All About Engines

The Gas Engine 187 eccentric without the strap. One for each valve— air, gas; and exhaust—is fixed on a shaft parallel to the engine bed, and driven from the main shaft by bevel wheels or by worm gearing at half the speed. This ensures that each valve shall be opened once in every two revolutions of the main shaft. In small engines the cams may be in direct contact with the valve spindles, but the usual plan is for the motion to be communicated through levers. Other kinds of valve have been tried, but none are so satisfactory as these. Slide valves do not work well under the high temperatures which occur during explosion, and rotary valves are not easily adjusted for wear. Ignition of the mixed gases in the original Otto engine was effected by means of a hot tube. The tube was closed at one end, the open end communicat- ing with the interior of the cylinder. It was kept hot by a flame which played upon it all the time the engine was working. At each compression stroke the mixture of gases was forced into the tube and became ignited. Sometimes the ignition occurred prema- turely, but not as a rule, because there always re- mained some of the spent gases from a previous explosion. For many years it was the only plan which worked satisfactorily, but in recent years it has been replaced entirely by the electric spark. For producing the electric spark there are two methods—an induction coil and a magneto machine or small dynamo. Both are good, but the magneto is cheaper than a really good coil and less liable to get out of order than a cheap one. Moreover, for the