Is the Otto Cycle Reversible with Ideal Gas Assumptions?

In summary, the discussion covers the concept of reversible isochoric expansion and compression, and whether the Otto Cycle is reversible. It is noted that the efficiency of all reversible engines between two heat reservoirs would be the same, but since the Otto Cycle's efficiency is lower than Carnot Cycle's, it cannot be reversible. The question of resolving this problem is raised, and it is suggested that the Otto Engine must operate between two heat reservoirs in an irreversible process. The reasoning behind this is discussed, with the conclusion that while a real Otto cycle is not reversible, the ideal air standard Otto cycle can be considered reversible. The concept of constant volume heat addition is also mentioned, and it is noted that this process is reversible between two heat reservoir
  • #1
Harmony
203
0
Theoretically, we can have reversible isochoric expansion and isochoric compression. Is Otto Cycle reversible then?

I read that the efficiency of all reversible engine between two heat reservoir would have the same efficiency. However, since the efficiency of an Otto Cycle is lower than Carnot Cycle, it cannot be reversible.

How do I resolve this problem? Thanks in advanced.
 
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  • #2
Are you referring to the process whereby the hot combustion products are allowed to expand, cool and perform work? Or to the preceding different process whereby chemical potential is let change directly into thermal energy, in a spontaneous (and somewhat uncontrolled in comparison to fuel cells) manner?
 
  • #3
I refer to the first process, but a hypothetical ideal one where it is reversible. After some thought, I think that in order for an ideal gas to expand under constant volume and yet be reversible, there will have to be many heat reservoir, with infinitesimal small difference in temperature. Hence the reversible Otto Cycle will not be operating between two heat reservoir.

As for an Otto Engine operating between two heat reservoir, the process must be irreversible.

Is my reasoning correct?
 
  • #4
Well a real Otto cycle is in no way reversible as you have the messy business of combustion and pumping losses.

However the ideal air standard otto cycle can be considered reversible as each of the 4 processes are reversible. I don't get what you mean when you say that the gas expands under constant volume (it just seems a funny way of describing the process), are you referring only to the increase in pressure at heat addition/rejection? As contant volume heat addition is reversible between two heat reservoirs.

I'm in the process of moving back home to all my books are packed so I can't read up to check.
 
Last edited:

Related to Is the Otto Cycle Reversible with Ideal Gas Assumptions?

1. What is the Otto Cycle?

The Otto Cycle is a thermodynamic cycle used in spark-ignition internal combustion engines to convert fuel into mechanical energy. It consists of four processes: isentropic compression, constant volume heat addition, isentropic expansion, and constant volume heat rejection.

2. What is the theory behind the reversibility of the Otto Cycle?

The Otto Cycle is considered reversible because the four processes that make up the cycle are theoretically reversible. This means that if the engine were to run in the reverse direction, it would still follow the same path in the opposite direction.

3. Why is the reversibility of the Otto Cycle important?

The reversibility of the Otto Cycle is important because it allows for the calculation of the maximum possible efficiency of an engine. In a reversible cycle, the efficiency is equal to the ratio of the work output to the heat input, known as the Carnot efficiency.

4. Is the Otto Cycle actually reversible in practice?

No, the Otto Cycle is not reversible in practice. In real engines, there are inefficiencies such as friction and heat loss that prevent the cycle from being completely reversible. However, the concept of reversibility is still useful for understanding the theoretical limits of engine efficiency.

5. How does the reversibility of the Otto Cycle affect the performance of an engine?

The closer an engine can operate to the ideal reversible cycle, the more efficient it will be. The efficiency of an engine is directly influenced by the level of irreversibility in its processes. In other words, the less energy is lost in the form of heat or work during the cycle, the more efficient the engine will be.

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