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;

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 Purchased and delivered to the carburetor in this

state. Atomized

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

IntakeCompression

PowerExhaust

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