Question

why the efficiency of actual cycle is lower than the efficiency of standard cycle

Answer 1

The difference in form consists of a different profile curves in compression expansióny in replacing introduccióny rectilinear subtraction of heat by curved lines and to the rounding of the , acute angles. The reasons for these differences are based on the following reasons:

Heat losses. In the theoretical cycle are null, but very sensitive, however, in the actual cycle. As the cylinder is cooled to ensure the smooth operation of the piston, a certain portion of heat from the fluid is transferred to the walls. Expansion compresióny lines are therefore adiabatic but polytropic with exponent n, k different. Because the fluid undergoes heat loss evidently has: for expansion, n> k, and for compression nfigura.

Combustion is not instantaneous. In the theoretical cycle, it is assumed that the combustion takes place at constant volume, is therefore instantaneous in the real cycle, however, combustion takes some time. If ignition take place precisely at TDC, combustion would occur while the piston moves away from that point, and serious pressure value lower than expected, with the corresponding loss of useful work .

It is therefore necessary to advance the ignition so that combustion can take place, for the most part, when the pistonis in the proximity of PMS This produces a rounding of the theoretical line 2-3 heat input and therefore a loss of useful work represented by the area B. But this loss is of markedly lower amount that would not advance the ignition.

Time of opening of the exhaust valve. In the theoretical cycle had also assumed that the heat removal occurred instantaneously



in the PMI in the real cycle heat removal takes place in a time to that part of the gas escaping from the cylinder before the piston reaches BDC so that its pressure drops close to the value of the externalpressure at the beginning of the ejection stroke. This causes a loss of useful work arrea represented by C, loss is however less than that which would have without the advance of the opening of the exhaust valve.

The causes of the differences in the values ​​of the maximumtemperature presióny are:

Increased fluid specific heats with temperature. As we know, so the specific heat at constant pressure and the corresponding cp constant volume cv of a real gas, grow with the temperature, but such that their difference remains constant, ie cp - cv = AR, therefore increasing the temperature lessens the value of relaciónk = cp / cv. From which it follows that the values ​​of the maximum temperature presióny are always less than those that would be achieved in the case where the specific heats remained constant as the temperature varies. This fact is also taken into consideration when drawing air theoretical cycle, but in the actual case, the combustion products have higher specific heat than air, and therefore, presióny values ​​are the maximum temperature in the real cycle, corresponding to less than the theoretical cycle. For this reason, the surface and thermal efficiency are decreased.

Dissociation in combustion. The combustion products are essentially CO2 and H2O, as well as other compounds such as CO, H2 and O2. Dissociation of these products is a reaction that takes place with heat absorption, the maximum attainable temperature is lower and lose a certain amount of work. But as the temperature decreases during the expansion, there is a decrease in the dissociation reaction. Consequently, this phase ensues with the partial annealing heat development. Decreases the value of the polytropic exponent of the expansion, which should be May than k by heat loss through the walls of the cylinder and approaches the polytropic compression, for Thus, you get a partial recovery of lost work before.

The actual cycle presents, finally, another important difference when compared to the theoretical cycle, during the suction stroke, the pressure in the cylinder is lower than that is during the exhaust stroke. Except in special cases, in the course of the suction pressure is less than atmospheric, whereas in the exhaust is higher. Is created, therefore, the diagram indicated negative surface (D, in the figure), which corresponds to lost work. The effort made by the engine to effect the escape aspiracióny called pumping work and this, usually within the work missed due to friction.