Calculating mechanical eff. @ ICE

[Mr. Red]

Hi guys,

I have been trying to model mechanical eff. of an ICE. I have two methods for that one using:

$$\eta$$_m = $$\frac{bp}{bp+fp}$$

where bp = brake power
fp = friction power

This method did not work well because of bad assumptions in fp. So I decided to calculate the bp_required from back calculating the power from the tires. And using that to calculate the %load on the engine however the load values are too high so there should be a problem with bp_required. I calculate bp_required through coast-down test. What can be the reason of high bp_required? Do you have any idea or suggestion to check?

Thanks,
- Red

 

[Mech_Engineer]

If you know the fuel flow to the engine at a specific horsepower rating you can calculate the total chemical power available in the gasoline (volume/time times energy/volume = energy/time = power) and use that number to determine the ratio between the theroetical maximum power output and the measured engine output.

If the fuel flow isn't directly measured, you could make an estimate based on the volumetric flow efficiency of the engine, the manifold pressure (could be estimated to be atmospheric pressure on a naturally aspirated engine at full throttle), and the Air/Fuel ratio in the cylinder. It's important to note that most engines at full throttle push an air/fuel ratio of 12.0:1 rather than 14.7:1, to develop maximum power.