1. TRANSPORTATION TECHNOLOGY
SMALL ENGINES
TRANSPORTATION TECHNOLOGY

2. Key Words Atomized Cooling subsystem Fuel subsystem Bore
Crankpin journal
Governing mechanism
Bottom Dead Center (BDC)
Crankshaft
Governing subsystem
Cylinder
Cam Lobe
Electrical subsystem
Idle
Camshaft
Engine ring
Idle bypass circuit
Choke
Exhaust stroke
Inertia
Combustion chamber
External combustion engine
Intake stroke
Compression ratio
Internal combustion engine
Feeler gauge
Compression stroke
Flywheel
Key
Compression test
Four-stroke cycle engine
Lubrication system
Connecting rod
Cooling fin
Fuel-air charge

3. Key words Margin thickness Spark plug gaper Mechanical efficiency
Spark test
Stroke
Micrometer
Thermal efficiency
Multigrade
Thermostat
Piston
Throttle
Power stroke
Top dead center (TDC)
Practical efficiency
Radiator
Two-stroke cycle engine
Rated horsepower (rhp)
Valve lifter
Rich mixture
Venturi
Society of Automotive Engineers (SAE)
Volumetric efficiency
Water jacket
Spark plug

4. History
Over 100 years ago, internal combustion engines began to replace external combustion engines (steam engines) as the major source of power for vehicles.
Internal combustion engines produce heat inside the cylinder containing the piston.
ICE engines are more efficient and reliable than external combustion engines, which generate heat in a boiler or other device outside the cylinder.
Produce much more power
The cylinder, more correctly identified as the cylinder bore, is a hole in the block of the engine that directs the piston during movement.

5. Engine Theory
There are 2 main type of small gas engines, the four-stroke cycle engine and the two-stroke cycle engine.
The two types perform the same function– converting chemical energy into mechanical power.
They differ considerably in their methods of operation.
The Difference

6. The four-stroke cycle engine
Automobiles use four-stroke cycle engines as their power source.
The automobile engine has 4, 6, 8, 10, or 12 cylinders, which are coupled to one crankshaft.
The crankshaft is an engine component that converts the reciprocating motion of the piston and rod assembly into rotary motion.
It also is the shaft that powers the load.
Most small gas engines have only one cylinder powering the crankshaft
The Piston is a cylindrical engine component that slides back and forth in the cylinder when propelled by the force of combustion.

7. Four-stroke cycle engine
A stroke is the movement of the piston from the bottom limit of its travel to the top limit.
The four-stroke cycle engine’s stroke are the following:
Intake Stroke- this is the downward stroke of the piston that begins the process of producing power.
this movement creates a partial vacuum.
The force of this vacuum draws air through the carburetor.
Liquid fuel is drawn into the carburetor at the same time and atomized (broken into small droplets) to mix with the air.
The mixture is called the fuel-air charge.
It flows into the cylinder through the intake valve.

8. Four-stroke cycle engine
Compression Stroke- this is an upward movement of the piston and connecting rod assembly.
The connecting rod is an engine component that connects the piston with the crankshaft.
The fuel-air charge is typically squeezed to about 1/9th of its original volume.
When the piston is as low in the cylinder as it can go, it is said to be at bottom dead center (BDC).
When the piston is as high in the cylinder as it can go, it is said to be at top dead center (TDC).
The Compression Ratio of an engine is the mathematical relationship at BDC and the volume available in the cylinder with the piston at TDC.

9. Four-stroke cycle engine
Power stroke- this is the stroke in which power (mechanical movement) is transferred from the piston to the connecting rod and then to the crankshaft.
As the piston approaches TDC on the compression stroke, the spark plug fires.
It takes a fraction of a second for the gases in the combustion chamber to ignite and expand.
This allows the piston to move past TDC, so the expanding gases will push down on the piston with tremendous force.

