Slide Valves by Professor Green
Slide Valves
by B. M. Green
July - August 1954
When I started some years ago, to build a miniature steam locomotive I decided to design it from the ground up using good engineering, or model engineering, principles. I selected slide vales, chiefly because of ease of manufacture. The next step was an investigation of suitable port widths and lengths and desirable areas of steam passages. Perhaps others would be interested in what I found.
Of two chief sources of information, one was "Slide Valve Gears" by Frederick A. Halsey, associate editor of American Machinist for many years. This little book was first published in 1889 when the interest in steam engines and in the design of slide valves was at its height. My copy is the thirteenth edition, dated 1920, and the book went our of print a few years later. It contains much of interest to the steam engine enthusiast. The other chief source was the writings and designs of our good friend "L.B.S.C.". he has never said much about how he designs his engines and i trust he will forgive me for analyzing a few of his designs to determine average proportions. There are also many articles by experienced designers of miniature locomotives in the old and lamented Modelmaker.
The slide valve is, of course, the means by which live steam is admitted to the engine cylinder and released from it at the proper times. If one plots pressure in the cylinder against piston position a diagram is obtained called a "card." Cards can be drawn for actual engines by an instrument called an indicator. Idealized cards for one end of an engine cylinder are shown in Figure 1. Figure 1 (a) shows what happens when an engine is operating at about 75% cut-off and 1 (b) illustrates the situation for about 25% cut-off. Since work equals force times distance the area within the complete curve times the piston area measures the work done by the steam during one stroke. it is evident that the work done at the earlier cut-off is less than that done at the later valve but, since a locomotive is only "hooked up" at the higher speeds, the work done per minute, or horsepower, may be greater for the earlier cut-off.
The "events" of a card are: *(1) Admission, when the valve opens to steam; (2) cutoff, when the valve closes to steam; (3) Release, when the valve opens to exhaust; (4) Compression, when the valve closes the exhaust. Looking again at Figure 1, the space from zero to the beginning of the stroke is proportional to the clearance volume in the cylinder, that is, the volume between piston and cylinder head plus the volume of the steam occupying it does no work on the piston and so is wasted. It is evident then that clearance volume should be as small as possible without getting the passage so small that there is a large pressure drop. The line marked "Atm" represents atmospheric pressure and the actual exhaust pressure will be a little steam passage up to the valve face. Clearance volume is expressed as a percentage of the stroke volume and in fullsize engines may be as much as 20%. The higher because of resistance at the blast nozzle. Halsey shows cards taken on locomotives which indicate a back pressure of about five pounds.
Comparing the card in Figure 1 (b) with that in Figure 1 (a) it will be noticed that cut-off and release are earlier in the forward stroke and compression and compression is earlier in the return stroke. These limitations of the plain slide valve must be accepted. The early release represents a waste of steam. The early compression means that more steam is trapped and compressed in the cylinder at the end of the return stroke. The resulting higher pressure may be a good thing because it aids in absorbing the energy of the faster moving piston and bringing it to rest at the end of the stroke.
Now, what about dimensions of ports and passages? Halsey has quite a bit to say about assumed average velocities of the steam through the ports and passages and this may be a good way when designing single speed stationary engines but he admits, himself, that the method can't be applied to locomotives with their widely varying piston speeds and cut-offs. A better way, for us at least, is probably to analyze a few locomotives and determine some proportions. Start by scaling the bore and stroke of the engine we wish to follow. Our bore is likely to be smaller than scale because our cylinder head screws are usually over scale and so require thicker cylinder walls. When the cylinder diameter is decided upon we can express port widths and lengths in terms of this diameter.
Figure 2 represents the port face for a cylinder using a slide valve. L is length of port (across the cylinder); S is steam port width; E is exhaust port width; and B is thickness of bridge wall. Let D be cylinder diameter. Halsey gives the dimensions for nineteen locomotives and also proportions of average stationary engine practice. Take first, the port length, L. In Halsey's locos L. varies from 0.85D to 1.0D with an average of 0.9D. In stationary engine practice L in often 0.75D. Halsey states that the ports of British locomotives are smaller for a given size of cylinder than are those of American engines and thinks that may be one reason why British engines are claimed to be more economical. (When we write). I have no data on British engines but I did investiage four of L.B.S.C.'s engines and in these L averages 0.57D. This small ratio may be partly due to over-size valve chest screws which reduce the available width inside the chest. However, a large L seems desireable. The steam pressure in the cylinder should rise rapidly as possible when the valve is just opening to steam and the longer the port the greater the available port area. I am not arguing for lengths greater than L.B.S.C's but the reasoning indicates why some amateur designs using a single drilled hole for a steam port will not pull anything.