A more attractive system of control is to have the turbines drive electric generators and then use the electric power to drive the propellers through motors mounted on the propeller shafts. The electric power can easily be controlled from the bridge and the propellers may be reversed by reversing the motors. However, the disadvantage of the electric system is that it occupies a great deal of space, particularly in plants running over 100,000 horsepower. CHAPTER X BURNING FUEL IN THE ENGINE CYLINDER ONE OF the handicaps of steam power is that the heat produced by the combustion of fuel is not used directly to drive the piston. A large part of the heat energy in coal goes up the chimney or is wasted by radiation from the furnace walls. With anthracite coal in the furnace the loss may be as low as 22 per cent in the very best types of boilers. In the engine other serious losses occur, so that in the best condensing reciprocating steam engines the power delivered is only 10 to 16 per cent of that stored in the fuel. In locomotives the efficiency is as low as 4 to 6 per cent. In turbines the steam is used to better advantage and the actual power delivered may run up to 20 per cent of that in the fuel. In internal-combustion engines the furnace and boiler losses are largely overcome by burning the fuel right in the cylinder, where the heat energy of the combustion may be utilized directly upon the piston. There are other losses, however, so that the most efficient gasoline engines deliver only 28 per cent of the energy in the fuel, and coal-gas engines may run up to 31 per cent. A wide range of fuels may be used in an internal-combustion engine. They may be either gaseous, liquid, or even solid. In stationary engines hydrogen, coal gas, natural gas, blast-furnace gas, and producer gas are employed to advantage. Volatile fuels, such as alcohol, bensol, gasoline, and kerosene, are turned into a mist or vapor and then burned as a gas. In engines of the Diesel type any liquid fuel ranging down to thick crude oils may be employed, and in one type of engine, which, however, has not proved commercially practicable, coal dust is burned in the cylinder. BURNING AIR IN THE AUTOMOBILE ENGINE Owing to the widespread use of automobiles, the general public is better acquainted with internal-combustion engines than with steam engines. Nevertheless, there are many elementary facts in connection with gasoline engines that are not generally known. The average motorist probably does not realize that he burns far more air in his motor than gasoline, and he probably does not understand why it is that the mixture of gasoline vapor and air must be compressed before it is ignited, or why only one out of four strokes of the piston is a power stroke.