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Twin cylinder gas engine of the Acme Machine Co., Glasgow, Makers

Reprinted from The Engineer, 1890.

Twin cylinder gas engine of the Acme Machine Co., Glasgow, Makers

This gas engine differs from any that have preceded it, in that it has two cylinders which are always freely in communication without the intervention of valves of any kind; and further in that the Maker, and apparently with success, has dared to transmit the energy of the impulse due to the explosive combustion in one cylinder, to the crankshaft of the other, by means of the teeth of spur wheels. The action of the engine is very simple, and the arrangement adopted secures a long range of expansion, and therefore a low exhaust pressure. The engine shown above is 16-horse power, capable of working to about 22 horse power on the brake. It was brought out in 1888 and then shown in the Glasgow Exhibition.

These engines use two cylinders, pistons, and shafts. The two shafts are connected by toothed wheels, which are geared in the ratio of 2 to 1. The piston of the cylinder, which is connected to the slow moving shaft, sweeps a less volume than the other does, besides making only half the number of strokes. This smaller volume is secured either by shortening the crank or lessening the diameter of the cylinder, or by the two combined. The two wheels are engaged so that when the fast moving piston is at its outer and inner dead points, the other is distant from its dead point by a distance corresponding to a motion of about 45 degrees of its crank, an amount of travel corresponding roughly to one-seventh of the whole stroke. This piston regulates the firing and the exhaust by having the firing tubes inserted through the cylinder at about one-seventh of its stroke from the inner dead point, and having ports opening from the cylinder at the outer seventh. Thus only one valve is required, namely, an automatic lift valve for admitting the charge of gas and air, and for preventing the formation of a partial vacuum in the cylinders when the engine misses an explosion by being governed.

The cycle is as follows:
Suppose the engine to have completed an exhaust. The main (large) piston is now quite in, and the other returning in, having just closed the exhaust pipes. Let V be the volume swept by the main piston, v that by the other, and C the amount of clearance if both pistons could be fully in at the same time. We start with a volume C + 6/7v.

(1) The main piston takes an out-stroke, and the other returns in from about 1/7 to 6/7 of its in-stroke. The volume now is V + C + 1/7v, that is, the intake has been: V - 5/7v.

(2) The main piston makes an in-stroke, and the other completes the remaining portion of its in-stroke and makes about 1/7 of its out-stroke, where the firing tube is inserted. The compressed volume now is C + 1/7v, and firing takes place.

(3) The main piston makes an out-stroke, and the other also travels out for about 5/7 of its out-stroke, thus expanding to a volume: V + 6/7v + C.

(4) The exhaust ports are now uncovered by the left-hand piston, and whilst the main piston returns, this completes the last 1/7th of its out-stroke, and returns 1/7th in, thus completing the cycle.

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