Engine Systems

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Five (5) Engine Systems

All engine parts and functions can be divided into five (5) systems.

1. Compression

2. Fuel

3. Electrical

4. Cooling

5. Lubrication

3

Compression System

The compression system includes all of the parts that create, contain and manage the engine compression.

Block

Piston

Piston rings

Cylinder head

Cylinder bore

PartsValves

Valve springs

Connecting rod

Crankshaft

Gaskets

4

Adiabatic Process

• A process in which heat is derived from the process itself. During compression heat is produced from the work applied by the

piston. The greater the work, higher compression, the greater the heat

produced.

• Changes in the charge As the air-fuel mixture is compressed the molecular forces produce

heat. As the temperature increases gasoline molecules become more

active. This results in additional heat to the air-fuel charge.

• Results of heating Small droplets of gasoline are vaporized. Larger droplets are broken apart. Reduced energy required to maintain combustion.

5

Compression Problems

• Two possible problems:– Inadequate compression

– Excessive compression

• Inadequate compression– Commonly caused by leaks

– Maverick air: undesirable air entering the engine through leaks.

• Excessive compression– Harder starting

– Engine performance problems

– Detonation

– Preignition

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Detonation

• An undesirable engine condition in which pockets of fuel start to burn at about the same time as the spark plug fires.

• Multiple pressure fronts collide • Sometimes called knocking, spark knock or pinging.• Causes large pressure differentials in the combustion chamber.• Can cause engine damage.

PreventionRemove any cause

Causes1. Increased compression2. High temperatures3. Lean fuel/air mixture4. Advanced ignition timing5. Lower octane fuels

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Preignition

• Fuel starts to burn before the spark plug fires.• Decreases engine performance and produces and audible

pinging or knocking sound in the engine.• Increases the peak combustion pressure in the cylinder.• Increases internal temperature.• Will cause engine parts like pistons, connecting rods and

crankshafts to fail.

Causes1. An overheated spark plug

2. Glowing carbon deposits

3. Over heated exhaust valve

4. A sharp edge in the combustion chamber or on top of a piston

5. Sharp edges on valves that were reground improperly

6. A lean fuel mixture.

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Valves

• Control flow of air-fuel into and exhaust gases out of the cylinder.

• Types– One piece– Two piece– Projection-tip

• Valve hardfacing• Valve stem surface treatments• Valve head design

– Interference interface angle– Face can be resurfaced

• Valve dynamics– Most valves rotate slightly each

time they open and close.– Rotation improves temperature

distribution– Rotation helps clean valve

interface.

-- Rotation can be enhanced through use of valve rotators

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Valve Guide

• Controls the position of the valves– Subject to fluctuations in temperature,

chemical corrosion, ingestion of foreign material and for the exhaust valve, high temperatures.

– Must provide a predictable and consistent clearance between itself and the valve stem.

– Can be aluminum, brass or sintered iron.

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Valve Seats

• Mate with valve face to seal combustion chamber.

• Metal to metal seal• Usually insert• Can be resurfaced

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Pistons

• May use elliptical shape Elliptical when cold Diameter at piston pin bore

expands more that thin edge of piston.

Round when hot

• Acts as moveable end of the combustion chamber and must withstand pressure fluctuations, thermal stress, and mechanical loads.

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Piston--cont.

• Some piston pins are offset– Piston must be orientated

correctly in bore.

• Windows are used in oil ring groove to allow excess oil to return to crankcase.

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Ring Grooves

• Ring grooves are machined grooves in the piston designed to hold the rings.

• Ring lands are the areas of the piston between the ring grooves.

• The clearance between the rings and the ring lands is critical.

• During overhaul the grooves should be cleaned with a ring groove cleaner.

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Piston Rings

• The job of the rings is to fill the space between the piston and the cylinder walls.

• The combustion chamber is sealed by a thin film of oil between the rings and the piston and between the rings and the cylinder wall.

• Usually constructed of cast iron.• The total number of rings per

piston can vary, but there are three types of rings on each piston.

Compression

Wiper

Oil

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Piston Rings--cont.

• Wiper ring–Meters oil film on cylinder walls

–Must be installed correctly.

• Oil ring– Constructed of two thin rails with holes or

slots cut inbetween.

– Has the highest pressure against the cylinder wall of the three rings.

