Considering for a moment just the small strip efdc of the diagram we see that it is easy to find a rectangle abcd equal in area to it. Now the height of this rectangle will be the average value of the force while it traversed the space cd, and hence the area of the rectangle abcd gives the work done by the force in passing from c to d. Similarly by dividing up the whole diagram we would obtain a number of little rectangles each equal in area to the magnitude of the work done from point to point. Thus the whole area ABCD gives the whole work done. To measure the work done in an engine cylinder we must use some form of indicator. An indicator is an instrument which traces out a diagram on which abscissæ (or horizontal distances) represent displacements of the piston and ordinates (or vertical distances) represent the pressures acting on the piston.
Fig. 69.—Petrol Engine Indicator Diagram. Four-stroke Cycle.
Ordinary steam engine indicators with pencil motion and paper drum are not suitable for use with fast running petrol engines. The moving parts of these indicators are too heavy and their springs too sluggish in action to keep correct time with these high speed engines. Again, there is too much friction between the pencil and the paper drum, as well as in the lever joints. Therefore special indicators have to be used, in which the diagram is traced out by a beam of light reflected from a mirror on to a ground glass screen or photographic plate. One corner of the mirror is tilted in time with the movement of the engine piston by means of a special reducing mechanism, and another corner of the mirror is tilted in a direction at right angles to the first by means of a very short thin rod kept in contact with a metal diaphragm subjected to the pressure of the gases in the engine cylinder. A beam of light is thrown on to the mirror from a lamp, and after reflection traces out the diagram on the screen or plate. Such an instrument would generally be described as a manograph. An indicator diagram from a four-stroke engine is shown in Fig. [69]. The line ABC represents the suction stroke of the piston during which the pressure of the gases in the cylinder falls a little below that of the atmosphere. Atmospheric pressure is shown by the height of the line LL above the base, or line of zero pressure (perfect vacuum). The inlet valve can be opened at B and closed at D after the crank has turned the bottom dead-centre and begun the compression stroke. The line CDE represents the compression stroke of the engine, during which the gases are compressed and their pressure rises. The height of the point E above the line LL gives the compression pressure to the scale of the diagram. Ignition occurs at E, and results in an instantaneous rise of pressure to F due to the explosion, which is, however, quickly followed by expansion to G. The exhaust valve opens at G, the gases are released, and the pressure falls still further to point H. The line HA represents the exhaust stroke of the piston, and the exhaust valve would be closed after the crank had passed its upper dead-centre and commenced the suction stroke. The distance marked (x) on the diagram measures the clearance volume (or volume of the space above the piston containing the valves and referred to as the combustion chamber) to the same scale that the length of the diagram measures the volumetric displacement of the piston. The volume traced out by the piston during any working stroke is measured by multiplying the area of the piston in square (centimetres/inches) by the length of the stroke in (centimetres/inches) the product giving us the capacity of the cylinder in cubic (centimetres/inches). The area of the diagram HEFG gives the work done during one cycle of operations, and the area of the small diagram ABCD gives the work lost in taking in and expelling the charge. The small area should be subtracted from the large one to get the useful work done per cycle of operations. The area of the diagram HEFG may readily be obtained by finding its vertical height at a number of equidistant points, and from these measurements ascertaining the average or mean height of the diagram. The average height of the diagram (in inches) multiplied by its length (also in inches) gives the area in square inches.
Fig. 70.—Indicator Diagram from a Two-stroke Engine.
The average or mean height of the diagram also gives what we term the mean effective pressure acting on the piston, and constitutes the Pe of the indicated horse-power formula above. The area ABCD is always small and generally neglected with four-stroke engines. There are two separate diagrams for a two-stroke engine. The diagram for the working cylinder is A1B1C1D1 in Fig. [70], and that for the crankchamber is E1F1G1H1. The effective work done per cycle is measured by the difference in the area of these two diagrams. The piston uncovers the exhaust port at B1 and closes it again at C1; it uncovers the inlet port at F1 and covers it again at G1. From F1 to G1 the charge is being delivered from the crankchamber to the working cylinder. The area of the loop E1F1G1H1 is larger than the corresponding portion of the four-stroke diagram and should not be neglected.