374 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[November, 



ON LONG AND SHORT STROKE STEAM ENGINES. 

 By John Seaward, C. E. 



A ))oinilui- notion has for a considerable timfi ]);>.st prevailed, that a 

 long stroke engine is much superior to a short stroke engine ; and it 

 will consequently be found lliat the practice of most, if not all engineers, 

 is greatly regulated by this idea. On very careful consideration, how- 

 ever, it does not appear that this alleged superiority can be satisfac- 

 torily proved. That a long stroke engine, under certain circunistancrs, 

 may be much more advantageously emi)loyed than a sliort one, is un- 

 doubtedly true, but considering the steam engine ptr se, that is without 

 reference to adventitious or extraneous circumstances, it would be 

 difficult to show that the former has any advantage whatever over the 

 latter. 



For let a careful comparison be made of a long stroke engine with a 

 short stroke eneine ; let there be two beam engines of thirty liorses 

 power eacli, both equally well made, but the one having a stroke of 

 eight feet, while the stroke of the other is only four feet, the cylinder 

 of the latter being double the area of that of tiie former ; it being un- 

 derstood that both engines shall make the satjie number of revolutions 

 per minute ; tlje steam passages and valves to be of the same area and 

 capacity ; and the two engines in all other respects to be well propor- 

 tioned and made without any limitation as to space or weight. 



Now as regards the mere mechanical effect of the moving power 

 0'. e. of the steam) it is jjerfectly clear that it must be precisely the 

 same in both engines, because the same volume of steam must produce 

 the same mechanical effect whether it is let into a long narrow cylinder 

 or into a short wide one ; therefore, if there be found any difference 

 ill the efficient duty or economical working of these two engines, that 

 difference must arise from circumstances quite unconnected with the 

 mechanical effect of the steam power. 



The only circumstances which really can make any essential diffe- 

 rence in the efficient duty or economical working of these two engines 

 are these : — First, the greater or smaller quantity of friction in the 

 various jiarts of the machines. Second, the greater or lesser radiation 

 of heat from the cylinders and passages; third, the greater or smaller 

 loss of steam bv the clearance of the piston at the top and bottom of 

 the cylinder. Fourth, the inertia and the impulse of the parts of the 

 machine in motion on the surrounding air. 



First, then of the/iictioii. It will be found in the working of a well 

 made engine of the proportions of the short stroke engine under com- 

 parison, that more than four-fifths of the whole friction are due to the 

 packings of the piston and air pump bucket, and of the piston rod and 

 bucket rod,* and less than one-fifth to the main gudgeons, the end 

 gudgeons, the crank pin and other moving joints about the engine. 

 But the friction of the piston packing will vary as the circumference 

 of the piston, multiplied into the distance which the piston travels. 

 Now in the long stroke engine the piston supposing it to be 30 inches 

 diameter, will move eight feet, and the friction of the packing be 

 therefore as 24, while in the short stroke engine the piston will be 

 about 42-4 inches diameter, will move only four feet, while the friction 

 of the jiacking will be only as 17. In the same way it can be shown 

 that the friction caused by the packing of the air pump bucket, of the 

 piston rod, and of the bucket rod, is also respectively in the ratio of 

 24 to 17, in the two engines. With respect again to the friction due 

 to the main and end gudgeons, cSrc, it is clear that it will be less in the 

 long stroke engine, because in the latter engine, the force acting upon 

 these parts will be one-half what it is in the short stroke engine. As- 

 suming therefore lUO to be the whole quantity of friction in an ordinary 

 engine then, SO of these parts in the short stroke engine, will be due 

 to the piston, air |iunip, bucket, &c., while in the long stroke engines 

 the friction of these parts will be as 1 13 that is = f^ X SO, but the 

 friclion on the main and end gudgeons in the former engines will be as 

 20, and in the latter only 10, making the total friction in the short 

 stroke engine 100, and in the long stroke engines 123, or one-fourth 

 more. 



Second. — The radiation of hat will be in proportion to the extent 

 of surface, but the surface of the long stroke cylinder, is much greater 

 than that of the short cylinder, whence it follows that the loss by radia- 

 tion in the former, must be greater than in the latter. 



Third. — The cliuruvce (if the pinton at the top and bottom of the 

 cylinder, which will evidently be greater in the short stroke engine 

 than in the long stroke engine. Because the area of piston in the 

 former is double that of the latter, some persons would be disposed to 

 say, that the loss by clearance in the former must be double what it is 

 in the latter ; but this is not quite certain, for it is not required to give 

 so much clearance in a 4 feet stroke cylinder as it would be advisable 



'llic friction of the slide is nut included, as that «ill obviously be the 

 same in both engines. .See remarkb on Friction at the end. 



to give in an 8 feet stroke cylinder, the reason of which is obviously 

 that the spring and elasticity of the parts in the long stroke engine, 

 must be much greater than in the short stroke engine, and that they 

 must therefore require more clearance. However, it is probable that 

 there would be more loss in the latter engine than in the former. 



