Mechanics: The Science of Machinery

by A. Russell Bond

COMBINED STEAM AND OIL ENGINE

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.

FIG. 53.—SECTION THROUGH THE CYLINDER AND WATER JACKET OF THE STILL ENGINE

FIG. 54.—DIAGRAMMATIC VIEW OF THE STILL ENGINE

A diagrammatic representation of a Still engine of the two-cycle heavy-oil type is shown in Figure 54. This is shown with an auxiliary boiler heated by an oil burner. The piston of this engine has a sleeve which fits into an annular steam cylinder. The latter is an extension of the combustion cylinder. Surrounding the steam and combustion cylinders are the steam and water jackets. The drawing shows the piston at the end of its downward stroke, having just been driven down by the gases of combustion. In this position a scavenging-air port at the left is uncovered and the burnt gases are swept out by a blast of air, passing out of the exhaust. This consists of a bank of tubes or flues which pass through a water heater. The gases leave the cylinder with a temperature of 900 degrees Fahrenheit and they issue from the exhaust with a final temperature of only 150 degrees Fahrenheit. Steam now enters the annular steam cylinder from the steam jacket, through the port shown at the lower right-hand side, forcing the piston up and compressing the charge of air in the cylinder. Then oil is sprayed in through a nozzle, not shown in the drawing, and is ignited by the heat of the compressed air as in a Diesel engine and the piston is driven down again by the gases of the combustion. The water surrounding the combustion cylinder comes from the lower part of the boiler and flows by the exhaust flues. It is maintained at a temperature of 350 degrees in the water jacket, and here is turned into steam by the heat of the combustion. Thence it passes into the steam dome of the boiler. On the downward stroke of the piston a slide valve, shown at the lower right-hand side, moves down and connects the steam port with an exhaust port through which the steam flows out into a condenser. Water from the hot well of the condenser enters the system at the combustion exhaust. The arrows show the course of the steam and water and the temperature at various points is given. As may be noted there are two systems of steam and water circulation: one from the bottom of the boiler through combustion exhaust heater to the water jacket and back to the top of the boiler; and the other from the steam dome of the boiler to the steam jacket, to the steam cylinder, to the steam exhaust, to the condenser and back again from the condenser into the combustion exhaust heater, whence it enters the boiler by way of the water jacket.

LATENT HEAT

When water turns into steam a certain amount of heat is absorbed, which does not show in the thermometer. This is known as latent heat. If cold water is placed in a kettle and is then heated the thermometer will gradually rise until the water reaches the boiling point. Then there will be no further rise of temperature, although heat is still applied to the kettle, until all the water is turned into steam, after which the thermometer will begin to show a rise of temperature, showing that the steam is beginning to grow sensibly hotter. In the engine, shown in Figure 54, a boiler pressure of 120 pounds gauge pressure is maintained, and therefore water will not boil until it reaches 350 degrees Fahrenheit. The water in the water jacket registers 350 degrees and is therefore far hotter than it could possibly be in the open atmosphere. However, the heat from the combustion does not raise the temperature of the water, but expends its energy in converting the water into steam. The cylinder wall is kept at a temperature of at least 350 degrees all the time, so that the air that enters the cylinder gathers heat from the cylinder as well as from its own compression, and its temperature at the end of the compression stroke is higher than it would be in a cold cylinder, thereby insuring the ignition of the fuel when it is sprayed in.