Although this country has lagged somewhat behind Europe in adopting large gas engines, there is evidence that this state of affairs will not exist very long, for a number of firms are now in the field building gas engines from 1,000 to 6,000 horse-power capacity. The uses to which these large engines are put are about equally divided between the operation of blow ing engines for blast furnaces and the driving of dynamos for general power distribution. While the gas engine in the larger sizes is thus used extensively for the generation of electric light and power, a growing tendency is observed to use the gas engines direct as motors. A num ber of railroad and other machine shops have been equipped with moderate sized gas engines suitably located about the works, and in addi tion, thousands of horse power are used in the smaller sizes for a wide variety of purposes, including village waterworks, isolated lighting stations and manufacturing plants of all kinds.
The gas engine, usually adapted to consume gasoline or other oil fuel, has invaded the agri cultural field and is being used to an increasing extent for a variety of purposes and in many forms, not the least important of which is the gas tractor, which is especially adapted to agri cultural service in that it is conveniently mobile in the field and on the road, nor is it burdened with steam boiler nor gas producer.
In the field of transportation in addition to its use in the familiar automobile and motor truck, the -oil engine constitutes the power plant in motor cars, in submarine vessels, in aero plane service and also to a limited though in creasing extent in railway motor cars.
Gas Engine Central Power Stations.— With the possibilities of high thermal efficien cies still higher development of cheap fuel gas processes may be expected that will bring the gas engine into close competition with the elec tric motor for power purposes, for it will doubtless be found that gas transmitted from a central gas-making plant at a manufacturing works into engines located nearby, combined with the recovery of by-products from the fuel will produce a very cheap form of power. Where large amounts of power are involved, as in the various electro-chemical industries, it is quite feasible to locate such works near a cheap fuel supply; in which case the recovery of ammonia and liquid hydrocarbons from the solid fuel, and the use of the residual gas in internal combustion engines combined with the generation and distribution of electrical energy should effect a material saving in the utilization of power over any existing method. It is not to be presumed that the gas engine will displace either the electric motor or the steam engine; each has its legitimate sphere of usefulness, and each will be more highly developed as the result of direct competition. Yet the economies al
ready obtained indicate that the field of the gas engine will be gradually extended into that of the steam engine and the electric motor.
Steam Many of the questions involved in this consideration are at the present time in a transitional stage. The reciprocating steam engine has reached a high state of de velopment, but it is not probable that it has attained its highest degree of perfection. While an economy of less than eight and one-half pounds of steam per horse-power hour has been obtained, even better results may be anticipated; the use of high pressure super-heated steam in compound, jacketed engines involves more per fect lubrication, and this may demand modifica tion in existing valve types; however this may be, the outlook is promising for still higher efficiency.
Experiment demonstrates that both piston engines and steam turbines gain more in effi ciency with a reduction in back pressure than they do with a corresponding increase in initial pressures; and further that steam turbines gain more than reciprocating engines. Superheat is another important element in steam engine and turbine economy. A gain in thermal efficiency very nearly proportional to the superheat is to be expected, but since maintenance costs in crease rapidly beyond a moderate degree of superheat practice has confined itself to values of about 100°; occasionally going as high as 200° or 250° as the economic limit for recipro cating engines. These amounts depend upon the boiler pressure so that the initial tempera ture of the steam usually runs from 450° to 550° F. In steam turbines, owing to the ab sence of rubbing surfaces in contact with the steam, the superheat may be higher and tem peratures of 650° F. are not uncommon. With back pressures reduced to less than one-half pound per square inch absolute which are now in use, it is reasonable to assume that any in creased gain in efficiency in the future must result from higher initial temperatures and pressures. In this respect it is significant to note that steam boilers have recently been con structed for initial pressures of 400 pounds per inch, nch, and even 600 pounds pressure is being considered. At the same time high de grees of superheat have recently been adopted in special cases in which final temperatures of 800° F. and 1,000° F. have been attained. Whether this will mean cheaper power than can be obtained in other ways will depend upon many conditions. In any case, and especially with intermittent or variable loads, it is not so much a question of maximum efficiency as it is economy of operation.