GAS AND OIL ENGINES.
Lower right-hand figure, a very early type of commercially successful gas engine. It has a "free" piston, an arrangement that was first proposed for a gas engine in 1857, but only brought into practical form by Langen & Otto under their patent of 1866. Upper figure, the gas engine patented by Lenoir in 1860, one of the very first practically successful engines. Lower left-hand figure, a sectional view of a modern gas engine of the type used as the motor of the automobile.
"(a) The piston is lifted about one-tenth of its travel by the momentum of the fly-wheel, thus drawing in a charge of gas and air.
"(b) The charge is ignited by flame carried in by a slide valve.
"(c) Under the impulse of the explosion, the piston shoots upward nearly to the top of the cylinder, the pressure in which falls by expansion to about 4 lbs. absolute, while absorbing the energy of the piston.
"(d) The piston descends by its own weight and the atmospheric pressure, and in doing so causes a roller-clutch on a spur-wheel gearing with a rack on the piston-rod to engage, so that the fly-wheel shaft shall be driven by the piston; during this down-stroke the pressure increases from 4 lbs. absolute to that of the atmosphere, and averages 7 lbs. per square inch effective throughout the stroke.
"(e) When the piston is near the bottom of the cylinder, the pressure rises above atmospheric, and the stroke is completed by the weight of the piston and rack, and the products of combustion are expelled.