How Many Small Engines do you have at home? Do you understand how they work and what makes them run? This presentation is from Virginia Tech and has not been edited by the Georgia Curriculum Office.

Small Gasoline Engines: b Principles of Operation 4-stroke and and2-stroke Zach Olinger Spring 2002

Name some uses of small engines. b chainsaws b lawn mowers b weed eaters b four wheelers b wood splitters b garden tillers

There are many things that we use on a regular basis that are powered by small engines. Small engines make our lives easier.

There are two types of small gasoline engines that are common today: b Four- cycle engines b Two- cycle engines

The Four-Cycle Engine

The four-cycle engine operates on a series of four “strokes” or piston movements

The piston operates in an up and down, or back and forth motion within a cylinder. Piston Cylinder

The piston is connected to a crankshaft that converts the up and down motion to a rotary motion, which powers the implement. crankshaft:

The crankshaft extends through the crankcase and is attached to a flywheel at one end and a blade, gear, transmission or pulley at the other end. b Crankcase

The cylinder is capped with a thick plate called a Head, which seals the cylinder. b Cylinder Head b Head Bolts

Engine Block & Valves b Within the block are two valves: Intake and Exhaust

b The Intake Valve lets the fuel mixture into the combustion chamber. b The Exhaust valve lets exhaust fumes out.

The valves operate by means of a Camshaft. Which is powered by a gear on the crankshaft As the camshaft turns, the lobes push open the valves with precise timing so that intake and exhaust occur at the right instant.

The Carburetor provides the fuel/air mixture to the engine. Gas from the fuel tank enters the carburetor, where it is vaporized and mixed with the correct amount of air.

In the engine, the mixture is ignited, and the burning gases provide the force to drive the piston. b Ignition in the combustion chamber occurs in the form of an electrical spark from the spark plug.

The spark at the spark plug comes from a process known as electrical induction. b This begins with the flywheel.

As the flywheel magnet passes the armature coil, a low-voltage current is induced in a primary circuit. When the circuit is opened, the decaying magnetic field sends a high voltage charge through a secondary circuit to the spark plug.

In a four-stroke engine, moving parts are lubricated by motor oil. In most small engines, the oil is located in the bottom of the crankcase in the oil sump, and is distributed throughout the engine to vital parts by means of a oil slinger or oil dipper depending upon the crankshaft position. In most small engines, the oil is located in the bottom of the crankcase in the oil sump, and is distributed throughout the engine to vital parts by means of a oil slinger or oil dipper depending upon the crankshaft position.

Strokes of a Four-Cycle Engine b It takes four strokes (piston moving within the cylinder) to complete one cycle. These are: 1. Intake Stroke 2. Compression Stroke 3. Power Stroke 4. Exhaust Stroke

Intake Stroke: b The piston moves downward in the cylinder, creating a vacuum or area of low pressure. b The intake valve opens allowing a mixture of air and atomized (vaporized) fuel to flow into the combustion chamber.

Intake Stroke: KEY: A = Intake passage B = Intake valve C = Spark plug D = Exhaust valve E = Exhaust passage F = Piston G = Piston pin H = Connecting rod I = Crankshaft J = Crankcase K = Combustion chamber

Compression Stroke:* b Piston moves upward in the cylinder, compressing the air/fuel mixture into the space between the top of the piston and the cylinder head. This area is called the combustion chamber.

Compression Stroke: KEY: A = Intake passage B = Intake valve C = Spark plug D = Exhaust valve E = Exhaust passage F = Piston G = Piston pin H = Connecting rod I = Crankshaft J = Crankcase K = Combustion chamber

Ignition b For the engine to operate properly, ignition of the compressed fuel/air mixture must occur with precise timing, slightly before the piston reaches TDC (Top Dead Center).

Power Stroke b The burning and expanding gases drive the piston downward. b Both valves remain tightly closed

Power Stroke: KEY: A = Intake passage B = Intake valve C = Spark plug D = Exhaust valve E = Exhaust passage F = Piston G = Piston pin H = Connecting rod I = Crankshaft J = Crankcase K = Combustion chamber

Exhaust Stroke b The piston moves upward, the exhaust valve opens and forces the exhaust fumes out of the cylinder.

Exhaust Stroke: KEY: A = Intake passage B = Intake valve C = Spark plug D = Exhaust valve E = Exhaust passage F = Piston G = Piston pin H = Connecting rod I = Crankshaft J = Crankcase K = Combustion chamber

b At the end of the exhaust stroke, when the piston reaches TDC, the intake valve begins to open and the piston begins moving downward, beginning a new cycle.

