2. 7
Measuring Engine Performance

3. Learning Objectives Define engine performance.
Define and compute bore and stroke.
Understand the concept of energy and differentiate between kinetic and potential energy.
Understand the concepts of force and pressure.
Explain the concepts of work, power, and torque.
Understand how levers and belt-and-pulley, chain-and-sprocket, and gear systems provide mechanical advantage.

4. Learning Objectives
Calculate an engine’s displacement and compression ratio.
Differentiate between the various types of engine horsepower.
Define and calculate engine torque.
Explain volumetric efficiency, practical efficiency, mechanical efficiency, and thermal efficiency.

5. The Engine Small gas engines classified as:
Heat engines
Internal combustion engines
Reciprocating engines
Engine operation relies on inertia

6. Basic Terminology Performance Engine bore Stroke Top dead center (TDC)
Bottom dead center (BDC)
Crank offset
Square
Over square
Under square

7. Basic Terminology

8. Science of Engine Performance
Energy
Force
Pressure
Work
Power
Torque

9. Energy Capacity to perform work Different classifications
Potential energy
Kinetic energy
Different classifications
Mechanical, chemical, thermal, electrical
Cannot be created or destroyed
Can be transformed

10. Force Pushing or pulling of one body on another Centripetal force
Centrifugal force
Many forces at work in engine
Tensile stress

11. Pressure Measured in psi Formula: Formula for area of circle
Pressure = Force/Area
Formula for area of circle
r2 or .7854D2
r = radius
D = diameter

12. Example of Calculating Force from Pressure
Cyl. pressure=125 psi
Piston diameter=3
Piston area=7.0686in2
Total Force
= 125 psi  in2
= lb

13. Work Formula: If force produces no movement, no work done
Work = Force  Distance
If force produces no movement, no work done
Measured in ft-lbs

14. Levers and Mechanical Advantage
Change distance between force and load
More work done per unit force
Used to open and close valves
Formula:
MA = ED/RD
MA = mechanical advantage
ED = effort distance
RD = resistance distance

15. Leverage

16. Power Rate at which work is performed Formula:
Power = Work/Time
Standard unit is horsepower
550 ft-lb/1 sec
33,000 ft-lb/min

17. Torque Ability of force to cause rotation Formula:
Torque = Force  Radius
Measured with Prony brake

18. Prony Brake

19. Wheel and Axle Similar to lever Provides mechanical advantage Formula:
ME = R1/R2
ME = mech. advantage
R1 = radius (load)
R2 = radius (driven)

20. Pulley, Gear, and Chain Drive Systems
Can increase speed or torque
Ratio of teeth on gears
Ratio of sizes of pulleys
Ratio of teeth on sprockets
Small driving large increases torque
Large driving small increases speed

21. Measurements of Engine Performance
Three ways of quantifying performance
Power
Torque
Efficiency

22. Engine Displacement Measure of cylinder volume
Cylinder displacement formula:
 D2  Stroke
Engine displacement formula:
Cylinder displacement  Number of cylinders

23. Compression Ratio Comparison of cylinder volume at TDC and BDC
Typically 5:1 or 6:1
As high as 9:1 or 10:1 in some engines

24. Compression Ratio
(Briggs and Stratton Corp.)

25. Engine Horsepower Formula: Different types of horsepower
Horsepower = Rate of Work / (550 ft-lb/sec)
Different types of horsepower
Measured in different ways

26. Brake Horsepower (bhp)
Measured at crankshaft
Prony brake
Dynamometer
Increases with engine speed
Drops off at very high speeds

27. Calculating Brake Horsepower
Formula:
bhp = (R  L  W)/(5252 ft-lb/min)
R = engine speed
L = length from center of flywheel to scale
W = force registered on scale

28. Indicated Horsepower (ihp)
Mean effective pressure (mep)
Formula:
ihp = PLANK/33,000
P = mep (psi)
L = piston stroke (ft)
A = cylinder area (in2)
N = rpm/2 (4-stroke)
K = number of cylinders

29. Frictional Horsepower (fhp)
Frictional loss
Depends on engine design
Varies with engine speed
Generally about 10%
Formula:
fhp = ihp  bhp

30. Corrected Horsepower Standard brake horsepower
Dry air, 60F, 29.92 Hg
Conditions affect horsepower
3.5% loss per 1000 above sea level
3.5% loss per 1 Hg drop in pressure
1% loss per 10F increase in temperature
10% loss if head temp increases by >200F

31. Rated Horsepower Determines maximum load for continuous operation
Typically 80% of maximum bhp
(Briggs and Stratton Corp.)

