Diagram pressures depending on the angular displacement of the shaft for 4-stroke engine
Knowing the cycle indicated immediate is plot the pressures in the cylinder depending on the angular displacement of the crank, instead of according to the volumes or the reciprocating movements of pistons n, taking into account the kinematic relationship for this latter linking with the rotation of the shaft.
This diagram is, as we will see, for calculating loads on bearings, but we anticipate the study to gain more familiarity with the friction on the cycles.
In the figure is shown the diagram on a 4-stroke engine. As we know, in the real cycle transformations are not verified, as assumed in the theoretical cycle, between the limits represented by dead spots. Therefore, the phases of the cycle are different from each other and differ, at a time corresponding to the piston stroke. Such event or circumstance is shown in Fig.
Examine how they vary pressure values during the development cycle
A) Admission. At the beginning of the suction stroke inside the cylinder 1,2 is at a pressure slightly above atmospheric pressure for not yet completed the exhaust phase. When the piston is at point 2, in the race for the PMI, sucks some air or gas mixture through the intake valve is open promptly. As we have seen, for most of this phase there is a lower than outside pressure, due to the resistance encountered by the gas in the ducts. This causes the depression called aspiration, which is much more intense the higher the gas velocity, because of the increased strength that this fluid has to beat its way through these ducts. Obviously, this work represents passive phase.
in point 3 When the piston begins its return stroke, the temperature in the cylinder is still in depression, which is why, despite the opposite movement of the piston, Continuous fluid introduction through 4, point where it equals the internal pressure and atmospheric pressure. At this point you should close the valve. If the intake pipe is long, it can use the effect of the inertia of the gas column, to continue the admission after delaying section 4, for this purpose the valve closure. Point 4 is started, then, the real compression.
B) Compression. Cargo compression occurs as a result of piston movement in phase 4-6. Given that the combustion takes some time to perform, in order to achieve the best useful development phase (combustion and expansion) is performed before TDC ignition Point 6 'gives the value' no pressure up on.
C) Combustion and expansion. With the ignition in Section 5, a little before the end of the compression phase starts combustion, which causes a sudden rise in temperature and pressure reaches its maximum value in the Item 7. The combustion ends when the piston has traveled as part of the race. Next examine in more detail the development of combustion in two different cases of spark ignition and compression
After the combustion, the expansion occurs. The volume increases and the pressure undergoes a rapid decrease or diminution, also caused in part by the transfer of heat to the cylinder walls. The expansion should continue where possible to maximize the useful phase, ie, to the vicinity of PMI, but, in practice, to facilitate the expulsion of gases is interrupted anticipated opening it with respect to the bottom dead-exhaust valve in 8.
D) Escape. Gases, that the moment of opening of the exhaust valve is higher than atmospheric pressure, violently discharged outside. In this first period of the phase, which runs almost at constant volume (spontaneous escape), the pressure drops rapidly, and in 9, when starting the exhaust stroke is slightly above atmospheric, with a tendency to drop even more during the first part of this race. Can occur if the exhaust passages are long, which, due to the inertia of the gas column, is produced in 10 intense depression. In 11 starts the second phase period: the piston expels gases still occupying the cylinder. This period runs with pressure slightly above atmospheric (pressure in the exhaust) by the resistance to be overcome by the gases flow through the valve and the exhaust ducts and is therefore positive work. The piston can not, however, pass all the gas, because some of them occupies the combustion chamber. In 1, the end of the exhaust stroke, the pressure is still a value slightly above atmospheric, for this reason phase extends to point 2. Meanwhile has started opening 12 the intake valve, in such a way that in 2 is already fully open at this point and provides maximum passage section for the new phase SUCTION n. Thus begins a new cycle will be repeated regularly.
In the figure, the hatched area represents the useful work, while indicating marked with double striped passive work, and the dashed line, the pressure up when no combustion .
Last review: February 1, 2016