What is the indicated diagram?
The indicated diagram is the diagram that represents the real cycle of a heat engine (a diesel engine or an Otto engine).
The actual cycle of a thermal engine reflects the effective operating conditions. These conditions are identified with the diagram of average pressures in the cylinder in correspondence to the different positions of the piston. The pressure shown in a mean pressure diagram is the so-called average pressure indicated.
Study of the indicated diagram
Stop studying the internal combustion thermal motors requires knowing the indicated diagram of a motor well.
Measuring the surface of the diagram gives the indicated average pressure. Knowing the indicated average pressure, and taking into account the displacement of the thermal engine and the number of useful strokes in the unit of time, the indicated power is obtained, that is, the power developed in the cylinder.
In practice, to calculate the indicated power, the power absorbed by the friction is added to the power measured with the brake, which is evaluated, in turn, by turning the motor without combustion.
Indicators are used for the study in the test rooms and to check the regularity of the cycle. Since the form of the latter depends on the way in which the transformations are carried out in the motor, the irregularities of operation can be studied by means of a close inspection of the indicated cycle.
Study of the indicated diagram in a 4-stroke engine
Let us examine, then, in its main details, the indicated diagram taking as reference the 4-stroke engine.
Many of the arguments that are made are valid for both the Otto engine and the diesel engine, given that the shape is similar in both, since only the values of the pressures and the temperatures are different in them. The figure above shows the indicated diagrams of an Otto motor: one, with full opening, that is, with the butterfly valve, completely open; the other, with only partial opening, that is, with the semi-closed butterfly.
As we already know, the surface in white is positive and the striped, negative; the latter represents, in fact, the work lost by pumping during the escape and aspiration phases.
When the butterfly is fully open, the resistance to the passage of air is minimal and enters as much mixture as possible. The positive surface that represents useful work is, therefore, maximum. When the butterfly valve is partially closed, less amount of mixture enters; therefore, the work done by the fluid is less, and the positive surface is reduced.
The effect of the partialization by means of the butterfly valve is totally opposite on the scratched surface. When the butterfly valve is open, the resistance to the passage of air is minimal, and, if the conduit is well designed, the pressure in the cylinder will be very close to atmospheric pressure.
When the butterfly valve is partially closed, the resistance to the passage of the mixture is considerable; it does not enter with the speed required by the increase in volume in the cylinder when the piston slides and, therefore, a depression is created.
In the first case, the work lost by pumping is minimal, and the negative surface is very small; in the second case, the lost work is remarkable, and the negative surface, very large.
The position of the carburetor butterfly greatly influences, therefore, the loss of work by pumping, which does not happen in the diesel engine, because there is no throttle valve in them.
In the previous figure the point corresponding to the ignition of the load is indicated. As we have already indicated, this ignition must be carried out before the P.M.S., so that the combustion, which needs a certain time to be carried out, resembles as much as possible the theoretical phase. This means that the ignition must be done at almost constant volume. The ignition time is experimentally established by finding, by means of tests, the value corresponding to the maximum power that can be obtained without reaching the detonation or violent operation of the engine.
The moment at which the spark jumps corresponds to the point at which the pressure equals half of that reached in the P.M.S. If the spark jumps late, almost all combustion develops after P.M.S. The maximum pressure is reached when the piston has slid markedly towards the P.M.I and its value is, therefore, lower than normal. The surface of the cycle is reduced, as shown in the previous figure (a). The same deformation of the indicated diagram occurs in the case of a slow combustion. When the spark leaps out of the spark plug in advance, combustion occurs largely before the P.M.S., the maximum pressure is higher than normal and the cycle appears deformed, as can be seen in the previous figure (b).
From the examination of the indicated cycle, it is also easy to deduce whether the ducts and the suction and exhaust valves are well designed and well proportioned, and whether the opening times of the valves have been chosen correctly.
In fact, if the air or mixture ducts are insufficient and offer excessive resistance, or the principle of opening the suction valve has been delayed, the pressure in the cylinder during the suction stroke is less than normal and, therefore, increases the pumping work as shown in figure (a).
If, on the other hand, it is the exhaust ducts that offer abnormal resistance to the passage of gases, or the opening principle of the exhaust valve is delayed, the pressure in the cylinder is higher than normal during the stroke. of expulsion; for this reason, not only does the pumping work increase, as shown in figure (b), but the amount of combustion gases remaining in the cylinder at the end of the stroke is also excessive and, therefore, excessive also the dilution of the fresh charge that is introduced during the next aspiration phase.
As a consequence, we will have a decrease in the value of the maximum pressure and in the values of the pressures throughout the work phase. This defect is also cause of reduction in the surface of the indicated diagram, that is, of a loss of useful work.