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Measuring Engine Performance

Measuring Engine Performance. ME 115 Laboratory Spring 2008. Otto Cycle Review. Cengel & Boles, Thermodynamics: An Engineering Approach, 2006. Common terms used to compare engine performance. Brake power (bp): net power output of an IC engine Torque: A force acting through a radius

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Measuring Engine Performance

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  1. Measuring Engine Performance ME 115 Laboratory Spring 2008

  2. Otto Cycle Review Cengel & Boles, Thermodynamics: An Engineering Approach, 2006.

  3. Common terms used to compare engine performance • Brake power (bp): net power output of an IC engine • Torque: A force acting through a radius • RPM: engine speed, in rotations per minute • Specific fuel consumption (sfc): rate of fuel consumption per unit of brake power

  4. Mean Effective Pressure • MEP: a fictitious pressure that, if acted on the piston during the entire power stroke, would produce the same amount of net work as that produced during the actual cycle (Cengel & Boles, 2006) • If the MEP goes up, the cylinder volume can go down and still achieve the same power output

  5. Mean Effective Pressure, cont. • Indicated MEP (imep): uses the total power output minus the power needed for the intake and exhaust stokes (indicated power) • Brake MEP (bmep): the power used to overcome friction in the cylinder is also subtracted; this term is used more often than the imep

  6. Brake Thermal Efficiency • Brake thermal efficiency: brake power/rate of heat output for complete combustion • Brake thermal efficiency=indicated thermal efficiency* mechanical efficiency • Mechanical efficiency: related to the amount of power used to overcome friction

  7. Carnot Efficiency • To see how well our engine is doing, we can compare our brake thermal efficiency to the Carnot efficiency • Remember that the Carnot efficiency is the best we can do! • h=1-(Tlow/Thigh), where T’s are in absolute scale • We could estimate Thigh as our exhaust temperature • Tlow is our ambient temperature

  8. Engine Irreversibilities • Heat transfer from the cylinder wall during compression: will this irreversibility be larger for slow or fast engine speeds? • Pressure losses across the valves: will this irreversibility be larger for slow or fast engine speeds? • Frictional work due to sliding ring seals and other rotating components: will this irreversibility be larger for slow or fast engine speeds?

  9. Energy Efficiency Variation with Engine Speed Decher, 1994, Energy Conversion: Systems, Flow Physics and Engineering.

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