Page:Popular Science Monthly Volume 18.djvu/502

486 in two complete strokes, but whether a fresh supply is taken or not is determined by the governor, which acts to maintain a constant speed under varying loads. It is of the ordinary ball form, and is placed in the cup-shaped receptacle pendent from the cylinder. It actuates by its movement a lever controlling the gas-valve, so that this is opened and closed in accordance with changes in the speed. The regulation is delicate, and the speed nearly if not quite as uniform as in a steam-engine. The speed can be changed at will by increasing or diminishing the amounts of air and gas which may be drawn in each time. An automatic device is provided, which closes the gas-valve, should the engine by any accident stop in a position in which this would be left open. The oiling is committed almost entirely to the engine itself, the only work required in this connection being the filling of the oil-cups. They are placed upon the top of the cylinder, and by means of the small shaft and pulley driven from the lay-shaft deliver a given number of drops of oil to the slide-valve, cylinder, and piston at each revolution. The exhaust is rendered noiseless by being passed into a chamber, from which it escapes into the atmosphere under slight pressure. The cylinder is water-jacketed to keep it cool, the circulation of the water being maintained by the heat received, the warmer water rising to the supply-tank and the cool taking its place.

As before stated, the engine is very economical of gas. The amount used per hour per indicated horse-power is stated by the makers to be twenty-one and a half cubic feet, which, with gas at two dollars a thousand, is a trifle above four cents. In first cost the engine is somewhat more expensive than a good steam-engine, including boiler, of the same power, the price ranging from five hundred dollars for the two horse to eight hundred and fifty for that of seven horse-power. The former occupies a floor-space of about three feet by seven, and weighs fourteen hundred pounds, and the latter covers somewhat more space, and is of double the weight.

The heat generated by the combustion of the gas has been very fully utilized in this engine, but not to the greatest extent practicable. A certain portion of it is carried off by the water in the jacket, and is therefore wasted. If, instead of being allowed to escape without doing any useful work, it was employed to convert a small quantity of water injected into the cylinder into steam, overheating of the cylinder would be prevented, and at the same time this heat would be utilized. Besides the power gained, the use of steam is of value in giving a more sustained pressure on the piston and in lubricating the cylinder. Numerous attempts have been made to realize its advantages, both in hot-air and gas engines, but in most cases with no considerable gain in economy. The engine of Hugon, mentioned above, employed it, but apparently with little advantage.

Quite recently a gas-engine has been brought out in which the difficulties seem to have been mostly overcome, and which appears to