A Catechism of the Steam Engine

by C. E. John Bourne

216. Q.--How do you ascertain the power of high pressure engines?

A.--The actual power is readily ascertained by the indicator, by the same process by which the actual power of low pressure engines is ascertained. The friction of a locomotive engine when unloaded is found by experiment to be about 1 lb. per square inch on the surface of the pistons, and the additional friction caused by any additional resistance is estimated at about .14 of that resistance; but it will be a sufficiently near approximation to the power consumed by friction in high pressure engines, if we make a deduction of a pound and a half from the pressure on that account, as in the case of low pressure engines. High pressure engines, it is true, have no air pump to work; but the deduction of a pound and a half of pressure is relatively a much smaller one where the pressure is high, than where it does not much exceed the pressure of the atmosphere. The rule, therefore, for the actual horse power of a high pressure engine will stand thus: square the diameter of the cylinder in inches, multiply by the pressure of the steam in the cylinder per square inch less 1-1/2 lb., and by the speed of the piston in feet per minute, and divide by 42,017; the quotient is the actual horse power.

217. Q.--But how do you ascertain the nominal horse power of high pressure engines?

A.--The nominal horse power of a high pressure engine has never been defined; but it should obviously hold the same relation to the actual power as that which obtains in the case of condensing engines, so that an engine of a given nominal power may be capable of performing the same work, whether high pressure or condensing. This relation is maintained in the following rule, which expresses the nominal horse power of high pressure engines: multiply the square of the diameter of the cylinder in inches by the cube root of the length of stroke in feet, and divide the product by 15.6. This rule gives the nominal power of a high pressure engine three times greater than that of a low pressure engine of the same dimensions; the average effective pressure being taken at 21 lbs. per square inch instead of 7 lbs., and the speed of the piston in feet per minute being in both rules 128 times the cube root of the length of stroke. [1]

218. Q.--Is 128 times the cube root of the stroke in feet per minute the ordinary speed of all engines?

A.--Locomotive engines travel at a quicker speed--an innovation brought about not by any process of scientific deduction, but by the accidents and exigencies of railway transit. Most other engines, however, travel at about the speed of 128 times the cube root of the stroke in feet; but some marine condensing engines of recent construction travel at as high a rate as 700 feet per minute. To mitigate the shock of the air pump valves in cases in which a high speed has been desirable, as in the case of marine engines employed to drive the screw propeller without intermediate gearing, India rubber discs, resting on a perforated metal plate, are now generally adopted; but the India rubber should be very thick, and the guards employed to keep the discs down should be of the same diameter as the discs themselves.

219. Q.--Can you suggest any eligible method of enabling condensing engines to work satisfactorily at a high rate of speed?

A.--The most feasible way of enabling condensing engines to work satisfactorily at a high speed, appears to lie in the application of balance weights to the engine, so as to balance the momentum of its moving parts, and the engine must also be made very strong and rigid. It appears to be advisable to perform the condensation partly in the air pump, instead of altogether in the condenser, as a better vacuum and a superior action of the air pump valves will thus be obtained. Engines constructed upon this plan may be driven at four times the speed of common engines, whereby an engine of large power may be purchased for a very moderate price, and be capable of being put into a very small compass; while the motion, from being more equable, will be better adapted for most purposes for which a rotary motion is required. Even for pumping mines and blowing iron furnaces, engines of this kind appear likely to come into use, for they are more suitable than other engines for driving the centrifugal pump, which in many cases appears likely to supersede other kinds of pumps for lifting water; and they are also conveniently applicable to the driving of fans, which, when so arranged that the air condensed by one fan is employed to feed another, and so on through a series of 4 or 5, have succeeded in forcing air into a furnace with a pressure of 2-1/2 lbs. on the square inch, and with a far steadier flow than can be obtained by a blast engine with any conceivable kind of compensating apparatus. They are equally applicable if blast cylinders be employed.

220. Q.--Then, if by this modification of the engine you enable it to work at four times the speed, you also enable it to exert four times the power?

A.--Yes; always supposing it to be fully supplied with steam. The nominal power of this new species of engine can readily be ascertained by taking into account the speed of the piston, and this is taken into account by the Admiralty rule for power.