Small Engine Operation

Objectives Identify the main engine components of a 4 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 pieces Cylinder block Crankcase

Cylinder Block Consists of the following components: Cylinder bore Cooling fins Valve 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 and supports the crankshaft In a four-stroke engine the crankcase also acts as oil reservoir for lubrication of engine components

Crankcase components Sump Horizontal shaft engines do not have a 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 Con’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 Con’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 L-head engine 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 To 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 Wiper Oil 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

Flywheel Helps maintain engine power and speed between power strokes Cooling fins also provide air circulation during operation

Valving system Includes valve components as well as the cam shaft

L-Head

OHV