Engine Performance

Question 1

Work

Answer 1

Work done = force x distance

Question 2

Power

Answer 2

Rate of work being done
P = work done/time taken
Measured in watts
Or horsepower (1= 550 ft.lb per second)

Question 3

Horsepower to watts

Answer 3

1 hp = 746 watts

Question 4

Torque

Answer 4

T = F x R
Measured turning motion
Measured in newton metres
More radius means more torque

Question 5

Engine torque

Answer 5

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

Question 6

Prop torque

Answer 6

Drag force on prop
At constant rpm balances engine torque

Question 7

Work done by torque

Answer 7

F x R x Q
q is angular displacement

Question 8

Brake power

Answer 8

Actual power delivered to propeller
T x angular speed

Question 9

Indicated power

Answer 9

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

Question 10

Net Indicated Mean Effective Pressure IMEP

Answer 10

Average pressure during the power stroke minus the average pressure during other three strokes

Question 11

Friction power

Answer 11

Power lost while overcome friction and driving engine accessories
Increases with rpm
Indicated power - brake power = friction power

Question 12

Rated power

Answer 12

Brake power developed in good condition under standard conditions at stated rpm

Question 13

Rated or critical altitude

Answer 13

Altitude at which rated power is developed at full throttle
Normally aspirated engines rated altitude close to msl

Question 14

Engine efficiency

Answer 14

Ratio betweeen actual power delivered and potential power with no system losses
Thermal mechanical and volumetric efficiency

Question 15

Thermal efficiency

Answer 15

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)

Question 16

Volumetric efficiency

Answer 16

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

Question 17

Mechanical efficiency

Answer 17

Part of thermal efficiency
Hm = brake power/ indicated power %
At most efficient rpm can approach 90%

Question 18

Distribution of power

Answer 18

30% useful work
5% friction
20% cooling
45% exhaust

Question 19

Prop efficiency

Answer 19

Np = thrust power / brake power %
Depends on TAS rpm and pitch
Generally least efficient at low airspeed

Question 20

Diesel engine

Answer 20

Internal combustion
High CR
High thermal efficiency can exceed 50%
Can run on jet A1

Question 21

compression ratio

Answer 21

(swept volume + clearance volume)/ clearance volume