Calculating Power and Instantaneous Power - What's the difference?

[Zaraphrax]

Hi all,

I'm not a mechanical engineer, so please forgive my ignorance if this is obvious.

I have a set of data that I'm working with for a piston which gives me the crank angle and internal pressure inside the cylider as the piston cranks. I've been given stroke length, cylinder bore & that the measurements were taken at 1200 RPM.

I've been asked by a friend to write a MATLAB script which calculates and plots the power and the instantaneous power. However I cannot understand what the difference is between the two.

I know that power can be calculated using P = F * v (from which I've half figured out that F is the force applied to the piston multiplied by the pressure and V is the velocity, which would need to be in radians per second in our case). What to do with instantaneous power has me totally stumped, though.

Any help would be greatly appreciated.

 

[K^2]

That is instantaneous power. I'm guessing he might also want to have RMS power over a full revolution.

 

[Zaraphrax]

That is instantaneous power. I'm guessing he might also want to have RMS power over a full revolution.

Oh wow, that certainly makes more sense. How does one go about calculating the RMS power?

 

[K^2]

Actually, now that I think about it, you definitely want ordinary average, not RMS. Especially, if you have only a few cylinders.

Just take an average of instantaneous powers over some time interval. Ideally, you should take a whole number of revolutions as your interval. If it works well with one, I'd go with that. If it doesn't, try taking a few more. Depends on what exactly will be going on with the simulation, how stable it will be, etc.

By the way, if you are using radians/s for V, you should be using torque instead of force. It's either force * linear velocity, or torque * angular velocity for instantaneous power.

 

[PJC01]

Hi there,

I can explain the difference between power and instantaneous power for you.

Power is the rate at which work is done or energy is transferred. It is calculated by dividing the work done by the time taken to do that work, or by multiplying force by velocity. In your case, the power of the piston would be the force applied to the piston multiplied by its velocity, which would give you the power output of the piston at a specific point in time.

Instantaneous power, on the other hand, is the power at a specific moment in time. It is calculated by taking the derivative of the power function with respect to time. In simpler terms, it is the power output at a specific instant, as opposed to an average over a period of time.

In your MATLAB script, you will need to calculate the power at each point in time, which will give you the average power over the entire cycle. To calculate the instantaneous power, you will need to take the derivative of the power function and plot it at each point in time, which will give you the power output at that specific instant.

I hope this explanation helps. Let me know if you have any further questions. Good luck with your MATLAB script!