Bai7200:
...Use a Hydraulic pump as a brake, and a loadcell to calculate the power. This setup should be smaller and the Hardware will have a smaller "footprint".
The second i´m in the dark a bit. Can Arduino handle this and how can i make it calculate the output power? The hardware part of it is easy, but i have no clue how to get Arduino to calculate output power.
As I understand it, you will use the engine to directly drive a hydraulic pump, and in restricting the flow-rate of the pumped oil you will brake the engine to the required engine speed. The reaction to this will be measured with a load-cell, and it's output will be scaled an read as torque.
There is a relationship between torque and power, but I can't remember what it is!
Remember that using this method will put most of this power into the oil as heat, think how you will keep the oil temperature stable.
Measuring the engine speed (rpm) should be easy with an optical or magnetic pick-up. I'm afraid I'm not fully up to speed with the coding for this, but I suggest something like:
- Take a reading of millis() when the pickup is sensed, and set the revcount to 1.
- Next revolution revcount = revcount + 1 (assuming one revolution per pickup pulse)
- Read millis() when it revcount = 100.
- Time taken to do 100 revs is millis-now minus millis-then.
- Engine speed is 60 divided by time taken for 100 revs, times 100,000 in revs per minute.
The reason that I suggest taking the time for 100 revs is that an engine operating at 6000 rpm does 100 revs per second, or 1 rev per 10 milliseconds. Given that millis() returns the number of milliseconds since the last reset, the resolution will be +/- 600rpm if you tried to read it every revolution. This way the average of 100 revs gives an update once per second at 6000 rpm, but at a resolution of about +/- 6rpm.
That said, 6000 rpm is relatively sedate for a high performance two-stroke... 100cc kart engines typically peak at 18-20,000 rpm!
Measuring the torque, and then on to power should be straightforward. The load-cell will be a set distance in metres from the centre of rotation, and it will output relative to the force applied to it (say in killogram). The torque will be Force in Newtons (kg x 9.81) x distance from centre in metres .... unit is Newton-metre (Nm).
Despite my cop-out over conversion from torque to power, I seem to remember that 1 hp is defined as 33,000 foot-pounds per minute, so I suppose you can substitute the imperial equivalents (feet and pounds) into this equation to get power in hp, or indeed use the metric figures to calculate kW. My brain won't let me do this at the moment.
Hope that helps, an that I haven't gone off at a tangent.
GM