Two Cycle Engines

b Two- cycle engines are typically used in smaller, hand-held equipment like chainsaws and string trimmers because they can be smaller and lighter than four- cycles. b Also: b Used in these types of equipment because unlike four-cycle engines, two cycle engines can operate on their side or even up-side down

b The two cycle engine completes its cycle of intake, compression, power, and exhaust with only two strokes of the piston. b It takes only one revolution of the shaft to complete the 2-stroke cycle.

Two cycle engines do not have an oil sump. Critical parts of the engine are lubricated by oil that is mixed with the fuel.

b Two-stroke engines do not have valves like four-strokes. b They have an exhaust port that is opened and closed by the moving piston, and a Reed valve to let air/fuel mixture in.

Summary b There are two major classifications of small engines –2-cycle –4-cycle b The four strokes in a four cycle engine are intakeintake compressioncompression powerpower exhaustexhaust

b Major parts and functions summary: With the piston moving down, air/fuel mixture from the carburetor enters the combustion chamber through the intake valve.With the piston moving down, air/fuel mixture from the carburetor enters the combustion chamber through the intake valve. An electrical charge induced from the magneto is sent to the spark plug, and ignites the mixture.An electrical charge induced from the magneto is sent to the spark plug, and ignites the mixture. The burning gases drive the piston downward while both valves are closed.The burning gases drive the piston downward while both valves are closed. The piston is connected to a crankshaft which converts the up and down motion of the piston to a rotary motion.The piston is connected to a crankshaft which converts the up and down motion of the piston to a rotary motion.

b The crankshaft is responsible for turning the camshaft, which raises and lowers the valves, the flywheel, and the lubrication system. b The momentum of the counterweights on the crankshaft carry the piston back up the cylinder, while the exhaust valve opens to expel exhaust fumes.

b The 2-cycle engines undergo the same events of intake, compression, power, and exhaust, but only takes 2 piston strokes to complete the cycle.

1. Crankcase By Katie Murray, UGA Agriculture Education Edited by Georgia Agriculture Education Curriculum Office May 2006 Small Engine Parts Four-Cycle Revised May 2007

Engine Block 1. Crankcase 2. Cylinder Block Crankcase Cover

Cylinder Block 2. Valves 1. Cylinder Bore 3. Cooling Fins Head gasket goes here

Cooling Fins-Cooling fins keep heat away from the cylinder block by increasing the surface area of the cylinder block and contacting the already existing cooler air for cooling efficiency. Spark Plug

Crankshaft Counterweights-balances the forces of the reciprocating piston and reduces the load on crankshaft bearing journals. Throw-measurement from the center of the crankshaft to the center of the crankpin journal. Determines the stroke of an engine. Throw = ½ stroke. Crankshaft-converts the linear motion of the piston into rotary motion. Crankgear-interlocks with the cam gear to turn the crankshaft

Crankshaft Magneto Journal PTO Journal Bearing Journal Crankpin Journal-attaches the connecting rod to the crankshaft

Piston Skirt Rings Piston Pin Bore Piston Head

Piston Rings 1. Compression Ring-seals the combustion chamber from any leakage during the combustion process. 3. Oil Ring-Wipes excess oil from the cylinder wall during piston movement. Openings in the ring returns the excess oil to the engine block. 2. Wiper Ring- used to further seal the combustion chamber and to wipe the cylinder wall clean of excess oil.

Ring Grooves Ring Lands

Connecting Rod- transfers motion from the piston to the crankshaft and functions as a lever arm Rod Cap Crankpin Journal Bearing Surface Piston Pin Piston pin Bearing Surface Piston

Valve Tappets-Rides on the camshaft and pushes the bottom of the valve stem to open the valve Valve Tappets

Cam Shaft- includes cam gear and cam lobes; driven by the crankgear 2. Cam Lobes- egg-shaped protrusion on the camshaft that moves a tappet to open a valve; controls the lift and duration of the opening and closing of the valves 1. Cam Gear- portion of the camshaft that interlocks with the crankgear. Timing Mark

Valve Spring Valve Stem Valve Head Valve Retainer Intake Valve- allows the air-fuel mixture to flow into the cylinder Exhaust Valve- allows exhaust valves to flow out of the cylinder

Flywheel Flywheel Nut

Magneto Armature Magneto Spark Plug Wire

Air Filter

Carburetor Carburetor- provides the proper mixture of air and fuel to the intake valve

Intake Manifold

Muffler Muffler Guard

Breather Assembly Breather Cover

Shields Cylinder Shield

Dipstick

Starter Housing

Gas Tank

Blower Housing Cover