GAS-ENGINE 



107 



Hame through the action of a special slide-valve. 

 l'nder tin- impulse of the explosion tin- piston rose 

 \\ith great velocity to the top of its stroke, being 

 free to rise without doing work on the engine shaft. 

 Tin' 1 turned gases then c<toled, and their pressure 

 fell below tluit of the atmosphere. The pi-ton wiis 

 therefore urged down by the. pressure of the uir, 

 and in i-Miiiin;: down it was automatically put into 

 gear with the shaft, and so did work, the products 

 of coinliiistion being expelled during tlie lust part 

 of the down stroke. Tlie engine was excessively 

 noi-\ , luit it took less than half the amount of gas 

 that had heeii taken by Lenoir. 



Otto's invention of 1876 again halved the 

 consumption of gas, and quickly raised the gas- 

 engine to the position of a commercially important 

 motor. Its success may he judged from the fact 

 that in 1889 there were some thirty thousand 

 engines of this type in use, of sizes which give 

 from 100 horse-power down to a fraction of 1 

 horse-power. In the Otto engine the cylinder is 

 generally horizontal and single-acting, with a trunk 

 piston, and it takes two revolutions of the crank- 

 shaft to complete a cycle of operations. During the 

 first forward stroke gas and air are drawn in, in the 

 proportion proper to form an explosive mixture. 

 During the first backward stroke the mixture is 

 compressed into a large clearance space behind the 

 piston. When the next forward stroke is about to 

 pegin, the compressed mixture is ignited, and work 

 is done by the neated gases during the second for- 

 ward stroke. The second backward stroke com- 

 pletes the cycle by causing the burned gases to be 

 expelled into an exhaust-pipe leading to the outer 

 air. The clearance space is, however, left full of 

 burned gases, and this portion of the previous charge 

 is allowed to mix with the fresh air and gas which 

 is drawn in during the first forward stroke of the 

 next cycle. Since only one of the four strokes 

 which are required to complete a cycle is effective 

 in doing work, a massive fly-wheel, running fast, is 

 used to furnish a large magazine of energy, and in 

 cases where exceptional uniformity of "speed is im- 

 portant as, for instance, in electric lighting it is 

 usual to have two heavy fly-wheels. A centrifugal 

 governor controls the engine by cutting off the 

 supply of gas when the speed exceeds a prescribed 

 limit. The cylinder is kept moderately cool by the 

 circulation of cold water in a water-jacket ; and 

 the usual means of igniting the charge is a slide- 

 valve, the construction of which is described below. 



The general appearance of an Otto engine, as 

 made by Messrs Crossley Brothers, is too well known 

 to need an extended description. It resembles 

 a single-cylinder horizontal steam-engine, heavily 

 built and mounted on a somewhat high bed-plate. 



Fig. 2. Section through Cylinder of Otto's Engine. 



In the smallest forms a vertical arrangement of 

 the cylinder is adopted, and for the largest 

 powers a pair of horizontal cylinders are set side 

 by side. Fig. 2 shows some of the principal 



details by a hori/ontal section through the cylinder. 

 The pi -i in. I', appears in the figure at tne back 

 end of its stroke, ami the spare A in the clear- 

 ance. Its volume is usually from two to three 

 fifths of the volume swept through by the pi-ion. 

 lillli in the water-jacket. C in the exhaust-valve, 

 which is opened by the action of a revolving cam 

 dining the seconu back-stroke of the cycle. The 

 slide valve, D, in made to slide backwards and for- 

 wards across the back end of the cylinder by means 

 of a connecting-rod driven by a short crank on the 

 lay-shaft, K, which is driven by bevel or (screw 

 gear from the main shaft, so that it turns once for 

 two revolutions of the main shaft. This valve 

 serves to admit gas and air, and also to carry an 



igniting ilame to the mixture after compression in 

 the cylinder. An igniting jet is kept burning at F, 

 behind the valve. In the valve tnere is a small 



chamber, G, supplied with gas, and as this passes 

 the jet it ignites and continues burning until by 

 the further movement of the valve the chamber, G, 

 communicates with the cylinder through the open- 

 ing H, by which time the back of the chamber is 

 closed. In a number of recent Otto engines the 

 ignition of the mixture is brought about in a differ- 

 ent way. There is a short tube closed at one end 

 and communicating at the other with the cylinder, 

 through a valve. The tube is kept red-hot by a 

 Bunsen-flame playing round it, and at the proper 

 moment a portion of the charge within the cylinder 

 is allowed access to the red-hot tube through the 

 valve. 



Fig. 3 is a copy of an indicator-diagram from an 

 Otto engine. AB is the first stroke of the cycle, 



Fig. 3. Indicator-diagram of Otto's Engine. 



and corresponds to the taking in of gas and air at 

 a pressure sensibly the same as that of the atmo- 

 sphere. BC is the compression stroke. At C igni- 

 tion takes place and raises the pressure quickly to 

 D. CDEB is the effective forward stroke, and the 

 exhaust-valve is opened for the escape of the waste 

 gases near the end of this stroke at E. The 

 expulsion of the gases goes on from B as the piston 

 moves back to A, and this completes the cycle. 



There are now a number of other successful gas- 

 engines which more or less resemble Otto's. In 

 Clerk's engine a similar cycle is performed, except 

 that there is an explosion at each forward stroke. 

 The waste gases escape through exhaust-ports near 

 the front end of the cylinder, which are uncovered 

 by the advance of the piston, and a displacer cylin- 

 der or pump immediately forces in a fresh mixture, 

 which is compressed dunng the return stroke. In 

 Andrew's (the Stockport) engine, and in Robson's 

 (made by Messrs Tangye), an impulse in every 

 revolution is secured by compressing the explosive 

 mixture in a pump, which in some cases is supplied 

 by using the front end of the working cylinder itself 

 for this purpose. In the ' Griffin ' engine ( Messrs 

 Dick, Kerr, & Co.) explosion occurs at both ends of 

 the cylinder, bnt only at every third stroke : the 

 cycle includes the drawing in and rejecting of a 

 ' scavenger ' charge of air, as well as the drawing 

 in and compression of the explosive mixture and 

 the rejection of the burned gases. A recent engine 



