Heat energy input from a hot source in a heat engine

[Franz Rojas Ayala]

Greetings!

I did the famous "Mass Lifter Heat Engine" experiment in which a mass is put at the top of a piston enclosed within a cylinder. The cylinder is connected to an aluminium canister of air and the air inside this canister expands and contracts the piston with thermal contact. We had 2 Heat reservoirs, one cold and one hot. The aluminium canister of air was placed in the cold reservoir until the height of the piston was almost zero and once the piston’s volume was at minimum, 200g of mass were added to the piston platform. The aluminium canister was subsequently moved from the cold reservoir to the hot reservoir and once it reached maximum height, the weight was removed. Finally, the canister was moved from the hot reservoir to the cold reservoir until it reached minimum volume, completing the cycle.

Basically, we made a Carnot cycle (2 adiabatic processes and 2 isothermal processes) and now in my report I have to find what the thermal efficiency was and compare it to the Carnot efficiency. Also I have to calculate the mechanical efficiency.

What I did:
- From the PV diagram I got the work from the cycle. Now I have the work performed by the gas in the heat engine.
- I have the maximum height the piston reached with the mass. Using mgh, I found the mechanical work.
- Got the Carnot Efficiency. I am saving that to compare it with the empirical thermal efficiency.

What I need to find:
- I need the Heat input from the hot reservoir to obtain the thermal efficiency.
- To calculate the mechanical efficiency, I need work output and energy input. I think the mechanical work is the work output, but I don't know what the mechanical energy Input would be.

What I have:
- I have Pressure and Volume every 0.2 seconds during the cycle.
- I have the temperatures of both reservoirs.
- I know the gas is Air, so I have Rgas.

Any help would be appreciated! Thank you!

 

[Mister T]

Do you know how to calculate the amount of heat energy transferred during an isobaric process?

 

[Franz Rojas Ayala]

Do you know how to calculate the amount of heat energy transferred during an isobaric process?

Hey! thank you for answering :)

I think I know. It should be m*cp*dT, right? If so, the problem is I don't have the mass of the gas.

 

[Mister T]

Is $C_P$ the specific heat or the molar specific heat?

 

[anorlunda]

Hey! thank you for answering :)

I think I know. It should be m*cp*dT, right? If so, the problem is I don't have the mass of the gas.

If you have the initial temperature, can you look up it's initial density. If you also know the volume, then you can compute mass.

 

[Franz Rojas Ayala]

Is $C_P$ the specific heat or the molar specific heat?

I referred to the Specific Heat capacity Cp

 

[Franz Rojas Ayala]

If you have the initial temperature, can you look up it's initial density. If you also know the volume, then you can compute mass.

Thank you for the answer :) Yeah I thought about that, but thing is when the volume reaches zero (full contraction) the mass becomes zero apparently...

 

[Mister T]

I referred to the Specific Heat capacity Cp

Is the value 29.1 $\mathrm{\frac{J}{mol \cdot K}}$?

Thank you for the answer :) Yeah I thought about that, but thing is when the volume reaches zero (full contraction) the mass becomes zero apparently...

You need to include the volume of the metal canister and connecting hoses.

 

[Franz Rojas Ayala]

Is the value 29.1 $\mathrm{\frac{J}{mol \cdot K}}$?
You need to include the volume of the metal canister and connecting hoses.

That is it! Thank you! Ill search the size of the canister. Thank you :)

 

[Mister T]

29.1 $\mathrm{\frac{J}{mol \cdot K}}$ is the molar specific heat.

 

[Franz Rojas Ayala]

29.1 $\mathrm{\frac{J}{mol \cdot K}}$ is the molar specific heat.

Thank you good Sir!

 

[Mister T]

It should be m*cp*dT, right?

So you now know that this is not the right formula?