The loss of steam by filling the passages and nozzles, as also by the 

 radiation of heat from those parts, must evidently be the same in both 

 engines. 



Fourth. — The inertia and impulse of the muring parts on the sur- 

 rounding air. The loss in a steam engine occasioned by tliese two 

 causes may not be very considerable ; indeed as regards what is called 

 tlie inertia of matter in the moving parts, it is doubtful wliether any 

 such source of loss really exists; however if it does exist, it is clear 

 that the amount of loss must vary in proportion to the momenta of those 

 jjarts of the machine which are in motion, but as the momenta must be 

 as the mass of matter in motion mnlti|)lied by the velocity, and as 

 these are evidently much greater in the long stroke than in the short 

 stroke engines, (because the parts in the former, are if any thing, of 

 greater weight than in the latter, and also move at a double velocity,) 

 it follows that whatever loss may arise from the inertia, must be much 

 greater (double ill the long stroke engine than in the short stroke 

 engine. With regard to the loss occasioned by the impulse of the 

 moving parts on the air ; it must be admitted that in very slow motions 

 it cannot he very important ; nevertheless with a material increase of 

 velocity this source of loss becomes serious ; it varies as the extent of 

 surface of the moving parts multiplied into the square of the velocity; 

 It is tolerably manifest however that the surface of the moving parts 

 in the long stroke engine, will be, if any thing, greater than in the short 

 stroke engine, and that the velocity oi' the former will be twice that of 

 the latter; therefore the loss by impulse on the air in the long stroke 

 engine, must be four times that in the shoit stroke engine. 



Beside the foregoing causes, it is doubtful whether there are any 

 others that can produce any material difference in the efficient duty 

 or economical working of a steam engine ; at least none that can in any 

 way influence the question now under consideration. In estimating 

 therefore, the advantages of the short and long stroke engines, we 

 have in favour of the former a diminution of loss occasioned by fric- 

 tion, by radiation, by inertia, and by impulse on the air; while on the 

 other hand, we have in favour of the long stroke engines, a diminution 

 of loss in the clearance of the piston at the top and bottom of the 

 cylinder. It may be difficult to strike an exact balance between these 

 several sources of loss; but there can be no doubt that in a steam en- 

 gine the loss by friction is much greater than the loss by all the other 

 causes before mentioned put together; and it is past dispute that the 

 balance of loss as regards these causes, is decidedly against the long 

 stroke engine. (The advantages offered by the short stroke engine 

 as regards diminution of space ami weight, although of vast importance, 

 are not here adverted to, because they form no part of the immediate 

 inquiry.) 



It may be objected that to select an engine with an 8 feet stroke 

 and a cylinder of only 22 feet diameter for comparison, is not a fair 

 proceeding, because an engine of such proportions is unusual ; and it 

 may be also asked whether, if the principle is further extended by 

 making the stroke only 2 feet, and again doubling the area of the pis- 

 ton, wliether the advantage would still be in favour of the short stroke 

 engine? 



To this it may be answered that although an engine of 8 feet stroke 

 and 22 feet diameter of cylinder, may be unusual in this country, it is 

 not so in America; in that part of the world, many engines are em- 

 ployed of very nearly the above proportions, for purposes of steam 

 navigation ; and in which engines it is not unusual for the piston to 

 travel at the rate of 300 or 400 feet per minute. Again, as regards 

 the carrying out of the principle by still farther reducing the length of 

 stroke, say to two feet, and increasing the diameter of cylinder pro- 

 ])ortionately, say to 5 feet* there is no doubt whatever that such an 

 engine would have precisely the same mechanical effect as either of 

 the other two ; but the balance of advantages would be against an en- 

 gine of such proportions; because it would be verging to an extreme 

 on one side as much as the 8 feet stroke engine may be thought ex- 

 treme on the other side. It may, however, be safely affirmed that the 

 principle applies most powerfully to the case where the diameter of 

 cylinder is the same as the length of stroke ; because in that case the 

 proportions are most favourable for the diminution of friction and of 

 radiation, and offer- the minimum of disadvantage under the several 

 heads of loss above enumerated. 



As it. is manifest, therefore, that in all particulars which more im- 

 mediately affect the beneficial employment or working of a steam en- 

 gine, the long stroke has no manifest superiority over the short stroke; 

 it may appear strange that so decided a preference should have 

 hitherto been given to the former by the generality of engineers.. 