10. Four-stroke cycle engine
Exhaust stroke- this is the final movement in the four-cycle process– an upward stroke of the piston.
The camshaft holds the exhaust valve open.
Movement of the piston forces the spent fuel-air mixture out through the exhaust valve.
As the piston clears TDC, the camshaft causes the exhaust valve to close and the intake valve to open.
A new fuel-air charge is drawn into the cylinder, beginning the four-stroke cycle again.
How it works

11. Engine Subsystems COOLING SYSTEM LUBRICATION SYSTEM MECHANICAL SYSTEM-
Removes excess heat from the engine.
LUBRICATION SYSTEM
Reduces friction so moving parts will not overheat and quickly wear out.
MECHANICAL SYSTEM-
Converts reciprocating (up and down) motion to rotary motion.
ELECTRICAL SYSTEM
Produces high-voltage electrical sparks to ignite fuel inside the engine
FUEL AND GOVERNING SYSTEM
Mixes air and fuel for proper burning inside the engine

12. Cooling system
The Cooling Subsystem is responsible for keeping the engine operating with a comfortable temperature range.
The temperature generated by burning gasoline in an engine can reach 4,000° Fahrenheit. Since steel melts at about 2,500° Fahrenheit, it is necessary to lower the heat of the engine before it does any damage.
Engines are cooled by:
Air cooling (Most small engines)
Water cooling

13. Air Cooling
Air cooling is the most popular method of cooling small engines because they are simple, inexpensive to manufacture, and easy to maintain.
The primary pars of an air-cooled system are:
Cooling fins on the head and block of the engine.
The cooling fins conduct heat from the combustion chamber and transfer it to the surrounding atmosphere.
Flywheel blades create a flow of air that cools the engine.
The sheet metal shrouds channel the airflow across the hottest parts of the engine, which are the cooling fins surrounding the combustion chamber.
Various sheet metal parts that enclose the engine
It is important that all sheet metal shrouds are in place, so the airflow produced by the rotating flywheel is channeled to the proper areas of the engine.
Process
Cooling fins increase the surface area of the engine, which means more air comes in contact with more hot surfaces.
Air is forced over the cooling fins by the vanes that were cast into the flywheel.
As the flywheel spins, the vanes blow air over and through the cooling fins.
The air carries away the heat from these surfaces to cool the engine.

14. Air Cooling

15. Liquid Cooling
Some larger engines may use a liquid cooling system that includes a water pump, radiator, and thermostat.
Since water is much denser than air, it can absorb and dissipate much more heat from an engine.
In this type of system, a water and antifreeze solution is pumped through water jackets, or spaces machined into the block of an engine, surrounding the cylinder
When the liquid heats up, a thermostat (temperature controlled valve) opens, allowing the liquid to flow to the radiator.
The Radiator is a heat exchanger that transfers the heat from the liquid to the surrounding environment.
The cooled water is pumped back to the engine block to absorb more heat from the engine and transfer it to the radiator.
How it works

16. Lubrication Subsystem
The lubrication system of a small gas engine includes the oil distribution mechanism, the oil seals, the piston rings, and the lubrication oil.
It is important that all moving parts within the engine are lubricated.
There are several ways to distribute oil to the working parts of an engine.

17. Two-stroke engine lubrication
In a two-stroke engine, lubricating oil is mixed directly with the engine’s fuel.
During operation, the oil and fuel are mixed with air and then drawn into the engine’s crankcase. The oil enters the crankcase as a fine mist.
The mist forms a thin film of oil on all the internal engine parts.
With each compression stroke, a new supply of oil is drawn into the crankcase.

18. Four-stroke engine lubrication
In a four-stroke engine, the bottom of the crankcase is filled with motor oil. From the crankcase, the oil is moved throughout the engine by one of two lubrication methods:
Splash lubrication
Pump lubrication

19. Splash lubrication
In a splash lubrication system, the connecting rod in the engine has an extension called an oil dipper.
As a connecting rod moves around the crankshaft, the dipper plunges into the oil in the crankcase (oil reservoir) and splashes it onto the internal engine parts.
The level of the oil in this system is very important. Too low and the dipper will not splash any oil. Too high and the oil may not splash enough to reach all internal parts.
If a dipper has been replaced, it is vital to properly torque the connecting rod bolts.
Undertorquing the bolts could cause the dipper and connecting rod cap to become loose, leading to catastrophic engine failure.
Example

20. Pump Lubrication
In this system, a pump draws in oil from the crankcase and then forces it under high pressure onto the crankshaft and connecting rod.
The pump is operated by a cam on the crankshaft.
When the cam moves up, oil is drawn into the pump by the pump piston. Then, when the cam moves down, the oil is forced out through the spray nozzle onto the crankshaft bearings.

