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 RPM: engine speed, in rotations per

minute Specific fuel consumption (sfc): rate of fuel

consumption per unit of brake power

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.

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 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

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! =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

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

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