1S43.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



49 



SELF-REGULATING EXPANSION SLIDE VALVE. 

 Fig. 1. 





7'"F\ 



Improvement in the steam engine is so much sought after, so many are 

 engaged in the pursuit, and economy in the consumption of fuel is a question 

 of so much importance, that no apology need be offered on presenting a plan 

 to attain that object, and which has been found here or elsewhere to be an 

 absolute improvement. 



The simple apparatus which I am about to describe, (for which I obtained 

 two patents abroad,) has been applied very successfully, and is now getting 

 into extensive use. I believe it to be unknown in this country, except to a 

 few persons to whom I have explained it ; and as it will on most occasions 

 be found to be useful, I propose to make it known through the medium of 

 your truly valuable columns, which being open to communications of the 

 kind, and much read, I should be happy to introduce this to your readers 

 through so respectable a channel. 



The advantage of using steam expansively does not require demonstration, 

 it is too universally acknowledged to admit of any doubt ; I must, however, 

 enter a little into the subject, to point out the benefit to be derived from 

 the application of my slide valve, but will endeavour to be as concise as 

 possible. 



An engine working without expansion, receives the steam on its piston 

 during the whole length of stroke, its speed being regulated by contracting 

 more or less the passage through the throttle valve, thereby to a certain ex- 

 tent wire-drawing the steam. The speed of the engine is effectually regu- 

 lated by this means, but a considerable quantity of steam is thereby thrown 

 away, as I will endeavour to show. 



It frequently happens that an engine is lightly loaded, and as the loss to 

 which I allude is comparatively greater with a light load than with a full 

 one, on account of the wire-drawing becoming more complete, I will take 

 for example an engine working with such a load as will require the orifice 

 through the throttle valve to be sensibly contracted, in order to keep down 

 the speed of the piston. 



When the engine passes over her centre, the motion of the piston is very 



slow, and the orifice of the throttle valve will allow the steam to rush into 

 the cylinder in sufficient quantity to exert its full pressure ; but as the speed 

 of the piston increases, the quantity of steam admitted becomes insufficient 

 to fill the space at full pressure behind the piston. 



The piston continues increasing in speed until it reaches the middle of the 

 cylinder, where it is the greatest, from that point to the end of the stroke 

 the speed decreases until the motion is reversed j there is necessarily a point 

 of the stroke at which the speed is so slow, that the quantity of steam ad- 

 mitted through the throttle valve will be proportionate to the speed of the 

 piston, and from that point until the end of the stroke, as the speed of the 

 piston decreases, the steam will accumulate in the cylinder, and the pressure 

 will increase; but at that moment the position of the leverage of the crank 

 is such, that the increasing pressure of the steam produces comparatively 

 little effect on the speed of the engine, and at the moment at which the 

 pressure reaches its maximum, the slide valve is reversed, and the contents 

 of the cylinder are thrown into the condenser. 



The quantity of steam thrown into the cylinder at the beginning of the 

 stroke is not lost, because it continues to act expansively on the piston, and 

 becomes a portion of that volume of steam which determines the speed of 

 the engine and the relative steam passage through the throttle valve ; hut as 

 I said above, the volume of steam thrown in towards the end of the stroke, 

 only serves to fill the cylinder uselessly at the moment when its contents are 

 about to be thrown iuto the condenser. 



If the engine happens to have a light fly-wheel, the evil is considerably 

 increased, because the speed of the engine will sensibly decrease towards 

 the end of the stroke, the orifice of the throttle valve will be enlarged by 

 the action of the governor, and an increased volume of steam will he ad- 

 mitted into the cylinder just in time to he thrown away. 



By working the steam expansively, the above-mentioned loss is avoided ; 

 and if the resistance to be overcome was constant — as for instance, to raise a 

 given quantity of water to a given height in a given time — then the fixed ex- 

 pansion would answer every purpose ; and this is, perhaps, the only instance 

 in which that can he said to be the case. 



Generally speaking, the load is variable, and when that is the case, the 

 point of the stroke at which the steam is cut off should also be variable, so 

 that the steam employed should exert its full pressure while it is being ad- 

 mitted to the piston, in order to produce the full effect of expansion from 

 the moment it is cut otf until the end of the stroke. 



For this to be carried out efficiently, the engine itself must determine the 

 point of the stroke at which the steam should be cut off, and the governor is 

 sufficient for the purpose. I think I may infer, that the valve hereafter de- 

 scribed, will be found useful for all engines which require a governor to 

 regulate their motion. 



The present system of advancing the eccentric, and constructing the 

 working valve, so that the steam is cut otf at about three fourths of the 

 stroke, is an immense improvement, but stops short of what is wanted, par- 

 ticularly for those engines which work with high steam. 



The elasticity of steam being subject to the same law that governs the 

 elasticity of atmospheric air, as determined by Mariotte, the elasticity being 

 proportionate to its density, then a volume equal to 200 under a pressure = 

 2, will be reduced to 100 under a pressure = 4, and will expand so as to 

 represent 400, the pressure being reduced to = 1. 



This being the case, let us suppose the length of the stroke of the cy- 

 linder of a steam engine divided into 20 equal parts, and that steam of four 

 atmospheres is acting upon the piston during the whole of the stroke ; the 

 consumption of steam will be represented by 20 x 4 = 80, and the sum of 

 the forces will also be 20 x 4 = 80 ; in this case the consumption of steam 

 will be as 1, and the power exerted will also be 1. 



Take the same cylinder, and admit steam of the same pressure during 

 xi = J of the length of stroke, the quantity of steam expended will be 

 15 x 4 = 60, and the sum of the forces will be 15 x 4 = 60 for the first 

 15 spaces, and 16-77 for the remaining o. 

 The consumption of steam will be 60 = 1. 

 The power exerted will be 60 + 16-77 = 76'77 = 1-27. 

 (See diagram No. 1.) 



Again, in the same cylinder, admit the steam only during J£ = J the 

 length of stroke, the quantity of steam used will be 10 x 4 = 40, and the 

 sum of the forces will be 10 x 4 = 40 for the first 10 spaces, and for the 

 remaining 10 spaces it will be 26-75. 

 The consumption of steam in this case will be 10 = 1. 

 The power exerted will be 40 + 26-75 = 6675 = 166. 

 (See diagram No. 2.) 



