An indicator diagram traces out the pressure and volume relationships in the cylinder of an engine on rectangular axes, and it can be used to estimate the work done by the engine per cycle. The indicator must move a vertical distance proportional to the pressure in the cylinder and the drum, as it rotates, provides a horizontal motion proportional to the change in cylinder volume.
The indicator is connected to a screwed cock which is in connection with the combustion chamber of the cylinder being monitored. When the engine cock is opened, the cylinder pressure also acts on the piston in the indicator cylinder, causing it to rise. This vertical movement is resisted by a spring, the strength of which is chosen to obtain a diagram illustrating the part of the cycle being investigated. The vertical movement of the piston is transferred to the pen mechanism by a parallel motion linkage.
The indicator card is held onto drum with spring clips. The drum is rotated against an internal spring pressure by a special wire reinforced, non-elastic cord, which, being wrapped around the drum several times, is led to a linkage operated by a cam, the cam throw being proportional to the stroke of the engine.
When the cord is attached to the cam mechanism and the indicator cock is opened, a P-V diagram is obtained by lightly pressing the stylus onto the card. From the area of this diagram and the stiffness of the indicator spring it is possible to calculate the indicated power output of the cylinder.
The average height of the diagram is found by measuring its area and dividing this area by the length of the card. The area can be found by using the mid-ordinate rule or similar, or by using a planimeter (an instrument used for tracing out and accurately measuring the areas of small diagrams such as from ordnance survey maps or plans of a factory etc.). The length of the diagram should always be the same as the throw of the cam, but should be measured in any case as any deviation from the ‘recorded’ length indicates a fault with the indicator mechanism. Too long a diagram would indicate that the drum spring is, perhaps, not tensioned sufficiently and the drum is skidding beyond its warranted travel; too short and maybe the drum is incorrectly set up and insufficient rotation is occurring. Similarly, faults on the cord or pulleys may become apparent if the measured card length deviates from the original set length. The faults mentioned above may not be immediately apparent so it is always necessary to measure the length as a cross check.
The spring scale is marked on the spring itself and is a measure of the pressure required to compress the spring through a quoted length (6 bar to a cm, for example).
The engine constant is the stroke of the engine multiplied by the cylinder bore area.
Apart from taking ‘power cards’, the standard P-V diagram, the indicator may be used to take cards 90 deg out of phase (draw cards). A draw card is merely a diagram taken with the drum rotation advanced by 90 deg to the main piston of the engine. This allows the injection period to be spread out across the centre of the card so that the combustion process can be examined more closely (the combustion process is very compact and very little other than the peak or firing pressure can be told from a power card.).
The same spring is used for both power and draw cards, and can also be used to measure the compression pressure. It is best to do this with the engine running slowly, as slow compression highlights leakage that could be associated with worn liners, piston rings or even a leaky exhaust valve. With the fuel shut off the relevant cylinder, the compression card can be taken with or without the drum rotating. If the drum is pulled slowly around by hand as the indicator responds to rising cylinder pressure, a series of vertical lines will be drawn on the card. The average height of these will provide the compression pressure for that cylinder. If the drum is rotated as before, by the power cam, a single rising curve should be obtained, and the height is the compression pressure. If a double line appears on the card the cam drive system is either advanced or retarded with respect to the crankshaft. This is usually sorted out by the engine builder, but, in time, a chain drive system may stretch and the resulting retardation of the cam shaft will show up as an open loop on a compression card. This loss of timing should be rectified immediately if efficient and economic running of the engine is to be maintained. Power cards taken when the cam shaft is hot ‘timed’ correctly to the crankshaft will give false indications of power, and any adjustment of fuel timing to ‘recover’ this timing will adversely affect the performance of the engine.
If the indicator spring is removed and replaced with a weaker spring the diagram will show what is happening over the low pressure part of the cycle more quickly. This diagram, drawn at the expense of the higher pressures in the cylinder, will dive some indication of the exhaust and scavenge processes and how effectively they are being carried out.
Before ‘taking’ a set of cards, ensure that the engine has completely warmed through, and that the engine speed and load are stable (i.e. avoid taking the cards in heavy weather).
Blow each cock through before attaching the indicator, because carbon particles entering the mechanism will adversely affect its operation. Ensure that the card will adversely affect its operation. Ensure that the card is correctly fitted and that the correct spring tensions are set. Allow the indicator and all the other units with an indicator that is progressively warming up. Lubricate the piston as necessary and take the card by pressing the stylus onto the paper. Draw an atmospheric line first, by closing the cock and rotating the drum under cam actuation (do not pull it by hand). The power card can then be drawn by opening the indicator cock and, again, pressing the stylus lightly onto the card. The atmospheric line is drawn using the cam system so that its length can be measured and checked against the original. Any deviation, as explained earlier, is a result of an indicator fault. The atmospheric line also gives a datum from which the height of the diagram can be measured to determine the firing pressure, or compression pressure as the case may be.
It must be accepted that an indicator is just an ‘indication’ of what is happening in a cylinder. Indicators do not give accurate measurements but, if used sensibly and with understanding, they can be a diagnostic tool for tracking down discrepancies in engine operations, either across the engine as a whole or between units. Modern technology has developed these systems so that electronic sensors can provide read outs not only of maximum pressures etc., but also of where in the cycle they occur. The pressure readings are given against crank angles so that late or early ignition can be monitored, and adjusted to optimise performance. These electronic systems derive the crank angle relationship from emitters attached to a rotating element (such as the flywheel). Their signals are then integrated with the pressure signal, the latter being generated from sensors fitted in the combustion chamber or from attachments to the indicator cock itself. In other words they are highly refined and more accurate developments of the mechanical indicator.