• Compression– Subject to to greatest amount

of chemical corrosion and highest temperatures.

– Transfers 70% of combustion heat from piston to cylinder walls.

CompressionRing

Wiper Ring

Oil Ring

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Cylinder Bore

• Three types– Cast aluminum

– Cast aluminum with cast iron sleeve

– Cast iron

• Usually use a cross-hatch finish to improves ring lubrication

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Crankcase Breather

• Maintains pressure in the crankcase at less than ambient pressure to assist in the control of oil consumption.

• Excessive blow by renders the breather useless.• Old engines vent to the atmosphere.• New engines vent to the carburetor.

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Compression Release

• Compression release systems are used to decrease effort required to start engine.

• Holds the exhaust or intake valve slightly open during starting, and then allows it to fully close once engine starts.– May be designed into camshaft and hold the valve open for a

short period of time on every compression stoke.

– May be mechanical. Engages during starting and disengages after the engine reaches operating speed.

Compression release pin

Fuel System

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Introduction

• The function of the fuel system is to store, meter, atomize, vaporize and start the mixing with the air.

• Fuel system parts:– Supply (tank)

– Lines

– Valves

– Filter

– Pump

– Carburetor

• Common Small Engine Fuels include:– Gasoline

– Diesel

– LPG

– LNG

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Combustion Chemistry

• Combustion: the rapid oxidizing chemical reaction in which fuel chemically combines with oxygen in the atmosphere and releases energy in the form of heat.

• Stoichiometric ratio– Ratio of air to fuel, by weight, where the most efficient combustion

occurs.

– Does not produce maximum horsepower.

• Lambda () excess air factor– A numerical value assigned to represent the stoichiometric ration of

atmospheric air to any hydrocarbon fuel.

– A of 1.0 is theoretically perfect ratio.

– Small engines use a of 0.6 to 0.8

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Volatility

• Volatility is the propensity of a liquid to become a vapor.• The volatility of gasoline changes with the seasons.

Low Volatility High Volatility

Poor cold weather operation Poor hot weather operation

Spark plug deposit buildup Vapor lock

Combustion chamber deposit buildup

Poor fuel economy

Poor cold starting Excessive fuel evaporation

• Vapor lock: the stoppage of fuel flow caused by internal pressure of fuel vapor bubbles.

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Vaporization

• Vaporization is the process converting a liquid to a vapor.– Requires heat

• The rate and efficiency of vaporization is improved when the liquid is reduced to small droplets (atomized).

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Oxygenated Gasoline

• Clean Air Act 1990 requires gasoline to be modified with oxygen additives in nonattainment zones.– Nonattainment zone: areas of the country that exceed ozone levels.

• Two common additives;– Alcohol

– Methyl Tertiary Butyl Ether (MTBE)

• Alcohol– Two types

• Ethanol: distilled from grains and sugar containing plants• Methanol: distilled form natural gas

– The addition of alcohol to gasoline increases the available oxygen during the combustion process.

– Up to 10% ethanol acceptable for Briggs & Stratton engines.

– Methanol should not be used in Briggs & Stratton engines.

• MTBE– Removed from marked because of health concerns.

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Engine Emissions

• An engine with a of 1.0 exhausts 12% water vapor and 14% carbon dioxide.

• Emissions are changed because additional chemicals are added to improve the performance of the gasoline.

Additive Function

Anti-icers Prevent fuel from freezing in lines

Anti-oxidants Reduce hum formation in stored gasoline

Corrosion inhibitors Minimize corrosion in fuel system

Detergents Reduce/remove fuel system deposits

Fluidizer oils Control intake valve deposits

Lead replacement additives Minimize exhaust valve seat wear

Metal deactivators Minimize effects of metals present in gasoline

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Engine Emissions--cont.

• Carbon Monoxide– Product of incomplete combustion

– The richer the air-fuel ration, the more CO is produced.

• Hydrocarbon emissions– Product of incomplete combustion

– Contains gasses not readily oxidized at normal engine operating temperature. Methane, ethane, acetylene, etc.

• Oxides of Nitrogen– Nitric oxide (CO)

– Nitrogen dioxide (CO2)

– Dinitrogen monoxide (N2O)

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Octane

• Octane: the ability of a fuel sample to resist engine knock and/or ping.

