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Engine Systems
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
2
Compression System
The compression system includes all of the parts that create,
contain and manage the engine compression.
Parts
Block
Piston
Piston rings
Cylinder head
Cylinder bore
Valves
Valve springs
Connecting rod
Crankshaft
Gaskets
3
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.
4
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
5
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.
Causes
1.
2.
3.
4.
5.
Increased compression
High temperatures
Lean fuel/air mixture
Advanced ignition timing
Lower octane fuels
Prevention
Remove any cause
6
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.
Causes
1.
2.
3.
4.
5.
6.
An overheated spark plug
Glowing carbon deposits
Over heated exhaust valve
A sharp edge in the combustion chamber or on top of a piston
Sharp edges on valves that were reground improperly
A lean fuel mixture.
7
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
8
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.
9
Valve Seats
• Mate with valve face to seal combustion
chamber.
• Metal to metal seal
• Usually insert
• Can be resurfaced
10
Pistons
• Acts as moveable end of the
combustion chamber and must
withstand pressure fluctuations,
thermal stress, and mechanical
loads.
• May use elliptical shape
 Elliptical when cold
 Diameter at piston pin bore
expands more that thin
edge of piston.
 Round when hot
11
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.
12
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.
13
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
Oil
Wiper
14
Piston Rings--cont.
• Compression
– Subject to to greatest amount
of chemical corrosion and
highest temperatures.
– Transfers 70% of combustion
heat from piston to cylinder
walls.
Compression
Ring
Wiper Ring
Oil Ring
• 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.
15
Cylinder Bore
• Three types
– Cast aluminum
– Cast aluminum with cast iron sleeve
– Cast iron
• Usually use a cross-hatch finish to
improves ring lubrication
16
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.
17
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
18
Fuel System
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
20
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
21
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.
22
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).
23
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.
24
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
25
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)
26
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.
27
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.
28
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)
29
Fuel System--Carburetor
30
Introduction
“Carburetor: the engine component that provides the required airfuel mixture to the combustion chamber based on
engine speed and load.”
Carburetors achieve this result using four common principles of
fluids.
31
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.
32
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.
33
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.
34
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.
35
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.
36
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
37
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.
38
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.
39
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.
40
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.
41
Complete Carburetor Old Style
Carburetor-Additional Features
• Several additional features have been tried/added to improve
carburetor performance.
–
–
–
–
Air bleeds
Fixed jets
Transition ports
Pilot jets
43
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.
44
Three Types of Briggs & Stratton Carburetors
Vacu-jet
• 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.
45
Three Types of Briggs & Stratton Carburetors--cont.
Pulsa-jet
• 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.
46
Pulsa Jet Parts
A.
B.
C.
D.
E.
F.
Fuel pump
Primary fuel tube
Primary fuel tank
Primary fuel tube check valve
Fuel screens
Secondary fuel tube check
valve
G.
H.
I.
J.
K.
L.
Secondary fuel tank
Secondary fuel tube
Secondary tank drain
Choke
High speed needle valve
Air horn (inlet)
47
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.
48
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.
49
Fuel System---Governor
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
51
Pneumatic Governor Operation-Engine Not Operating
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.
52
Pneumatic Governor Operation--Top No Load Speed
• 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.
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.
53
Pneumatic Governor Operation--Engine Under Load
•
•
•
•
•
•
•
•
•
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.
When the forces are balanced, the engine speed is constant.
54
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.
55
Mechanical Governor--cont
56
Electrical System
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.
58
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
59
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.
60
Terms-cont.
• 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.
61
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.
62
Direct Current
• In direct current the polarity
and the voltage stay
constant.
• Direct current is supplied
by batteries or rectifiers.
63
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.
Wire Size and Resistance
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
64
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.
2.
3.
4.
Series circuit
Parallel Circuit
Series-Parallel
Short Circuit
65
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.
66
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.
67
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.
68
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.
69
Ohm’s Law
• Ohm’s Law explains the relationship between voltage,
amperage and resistance.
• Law
E = IR
Emf (Voltage)
= Intensity (amp) x Resistance (
)
V
I=
R



70
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
71
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.
72
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
73
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.
74
Charging System--cont.
• The components of a charging system may include:
–
–
–
–
–
–
Coil
Magnets
Voltage regulator
Rectifier
Switches
Conductors
75
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
76
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.
77
Ignition system--Magneto Ignition
•
•
•
•
•
•
•
•
•
Magnets
Points (Breaker point only)
Trigger coil
Conductors
Spark plug
Condenser (Breaker point only)
Lamination stack
Primary winding
Secondary winding
78
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.
Spark plug
Condenser
Secondary Winding
Primary Winding
Points
Armature
Lamination stack
Magnetic field
Flywheel
magnets
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.
79
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.
80
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
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