nd few small cars use the two-stroke cycle first devised by Dugald Clerk in 1880. In this type of engine the movement of the piston admits the fuel and exhausts the burned gases by uncovering "ports" or holes in the side of the cylinder.Fuel and exhaust pass in and out of a four-stroke engine using a more sophisticated system of valves, controlled automatically by a camshaft driven direct from the engine's crankshaft. As the engine operates, the valves are successively opened and closed.The moment of ignition of the fuel must also be accurately controlled. This is done by a distributor, again mechanically connected to the crankshaft, which directs a current of electricity successively to each of the cylinders. This current "fires" a spark in the sparking plugs and the fuel is ignited.Otto's engines ran on coal gas, a perfectly satisfactory fuel but one that is difficult to store. The gas engine was greatly improved by the use of liquid fuels such as petrol (gasoline) made by refining crude oil. To turn petrol into a combustible vapor it is mixed with air to form a fine mist of droplets that can be drawn into the cylinders. The mixing is carried out in a carburetor.Unlike steam engines most internal combustion engines do not produce great power at slow speeds. The cylinders are small and each individual ignition stroke produces comparatively little power. To obtain a useful amount of work from such engine it must be run fast, to put the maximum number of ignition strokes into each second. Motor car engines commonly produce their maximum power at speeds of 5,000 revolutions per minute or more. The oscillating pistons and valve gear sets the upper limit on speed. Specially prepared engines, in which great attention has been paid to balance and smoothness, can be obtained more power by running into speeds of 12,000 rpm or more.The Economical Diesel EngineThe compression-ignition engine, designed by the German Rudolf Diesel (1858-1913) in 18...
