# Rankine and Otto cycles

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## Main Question or Discussion Point

Hello!

In ideal Rankine cycle, we say that during the isobaric expansion, the heat provided to the system is Qin = h3 - h2
However, in my opinion, it is Qin - W = Δh -> Qin - PΔV = Δh. Which is correct?

Secondly,
In Otto cycle, let's say you have calculated Wnet in kJ/kg. In order to be able to calculate Power, isn't it necessary to know mass flow rate?

Thanks!

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Chestermiller
Mentor
Hello!

In ideal Rankine cycle, we say that during the isobaric expansion, the heat provided to the system is Qin = h3 - h2
However, in my opinion, it is Qin - W = Δh -> Qin - PΔV = Δh. Which is correct?

Secondly,
In Otto cycle, let's say you have calculated Wnet in kJ/kg. In order to be able to calculate Power, isn't it necessary to know mass flow rate?

Thanks!
The answer to the question about the Rankine cycle is that the first equation is correct (and yours is wrong).

The answer to the question about the Otto cycle is yes and no. It is acceptable to express the power as Wnet in kJ/kg if you also know that all you need to do to get the actual power is multiply by the mass flow rate.

The answer to the question about the Rankine cycle is that the first equation is correct (and yours is wrong).

The answer to the question about the Otto cycle is yes and no. It is acceptable to express the power as Wnet in kJ/kg if you also know that all you need to do to get the actual power is multiply by the mass flow rate.
Why my equation is wrong?
Isn't 2->3 a constant pressure process where the volume expands?
Isn't there work generated? Why we don't count it?

Chestermiller
Mentor
Why my equation is wrong?
Isn't 2->3 a constant pressure process where the volume expands?
Isn't there work generated? Why we don't count it?
It is counted. It’s lumped in with the enthalpy.