Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(35) General Description.

A brief description of the indicator as a means of recording the pressures in the cylinder of a simple heat engine in relation to the piston position was given in paragraph (6), Chapter I, and as this instrument is so peculiarly adapted to locating the events taking place in the cylinder we will devote some space on its application to the practical gas engine cycles described in the preceding chapter. Since each event in the cycle is accompanied by a corresponding increase or reduction in pressure, the beginning or end of an event will be indicated on the diagram by a change in the vertical height of the curve above the atmospheric line, at some particular piston position. The piston position will be in the same relation to the total stroke as the pencil position will be to the horizontal length of the card.

If the event, for example, as indicated by a drop in pressure, be at the center of the card, it will show that the drop in pressure took place when the piston was in the center of the cylinder or at mid-stroke. Should the pressure change at a point one-quarter of the card length from the starting point of the pencil, it shows that the event took place in the cylinder when the piston had accomplished the first quarter of its stroke, and so on. It should be noted that horizontal distances on the indicator card denote piston positions, and the vertical distances, pressures.

As explained in a former paragraph the length of the vertical lines represents certain definite pressures, each inch of length representing so many pounds as per square inch, the exact amount per inch depending on the indicator spring strength or adjustment. To make this point clear, all of the indicator diagrams shown in this chapter will be provided with a scale of pressures at the left of the diagram by which the pressure at any point may be accurately measured off for practice. It should be noted that points on the curves which are above the atmospheric line represent positive pressures above the atmosphere, and that the points lying below the atmospheric line represent partial vacuums which may be expressed as being so many pounds per square inch below the atmosphere. The vacuum pressures indicate the extent of the “suction” created by the piston when drawing in a charge of air and gas.

Straight vertical lines show that the increase of pressure along that line has been practically instantaneous in regard to the piston velocity, for if the pressure increased at a slow rate this line would be inclined toward the direction in which the piston was moving, as the piston would have moved a considerable distance horizontally while the pencil was moving vertically. This inclination of the vertical line gives an idea of the rate at which the pressure increases in relation to the piston speed, the greater the inclination, the slower is the rate of pressure increase. Straight horizontal lines that lie parallel to the atmospheric line denote a constant pressure or vacuum.

The rate at which horizontal lines descend or incline to the atmospheric line represents the rate at which the pressure increases or decreases, in respect to the piston position (not piston velocity). A steep curve represents a rapid expansion or compression from one piston position to the next. A waving or rippling line indicates vibration due to valve chattering or explosion vibrations. A straight inclined line shows that the pressure is decreasing or increasing in direct proportion to the piston position.