1. Small Gas Engines
Chapter 14

2. Overview
Identify differences between internal and external combustion engines
Understand 2-stroke vs. 4-stroke engines
Understand subsystems of small gas engines
Discuss procedures for assembling and disassembling small gas engines

3. Internal Combustion Engines (ICE) vs. External Combustion Engines (ECE)
External combustion engines: produce heat outside of the cylinder containing the piston
Often used boilers to create steam
Internal combustion engines: produce heat Inside of the cylinder containing the piston
More reliable than ECE
Produce more power than similar size ECE
Used to power MOST vehicles in the USA
Used in agriculture and construction industries

4. Continued…
Cylinder ( aka. cylinder bore): is a hole in the block that directs the piston during movement
The ICE began replacing the ECE about 100 years ago.

5. Engine Theory
All ICEs convert chemical energy into mechanical power and share common mechanical elements
Two main types of engines
Two Stroke
Four Stroke

6. 4-Stroke Engine
Can be any number of cylinders (1,2,3,4,6,8,10,12) and all are coupled to a single crank-shaft
Crank-shaft: converts the reciprocal motion of the pistons into rotary motion and powers the load
Piston: a cylindrical engine component that slides back and forth in the cylinder when propelled by the force of combustion.

7. 4-Stroke Engine (continued)
Stroke: the movement of the piston from the bottom limit of its travel to the top limit of its travel in the cylinder bore.
Require 4 strokes of the piston to complete one cycle
Intake Stroke
Compression Stroke
Power Stroke (combustion)
Exhaust Stroke

8. 4-Stroke Engines
Intake
Compression
Power
Exhaust

9. 4-Stroke Engine (1-INTAKE)
Intake Stroke: (downward) creates a partial vacuum drawing air into the cylinder through the carburetor where liquid fuel is atomized and mixed with the air (called a fuel-air charge).
Intake valve is open
Exhaust valve is closed
4-stroke graphic

10. 4-Stroke Engine (2-Compression)
Compression Stroke: (upward) Fuel-air charge is squeezed to about 1/10th of its original volume
Bottom Dead Center (BDC) when the piston is at its lowest point (crankshaft is rounding the bottom of its travel)
Top Dead Center (TDC) when the piston is at its highest point (crankshaft is rounding the top of its travel)
Compression ratio is mathematical relationship between BDC and TDC (ie: 10:1 compression)
Intake and exhaust valves are closed
4-stroke graphic

11. 4-Stroke Engine (3-Power)
Power Stroke: (downward) With piston near TDC the compressed fuel-air charge is detonated (by the spark plug)
Combusting gasses expand pushing down piston.
The connecting rod pushes down on the crank shaft causing it to rotate
Intake and exhaust valves are closed
4-stroke graphic

12. 4-Stroke Engine (4-Exhaust)
Exhaust: (upward) Piston moves from BDC to TDC pushing the spent fuel-air mixture out of the cylinder
Piston is moved up by momentum or by power stroke of another piston pushing on the crank shaft
Intake valve is closed
Exhaust valves is open
4-stroke graphic

13. 4-Stroke Engine Animation
ns/a/a6/4-Stroke-Engine.gif

14. 2-Stroke engine

15. 2-Stroke Engine Every upward stroke is a compression stroke
Every downward stroke is a power stroke
Intake and Exhaust stroke occur during the compression and power strokes
Every revolution of the crankshaft produces power
On a 4-stroke engine, it takes 2 revolutions

16. 2-Stroke Engines (Advantages)
2-stroke engines are more powerful for their size
Good at high RPM (revolutions per minute) applications
Simpler design than 4-stroke (less parts)
No valve train
No cam-shaft
Lighter than 4-stroke engines of comprable power
No oil reservoir
No valve train, cam, etc.
Can be operated at any angle (no oil reservoir)

17. 2-Stroke Engines
Intake and exhaust occur through ports on the side of the cylinder.
Oil is mixed with the fuel and burned in the combustion chamber.
Pressure from the moving piston pushes gas/air/oil where it needs to go.

18. 2-Stroke Engine (disadvantages)
Exhaust is dirtier than 4-stroke because oil is burned
They wear more quickly than 4-stroke because every other stroke is a power stroke
They don’t last as long
Mixing oil with fuel is inconvenient and if forgotten it will destroy the engine

19. 2-Stroke Engine Animations
html
Stroke_Engine_ani.gif

20. Engine Subsystems Many of them on all engines
All must perform properly for peak performance
Cooling subsystem
Electrical subsystem
Lubrication subsystem
Mechanical subsystem
Governing subsystem
Fuel subsystem

21. Cooling Subsystem Can be cooled by air or liquid Air cooled systems
Cooling fins increase surface area
Flywheel blades direct air across engine fins
Sheet metal shrouds direct the air
Liquid cooled systems
Water jackets surround cylinder walls
Water pumps move water through jackets to radiator
Radiator expose surface area to surrounding air
Thermostat allows/impedes flow of water to radiator

22. Lubrication Subsystem
Oil distribution mechanism
Oil seals
Piston rings
Oil

23. Lubrication Subsystem
ALL moving parts must be lubricated
Splash lubrication method
Better for small gas engines
“Oil dipper” attached to bottom of connecting rod flings oil up on bottom of pistons
Piston Rings
Oil ring: (bottom ring) limits the amount of oil that squeezes past the piston into the combustion chamber
Compression ring(s): (upper ring(s)) contain combustion, scrape oil off of cyl. walls back into crankcase.