• The octane rating required for an engine is based on the compression ratio.– Higher compression ratio requires higher octane.

– Higher compression ratios increase compression temperatures--increases chance of autoignition or the fuel.

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Octane--cont.

• Antiknock Index (AKI)– The number assigned to gasoline that indicates the ability to

eliminate knocking and/or pinging.

– Research octane number (RON)

– Market octane number (MON)

• RON: the octane number that affects engine knock at low to medium speed

• MON: the octane number that affects engine knock at high speed, performance in severe operating conditions and under load.

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Octane--cont.

• The AKI is posted on the gasoline pump.• The AKI can be raised or lowered by the use of additives.

– Tetraethyl Lead

– Alcohol

– Methyl Tertiary Butyl Ether (MTBE)

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Fuel System--Carburetor

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“Carburetor: the engine component that provides the required air-fuel mixture to the combustion chamber based on engine speed and load.”

Introduction

Carburetors achieve this result using four common principles of fluids.

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Four Fluid Principles

1. Fluids flow from areas of high pressure to areas of low pressure.

2. When there is no pressure difference--there is no fluid flow.

3. Fluids exert pressure of the same value throughout a system.

4. Fluid flow in a carburetor utilizes Bernoulli’s principle.– Air flowing through a narrowed

portion of a tube increases in velocity and decreases in pressure.

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Carburetor Operating Principles

• A carburetor is a tube attached to the intake port of the engine and open to the atmosphere.

• On the intake stroke a volume with little to no pressure develops in the combustion chamber.– Atmospheric pressure outside the engine--14.7 psi

– Low pressure in the combustion chamber--0 to slight vacuum.

• Result: air flows from outside to inside the engine.

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Carburetor Operating Principles

• As the air flows through the carburetor, the fuel is metered, atomized and vaporized.

• To have available fuel, the carburetor must have a source of fuel.

• In the float type carburetor this source is the fuel bowel.

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Carburetor--Venturi

• A pressure difference is also needed to cause the fuel to flow from the fuel bowel into the air stream.

• This is accomplished using a venturi, Bernoulli’s principle and a tube connecting the mouth of the venture to the fuel bowel.

• This is a functioning carburetor and it will operate an engine as long as it has a constant load and constant speed.

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Carburetor-Throttle

• Very few engines operate at a constant load and constant speed.

• To adjust the rate of fuel flow a throttle is used.

• When the throttle is in the closed position there is minimum air flow through the carburetor.

Less air flow = less pressure difference in venturi

Less pressure difference = less fuel flow

Less fuel flow = less speed.

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Throttle--cont.

• When the throttle is in the wide open position, there is maximum air flow through the carburetor.

• To provide a means to adjust maximum fuel flow, a needle valve was added to the orifice in the emulsion tube.

Maximum air flow = maximum pressure difference

Maximum pressure difference = maximum fuel flow

Maximum fuel flow = maximum speed & power

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Carburetor-Choke

• A carburetor with this design would function well under varying loads and speeds,

• Starting is a different condition• For starting an engine needs a richer fuel-air mixture.• This was accomplished by adding a choke.

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Carburetor-Choke--cont.

• Closing the choke increases the pressure difference between the fuel bowel and the venturi.

• Increased pressure difference = increased fuel flow

• Once engine starts the choke must be opened to prevent the engine from running too rich.

• A primer bulb has replaced the choke on most modern engines.

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Carburetor-Idle Circuit

• The addition of a choke/primer improved engine starting, but this carburetor still has a problem if the engine needs to idle.

• When the throttle is in the idle position, almost closed, the area with greatest restriction, and greatest pressure difference, moves from the venturi to the area between the throttle plate and the wall of the tube.

• This problem was solved with the addition of an idle circuit and idle needle valve.

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Carburetor-Float

• To have constant fuel flow with constant pressure difference the lift, distance from the top of the fuel to the top of the main nozzle, must remain constant.

• A constant level of fuel is maintained in the fuel bowel by the float, float needle valve and float needle valve seat.

Complete Carburetor - Old Style

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Carburetor-Additional Features

• Several additional features have been tried/added to improve carburetor performance.– Air bleeds

– Fixed jets

– Transition ports

– Pilot jets

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Carburetor Designs

• All carburetors have the same basic components. The design of any individual carburetor is determined by the operating conditions of the engine.

