INTERNAL COMBUSTION 71NGINE 171 INTERNAL COMBUSTION ENGINE engine have been comparatively recent, as it was only in 1878 that Otto placed the first important commercial gas en- gines upon the market. In his engine, as in the common motor of to-day, the gas was compressed before it was ig- nited. The two great classes into which in- ternal combustion engines may be di- vided are: done at approximately the end of the compression; fourth, the expansion of the gas, i-esulting from combustion; which forces the piston down ; and fifth and last, forcing out the burnt gases on the up stroke of the piston. This type of motor, which requires four strokes of the piston to complete its cycle, is known as a four-cycle motor, while some motors with special valve arrangements and £XHAU5T, VALVE SXHAU5T w SPARK RLUe HEAD PISTON RfNQb WR/ST P/fsf CYLINDER- CAM SHAFT CRANK -=fcl 3/ ^/,'^yACVE SPRINO n (i VALVE ROD irk , ._ /NLETFOR nilll/i FUEL MjxruRE VALVE HEA-O INLET VALVE TIMING Gear C/?A^NH SHAFT CROSS SECTION OF FOUR-STROKE GASOLINE INTERNAL COMBUSTION ENGINE A. Those in which the ignition of the fuel occurs at constant volume. B. Those in which the ignition of the fuel occurs at constant pressure. The common gasoline motor of to- day is of the first type, and usually op- erates under what is known as the Otto cycle: first, charging the cylinder with fuel, which is done on a down stroke of the piston; second, the compression of the fuel on the up stroke of the piston; third, the ignition of the fuel, which is deflectors go through a complete cycle in two strokes. Gasoline is the common fuel of this type of motor, although there are models designed to operate on natural gas, al- cohol, and kerosene. The fuel is mixed with air in a carburetor or similar de- vice. The entrance of the fuel into the cylinder, as well as the expulsion of burnt gases, is controlled by valves, which may be of the poppet and cam type, or of the sleeve variety. In a
