DOBBIE-MCINNES ENGINE INDICATOR

An engine indicator is an instrument used to measure and record the rapidly changing pressured occurring in an engine cylinder. To be of value it is usually necessary to know not only the magnitude of the pressures but also the position of the piston at which they occur. An indicator has two independent motions. The first is that of a pistion of a definite area acted upon by pressure in the cylinder. This piston actuates a pencil mechanism through connectinglinks so proportioned that the pencil will move up and down in practically straight lines rather than an arc of a circle. The pressure acting on the piston , is opposed by a carefully calibrated spring. The height of the line, traced by a pencil from a base line will indicate the pressure in the cylinder when measured with a suitable scale. This part of the indicator is then a recording pressure gauge. The second motion is that of a drum carrying the paper indicator card and oscillating in synchronism with the otion of the engine piston. The travel of the drum is at right angles to that of the pencil. Thus any point in the length of the diagram represents the position of the piston in the cylinder. The various springs are used with the intrument and are calibrated so that the indicator can be used over a wide range of pressures.