Small Engine Theory
What
is the purpose of a Small
Engine?
When does a small engine Get
classified into a large engine?
Internal Combustion Engine



Internal: To happen inside
Combustion: Burning of some sort of fuel and
air.
Engine: Converts heat energy into a useful
mechanical energy.
Four Main Parts
Piston
Connecting Rod
Cylinder
Crankshaft
Piston





A plunger that slides in the cylinder
converting heat energy into mechanical
energy
Light weight material (Aluminum alloy) so it
can spin at high revolutions per minute.
Must withstand pressures up to 2,000PSI
Must withstand heat up to 1,200 F
Must not expand
Connecting Rod




Used to connect the piston to the crankshaft
I-Beam construction, saves weight and
makes the rod stronger.
Connecting rod cap is bolted to the end with
the use of two hardened bolts.
Piston pin connects piston to the connecting
rod
Cylinder
Made from Aluminum alloy or Cast iron
TWO PARTS OF THE CYLINDER ARE:
 Cylinder Head
 Cylinder Block

Cylinder Head


Houses the combustion chamber and the
spark plug.
I-Head (OHV) Houses:all the above plus
valves, seats, valve guides, intake and
exhaust ports, rocker stud, pushrod guide
plate.
Cylinder Block





The foundation for all moving parts.
Houses the cylinder: the area the piston
moves up and down in.
Houses the crankshaft
Makes-up the crankcase, area where the
crankshaft rotates.
Houses the following in a L-head engine;


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Valves
Valve Seats
Intake and Exhaust Ports
Valve Springs and Retainers
Tappets
Camshaft
Crankshaft



Made of nodular steel.
Changes Reciprocating motion into rotary
motion.
Parts




PTO JOURNAL
ROD JOURNAL
MAGNETO JOURNAL
COUNTER-WEIGHTS
FLYWHEEL



Carries the crankshaft through the three non
power producing strokes.
A heavy weight; takes a lot of energy to start
and stop
Provides the crankshaft with inertia.
COMBUSTION CHAMBER


Area in the cylinder head where fuel and air
are burned.
Spark plug is always located in the
combustion chamber.
Fuel


A combustible that will burn effectively in the
combustion chamber
Types




Gasoline (petro)
Diesel
Propane
Compressed natural gas
Fuel Stages

Liquid


Atomized


Purchased and delivered to the carburetor in this
state.
Carburetor mixes with air in this state. To the
proper stochiometric ratio of 14:1 air to fuel
Vaporized

Fuel becomes a gas in the combustion chamber
on the compression stroke
Valve train

All components that allow for the transfer of
fuel and air both fresh and burnt.
Valves



Open and close the combustion chamber.
Seals the combustion chamber.
Allows for the transfer of fuel and air.
Valve seats


Inserts in the block or head that allow the
valves face to rest on, and to cool and seal
the combustion chamber.
Made from high carbon steel to withstand
high heat and pressure.
Valve guide

Tube like structure in the head or block that
guides the valve, and is concentric with the
valve seat.
Valve spring

Used to keep the valve closed.
Valve Retainers

Used to retain the valve spring onto the
valve.
Camshaft



Used to open and close the valves at the
proper time (phasing), for a specific length of
time (duration).
Driven by the crankshaft.
Gear, chain or belt driven
Tappets/ Lifters

Used to transfer the motion of the camshaft
to the valve (L-head) or the push-rod (ohv)
Passageways
Fuel and Air must pass into and out of the
combustion chamber.
Fresh fuel and air is called INTAKE
Burnt fuel and air is called EXHAUST
Cylinder head passageways are called PORTS
MANIFOLD passageways are called
RUNNERS
Four stroke cycle
Intake
Compression
Power
Exhaust
Intake stroke
Piston moves down from TDC to BDC.
Cylinder creates a vacuum.
Fuel and Air rush into the cylinder when the
Intake Valve opens.
Cylinder is now filled with the fresh fuel and
air charge.
Exhaust valve stays closed.
Compression
Piston moves from BDC to TDC.
Fuel and Air is squeezed so tight that
heat is generated.
Fuel becomes Vaporized.
Intake and Exhaust valves are closed.
Power
Piston moves from TDC to BDC.
Fuel and air is Ignited by the spark plug.
Fast burning of the fuel creates a
tremendous heat and pressure increase.
Pressure forces the piston down with 2000
PSI of Force.
Both the Intake and Exhaust valves are
closed.
Exhaust




Piston moves from BDC to TDC
Exhaust valve opens,
Intake stays closed
Burnt fuel and air is forced out of the exhaust
Ports and exhaust manifold Runners.
Four stroke cycle
HEAD AND BLOCK
CLYINDER BLOCKS
ALUMINUM BLOCKS
CYLINDER SLEEVES
GREY IRON
CYLINDER HEADS
Alloys
RECIPROCATING
PISTONS RINGS AND RODS
PRESSURE AND TEMPERATURE
PRESSURE AND VOLUME
ENERGY
CAMSHAFT
CAM PHASING
COMBUSTION CHAMBER
TURBULANCE
GASKETS