Calculating Efficiency of Otto Cycle

[mblair90]

My team has recorded information on a single cylinder Otto cycle from a lab we have done. We are supposed to compare the theoretical efficiency of the cycle to the experimental efficiency. Using a given compression ratio of 3 and the ratio of specific heats for the fuels we used we were able to come up with theoretical efficiencies of 20% and 13.3% (from eff. = 1 - 1/r^(gamma - 1)). The question at hand is how to calculate the actual experimental efficiencies. What we did exactly was get the system running at a steady RPM then slowly apply a load to the system in order to measure horsepower. We recorded the RPM, HP, torque, air/fuel ratio, exhaust gas temperature, and cylinder head temperature during the runs. Would the equation eff = 1 - (T4-T1)/(T3-T2) be applicable to find the actual efficiency?

 

[edgepflow]

Yes, eff = 1 - (T4-T1)/(T3-T2) should give you the actual efficiency.

 

[brewnog]

No, that would give you the efficiency of the ideal cycle.

Your actual efficiency can be calculated from the power absorbed by your dyno and the amount of energy you're putting in as fuel.

 

[mblair90]

Any info on how to calculate the energy from fuel?

 

[brewnog]

The heat you're putting in as fuel is the calorific value (kJ/kg) multiplied by the fuel mass consumption.

Get your calorific value off the fuel supplier (or approximate it for 'shop' fuel).

 

[edgepflow]

No, that would give you the efficiency of the ideal cycle.

Your actual efficiency can be calculated from the power absorbed by your dyno and the amount of energy you're putting in as fuel.

Correct, my mistake. That expression comes out of an ideal cycle.

I was going to suggest figuring the actual energy from fuel, but in the back of my mind I was thinking they may not be set up to measure fuel consumption rate. A simple flow meter would do, however, or even change in tank level per unit time.