Mathematical and Physical Papers. 213 



strokes, and, both together, an impulse for each stroke, or two for 

 each revolution. This places it on a par in this respect with a 

 single-crank steam-engine, and the governing features are good for 

 any service. In the largest sizes it is made still better, however, 

 by having two cranks and rods, with two pairs of cylinders, the 

 cranks being set at ninety degrees with each other, by which 

 method four impulses per revolution are secured. Engines of the 

 single-crank type are made in powers ranging from 200 to 2000 

 horse-power, and of the double-crank type from 400 to 4000 horse- 

 power. Since very high temperatures are produced in all gas- 

 engines during combustion, sometimes as high as 2800 or 3000 

 degrees Fahrenheit, which is above the melting-point of cast iron, 

 water circulation in all parts exposed to the heat is necessary, and 

 in the double-acting engines this cooling of the enclosed pistons 

 presents some difficult features in design. 



The Diesel oil-engine is built in vertical form, with either two 

 or three cylinders and sets of moving parts, and in general appear- 

 ance resembles a marine steam-engine. Each cylinder is single 

 acting, giving one impulse in two revolutions, so that the engine as 

 a whole gives either one or one and one-half impulses per revolution, 

 which insures good governing features. The maximum tempera- 

 ture in the cylinder is not far from 1400 degrees Fahrenheit, which 

 is that resulting from the preliminary compression, since the ad- 

 mission of fuel oil is so regulated that it burns during expansion 

 without rise in temperature. The engine is built at present in 

 sizes ranging from 75 to 225 horse-power, but may be readily 

 doubled up on the same shaft and so double the range of power. 



As efficient converters of heat into mechanical energy these en- 

 gines may well be studied in comparison with the steam-engine, 

 since it is with the latter that they come into competition. In this 

 comparison a compound condensing steam-engine will be taken as 

 the type representing the class in large manufacturing plants. and 

 for power-station service. A good type of steam-boiler will utilize 

 from 70 to 75 per cent, of the heat in the fuel, as equipped and 

 operated in first-class establishments. The engine of the kind in 

 question will utilize from 14 to 15 per cent, of the heat coming 

 to it in the steam in general service, or 10 to 11.2 per cent, of the 

 total heat value of the fuel burned. The bulk of the remainder of 

 the heat is carried away in the cooling water in the condenser 

 and up the flue from the boilers. The gas- and oil-engines, on the 

 other hand, work with a thermal efficiency of from 17 to 25 per 

 cent., with possibilities of still better results. 



