Small Engine Operation Objectives Identify the main engine

Small Engine Operation

Objectives �Identify the main engine components of a 4 cycle engine �Identify the main engine components of a 2 cycle engine

Engine Block � Main Structure of an engine � Supports and maintains alignment of internal and external components � Can be produced as one or two piece unit � Consists of two • Cylinder block • Crankcase pieces

Cylinder Block �Consists of the following components: �Cylinder bore �Cooling �Valve fins train components

Cylinder block

Cylinder bore �Hole in the engine block that houses and aligns the piston in the engine block �Bore- is the diameter of the cylinder bore �Stroke- is the linear distance that a piston travels from TDC to BDC in the cylinder bore

Displacement �Volume that a piston displaces when traveling from TDC to BDC �When bore and stroke are know displacement fro single-cylinder engine is found by �D=0. 7854*Bsquared*S

Displacement �D=0. 7854*B squared*S �D=displacement � 0. 7854= constant �B squared= bore squared in inches �S= stroke in inches

Practice �What is the displacement of a single cylinder engine that has a 2. 5” bore and a 2”stroke? �D=0. 7854* B squared* S

Answer �D= 0. 7854* (2. 5*2. 5)*2 �D=0. 7854*6. 25*2 �D=9. 82 cu in.

Multiple Cylinder Displacement �To calculate the displacement of multiple cylinder engine; �Find the displacement of a single cylinder and multiply by the total number of cylinders �The larger the displacement the more power produced

Cooling Fins �Used on an air cooled engine �Thin cast strip on block that dissipates heat away from engine cylinder �Increase the surface area of cylinder block to which ambient air has contact �Also cast into the flywheel • Act as fan blades providing air circulation

Crankcase �Houses �In and supports the crankshaft a four-stroke engine the crankcase also acts as oil reservoir for lubrication of engine components

Crankcase components �Sump • Removable part of engine crankcase that serves as an oil reservoir and provides access to internal parts �Horizontal shaft engines do not have a sump • The crankcase and engine block serve as an oil resivoir

Crankcase Componenets �Crankcase cover • Provides access to internal parts • Supports the crankshaft �Crankcase breather functions as a check valve releasing pressure created by the cylinder moving towards BDC

Cylinder block construction �Cylinder blocks are commonly constructed from: �cast aluminum alloy � cast iron �or cast aluminum alloy with cast iron cylinder sleeves

Cast Aluminum Alloy �Pro’s • Cheap to produce • Lightweight • Dissipate heat more rapidly than cast iron �Con’s • More prone to heat distortion • Less resistant to wear

Cast Iron �Pro’s • Last longer more resistant to wear • Can handle higher temps �Con’s • Heavier • More expensive to produce

Cylinder Head �Component that is fastened to the end of the cylinder block farthest from the crankshaft �Head gasket is placed between cylinder head and cylinder block • Usually constructed of soft metals and graphite layered together • Used to completely seal the combustion chamber

Cylinder Head �In some two-stroke engines the cylinder head and block are made as one piece otherwise know as a jug. �This single cast unit provides the maximum strength �Eliminates possible leaks in combustion chamber

Cylinder Head Design �OHV- overhead valve • Valves and related components are housed in the cylinder head �L-head engine • Valves and related components located in cylinder block �DOV-direct overhead valves

Crankshaft �Converts the up and down motion of the piston into a rotary motion �Orientation of the crankshaft in small engines is either • Horizontal or vertical shaft

Crankshaft Components �Crankpin journal �Throw- measurement of distance between crankshaft and crankpin journal • Throw is equal to one half the stroke �Bearing journal �Counterweights �Crank gear �and power take off (PTO)

Bearing journal Crankpin journal Crank gear Bearing journal Throw PTO counterweight

Piston and Piston Rings �Component that is fitted into the cylinder bore �Pistons are usually built out of cast aluminum allow �Proper tolerance must be maintained to compensate for the expansion of metals under extreme heat

�To little and an engine might seize much and proper compression will not be met

Piston componenets �Piston head �Piston pin �Skirt �Ring grooves �Ring lands �Piston rings

Piston Head �The shape of a piston head is either �Flat �Contoured • Irregular • Domed • Dished

Piston rings �Usually composed of cast iron to hold their structural integrity �The most common types of rings found in small engines include • Compression ring • Wiper ring • Oil ring

Piston rings � Compression • Located closest to the top of the piston • Seals the combustion chamber � Wiper • Ring with a tapered face • Further seals combustion process/wipes cylinder wall clear of oil � Oil • Located closest to crankshaft • Wipe oil off cylinder walls return it to oil reservoir � 2 stroke engines do not need an oil ring

Piston ring components

Connecting Rod �Transfers movement from piston to the crankshaft �Includes the following pieces �Piston pin (wrist pin) w/pin clips �Connecting rod �Rod cap w/rod cap screws