1853] 



ERICSSON'S ENGINE. 



251 



of tlie external air, the inner surface being exposed towards a 

 highly heated enclosure will be less ati'eeted. It is to be ob- 

 served th.it the temperature (if the external surface of the regene- 

 rator cannot at any time be greater than that of the air escaping 

 through the pipe i, and that the temperature of the internal 

 surface can never be less than that of tbe air issuing from this 

 surface, on its passage into the working cylinder, or rathei', 

 heating chamber. 



Ihe preiiaration necessary for starting the engine cnsists in 

 " keeping up a slow fire in the furnaces, for about two hours, 

 until the various parts contained within the brick work shall 

 have become moderately heated, and then charging the receiver 

 with air by means of a hand-pump," until the gauge sho\vs a 

 pressure of about 8 pounds above that of the external air. The 

 upper valve, g, is then opened by a starting bar, and the com- 

 pressed air flows into the working ejlinder, and begins the work 

 of raising the piston. 



We are now prepared to enquire into the 



THEORY OF IHE MOTIVE POWER 01' THE ENGINE. 



I will first state a few principals which it is important should 

 be kept in view. 



1. The expansive force of the heated air under the woiking 

 piston must be somewhat less than that of the compressed air in 

 the receiver ; otherwise the air in the receiver would have no 

 tendency to flow from it into the heating chamber. The differ- 

 ence may not amount to more than a few ounces; it depends 

 upon the obstructions to the -free flow of the air and the relative 

 size of the aperture of communication and heating chamber. 



When the air is flowing from the supply cylinder into the 

 receiver, its elastic force must exceed that of the air in the re- 

 ceiver; for the additional reason, beside that just stated, tliat the 

 ^■ales in the supply piston would close if no such difference of 

 pressure existed. 



In seeking to determine the power of the engine, I shall how- 

 ever disregard the inequality of pressure and suppose the expan- 

 sive force of the air to be the same in the working and supply 

 cylinders as in the receiver, so long as the communications be- 

 tween them are open. 



3. Since the two connected pistons are of unequal size, and 

 the elastic force of the air pressing upon them the same or nearly 

 the same, the entire upward pressure exceeds the downward pres- 

 sure, and the two pistons are urged up with a force equal to 

 the difference of these pressures. This statement is here made 

 with respect to the actual pressures subsisting when the commu- 

 nications are open. We shall see hereafter that it might also be 

 made in regard to the mean effective pressures throughout the 

 stroke. 



4. In the engines of the Ericsson the cut off is introduced at 

 the I stroke, and therefore the space underneath the woi'king 

 piston into which the air is admitted from the receiver, before 

 the cut off valve is closed, is equal in volume to the interior of 

 the supply cylinder. It will soon be seen that this is in accord- 

 ance with a general principle, the adoption of which is essential 

 to the most etKcient operation of the present form of engine. 



6. When the engine has reached its permanent working state, 

 the quantity of air admitted into the working cylinder, each 

 upward stroke of the piston, cannot exceed the quantity forced 

 into the receiver, from the supply cylinder, duiing the same 

 interval. In fact it must be less, by reason of the w^aste from 

 leakage and clearance. 



Now it will be perceived that if this quantity of air, after being- 

 admitted into the working cjdinder, as just supposed, retained 

 the same teinperature, its elastic force would hr^ the same as that 



of the external air (15 lbs. say, per square inch) since the same 

 quantity originally filled the supply cylinder, at this pressure. 

 But if we suppose the temperature to be elevated 480 ° , or 

 thereabouts, by the heat deri\ed from the regenerator and the heat- 

 ing chamber, "its elastic force would be doubled, or amount to 

 30 lbs., per square inch. To realize this supposition the com- 

 pressed air in the receiver must therefore have an expansive 

 force of o\-er 30 lbs., or 15 lbs., above the atmospheric pressure. 

 If the working temperature in the lower cylinder were 334 ° 

 abo\ e the temperature of the externtd air instead of 480 ° , 

 then the pressure in that cylinder, and of necessity therefore in 

 the receiver, would be 12 lbs., above tbe atmospheric pressure, 

 (i. e. fifths of 15 lbs.) It will be seen then that the working 

 pressure in the i-eceiver and the working temperature in the 

 principal cylinder are necessarily connected together — that the 

 one determines the other. 



It is here supposed that there is no leakage or clearance, ^ but 

 the feet is otherwise ; and therefore the quantity of air admitted 

 into the working cylinder, each ascending stroke, is less than 

 that which is expelled from the supply cylinder into the receiver. 

 If we suppose tbe pressure in the receiver to be 8 lbs., above the 

 atmospheric pressure, and that the leakage and clearance, at this 

 pressure amounts to \, then f of the air furnished by the supply 

 cylinder will enter the working cylinder, and its elastic force, 

 for the f stroke would be reduced to 11 jibs, (f of 15 ibs.,) by 

 the expansion, if the tempei'ature remained unchanged, but the 

 480 ° of additional heat will augment this to 22tJ- lbs., or ISlbs., 

 + 7;j ibs. Now 8 ibs. above the atmospheric, is the actual 

 ^vo^kino• ]-iressure of the engines, we may conclude therefore, that 

 if the woiking temperature is 480 ° above the atmospheric tem- 

 perature or a little less, the waste from leakage and clearance, 

 during 'Jie double s'l-oke, inns' amount to nearly \. The actual 

 working temperature is undoubtedly less than this, but how much 

 I ha\'e not been able to ascertain with certainty. The actual 

 leakage is therefore less than i, but its exact amount cannot at 

 present be determined. According to the newspaper accounts 

 the working temperature, on the trial tri]), was about 450 ° , 

 or 418 ° above the temperature of the air (taken at 32 ° .) This 

 would make the waste, from leakage and clearance, about \. It 

 undoubtedly lies between ^ and i. 



Working at a given temperature, and with a given cut off, 

 the leakag-e will determine the working pressure. To show this 

 suppose the elevation of temperature to be 480 ° , and the leak- 

 ao-e i at a pressure of 8 ibs., shown by the receiver-guage ; then 

 at 12 ibs. pressure the leakage, if we disregaid the clearance 

 which is comparatively small, -would be fths, and the elastic 

 force of the air in the working cylinder would be reduced from 

 7-2- ibs. to 3 J lbs. If the communications remained the same, 

 so" great a difference of pressure between the receiver and the 

 cylinder couLl not be realized ; an a^lditional quantity of air 

 would flow out of the receiver, and this would go on for each 

 successive stroke until the pressure in the receiver was reduced 

 to 8 ibs. or thereabouts, when the pressure in the cylinder would 

 be 1^ ibs., and the engine would be nearly in its permanent 

 w^orking condition. 



From this cause, (\'iz., the leakage,) mainly, as it would seem, 

 the expected pressure of 1 2 ibs. has not been obtained in the 

 working of the engines of the Ericsson. This is in fact the 

 reason assigned by the buildei-s of the engines, for the fact that 

 no higher "pressure than 8 ftis. has yet been reahzed. 



There is another mode of presenting the theory of the motive 

 power of the caloric engine. Suppose that the constant pressure 

 in the receiver is 15 ibs. + 15 ibs. On this supposition air will 

 begin to pass from the supply cylinder into the receiver, at the 

 end of the ^ stroke, or thereabouts, and will continue to flow to 

 the end of tlie stroke, at a pressure a little above this. At the 

 end of the 4 stroke of the supply piston the body of air which 



