All About Engines

244 All About Engines rate as the crank shaft in order that the exhaust valve shall be open every second stroke of the piston. The double-acting engine acts in the same way, but fuel is admitted at each end of the cylinder in turn. A trunk piston is here inadmissible, and there must be a front cover to the cylinder. The principal difficulty of making two-stroke and double-acting Diesel engines arises from the tendency to overheating, and consequent jamming of the pistons. With one “ combustion ” occurring every two revolutions the cold water in the cylinder jackets keeps the pistons cool enough, but with combustions occurring twice or four times as frequently the piston itself must have hollow spaces through which water is constantly circulating. This necessitates jointed tubes leading from the engine frame to the cross head and then up into the piston. The Mirrlees Diesel engine is made with single cylinders of 50, 80, and 125 horse-power, and from these sixteen different engines from 50 to 750 horse- power can be built. It is also made as a high-speed engine in nine sizes between those limits. It is used for driving machinery in factories, for pumping, and for driving dynamos, and it has also been applied to a rolling mill—a very severe test for any engine on account of the great variations of load. For fac- tories in which steam is not required for other pur- poses than power the engine has been found very suitable, and even in textile mills where steam is required to charge the atmosphere with moisture it has been employed.