Next we analyze the diagram of the pressures of a real cycle as a function of the angular displacement of the axis for a 4-stroke engine.
Knowing the indicated cycle, the immediate thing is to draw the diagram of the pressures in the cylinder of the thermal engine as a function of the angular displacement of the crank, instead of doing it as a function of the volumes or the alternative movements of the piston. In doing so, we take into account the kinematic relationship that links the latter with the rotation of the axis.
In the real cycle the transformations are not verified between the limits represented by the dead spots. Therefore, the phases of the cycle are different from each other and differ, at the same time, from those corresponding to the piston stroke.
Let us examine how the pressure values vary during the development of the 4-stroke reciprocating engine cycle and the thermodynamic reactions produced.
The pressure in the first half: admission
At the beginning of the suction stroke 1,2 the interior of the reciprocating engine cylinder is at a pressure slightly higher than atmospheric because the exhaust phase has not yet finished.
When the piston is at point 2, on its way to the PMI, it sucks a certain amount of air or gaseous mixture of air and gasoline or diesel fuel through the suction valve, opportunely opened.
During almost all this phase there is a lower pressure than the external one, due to the resistance that the gas finds in the pipes. This causes the so-called aspiration depression.
This depression becomes more intense the higher the speed of the gas, due to the greater resistance that this fluid has to overcome as it passes through said ducts. As is evident, this phase represents passive work.
When at point 3 the piston starts its return stroke, the environment in the cylinder of the endothermic motor is still in depression; For this reason, and despite the contrary movement of the piston, the introduction of the fluid continues up to 4.
In point 4 the internal pressure and atmospheric pressure are equalized. At this point the suction valve must be closed. If the intake duct is long, the effect of the inertia of the gas column can be used to continue the intake after point 4, delaying the closing of the valve.
In point 4, then, the true compression begins.
Second time: compression
Compression of the load occurs as a consequence of the movement of the piston in phase 4-6. Taking into account that the combustion requires a certain time to be carried out, in order to achieve the best development of the useful phase (combustion and expansion) the ignition is carried out before the PMS Point 6 'gives us the maximum value of the pressure without switched on.
Third time: combustion and expansion
With the ignition at point 5, shortly before the end of the compression phase, combustion begins, which causes a sudden rise in temperature and pressure that reaches its maximum value at point 7.
Combustion ends when the piston has already covered part of the stroke. We will now examine in more detail the development of combustion in the two different cases of spark ignition and compression ignition,
Finished the combustion, the expansion occurs. The volume increases and the pressure experiences a rapid decrease or decrease, also caused, in part, by the transmission of heat to the cylinder walls.
The expansion should be extended whenever possible to take full advantage of the useful phase, that is, until the proximity of the PMI, but, in practice, to facilitate the expulsion of gases, it is interrupted with the anticipated opening with respect to the bottom dead center. - from the exhaust valve at point 8.
Fourth time: escape
The gases, which at the time of the opening of the exhaust valve is at a pressure higher than atmospheric, are violently discharged to the outside. In this first period of the phase, which runs almost at constant volume (spontaneous exhaust), the pressure drops rapidly, and at point 9, when the exhaust stroke begins, it is slightly higher than atmospheric, with a tendency to drop even more during the first part of this race.
It can happen, if the exhaust ducts are long, that, due to the inertia of the gas column, an intense depression occurs. At 11 the second period of the phase begins: the piston expels the gases that still occupy the cylinder.
This period elapses with pressure slightly higher than atmospheric pressure (overpressure in the exhaust) due to the resistance that the gases have to overcome when passing through the valve and the exhaust ducts and therefore represents positive work. The reciprocating engine piston cannot, however, expel all gases, because a part of them occupies the combustion chamber.
In 1, at the end of the exhaust stroke, the pressure is still slightly higher than atmospheric; for this reason the phase is prolonged to point 2.
Meanwhile, the opening at 12 of the intake valve has started, so that at 2 it is already fully open, and at this point offers the maximum passage section for the new suction phase. Thus begins a new cycle, which will be repeated regularly.