The actual cycle of a thermal engine is the one that reflects the effective conditions of its operation. These conditions are identified with the diagram of mean pressures in the cylinder corresponding to the various positions of the piston. The pressure shown in a mean pressure chart is the so-called average pressure indicated.
The indicated diagram is the graph that represents this cycle.
Simplified indicator operation
The indicator is an apparatus that serves to obtain the indicated diagram of the cycle of a thermal engine.
In the indicator of the indicated diagram, a small cylinder provided with a piston held by a spring communicates with the combustion chamber of the engine cylinder by means of a tube. The piston rod acts on a system of levers that form an amplifying quadrilateral. The longest lever arm of the quadrilateral is provided at its tip with a stylet.
The drawing shown below shows schematically, as the diagram indicated by the indicator is drawn. The schematized apparatus serves to show how an indicated diagram is obtained, but it is not realizable in practice. The indicators are, in fact, more complicated devices, used by specialized technicians.
The gas pressure is transmitted through the cylinder. The pressure acts on the piston and, overcoming the spring load, moves the piston to a length proportional to the pressure value. The stylus draws, therefore, vertically a line of length proportional to the pressure acting on the piston. As the entire indicating device is fixed to the engine piston, the stylus moves linearly with it, and its horizontal position corresponds at each point to that of the engine piston.
The curve drawn by the stylet is thus referred to two coordinate axes. The abscissa represents the spaces covered by the piston and, therefore, the volumes. The ordinates represent the pressures.
Example in thermal motor of cycle Otto and 4 times
Suppose the engine is an Otto engine (chipa ignition engine) and that it is a 4-stroke engine. Likewise, we will assume that the operating conditions are approximately theoretical.
Slide the piston from the top dead center (PMS) to the bOttom dead center (PMI) and vice versa, with the valve open. We assume that the piston offers no resistance to the passage of gas. The pressure in the cylinder remains the same as atmospheric pressure. During this movement the stylus is drawing a horizontal segment AA, of length equal to the stroke of the piston. The drawn segment represents the pressure diagram for the suction and exhaust phase.
The valve is subsequently closed during the compression stroke. The stiletto describes the AB curve. At the end of the compression stroke, combustion is verified and, consequently, there is a sudden increase in pressure that draws the almost vertical line BC to the stylus. Successively, during the expansion run, the stylus describes the CD curve. Shortly before finishing the expansion stroke, the exhaust valve is opened, the pressure drops to a value very close to the atmospheric one and, therefore, the stylet traces the small, almost vertical, DA feature.
Comparison of two real cycles of Otto engine and diesel engine
Below we show the representation of two typical real cycles of Otto engine and diesel engine of equal unit displacement.
To facilitate the comparison between the two real cycles, the diagrams have been drawn overlapping. The axis of the pressures for the Otto cycle, as a consequence of the difference in volume Vc of the combustion chamber. In fact, at equal unit displacement Vp, the compression ratio of the diesel engine being higher than that of the Otto engine, the volume Vc of the combustion chamber is smaller.
The surface 1 2 6 1 '1 represents the negative work due to pumping in the suction and exhaust phase. The surface 2 3 4 5 6 2 represents the positive work. Its difference is useful work. By dividing the area corresponding to the useful work carried out by the fluid, by the length of the stroke, or by the displacement Vp according to the scale chosen for the axis of the abscissa, the value of the indicated average pressure (pmi) is obtained.
If we want to assess the difference in useful work between the theoretical cycle and the indicated cycle, we will establish the comparative relationship between the corresponding surfaces. By dividing the surface of the indicated cycle by the respective one of the theoretical air cycle, the indicated performance is obtained.