32. Correction Factor Three corrections multiplied together
Temperature correction
Pressure correction
Humidity correction
Corrected horsepower formula:
Corrected hp = correction factor  test hp

33. Temperature and Pressure Correction
Ideal temp: 60F
Ideal press: 29.92 Hg
Correct for test conditions
Test temp: 90F
Test press: 28.5 Hg
(Go-Power Corp.)

34. Humidity Correction
(Go-Power Corp.)

35. Engine Torque Increases with engine rpm
Reaches maximum volumetric efficiency
Occurs at different speed in different engines
Piston moves faster than cylinder can be filled
Drops as speed continues to increase
Drops off at lower rpm than horsepower

36. Torque and Horsepower
(Clinton Engine Corp.)

37. Volumetric Efficiency
Theoretical volume vs. actual volume
Changes with engine speed
Increases with rpm to max, then falls off quickly
Can be affected by many factors

38. Volumetric Efficiency

39. Practical Efficiency How efficiently engine uses fuel
Take into account all losses

40. Mechanical Efficiency
Typically about 90%
Frictional losses about 10%
Formula:
ME = bhp/ihp
ME = mechanical efficiency
bhp = brake horsepower
ihp = indicated horsepower

41. Thermal Efficiency Percent of heat used to drive piston down
Heat losses
Cooling
Exhaust
Lubrication
Typically about 20% to 25%

42. Thermal Efficiency Formula: TE = (bhp  33,000 ft-lb/min)/C  FHV  W
TE = thermal efficiency
C = 778 Btu/ft-lb
FHV = fuel heat value
W = weight of fuel burned/min

43. What does the term engine performance refer to
What does the term engine performance refer to? How efficiently an engine performs work.

44. An single cylinder engine has a bore of 3. 5 and a stroke of 2. 625
An single cylinder engine has a bore of 3.5 and a stroke of 2.625. What is the engine displacement? Displacement =  (3.5 in)2  in =  in2  in = in3

45. Is expanding combustion gas an example of potential energy or kinetic energy? Kinetic energy

46. At the top of the compression stroke, cylinder pressure is 80 psi
At the top of the compression stroke, cylinder pressure is 80 psi. If the engine has a bore of 3.25, what is the downward force on the piston? Force = pressure  area Force = 80 lb/in2  .7854(3.25in)2 Force = 80 lb/in2  .7854( in2) Force = lb

47. An engine lists a 200 lb load a distance of 50 ft
An engine lists a 200 lb load a distance of 50 ft. What additional information is needed to calculate the power of the engine? The amount of time it took to lift the weight.

48. The output gear on an engine’s output shaft has 10 teeth
The output gear on an engine’s output shaft has 10 teeth. It drives a 30-tooth gear on the implement’s input shaft. What are the speed and torque of the implement’s input shaft in relation to the engine’s output shaft? Speed of the input shaft is 1/3 of the output shaft, and torque of the input shaft is 3 that of the output shaft.

49. An engine’s cylinder volume at BDC is 27 in3
An engine’s cylinder volume at BDC is 27 in3. If its compression ratio is 6:1, what is its cylinder volume when the piston is at TDC? 4.5 in3 (or 27 in3/6)

50. Which type of horsepower rating is based on the theoretical maximum power the engine can develop? Indicated horsepower

51. Why does engine torque begin to decrease after the engine reaches a certain speed? The piston moves so quickly that there is not enough time to completely fill the cylinder with air-fuel mixture.