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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

Measuring Engine Performance

ME 115 Laboratory Spring 2008


Otto cycle review
Otto Cycle Review

Cengel & Boles, Thermodynamics: An Engineering Approach, 2006.


Common terms used to compare engine performance
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


Mean effective pressure
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


Mean effective pressure cont
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


Brake thermal efficiency
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


Carnot efficiency
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


Engine irreversibilities
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?


Energy efficiency variation with engine speed
Energy Efficiency Variation with Engine Speed

Decher, 1994, Energy Conversion: Systems, Flow Physics and Engineering.


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