How to calculate angular acceleration of crankshaft?

[Angus Keiller]

Hello. I am currently undergoing a university project and require the angular acceleration of a crankshaft? I only know the angular velocity from the RPM and I have strongly considered that it is not possible. Just looking for some reassurance? Cheers

 

[sophiecentaur]

If you are referring to the (short term) mean change of angular velocity as the engine revs increase as the car accelerates then that would relate directly to the angular velocity information and the car's performance figures. A smart phone acccelerometer would give you acceleration to a reaonable accuracy. You could get an idea about the instantaneous acceleration (within the cycle) if you knew the pressure in all the cylinders (over time) and the masses of all the pistons plus the moment of intertia of the flywheel. I imagine that would be a 'small' value, by design, as it would represent a source of vibration and the flywheel's job is partly to suppress it.

 

[Angus Keiller]

The engine has been overhauled and is currently in pieces haha. The primary reason for this is to establish the torque of the crankshaft and from that I can derive the pressure in the cylinder. The pressure will change rapidly as the piston moves up and down obviously. Or could I derive the pressure from the molecular weight etc and work down instead of up?

 

[sophiecentaur]

This sounds very hard and very basic. The pressure in the cylinder will depend upon the fuel air mixture and how well the engine breathes etc etc. I wouldn't know where to start without approaching an enthusiast group - who would probably be delighted to get involved with a project like yours.
But this is a University project so why not ask your supervisor? To be honest, I am not sure, from what you write, whether this is a theory based exercise or a measurement question. Will you have a test bench to run the engine on?

 

[Angus Keiller]

What I have now done is estimated the pressure in the cylinder at different crank angles assuming atmospheric pressure when the piston is at the bottom of its stroke (180 degrees) for that very brief period of time while the inlet valve opens. I have then calculated a fuel-air gas mixture of mass 1.83kg (ratio 15:1) and established a density at the different volumes inside the cylinder. I am now trying to establish the torque in the crankshaft from the pressure on the piston! (Fingers crossed)
It is a mixture of theory and practical from which I am now working on the theory. The engine will not be tested before the project is due which is a bit annoying but since its a rover v8... something to look forward to ;)