21. Lubrication Subsystem
Lubrication of the cylinder wall is necessary.
The engine rings serve to limit the amount of oil that makes its way into the combustion chamber.
To much oil entering the combustion chamber creates excessive emissions, usually in the form of thick smoke, and it is also can ruin spark plugs, causing the engine to stop running all together.
The oil ring is the bottom ring on the piston.
It is designed to allow a small amount of oil to make its way through the matching holes in the piston, and out to the cylinder wall.
The compression rings usually have a groove or a bevel to scrape excess oil off the cylinder wall and send it back down into the block where it is stored.
Oil seals serve a similar function on the valve stems.

22. Lubrication Subsystem Maintenance
Lubrication maintenance on small gas engines includes the following:
Changing the oil at regular intervals
Wiping debris and sludge from the bottom of the oil reservoir every few oil changes, since most small gas engines do not contain oil filters.
Ensuring that rings and seals are installed correctly during the engine reassembly.
Inspecting seals for wear

23. Lubrication subsystem functions
Protect the internal engine parts from corrosion.
Cleanse the engine of foreign matter by transferring it to the engine block, where the foreign matter settles to the bottom of the oil reservoir and can do little harm.
Seal the engine by filling the small space between moving parts, such as the piston rings, and the cylinder wall.
Cushion moving engine parts from the tremendous force of the power stroke on combustion.
Improve fuel economy.

24. MECHANICAL SYSTEM
Converts the force of the expanding gases during combustion into mechanical power, delivering the power to the crankshaft.
Most of the moving parts of a small engine are part of the mechanical system.

25. PARTS OF THE MECHANICAL SYSTEM
CYLINDER BLOCK
CYLINDER HEAD
PISTON
PISTON RINGS
CONNECTING ROD
CRANKSHAFT
FLYWHEEL
VALVES
CAMSHAFT
BEARINGS

26. CYLINDER
The cylinder block is main body of the engine. It contains the cylinders.
The cylinder head seals the top of the cylinder. It holds the spark plug and is directly above the combustion chamber

27. Piston/ Piston rings Piston Piston Rings
A cylinder-shaped device that slides back and forth inside a cylinder in a gasoline piston engine to create the pressure under which combustion occurs.
Piston Rings
Metal rings in an engine that press against the wall of each cylinder. Piston rings consist of two compression rings on the top (designed to prevent combustion gases from leaking past the piston) and one oil ring on the bottom(designed to keep the oil that is used to lubricate the engine out of the combustion chamber).

28. Piston/ Piston rings
Piston Head
Compression Rings
Oil Ring

29. Connecting Rod
The connecting rod is a metal rod that transfers force from the piston to the crankshaft.
The connecting rod attaches to the piston by a piston pin. The piston pin fits through a hole in one end of the connecting rod. The other end of the connecting rod attaches to the crankshaft. This end is a separate piece called the rod cap that secures the rod to the crankshaft.

30. Connecting Rod
Piston Pin Hole
Bolt
Rod Cap
Bolt

31. Crankshaft
The crankshaft is a metal rod that converts the reciprocating motion of the pistons into rotary motion.
Counter Weights
- balance the crankshaft so it spins smoothly
Crank Pin/ Journal
- connecting rod and rod cap fit around here

32. Function of connecting rod and crankshaft
During an engine’s power stroke, the pressure created by the combustion of fuel pushes on the piston. This pressure forces the piston and the connecting rod down. The connecting rod pushed on the crankshaft, causing it to rotate. Until the next power stroke, the crankshaft and the connecting rod move the piston. The crankshaft pushes the piston up on the exhaust stroke, pulls the piston down on the intake stroke, and pushes the piston back up on the compression stroke.

33. Flywheel
The flywheel is a heavy disk-shaped weight that is attached to one end of the crankshaft.
The momentum of the spinning flywheel keeps the crankshaft turning between power strokes.
Without the flywheel to keep the crankshaft turning, the engine would simply stop shortly after the power stroke.

34. Flywheel (Cont.)
In small engines, the flywheel is also part of the cooling system. Special vanes are cast into the flywheel when it is made. The vanes blow air over the engine to help keep it from over heating.
The flywheel contains a magnet that is used as part of the ignition system.