24. Lubrication Subsystem
Oil
Protects internal parts from corrosion
Cleans engine for foreign matter and allowing it to settle into the oil reservoir (crankcase or oil pan)
Seal the engine by filling small spaces between moving parts (ie: piston rings and moving parts)
Cushion moving parts from the power stroke
Improve fuel economy by reducing friction
Viscosity: measures resistance to flow (thickness)
Developed by the
Society of Automotive Engineers (SAE)

25. Oil Viscosity Chart

26. Mechanical Subsystem
Converts the force of the expanding gasses during combustion into mechanical power and delivers it to the crankshaft
Engine block (housing for all components)
Piston
Piston pin (aka: Wrist pin)
Connecting rod
Crankshaft (crankpin journal)
In a 4-stroke engine the crankshaft also powers the camshaft and valvetrain.

27. Mechanical Subsystem

28. Mechanical Subsystem: Camshaft
Opens and closes valves by pushing on rods called lifters (some are adjustable for cam wear)

29. Mechanical Subsystem: Flywheel
Heavy metal disk attached to the Crankshaft
Inertia of the rotating engine created by power stroke helps the engine coast through the exhaust, intake and compression stroke
Smoothes out the power produced by the engine so it does not continually speed up and slow down

30. Mechanical Subsystem
This system takes the most wear (usually not visible)
Measurements are made in critical areas for wear and for warpage
Micrometers
Feeler gauges (AKA: thickness Gauge)

31. Electrical Subsystem Produces the current that fires the sparkplug
Permanent magnet in the flywheel
Magnet passes the armature as flywheel spins creating low voltage
Converted to high voltage in the ignition coil
Spark jumps the gap in the spark plug to ignite fuel/air charge

32. Electrical Subsystem Timing Spacing of armature Sparkplug
Shear pin (key) keeps flywheel aligned on the crankshaft so spark is produced before TDC
Spacing of armature
Too close will rub on flywheel
Too far produces weak spark
Sparkplug
Must be “gapped” properly using feeler guage

34. Governing and Fuel Subsystem
Work in conjunction with one another
Governing system is designed to keep the engine running at the desired speed regardless of load
Fuel subsystem is responsible for creating the fuel/air mix used to power the engine and deliver it to the combustion chamber
Carburetor
Fuel injectors

35. Fuel Injected System
Fuel is pressurized and sprayed into the cylinder before TDC
Very common on cars and trucks with gas or diesel engines
Regulated by computers in modern cars to achieve maximum performance with minimum emissions

36. Carbureted System Very common on small engines and older cars
Fuel vapor is drawn through the carb by the air that rushes past it (by the intake stroke)
This occurs in the venturi.
Venturi Effect states that pressure decreases as velocity increases.

37. Governing System Definitions
Venturi: Narrow restricting section of carburetor where air speeds up and drafts the fuel vapor along with it into Cylinder
Choke: Plate-like device (usually) that varies the amount of air that can enter the carb.
Throttle: plate-like device located in back of venturi that regulates amount of fuel air mix entering the cylinders.
Load: condition under which an engine runs when it does work
Choke plate and Throttle are open

38. Governing System Definitions
Idle: the condition an engine will run under when it is warmed up to temperature and NOT under load
Choke is open
Throttle is closed
Idle Bypass Circuit: small passageway that allows some air/fuel mix to escape around the throttle plate to keep engine running

39. Measuring, Testing and Troubleshooting
All complex machines need maintenance, periodic testing and troubleshooting to run their best
Emissions testing
Temperature regulation
Tune-ups
Air filter changes
Oil changes
Etc.

40. Efficiency
Volumetric Efficiency: measures how well the engine “breathes.” Measure of how much fuel air mixture is drawn into cylinders with the amount that could be drawn in.
Mechanical efficiency: Percentage of power developed in the cylinder compared to the power that is actually delivered to the crankshaft

41. Efficiency
Thermal Efficiency: (aka heat efficiency) measure of how much heat is actually used to drive the pistons downward.
Only about 25% is used to drive the piston downward, the rest is lost.
Practical Efficiency: simple measure of how efficiently an engine uses its fuel supply
If used for motive power it is measured in MPG
Takes into account all losses of efficiency
friction
Drag
Thermal loss, etc

42. Horsepower
Developed as a means of comparing the power produced by James Watt’s steam engine to the amount of work a horse could do.
550 foot-pounds per second
Horsepower capability is affected by
Bore: diameter of the piston
Stroke: Distance from TDC to BDC
Frictional loss: within the engine (frictional vs non- frictional bearings)

43. Dynamometer

44. Horsepower Terminology
Brake Horsepower (bhp): the hp available for use at the crankshaft. Increases with engine rpm then decrease when engine is revved to high
Indicated horsepower (ihp): Theoretical term. Measure of the power developed by the fuel air charge upon ignition

45. Horsepower Terminology
Frictional Horsepower (fhp): represents the part of the potential hp lost due to friction within the engine
ihp-bhp=fhp
Rated horsepower (rhp): usually represents about 80% of the engines bhp because engines should not be run at full capability all the time (the sticker rating)