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

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

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

The valves operate by means of a Camshaft 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. Lobes Camshaft Camgear

In the engine, the mixture is ignited, and the burning gases provide the force to drive the piston. 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. This begins with the flywheel. Flywheel magnet

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. oil sump

Two Cycle Engines

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. Also: Used in these types of equipment because unlike four-cycle engines, two cycle engines can operate on their side or even up-side down

The two cycle engine completes its cycle of intake, compression, power, and exhaust with only two strokes of the piston. 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.

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

Engine Block 2. Cylinder Block 1. Crankcase Crankcase Cover

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

Spark Plug 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.

Crankshaft 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. Counterweights-balances the forces of the reciprocating piston and reduces the load on crankshaft bearing journals. 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 Magneto Journal PTO Journal Bearing Journal

Piston Rings Skirt Piston Head Piston Pin Bore

Piston Rings 1. Compression Ring-seals the combustion chamber from any leakage during the combustion process. 2. Wiper Ring- used to further seal the combustion chamber and to wipe the cylinder wall clean of excess oil. 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.

Ring Grooves Ring Lands

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

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

Cam Shaft- includes cam gear and cam lobes; driven by the crankgear 1. Cam Gear-portion of the camshaft that interlocks with 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 Timing Mark

Intake Valve- allows the air-fuel mixture to flow into the cylinder Valve Spring Valve Retainer Valve Stem Valve Head 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 Spark Plug Wire Armature Magneto

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