Question

Question 1 a) Define the idealizations and simplifications in the analysis of power cycles b) List and explain air-standard assumptions for gas power cycles c) Compare the actual and ideal cycles in spark-ignition engines and with proper (5 marks) (8 marks) illustration and P-v diagrams (12 marks)

Answer 1

A)The ideal cycles are internally reversible, but, unlike the Carnot cycle, they are not necessarily externally reversible. That is, they may involve irreversibilities external to the system such as heat transfer through a finite temperature difference. Therefore, the thermal efficiency of an ideal cycle, in general, is less than that of a totally reversible cycle operating between the same temperature limits. However, it is still considerably higher than the thermal efficiency of an actual cycle because of the idealizations utilized.

The idealizations and simplifications commonly employed in the analysis of power cycles can be summarized as follows:
1. The cycle does not involve any friction. Therefore, the working fluid does not experience any pressure drop as it flows in pipes or devices such as heat exchangers.
2. All expansion and compression processes take place in a quasi equilibrium manner.
3. The pipes connecting the various components of a system are well insulated, and heat transfer through them is negligible.

B) AIR STANDARD ASSUMPTIONS

The actual gas power cycles are rather complex. To reduce the analysis to a manageable level, we utilize the following approximations, commonly known as the air-standard assumptions:

1. The working fluid is air, which continuously circulates in a closed loop and always behaves as an ideal gas.

2. All the processes that make up the cycle are internally reversible.

3. The combustion process is replaced by a heat-addition process from an external source

4. The exhaust process is replaced by a heat-rejection process that restores the working fluid to its initial state.

C) ACTUAL AND IDEAL CYCLES IN SPARK - IGNITION ENGINES.

Actual cycle:

Process 0-1 : Suction of air (Piston moves from TDC to BDC) .
Process 1-2 : Compression of mixture (Non-adiabatic) .
Process 2-3 : Combustion of mixture (I non-isentropic) .
Process 3-4 : Expansion stroke (Non-isentropic) .
Process 4-0 : Exhaust of burnt gases .

* IV ( Inlet Valve)and EV(Exhaust Valve) open/close before and after dead centres.

* Valves are opened and closed by cam mechanism
* Valves will bounce on its seat if closed abruptly
* Opening/closing of valves spread over a certain crank angle.

* IV opens 20° before TDC.
* IV closes 35° after BDC to take the advantage of momentum of rapidly moving gases (Ram Effect).
*Ignition occurs 35° before TDC; this is to allow the time delay between the spark and commencement of combustion.
*EV opens at 35° before BDC; else pressure will rise enormously, and the work required to expel the gas will increase.
*EV closes at 10° after TDC; this is to increase the volumetric efficiency.

Actual P-V diagram is as follows:

IDEAL CYCLE:

processes:

*Process 0–1 a mass of air is drawn into piston/cylinder arrangement at constant pressure.
*Process 1–2 is an adiabatic (isentropic) compression of the charge as the piston moves from BDC to TDC.
*Process 2–3 is a constant-volume heat transfer to the working gas from an external source while the piston is at top dead center. This process is intended to represent the ignition of the fuel-air mixture and the subsequent rapid burning.
*Process 3–4 is an adiabatic (isentropic) expansion (power stroke).
*Process 4–1 completes the cycle by a constant-volume process in which heat is rejected from the air while the piston is at BDC.
*Process 1–0 the mass of air is released to the atmosphere in a constant pressure process.

P-V diagram of ideal cycles is as follows: