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

1. Jul 24, 2010

### 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.

2. Jul 25, 2010

### K^2

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

3. Jul 25, 2010

### Zaraphrax

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

4. Jul 25, 2010

### 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.