35. valves
Valves are engine parts that control the flow of gases in and out of a cylinder.
In a single-cylinder, four-stroke engine, two mushroom-shaped valves are located near the top the cylinder, just below the cylinder head.
The intake valve opens to allow the air-fuel mixture to enter the cylinder during the intake stroke.
The exhaust valve allows the exhaust gases to leave the cylinder after combustion has occurred.

36. Camshaft The camshaft is a metal shaft that the cams are attached to.
Cams are egg-shapped loves that are located under valves. The cams are what open and shut the valves.
The camshaft is driven by the crankshaft through gears. Lifters are connected to the cams which push up on push rods forcing the valve to open and close.
One cam operates the intake valve and the other operates the exhaust valve.

37. Valve assembly
Camshaft
Cam Lobe

38. Bearings
Bearings are devices that reduce friction between moving parts.
They are called bearings because they bear, or carry, a load. The most common type of bearing used in small engines is a friction bearing. Friction bearings are often used between connecting rods and the crankshaft.

39. Measuring The mechanical subsystem is subject to the most wear.
This wear is often not visible to the eye because it is only a few thousandths of an inch.
The ability to measure with micrometers, feeler gauges, hole gauges, and telescoping gauges is very important.
Micrometers are the basic precision measuring instruments used to check for wear points on engine parts.
Feeler gauges (thickness gauges) are thing strips of metal machined to a specific thickness, often measured in thousands of an inch.
The metal strips are used to verify a gap between two parts.
A feeler gauge that is to thick cannot enter the gap.
One that is too small moves around to freely in the gap.

40. Electrical subsystem
The electrical subsystem creates a carefully times electrical spark to ignite the air-fuel mixture within the cylinder.
It begins with the permanently mounted magnets within the flywheel. As the flywheel spins, the small amount of magnetism induces a low voltage in the armature (magneto) each time the magnets pass the armature.
This low voltage is then converted to high voltage in the ignition coil when the primary field collapses on the secondary field, causing the spark plug to fire.
Most single-cylinder engines use a magneto to generate electrical power and a spark plug to create a spark.

41. Magneto
The magneto is a special alternator used for producing high voltage electrical current.
As the flywheel spins, the flywheel magnet passes the magneto, creating an electrical current that is sent to the spark plug.
Magneto at work

42. Spark plug
A spark plug is a device designed to produce an electrical spark within a cylinder.
It consists of a terminal, an insulator, a shell, a center electrode, and a ground electrode.
Plugs must be gapped properly.
Over-gapping or under-gapping may result in failure to fire.
To properly gap the plug, use a spark plug gaper and bend the electrode as necessary.
The spark plug is screwed into the cylinder head and a heavy wire from the magneto is connected to the terminal. This wire is called the spark plug wire.

43. Keys for ignition to happen
The flywheel must be aligned properly on the crankshaft for spark to occur.
It is held in place by a small piece known as a key.
If no key is installed, or if the key breaks, the position of the flywheel on the crankshaft can shift.
If the flywheel shifts positions, the magnets will not be in the proper location at the time ignition is supposed to occur.
Also for spark to occur, the armature (magneto) must be located the proper distance from the flywheel.
If it is too far away from the flywheel (over-gapping) , a weak spark may result.
Under-gapping can cause the armature to rub against the flywheel and result in failure to produce a spark.
A good spark is essential to proper engine performance.
If the plug is cracked, the spark will jump through the ceramic insulator and over to the clock, rather than jumping between the spark plug and electrodes.

44. Diagnostic Procedures
Inspect the flywheel key to ensure that the flywheel is seated in the proper location.
Determine the proper armature air gap.
When reinstalling make sure the armature is properly spaced from the flywheel.
Remove and inspect the spark plug.
Look for cracks around the ceramic insulator ring.
Determine the proper spark plug gap, and then use a spark plug gaper and re-gap the plug as necessary.
Test for spark by disconnecting the plug wire and using a spark plug tester.
Be sure that a strong blue spark is being fed to the plug.
Can also put spark plug against piece of metal and pull on pull chord.

