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Engine Systems
Introduction
• Engines permeate our society.
• Labor costs for small engine are over $50.00 an hour.
• To operate a small engine efficiency you must be able to
troubleshoot engine problems.
• To be a good troubleshooter you must know the name and
function of all of the engine parts.
2
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
If an engine will not start, which one of the five system(s) is most likely the
problem?
If an engine has a lot of operating time, is hard to start and doesn’t seem to
be producing as much power as it once did, which one of the five
system(s) is most likely the problem?
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
Bearings
Parts
Valves
Valve springs
Connecting rod
Crankshaft
Gaskets
4
Compression Problems
• Two possible problems:
Inadequate
compression
Symptoms
Poor starting
Less power
Excessive
compression
Symptoms
Harder to crank
Detonation
Preignition
Cause(s)
Leaks
Cause(s)
Carbon buildup in
combustion
chamber
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.
Will cause engine damage.
6
Detonation
Causes
1.
Increased compression
2.
High temperatures
3.
Lean fuel/air mixture
4.
Advanced ignition timing
5.
Low octane fuels
How do you prevent detonation?
7
Preignition
•
•
•
•
•
Fuel starts to burn before the spark plug fires.
Increases the peak combustion pressure in
the cylinder.
Increases internal temperature.
Decreases engine performance and produces
and audible pinging or knocking sound in the
engine.
Will cause engine pistons, connecting rods,
crankshafts and other compression system
parts to fail.
8
Preignition
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 the piston
Sharp edges on valves that were
reground improperly
A lean fuel mixture.
How do you prevent preignition?
9
Piston Rings
• 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 in-between.
– Has the highest pressure against the
cylinder wall of the three rings.
10
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.
Should the crankcase breather be removed during regular servicing?
11
Compression Release
• Compression release systems are used to decrease effort
required to start engine.
• Holds the exhaust valve slightly open during starting, and
then allows it to fully close once engine starts.
Compression release pin
12
Fuel System
Introduction
• The function of the fuel system includes:
1
2
3
4
5
6
storing
metering
atomizing
vaporizing
mixing fuel and air
delivering fuel air mixture to intake manifold
What symptom(s) will an engine exhibit if the fuel system has failed?
14
Engines Fuels
• Common Small Engine Fuels include:
–
–
–
–
–
Gasoline
Diesel
LPG
LNG
CNG
Why are so many different types of fuel available?
15
Fuel system—cont.
• Fuel system parts:
–
–
–
–
–
–
Supply (tank)
Lines
Shut off valves
Filter
Pump
Carburetor
What is the function of each part?
16
Fuel system—cont.
• Additional facts about the fuel system:
– Fuel must be clean and good quality
– Air fuel mixture must be richer for starting
– Excess fuel washes lubrication off of the cylinder walls and
accelerates wear.
– Excess fuel dilutes the oil and accelerates wear.
– Air fuel mixture that is too lean will cause preignition
17
Volatility
• Volatility is the propensity of a liquid to become a vapor.
• The volatility of gasoline changes with the seasons.
Low Volatility
(summer)
High Volatility
(winter)
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
What is vapor lock?
18
Vaporization
• Vaporization is the process converting a liquid to a
vapor.
• Fuel must be vaporized because liquid fuel will not
burn.
• The rate and efficiency of vaporization is improved
when the liquid is reduced to small droplets (atomized).
What is required to vaporize a liquid fuel?
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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.”
Identify the parts on the carburetor and explain their
function(s)?
A
Newer carburetor
B
C
What is different?
D
H
E
F
G
21
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 (Venturi) increases in velocity
and decreases in pressure.
Name another application of Bernoulli’s
principle.
22
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 wants to flows from outside of to inside the engine.
What can
reduce the
amount or rate
of air flow?
23
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.
Why does the
carburetor have an
air vent to the fuel
bowel?
24
Carburetor--Venturi
•
•
•
•
•
A fluid will not flow without a pressure difference
A pressure difference is needed to cause the fuel to flow from the fuel
bowel into the air stream.
This is accomplished using a Venturi and a tube connecting the mouth of
the venture to the fuel bowel.
Add a jet to the emulsion tube and you have a functioning carburetor.
It will function correctly as long as it has a constant load and constant
speed.
How many
engines
operate at a
constant load
and constant
speed?
25
Carburetor-Throttle
•
•
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.
