Engine Performance Flashcards
Work
Work done = force x distance
Power
Rate of work being done
P = work done/time taken
Measured in watts
Or horsepower (1= 550 ft.lb per second)
Horsepower to watts
1 hp = 746 watts
Torque
T = F x R
Measured turning motion
Measured in newton metres
More radius means more torque
Engine torque
Turning effect of crankshaft that spins the propeller
Torque produces during power stroke
A single cylinder only produces torque every other rotation
Overall torque produced in a pulse manner
Prop torque
Drag force on prop
At constant rpm balances engine torque
Work done by torque
F x R x Q
q is angular displacement
Brake power
Actual power delivered to propeller
T x angular speed
Indicated power
Theoretical value always greater than brake power
Based on power developed within the engine cylinders
Calculation requires net IMEp to be determined
Indicated power = P L A (N/2) K
P= net imep
L= length of stroke
A= Area at the top of piston
N/2= Engine speed
K= number of cylinders
Net Indicated Mean Effective Pressure IMEP
Average pressure during the power stroke minus the average pressure during other three strokes
Friction power
Power lost while overcome friction and driving engine accessories
Increases with rpm
Indicated power - brake power = friction power
Rated power
Brake power developed in good condition under standard conditions at stated rpm
Rated or critical altitude
Altitude at which rated power is developed at full throttle
Normally aspirated engines rated altitude close to msl
Engine efficiency
Ratio betweeen actual power delivered and potential power with no system losses
Thermal mechanical and volumetric efficiency
Thermal efficiency
Fuel flow (kg/hr)/ brake power (w)
nBT is brake thermal efficiency = brake power/ fuel consumption (w per second)
Largely determined by compression ration
Higher CR means better nBT (limited by detonation)
Volumetric efficiency
Indicates effectiveness of induction
Nv = volume of charge/piston displacement %
Decreases with higher rpm
Higher with open throttle
Reduces as induction air gets hotter and humidor
Supercharged engines can reach 100% efficiency
Polished inlet ports can help efficiency
Mechanical efficiency
Part of thermal efficiency
Hm = brake power/ indicated power %
At most efficient rpm can approach 90%
Distribution of power
30% useful work
5% friction
20% cooling
45% exhaust
Prop efficiency
Np = thrust power / brake power %
Depends on TAS rpm and pitch
Generally least efficient at low airspeed
Diesel engine
Internal combustion
High CR
High thermal efficiency can exceed 50%
Can run on jet A1
compression ratio
(swept volume + clearance volume)/ clearance volume