• The more variable the load and speed the more complex the required carburetor design.

• Carburetors are also classified by the direction of the air flow.– Updraft

– Downdraft

– Sidedraft

• Some carburetors also use multiple barrels, venturi.

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Three Types of Briggs & Stratton Carburetors

• Carburetor attached to top of fuel tank.

• A single pickup tube is used between the carburetor and the tank.– Must use shallow fuel tank because

the main jet extends from the venturi to the bottom of the fuel tank.

• As the level of fuel in the tank changes, the fuel-air ratio changes.

• Not included in latest B & S repair manual.

Vacu-jet

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Three Types of Briggs & Stratton Carburetors--cont.

• Carburetor is attached to the top of fuel the fuel tank.

• Two tubes are used.• Primary is attached to fuel pump

to pump fuel from the main tank to the secondary tank.

• Secondary tube draws fuel from secondary tank to the venturi.

• The fuel pump is designed with excessive capacity, and the secondary tank has a drain

• The fuel in the secondary tank stays at a constant level.

• Not included in latest B & S repair manual.

Pulsa-jet

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Pulsa Jet Parts

A. Fuel pump

B. Primary fuel tube

C. Primary fuel tank

D. Primary fuel tube check valve

E. Fuel screens

F. Secondary fuel tube check valve

G. Secondary fuel tank

H. Secondary fuel tube

I. Secondary tank drain

J. Choke

K. High speed needle valve

L. Air horn (inlet)

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Three Types of Briggs & Stratton Carburetors--cont.

Flow-jet

• Different types and sizes are used.

• Most popular on modern engines.

• All use a fuel bowel and float system to maintain a consistent supply of fuel.

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Fuel Injection

• Fuel injection is the preferred method of metering the fuel in modern engines.

• Cost has limited use for small gas engines.

• BOSH has developed a system.

Fuel System---Governor

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Introduction

• The function of the governor system is to maintain the desired engine speed regardless of engine load.

• The governor is attached to the throttle on the carburetor and supplies a force that attempts to close the throttle.

• The governor spring is attached to the governor linkage and applies a force that attempts to open the throttle.

• A constant engine speed means these two forces are balanced.

• Small engines use two types of governors.– Pneumatic

– Mechanical

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1. When preparing to start an engine the throttle will be set to the run (choke) position.

2. The engine is stopped--there is no air flow.

3. No air flow means the governor will not be producing any force.

4. In the choke position the the governor spring produces the maximum force.

5. The throttle is wide open.

Pneumatic Governor Operation--Engine Not Operating

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• Once the engine starts, the throttle is moved to the run position.

• This sets the engine to operate at top--no load speed.

• When the flywheel starts to rotate, air starts flowing pass the the governor arm.

• Air movement produces a force on the governor vane which is then applied to the linkage.

• The force on the governor linkage stretches the governor spring and attempts to close the throttle.

Pneumatic Governor Operation--Top No Load Speed

When the force on the governor linkage equals the force produced by the spring, the throttle is held in a constant position and the engine runs at a constant speed.

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• When the engine load increases the engine speed is reduced.

• Less speed = Less air flow • Less air flow = less force• When the force produced by the

governor decreases, the force produced by governor spring is greater and the spring opens the throttle.

• Opened throttle = more fuel• More fuel = more speed• More speed = more air flow.• More air flow = more force• The governor and throttle spring are

constantly wrestling for control of the throttle.

Pneumatic Governor Operation--Engine Under Load

When the forces are balanced, the engine speed is constant.

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Mechanical Governor

• The mechanical governor operates on the same principles as the pneumatic governor.

• The difference is that the force to balance the governor spring is produced by rotating weights not a pneumatic arm.

• The weights are rotated by the governor gear which meshes with the crankshaft gear.

• As the governor spins the governor weights move out from the center shaft.

• The weights are mounted on a lever arm that pushes the governor shaft up as the weights move out.

• The higher the speed the greater the force produced.

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Mechanical Governor--cont

Electrical System

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Introduction

• Electricity is a predictable force, yet it is often challenging to service electrical systems because it can not been seen and there is the concern of electrical shock.

• Because almost all small engine electrical systems operate on 12 volts, the danger of severe electrical shock is reduced.