45. Fuel and governing subsystem
The governing subsystem and the fuel subsystem are described together because they work in conjunction with each other.
The governing subsystem is designed to keep an engine running at a desired speed, regardless of the load applied to the engine.
The governing system is made up by the governing mechanism.
The fuel subsystem is responsible for creating the fuel- air mixture used to power the engine and delivering that charge to the combustion chamber.
The fuel subsystem is made up of the:
Fuel tank
Air cleaner
Carburetor
Choke/throttle valve

46. Fuel tank
The Fuel tank is the container that holds the engine’s fuel supply.
Some small engines have a small hole in the tank cap. This hole allows air to enter the tank. Without the hole, a vacuum would develop as the fuel dropped. The vacuum would eventually prevent the fuel from moving into the engine.

47. Air cleaner
An air cleaner is a device that removes dirt and debris from air before it enters the engine.
Dirt can cause internal engines parts to wear.
Two common types of air cleaners:
Oil-foam
Contains a foam filter inside a housing. The foam is dampened with oil. As air is drawn through the air cleaner, the oil and foam trap and dirt particles before they enter the engine.
Dry-paper
contains a treated paper filter. This type of filter traps dirt in the fibers of the paper

48. Carburetor
The carburetor mixes the air from the air cleaner and fuel from the fuel tank in the proper proportion for combustion.
Carburetor parts:
Choke valve
Throttle valve
Fuel nozzle
Bowl
Venturi
Venturi Principle

49. How the carb works
Air from the air cleaner passes into the carburetor where it is mixed with fuel.
The air-fuel mixture then flows past the intake valve into the cylinder.
The carburetor uses fuel from a small container called a bowl. (fuel flows by gravity from the fuel tank into the bowl).
As the piston moves down on the intake stroke, it pulls air through the carburetor.
The air passes through a narrow section called a venturi.
Venturi is the narrow, restricted section of the carburetor, where air speed increases and drafts the fuel vapor along with it into the combustion chamber.

50. How the carb works (Cont.)
As the air passes through the venturi, it speeds up.
This increase in air speed causes a decrease in the air pressure within the venturi.
This allows atmospheric pressure in the bowl to force gasoline through a small pipe called a fuel nozzle.
Gasoline flows through the fuel nozzle into the venturi.
In the venturi, the gasoline mixes with the high-speed air to form a mist.
This mist, consisting of air and fuel, then flows into the engine, where it is compressed and ignited to produce a power stroke.
How the Carb works

51. Venturi Principle

52. Choke Valve
The choke is a plate-like device that varies the amount of air that can enter the carburetor.
When engine is “choked” (plate is mostly closed), more fuel vapor and less air are entering the combustion chamber.
The primary reason for choking an engine is to create a rich mixture (more fuel than normal, which is desireable to get the engine started and warmed up to temperature during a cold start
When the choke valve is closed, less air and more fuel enter the cylinder. This creates a rich air-fuel mixture that ignites more easily when the engine is cold

53. Throttle valve
The throttle valve controls the amount of the air fuel mixture that enters the cylinder. When the throttle valve is closed, the engine runs at slow speeds.
The throttle valve is similar in form to the choke valve but is located between the venturi and the cylinder.
This valve controls the amount of air- fuel mixture that enters the which means it also controls the speed and power produced by the engine.

54. Other Terms
Load is the condition under which an engine runs when it is called on to do work.
When an engine is running under load, both the choke and throttle are fully open.
Idle is the condition and engine will run under when it is warmed up to temperature and not under load (not moving).
When an engine is at idle, the choke is generally open, and the throttle is generally closed.
Idle bypass circuit is a small passageway that allows some fuel-air mixture to escape around the throttle plate and keep the engine running, even when the throttle is closed.

55. Governor Subsystem
A governor is a device used on many small engines to control the amount of fuel-air mixture that enters the combustion chamber.
Most types of governing mechanisms rely on the speed of the engine to determine whether more or less fuel-air mixture is needed.
The governor is directly linked to the throttle plate.
The faster the engine turns, the more the governor pulls the throttle towards its closed position, allowing less fuel-air mixture to enter.
When the engine begins to slow down, the governing mechanism moves inward, opening the throttle.
This allows more air-fuel mixture to enter the combustion chamber.
The most common type of governor is an air-vane governor.

56. Air-vane Governor
The air-vane governor consists of a spring-loaded piece of metal or plastic called an air vane. The air cane is connect to the throttle valve. As the flywheel spins, the air vane receives a stream of air created by vanes on the spinning flywheel.
An air-vane governor is operated by air blowing from the flywheel. When the flywheel slows down, the throttle valve is opened. When the flywheel speeds up, the throttle valve closes.