The Still engine is a combined steam and gas or oil engine. The heat that is ordinarily thrown away in the water jacket and in the exhaust is utilized to produce steam, and this steam is employed to return the piston to the top of the cylinder after the gases generated by the combustion of fuel have driven it down. While in previous experiments some steam had been developed from the exhaust gases it had not been found possible to generate steam in the water jacket because of the low temperature that had to be maintained. In the standard internal combustion engine the cylinder walls are made of cast iron, thick enough to withstand the heavy pressures to which they are subjected should the charge be ignited prematurely. Around them the water circulates under practically no pressure and so the wall of the water jacket is made comparatively thin. Because of the thickness of the cylinder wall the water has to be maintained at a comparatively low temperature so as to keep the interior of the cylinder from growing too hot. In the Still engine the cylinder wall is from one-third to one-fourth the thickness of the standard cylinder wall, while the water-jacket wall is of thick steel. The cylinder wall is formed with ribs which extend to the water-jacket wall, so that the latter will take care of any excessive loads due to premature ignition. Figure 53 shows how the cylinder and jacket of the Still engine compare with those of the ordinary motor. The thin cylinder wall and the ribs furnish a far better conduction of heat to the water which circulates between the ribs. The necessary cooling of the cylinder can be maintained with water at a much higher temperature than in the ordinary engine. [Illustration: FIG. 53.--SECTION THROUGH THE CYLINDER AND WATER JACKET OF THE STILL ENGINE] [Illustration: FIG. 54.--DIAGRAMMATIC VIEW OF THE STILL ENGINE]