The cylinder of the engine--assuming that it has only a single-acting one, placed with its axis vertical--consists of two parts; the upper hot part being lined with plumbago, fire-clay, or other refractory material, and the lower part kept cool by a water casing. The cylinder has a trunk piston working in the lower part, and on its upper side a shield that almost fills the hot part of the cylinder when the piston is at the extreme of its upstroke. The trunk-rod of the piston passes through a stuffing-box in the cylinder bottom, and is connected to a crank on the engine-shaft; and this (unless multiple cylinders are employed) carries a heavy fly-wheel. From the lower end of the cylinder there is a passage which, by means of a rotating or reciprocating slide, is alternately put in communication with inlets for gas and air (regulated by suitable cocks or valves) and with a strong receptacle. As the piston, makes its upstroke, air and gas are drawn into the annular space surrounding its trunk, and the mixed air and gas are compressed by the downstroke of the piston, and delivered into the receptacle, in which considerable pressure is maintained. The receptacle is made of cylindrical form, with a domed cover of thin sheet metal; so that in case of excessive internal pressure it can operate as a safety-valve to save the body of the receptacle from damage. From the upper end of the cylinder there is a passage that, by means of a rotating or reciprocating slide, is alternately put in communication with the receptacle and with a discharge outlet. In this passage are fixed a number of wire gauze screens or pieces of metal with interstices. These constitute a regenerator of heat, and also prevent a communication of flame from the cylinder to the receptacle. In the upper end of the cylinder or of the piston shield are provided electrodes which give an electric spark, or a platinum wire which is rendered incandescent by a current from an inductor or other source of electricity to ignite the combustible charge of the cylinder. After the engine has been for some time at work, the heat at the upper part of the cylinder may suffice for effecting ignition without provision of other means for this purpose. In combining such an engine with means for generating the combustible gas, a gas producer is employed. In this producer a current of heated air is introduced into the heart of a body of kindled fuel, and the gases produced--partly by distillation and partly by imperfect combustion of the fuel--are conveyed to the gas inlet of the cylinder or pump of the engine. As the gas in leaving the producer is hot, it is caused to pass through regenerating apparatus, to which it delivers a large portion of its heat before it reaches the engine, and the air which supplies the producer is made to pass through this regenerating apparatus so as to take up the heat abstracted from the gas.