All About Engines

All About Engines and the engine reverses. Usually the “ timing ” can be varied. In order to avoid a “ back fire ” when starting, it should be retarded ; but when the engine is running at full speed an increase of power is obtained by advancing the spark. With all single-cylinder engines and with some arrangements of double- cylinder engines, a flywheel is necessary to carry on in the intervals between the successive impulses. But, as a rule, no governor is required, since the petrol engine is AlR AMD Petrol Fig. 120,—Diagram of two-stroke petrol motor rarely used for any purpose in which the load is suddenly re- duced at a time when there is no one with his hand on the throttle valve or the brake. While the foregoing de- scription covers the maj ority of petrol engines there are a few variations from general practice to which refer- ence must be made. First as to valves. The poppet or mushroom type has the disadvantage of being noisy, especially if the “ lift ” is large, and there is a tendency