Low air flow = less pressure difference between the venture and fuel bowel
less pressure difference = less fuel flow
less fuel flow = less speed.
What happens when the throttle is opened?
Why?
26
Throttle--cont.
•
•
•
•
•
When the throttle is in the wide
open position, there is maximum air
flow through the carburetor.
Maximum air flow = maximum
pressure difference between
Venturi and fuel bowel
Maximum pressure difference =
maximum fuel flow
Maximum fuel flow = maximum
What controls the
speed & power.
position of the throttle?
A carburetor with this design would
function well under varying loads
and speeds,
• Older carburetors used a needle
valve to adjust the fuel flow into the
emulsion tube.
• Newer carburetors use a fixed
orifice.
27
Carburetor-Choke
• Engines require a richer
mixture for starting.
• This is accomplished by a
choke or primer bulb.
• When the choke is closed the
maximum pressure difference
occurs in the Venturi.
• Maximum pressure =
maximum fuel flow.
• Once engine starts the choke
must be opened to prevent the
engine from running too rich.
A primer bulb is used on some
engines.
28
Carburetor-Choke—cont.
• Briggs and Stratton newest engine uses a thermostatically
controlled choke.
• An air vane is attached to the choke lever and the lever on the
choke control.
• As the bimetallic strip in the thermostat heats up it moves the
lever and the air vane opens the choke.
29
Carburetor-Idle Circuit
•
•
•
•
The addition of a choke/primer improved
engine starting, but this carburetor still has
a problem if the engine needs to run at 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
carburetor wall.
No low pressure area in the Venturi means
no fuel flow.
This problem was solved with the addition
of an idle circuit and idle needle valve.
Many new walk behind mowers are single speed.
They are either running at full governed speed or off?
Will these carburetors have an idle circuit.
30
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 the same.
A constant level of fuel is
maintained in the fuel bowel
by the float, float needle
valve and float needle valve
seat.
31
Carburetor-Additional Features
• Several additional features have been tried/added to improve
carburetor performance.
–
–
–
–
Air bleeds
Fixed jets
Transition ports
Pilot jets
32
Three Types of Briggs & Stratton
Carburetors
• Vacu-jet
– Carburetor mounted on top of fuel
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.
33
Three Types of Briggs & Stratton
Carburetors--cont.
• Pulsa-jet
• Carburetor mounted on top of fuel tank.
• Carburetor includes a fuel pump that elevates fuel from the
main tank to the secondary tank.
• The combination of excess fuel pump capacity and secondary
tank drain keeps the fuel level in the secondary tank at a
constant level.
• Not included in latest B & S repair manual.
34
Pulsa Jet Parts
A.
B.
C.
D.
E.
Fuel pump
Primary fuel tube
Primary fuel tank
Primary fuel tube check valve
Fuel screens
F. Secondary fuel tube check
valve
G. Secondary fuel tank
H. Secondary fuel tube
I. Choke
J. High speed needle valve
K. Air horn (inlet)
35
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.
36
Governor System
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.
38
Mechanical Governor
•
•
•
•
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.
What will be the effect if the governor
doesn’t work, or because of slack in
the linkage the throttle plate doesn’t
move?
39
Mechanical Governor
The governor spring is
constantly trying to open
the throttle and the
governor is always trying
to close the throttle.
40
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.
42
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.
•
•
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•
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•
•
Electricity
Conductor
Electron
Free Electron
Voltage
Load
Current
Direct Current
Alternating Current
•
•
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•
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•
•
Polarity
Amperes
Resistance
Short circuit
Series Circuits
Parallel Circuits
Ohm’s Law
Magnetism
Induction
Solenoid
•
•
•
•
•
•
Diodes
Voltage Regulator
Battery
Primary winding
Secondary winding
Condenser
43
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
44
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
45
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.
46
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
47
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.
48
Charging System--cont.
• The components of a charging system may include:
–
–
–
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–
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Coil
Magnets
Voltage regulator
Rectifier
Switches
Conductors
49
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
50
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.
51
Magneto Ignition System Parts
•
•
•
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•
Magnets
Points (Breaker point only)
Trigger coil
Conductors
Spark plug
Condenser (Breaker point only)
Lamination stack
Primary winding
Secondary winding
52
Magneto Ignition system
• 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.
53
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.
54
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.
55
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
56
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.
57
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.
58
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
59
Questions
60