PKKSIDKXTIAI ALDRKSS SECTfON A. 21 



type of engine has proved a great success for certain classes of 

 work, and in the short time since it was placed on the market 

 several millions of brake horse-power have been built. 



In recent years an intermediate type of engine has been 

 produced, and has become popular, known as the semi-Diesel 

 engine, which combines to some extent the merits of the high 

 compression system of the Diesel engine and those of the low 

 compression oil engine. 



This very briefly covers the range of internal combustion 

 engines, which are found in everyday use. All are of the recipro- 

 cating type, but there is a pt)ssibility that a gas turbine may yet 

 emerge from the experimental stage and take practical shape. 



It has been shown that the internal combustion engine has 

 in a very short space of time developed in many directions. In 

 the course of its evolution many difficulties have been en- 

 countered. Some of these have been wholly surmounted, others 

 have only been partially overcome. Progress in some directions 

 has been very rapid ; in others slow. Thus, in the case of small 

 power imits up to four to five hundred horse-power, very rapid 

 development has taken place, and a very high degree of perfec- 

 tion has been attained, but with large power units many diffi- 

 culties, anticipated and otherwise, have not been fully overcome. 



Improvement in thermal efficiency has been very rapid. The 

 average efifective indicated thermal efficiency of gas engines is 

 about 35 per cent. Theoretically, it is possible to further increase 

 this efficiency by increasing the compression ratio, but practical 

 considerations place a limit on such an increase, and one of the 

 best authorities of gas engines considers it unlikely that a 40 per 

 cent, thermal efficiency will be exceeded in commercial practice. 

 So far as economy of heat is concerned, both gas and oil engines 

 have considerably surpassed the best steam engines, and within 

 certain prescribed limits the internal combustion engine now 

 entirely holds the field against its older rival, the steam engine. 



In the case of larger imits, more esjjecially l)last furnace 

 gas engines, the most recent improvement, as regards thermal 

 efficiency, consists in using the heat contained in the exhaust 

 gases of the gas engine for raising steam, and at a lecture 

 recently delivered by Professor Hubert, of Liege University, 

 before the Iron and Steel Institute in London, a large experi- 

 mental plant, installed bv the Cockerill Company at Seraing. in 

 Belgium, is described. In this plant the exhaust heat from four 

 gas engines developing an aggre^^aie of 5.000 P>.H.P. is utilised, 

 and it is stated that 55 per cent, of the heat of the waste exhaust 

 gases is removed by the boilers, thus increasing the thermal 

 efficiency of the gas engines by about 13 per cent. 



In the matter of fuel, the internal combustion engine secures 

 a very distinct advantage. The steam engine is dependent upon 

 a fuel by means of which water can be economically transformed 

 into steam in a boiler and furnace. In the best types of steam 

 plant about one pound of the best Welsh steam coal is required 



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