current engines show 12% and even more in the case of poppet valves.
The ideal engine, with zero clearance volume, working at 11% cut-off, operates along the cycle 1-2-3-4-5-1 and shows the area A as the incomplete expansion loss. Should 11% clearance volume be included, the cycle becomes 2'-3'-4'-5'-2', and the incomplete expansion loss (area B) becomes much greater."
A separate page of this website is dedicated to the subject of incomplete expansion losses.
Note: in the case of compound expansion engines, Porta recommends that a relatively large clearance volume be provided in the high-pressure cylinder(s) in order to avoid over-expansion at short cut-off working. He suggests a figure of around 16%, but notes that in the case of his 3-cylinder scheme, he adopts a figure of 35%. In the case of the low pressure cylinder(s) however, clearance volume should be as small as possible, as with single/simple expansion engines.
Several pages of this website include text and diagrams copied from Porta's "compounding" paper, including the pages covering condensation/wall effects, steam leakage, incomplete expansion and triangular losses. More specific references to his theories on compound expansion can be found on the α Coefficient and Compound Expansion pages.
Sincere thanks to Adam Harris of Camden Miniature Steam, publishers of "Advanced Steam Locomotive Development - Three Technical Papers" for allowing the sections of the book to be published on this website.