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POWER (Engine Performance)

Terms in this set (42)

TORQUE
• Turning effort
• Good indicator of an engine's ability to do work
TORQUE
T = F x L
Force doing work
Work/rev
2piT
Power
rate of doing work
Rotary Speed/Angular Speed
amount of angular rotation per unit of time
Rotary Power
product of work/rev and rotary speed
INDICATED POWER
power generated in the cylinder and received by the piston
INDICATED POWER (2-stroke)
IP = PLANn
INDICATED POWER (4-stroke)
IP = PLANn/2
Mean Effective Pressure
Equivalent constant pressure developed at the top of the piston during the power stroke
Mean Effective Pressure
Merely the height of the p-v diagram
Mean Effective Pressure
Area within the p-v diagram divided by the displacement volume of one cylinder
BRAKE POWER
power that is available at the engine output shaft/flywheel for doing useful work
BRAKE POWER
2piTN
FRICTION POWER
Part of the indicated power that is absorbed in the engine and is not available for useful work
FRICTION POWER
Includes the power required to overcome the friction in the moving parts of the engine plus pumping losses on the intake and compression strokes
FRICTION POWER
IP-BP
• Means for controlling torque
• Means for measuring torque
• Means for measuring speed
• Means for dissipating the power
Brake Power Measurement (Essential Elements)
DYNAMOMETER
Instrument for measuring engine
(brake) power
Electric Dynamometer
Eddy-Current Dynamometer
Hydraulic Dynamometer
3 Types Dynamometer
Electric Dynamometer
Use of generator to control the torque and dissipate the power
Eddy-Current Dynamometer
Use of magnetic field to control torque
Hydraulic Dynamometer
Use of water or oil pumped through orifices to absorbed engine power
Mechanical Efficiency
Fraction of IP that is delivered as useful power
Mechanical Efficiency
Ratio of BP to IP
Mechanical Efficiency
Varies from 75% to 90% depending on
the speed, load and other factors
Effm
BP/IP
Mechanical Efficiency (Factors)
• Losses due to friction in the moving parts such as crankshaft and connecting-rod bearings, pistons and
cylinders, valve mechanisms, and cooling and pump
• Losses involved in the induction of the fuel mixture and the exhaust of the residue.
THERMAL EFFICIENCY
Indicated Thermal Efficiency
Brake Thermal Efficiency
Indicated Thermal Efficiency
Fraction of power value of fuel that is converted to IP
Effith
IP/Power Value of Fuel

Note: Power must be converted into heat-energy units
or the fuel into mechanical power unit
Brake Thermal Efficiency
Fraction of power value of fuel that is converted to BP
Effbth
BP/Power Value of Fuel
Brake Thermal Efficiency
Overall efficiency of the engine in converting fuel power into useful work
Effbth
Effith x Effm
FUEL CONSUMPTION
Quantity of fuel consumed by the engine per unit time
FUEL CONSUMPTION
By volume or by mass
FCv
Vfuel/time
FCv
Vfuel/time
(Brake) Specific Fuel Consumption
Fuel consumption in relation to the amount of work that is being done by the engine

Good indicator of engine efficiency
SFCb
mfuel/time x BP = (specific density of fuel x FCv)/BP
FCm
mfuel/time = specific density of fuel x FCv
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