Pressure diagram for a 4-stroke engine
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 depending on the volumes or the reciprocating movements of the piston. In doing so, we take into account the kinematic relationship that links the latter with the axis rotation.
The diagram related to a 4-stroke engine is shown in the figure. In the real cycle the transformations are not verified between the limits represented by the dead points. Therefore, the phases of the cycle are different from each other and differ, at the same time, from those corresponding to the piston stroke.
First time: admission
At the start of the suction stroke 1,2, the interior of the reciprocating engine cylinder is at a slightly higher pressure than the atmospheric pressure because the exhaust phase has not yet been completed.
When the piston is in point 2, in its run towards the P.M.I, it sucks a certain amount of air or gas mixture of air and gasoline or diesel through the suction valve, opened opportunely. During almost all this phase there is a lower pressure than the outside, because of the resistance found by the gas in the ducts. This causes the so-called depression in aspiration. This depression is more intense the greater the velocity 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 in point 3 the piston starts its return stroke, the environment in the cylinder of the endothermic engine is still in depression; for this reason, and in spite of the opposite movement of the piston, the introduction of the fluid continues until 4. In point 4 the internal pressure and the 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 gaseous column can be used to continue the intake after point 4, thus delaying the closing of the valve.
In point 4, then, 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 P.M.S. Point 6 'gives us the maximum value of the pressure without ignition.
Third time: combustion and expansion
With the ignition in point 5, a little before finishing the compression phase, combustion starts, which causes a sudden rise in temperature and pressure that reaches its maximum value in point 7. The combustion ends when the piston He has already traveled part of the race. We will next examine in more detail the development of combustion in the two different cases of spark ignition and compression,
Once the combustion is over, expansion ensues. The volume increases and the pressure experiences a rapid decrease or decrease, also caused, in part, by the transmission of heat to the walls of the cylinder. The expansion should be prolonged when possible to make the most of the useful phase, that is, to the proximity of the PMI, but, in practice, to facilitate the expulsion of the gases, this is interrupted with the anticipated opening with respect to the bottom dead center - from the exhaust valve in point 8.
Fourth time: escape
The gases, which at the moment 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 escape), the pressure drops rapidly, and at point 9, when the escape race starts, it is little higher than atmospheric, with a tendency to descend 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 gaseous column, an intense depression occurs. In 11 the second period of the phase starts: the piston expels the gases that still occupy the cylinder. This period takes place with slightly higher than atmospheric pressure (overpressure in the exhaust) due to the resistance that the gases must overcome when passing through the valve and the exhaust ducts and therefore represents positive work. The piston of the reciprocating engine can not, however, expel all the gases, because a part of them occupies the combustion chamber.
In 1, at the end of the exhaust stroke, the pressure still has a value slightly higher than atmospheric; for this reason the phase is extended to point 2. Meanwhile, the opening at 12 of the intake valve has begun, so that in 2 it is already fully open, and at this point it offers the maximum passage section for the new aspiration phase. Thus begins a new cycle, which will be repeated regularly.
Last review: April 16, 2018