2. 34
Chapter
Small Engine Performance, Maintenance, and Troubleshooting

3. Chapter Outcomes Explain how engine performance is measured
List factors affecting an engine’s suitability for a particular application
Demonstrate proper engine maintenance procedures
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4. Chapter Outcomes
Describe the role that service information plays in the service and maintenance of an engine
Indentify the three basic things that an engine needs in order to run
List the sequence of checks used to systematically troubleshoot an engine
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5. Measuring Engine Performance
A group of measurements used to determine an engine’s suitability for different applications
The amount of work an engine can do in a given period of time and the efficiency with which the engine does that work
Areas of measurement include
Engine displacement
Compression ratio
Force, work, and power
Horsepower and torque

6. Measuring Engine Performance
Engine displacement
The volume of air and fuel the engine can draw into its cylinder(s) during an intake stroke
With one cylinder, displacement would be the volume displaced by the piston through its movement from TDC to BDC

7. Measuring Engine Performance
Engine displacement
To calculate the displacement of a single cylinder
(π/4) × D 2 × L = engine displacement
D = cylinder diameter
L = length of stroke

8. Measuring Engine Performance
Compression ratio
Ratio of the total volume of empty space above the piston at BDC compared to the volume when the piston is at TDC

9. Measuring Engine Performance
Compression ratio
Equals the volume above the piston when the piston is at BDC divided by the volume above the piston when the piston is at TDC
For example: Volume at BDC = 10 cubic inches
Volume at TDC = 1 cubic inch
Compression ratio is 10:1
Most small gasoline engines have a compression ratio between 6.5 to 1 and 9.5 to 1

10. Measuring Engine Performance
Force
An influence, such as a push or pull, that tends to change the motion of an object
Work
When force is applied to an object and causes displacement
Work = Force × Distance
Power
The rate at which work is performed in a given time frame

11. Measuring Engine Performance
Horsepower
A unit of power equal to 550 foot pounds per second
Measurement of work over time
Originally developed as a comparison measurement to prove the advantages of machines over animals
Torque
Rotating or twisting force
Equal to force applied across a distance
Commonly measured in foot pounds

12. Engine Selection Criteria to consider Horsepower rating Torque rating
Fuel efficiency
Crankshaft orientation
Physical engine size
Fuel system
Engine speed controls
Lubrication system type
Cooling system type
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13. Engine Maintenance
Crucial for efficient operation and engine longevity
Limits malfunctions and breakages
Preventive procedures
Follow manufacturer guidelines
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14. Engine Maintenance Lubrication system Check oil level
Change oil as needed to keep fresh in system
Use viscosity given by manufacturer
Use different oil as dictated by location and time of year
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15. Engine Maintenance Fuel system
Controls air/fuel mixture entering the engine
Includes fuel tank, lines, filter, and carburetor
Imperative to follow manufacturer’s fuel standards

16. Engine Maintenance Fuel system Keep tank and motor clean of debris
Replace filters at recommended intervals
Use manufacturer’s recommended ratios for fuel/oil mixtures for two-stroke engines
Drain fuel at the end of season or use fuel stabilizer
Keep tank full to minimize condensation in fuel
Use minimal ethanol content to prevent damage

17. Engine Maintenance Air system
Keep carburetor, which regulates fuel and air, clean and free of debris
Inspect, service, and replace air filters periodically

18. Engine Maintenance Cooling system
Keep engine and radiator clean of dirt and debris
Clean radiator fins often or as needed after work
Replace coolant as needed
Maintain proper antifreeze to water ratio

19. Service Information Keep manufacturer’s service literature on hand
Follow manufacturer’s guidelines for
Engine identification procedures
Service and repair procedures
Service intervals
Replacement parts
Wear tolerances
Carburetor settings

20. Engine Troubleshooting
Systematic diagnosis of engine problems
Must follow logical steps
1. Make sure the engine is getting fuel
2. Make sure the engine is getting air
3. Check for spark
4. Check the compression

21. Engine Troubleshooting
Checking the fuel supply
1. Make sure fuel supply is turned on
2. Check for fuel in the tank
3. Check if fuel is making it through the supply line to the carburetor
Make sure fuel filter is not clogged if applicable
4. Check vacuum or electric fuel pump
Make sure there is power or vacuum

22. Engine Troubleshooting
Checking the air supply
Check air filter
Clean by tapping out debris
Replace if needed

23. Engine Troubleshooting
Check the spark using a spark testing tool
If spark appears in the tester, the spark plug may be damaged
Check air gap in the plug
Replace if necessary

24. Engine Troubleshooting
If there is no spark
Check the air gap between flywheel and armature
Ensure flywheel key is present
Make sure all safety switches are correct
Test armature or ignition module
Spark plug wire
Connected to engine
Tester

25. Engine Troubleshooting
Checking the compression
At TDC, pressure must be present to ignite fuel
Use a compression gauge to test through spark plug hole
Compare to published data
Perform a leak-down test if needed
Use a flywheel bump test

26. Engine Troubleshooting
Common causes for compression loss
Loose spark plug or missing compression washer
Loose cylinder head bolts
Blown head gasket
Bad valve seat or face clearance
Insufficient lifter/tappet clearance
Warped cylinder head
Worn cylinder walls
Worn or broken piston rings

27. Engine Troubleshooting
Checking the carburetor
Make sure linkages are correct and in place
Make sure choke and throttle open and close as they should
Check governor
Do a full disassembly and cleaning according to the manufacturer
Replace damaged parts

28. SAE Connection Small Engine Troubleshooting and Repair
Do you like working with small engines?
Consider an SAE focused on servicing and repairing small engines, including
Weed trimmers
Lawn mowers
Rotortillers
Chain saws
Recreational vehicles