274 ANNUAL KEPOET SMITHSONIAN INSTITUTION, 1912. 



to cooling allows the gas to conserve its heat at the moment of the 

 explosion and, further, the absence of nooks assures a perfect removal 

 of the burnt gas. It should be noted that the concentric sheaths of 

 the Knight motor require very accurate working, wliich makes the 

 construction dehcate and raises the cost of repairs. 



Gas motors are not to be recommended for use on vessels because of 

 the danger of the formation of explosive gas mixtures in badly venti- 

 lated engine rooms. This objection does not apply to the Diesel 

 motor, mentioned in the Review for 1903. This motor is of interest for 

 war sliips because of the suppression of smoke, the reduction of per- 

 sonnel, as well as its augmented scope of action, which is some three 

 times as great as vnth the ordinary steam engine. 



Let me restate that the principle of the Diesel motor is to inject pro- 

 gressively the combustible liquid by means of a pump into air com- 

 pressed to 30 atmospheres. Because of the high temperature due to 

 the compression, the mixture spontaneously ignites, so that there is 

 no need of an igniter. This motor can use petroleum residues useless 

 for all other motors. Official tests made on submarines provided with 

 Diesel motors have shown thermal efficiencies reaching 32.5 per cent 

 for a horsepower of 75 and 42 per cent for horsepowers of 395, whereas 

 the best triple-expansion steam engines seldom surpass 13 per cent. 



The Maison Sulzer has recently constructed a locomotive of 1,000 

 horsepower driven by a 4-cylinder Diesel motor. 



Among the motors for aviation the king of the day is undoubtedly 

 the Gnome. This is a two-center rotating motor. The cylinders, 

 odd in number, turn about one of these centers while the pistons turn 

 about the other. The line of centers plays the part of a unique crank 

 which remains fixed. This arrangement does away with all the vary- 

 ing forces due to the inertia wliich are inevitable with motors using 

 fixed cylinders, and thus assures a very satisfactory equilibrium. 



On the other hand, the rapid rotation of all the c^dinders produces 

 an energetic ventilation and thus at the same time solves the problem 

 of cooling them. That result is obtained, it is true, thi'ough an 

 increased power consumption for this stirring of the air, wliieh may 

 amount to 10 per cent of the total power developed. We may add 

 that this rotation intimately mixes the combustible with the neces- 

 sary air, makes the cylinders fill very uniformly, circumstances very 

 favorable to a regularity of the explosions. 



Rotating motors have the objection that the oil is thrown against 

 the bases of the cylinders and toward the valves. They are also very 

 costly because of the especial care necessary in tneir construction and 

 their rapid wearing out. A rotating motor while working escapes 

 proper surveillance. The gjToscopic effects resulting from the 

 rapid rotation of the motor and possibly disturbing in the maneuver- 

 ing of the aeroplane do not seem to have troubled the aviators. 



