All About Engines

How a Modern Engine Works 9 the flywheel carries the crank over the dead centre, so that at two points in its revolution the flywheel alone keeps the engine working. For ninety-nine per cent, of the time the engine drives the flywheel, and for one per cent, the flywheel drives the engine. But it does more than this. Not only are there variations of pressure inside the cylinder, but the turning effort of the connecting rod alters with its position. Everyone who has turned the handle of a grindstone knows that the greatest effect is obtained when the push or pull is exerted at right angles to the crank. When this greatest effort occurs in the steam engine the flywheel is forced to turn more rapidly, and when the effort falls off the speed de- creases very slowly, because of the flywheel. The flywheel, therefore, smooths out the inequalities of motion and gives steadiness. It is like a policeman keeping a crowd always on the move. But the policeman does not work for nothing. His services have to be paid for. Some of the force exerted by the steam is employed in turning the heavy wheel. There is no increase, but a decrease, in the work done by the engine. The only thing that can be said in its favour is that the engine would not work without it. Let us now inquire how the steam is admitted to each end of the cylinder at the right moment to operate the piston. By referring to Fig. 6 it will be observed that the ports are partly covered by a sort of shallow box with the face downwards. This is the slide valve, and from its shape it is often called a “ D ” slide valve. The hollow in the under side