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Electrical Terms

• Before attempting to understand small engine electrical systems, it is important to know the terms and parts associated with electricity and the electrical systems.

• Electricity• Conductor• Electron• Free Electron• Voltage• Load• Current• Direct Current• Alternating Current

• Polarity• Amperes• Resistance• Short circuit• Series Circuits• Parallel Circuits• Ohm’s Law• Magnetism• Induction• Solenoid

• Diodes• Voltage Regulator• Battery• Primary winding• Secondary winding• Condenser

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Terms

• Electricity is energy created by the flow of electrons in a conductor.

• Conductor: a material that allows the free flow of electrons.

• Electron: one of three parts of atoms. Electrons have negative charge and rotate in orbits around the nucleus of the atom.

• Free Electron: an electron that is capable of jumping in or out of an orbit.

• Voltage: the amount of electrical pressure in a circuit.– Voltage is measured in volts (V).

– A voltage exists when there is an excess number of electrons at one terminal of a voltage source and deficiency of electrons at the other terminal.

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• Circuit: A complete path that controls the rate and direction of electron flow. The parts of a circuit include:– Voltage source– Pathway for electrons– Load or loads– Controls

• Current: the flow of electrons past a point in the circuit. It may be alternating or direct.–Alternating current: the flow of electrons reverses direction at regular intervals.–Direct current: the flow of electrons is in one direction.

• Polarity: the state of an object as negative or positive.• Amperes: the unit of measure for current flow.

Terms-cont.

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Alternating Current

• The voltage builds to a maximum value in one direction (polarity), decreases to zero and then builds to a maximum direction in the other direction.

• Alternating current is supplied by generators and alternators.

• How often this occurs is called the frequency.

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Direct Current

• In direct current the polarity and the voltage stay constant.

• Direct current is supplied by batteries or rectifiers.

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Resistance

• Resistance is opposition to the flow of electrons.• All circuit components have some resistance.• Forcing electricity through a resistance uses energy. The

energy is lost as heat.• Resistance is measured in units of Ohms ().• The amount of current flow and resistance in a circuit

determines the wire size for the circuit.

AWG Number Diameter /1000 ft (68 oF)

12 80.8 1.6

14 64.1 2.5

16 50.8 4.0

18 40.3 6.4

20 32 10.2

22 25.35 16.2

Wire Size and Resistance

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Circuits

• A complete path that controls the rate and direction of electron flow.

• Four terms are used to describe the different types of circuits:1. Series circuit

2. Parallel Circuit

3. Series-Parallel

4. Short Circuit

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Circuits--Series

• In a series circuit the electricity has no alternative paths.

• All of the electricity must go through all of the loads in the circuit.

• In the illustration the switch is in series with two loads that are also in series.

• All types of small engine electrical systems may have components in series.

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Circuits--Parallel

• In parallel circuits the electricity has alternative paths through the loads in the circuit.

• The amount of electricity that flows down either path is determined by the voltage and resistance of that path.

• In the illustration, a switch is in series with two loads that are in parallel.

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Circuits--Series-Parallel

• Circuits that have loads in both series and parallel.

• Not vary common in small gas engines.

• In the illustration load one is in series with loads two and three--which are parallel with each other.

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Circuit--Short Circuit

• A short circuit occurs when a low resistance circuit to ground develops.

• Low resistance means high current flow.

• Excessive current flow will damage electrical components if it is not stopped.

• Over current protection devices are used to protect the circuit when a short occurs.

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Ohm’s Law

• Ohm’s Law explains the relationship between voltage, amperage and resistance.

• Law

E =IR

Emf (Voltage) =Intensity (amp) x Resistance ()

I =V

R

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Magnetism

• “Magnetism is an atomic level force derived from the atomic structure and motion of certain orbiting electrons.”

• A Magnet field is an area of magnetic force created and defined by lines of magnetic flux surrounding a material in three dimensions.

• Magnetic flux: invisible lines of force in a magnetic field.

• Magnet: a material that attracts iron, cobalt or nickel and produces a magnetic field.– Permanent– Temporary

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Induction

• Induction: the production of voltage and current by the proximity and motion of a magnetic field or electric charge.

• With a conductor, either current, a magnetic field or motion can be produced as long as the other two are present.

• Magnetic field: When electricity passes through a conductor it forms a magnetic field around the conductor.

