Question

Are the processes that make up the Otto cycle analyzed as closed-system or steady-flow processes? Why?

Short answer

Answer: The Otto cycle should be analyzed as a closed-system process because no mass enters or leaves the cylinder during each of the four processes, and the mass of the air-fuel mixture and combustion products remains constant throughout the cycle.

Step-by-step solution

Understand closed-system and steady-flow processes

A closed-system process is one in which the system remains isolated from its surroundings, meaning no mass enters or leaves the system during the process. On the other hand, a steady-flow process occurs when a fluid flows continuously through a system, allowing mass to enter and exit the system, but the overall mass remains constant.

Analyze Process 1-2 (Isentropic Compression)

In this process, the piston compresses the air-fuel mixture inside the cylinder, increasing the pressure and temperature of the mixture. Since the piston is sealed and no mass enters or leaves the cylinder during the compression, this process can be considered as a closed-system process.

Analyze Process 2-3 (Isochoric Combustion)

During this process, heat is added to the system due to the ignition of the compressed air-fuel mixture. The volume of the cylinder remains constant while the pressure and temperature increase. Since the combustion occurs within the sealed cylinder and no mass enters or leaves the system, this process can also be considered as a closed-system process.

Analyze Process 3-4 (Isentropic Expansion)

During the expansion process, the high-pressure gas inside the cylinder expands, pushing the piston to transform the heat energy into mechanical work. Again, the piston is sealed and no mass enters or leaves the cylinder during the expansion. Therefore, this process can also be considered as a closed-system process.

Analyze Process 4-1 (Isochoric Exhaust)

In this process, the burned gases (products of combustion) are expelled from the cylinder at constant volume, preparing the engine for the next cycle. As no mass flow takes place during the actual process as it is instantaneous, this process can be considered as a closed-system process.

Conclusion

All the processes within the Otto cycle can be considered as closed-system processes. The reason is that no mass enters or leaves the cylinder during each process. The mass of the air-fuel mixture and combustion products inside the cylinder remains constant during the entirety of the Otto cycle processes.