All About Engines

The Gas Engine 199 numerous, and for anything over 200 horse-power engineers usually prefer to employ two or more cylinders. English makers for a long while arranged them side by side, while Continental firms preferred the horizontal tandem arrangement with one cylinder behind the other. All large gas engines are started by means of compressed air, which is produced by a separate engine of relatively small size. The flywheel is barred round until the piston and valves are in the correct position for starting, then compressed air is admitted, and as soon as the engine has fairly started the com- pressed air is cut off and the gas supply turned on. The absolute necessity of some such aid as this for large engines will be realised by anyone who has seen even a 20-horse-power engine started by hand. It is due both to the fact that with only one explosion in four strokes a very heavy flywheel is necessary to equalise the motion, and to the force required to compress the charge before an explosion can take place. When very large cylinders are necessary they are often cast in four or more parts and then bolted to- gether before being bored. All castings, and especially large ones of complicated form, are subject to strains which are set up during the process of cooling in the mould. This strain would disappear in time, especi- ally if the whole of the “skin” could be removed. But the shape of a gas engine cylinder is such that it cannot be machined all over, and while the strain is there it is a source of weakness.