• Current: When a conductor passes through a magnetic field or when magnetic field moves and/or varies in strength around a conductor, electrons are made to flow. A current is induced in the conductor.

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Five Small Gas Engine Electrical Systems

• Small engines may have one or more of five (5) electrical systems.

1. Charging

2. Ignition

3. Starting

4. Accessories

5. Safety

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1. Charging System

• Charging systems produces electrical to operate accessories and the replace electrical energy taken from a battery.

• Two different systems can be used.– Generator

– Alternator

• Generator produces DC.• Alternator produces AC. When DC is needed the current is

converted, rectified.• Some small engines use a stationary coil close to the flywheel.

When the flywheel magnets pass by the coil they induce a current in the coil.

• Other systems use stationary magnets and a rotating coil.• Conductors are sized for circuits with low current flow.

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Charging System--cont.

• The components of a charging system may include:– Coil

– Magnets

– Voltage regulator

– Rectifier

– Switches

– Conductors

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2. Ignition System

• The ignition system provides a high voltage spark in the combustion chamber at the proper time.

• Two types of ignition systems– Battery

– Magneto

• Battery– Battery systems transforms the battery voltage and fires the spark

plug at the correct time.

• Magneto– Magneto systems must produce the current, transform the voltage

and time the spark plug.

– Most small engines use the magneto system

• Two types of magneto systems:– Breaker point ignition

– Solid state (electronic) ignition

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2. Ignition System-cont.

• Breaker point ignition– Older system. Most manufacturers have replaced them with solid

state.

– Uses a set of points to break the primary circuit.

• Solid state ignition– Uses a transistor to break the primary circuit.

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Ignition system--Magneto Ignition

• Magnets• Points (Breaker point only)• Trigger coil• Conductors• Spark plug• Condenser (Breaker point only)• Lamination stack• Primary winding• Secondary winding

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Magneto Ignition System--Points

• As magnets in flywheel rotate past the magneto, the points close.

• The magnetic flux of the magnets in the flywheel induces a current in the primary coil.

• With current flowing in the primary circuit, a magnetic field develops around the primary coil.

• This magnetic field also surrounds the secondary coil.

• As the flywheel continues to rotate the breaker points open.

Condenser

Spark plug

Secondary Winding

Primary Winding

Lamination stack

Magnetic field

Points

Armature

Flywheel magnets

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Magneto Ignition System- Firing Spark Plug

• When the breaker points open the magnetic field produced by the current in the primary winding collapses.

• The collapsing magnetic field flows across the secondary coil which induces a current in the secondary coil.

• Because there is a 60:1 ratio of windings in the two coils, the voltage is transformed to the 10,000 and 15,000 volts needed to fire the spark plug.

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Magneto Ignition system

• As long as the flywheel is rotating and the ignition switch is on, the spark plug fires every time the magnets move past the magneto.

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Differences Between Breaker Point and Solid State Ignition System

• The solid state (electronic) ignition system replaces the mechanical points (switch) with an electronic switch.

• A trigger coil senses the presence of the magnets and opens the primary circuit.

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3. Starting System

• The purpose of the starting system is to use energy to turn the engine until it starts.

• System components may include:– Electrical source

– Starting motor

– Conductors

– Ignition switch

– Solenoid switch

• Two primary electrical systems.– Single switch

– Solenoid

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Staring Systems--cont.

• Single Switch– For systems with a single

switch the switch must be able to switch the current for the starting motor.

– Requires a heavy duty switch because starter motors drawn a lot of current.

• Solenoid– In this system the ignition

switch only switches the current that powers the solenoid.

– The solenoid has heavy duty contacts for switching the current to the starting motor.

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4. Accessories

• Small engines are used on machines that may require electricity to operate accessories.

• Accessories may include:– Lights – Electrical clutches– Electrical lift systems– Radio, etc.

• The conductors must be sized for the electrical load.• Each separate circuit should have overload protection.

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5. Safety

• It is common for small engines to be used on machines that may have one or more electrical safety systems.

• These systems are usually designed to stop the engine when activated.

• The electrical system is used because that is the easiest way to automate an engine stopping system.

• Safety systems can include:– Low oil switch

– Seat switch

– Anti after fire solenoid

– Deck switch

– Transmission switch

– Tilt switch

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Questions