2 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.
3 Five (5) Engine SystemsAll engine parts and functions can be divided into five (5) systems.CompressionFuelElectricalCoolingLubricationIf an engine will not start, which one of the five system(s) is most likely the problem?fuelIf 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?
4 Compression System Parts The compression system includes all of the parts that create, contain and manage the engine compression.ValvesValve springsConnecting rodCrankshaftGasketsBlockPistonPiston ringsCylinder headBearingsParts
5 Compression Problems Two possible problems: Inadequate compression SymptomsCause(s)LeaksPoor startingLess powerExcessive compressionSymptomsCause(s)Harder to crankDetonationPreignitionCarbon buildup in combustion chamber
6 DetonationAn 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.
7 How do you prevent detonation? CausesIncreased compressionHigh temperaturesLean fuel/air mixtureAdvanced ignition timingLow octane fuelsHow do you prevent detonation?Remove the causes
8 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.
9 How do you prevent preignition? CausesAn overheated spark plugGlowing carbon depositsOver heated exhaust valveA sharp edge in the combustion chamber or on top of the pistonSharp edges on valves that were reground improperlyA lean fuel mixture.Remove the cause(s)How do you prevent preignition?
10 Piston Rings Compression Wiper ring Oil ring Subject to to greatest amount of chemical corrosion and highest temperatures.Transfers 70% of combustion heat from piston to cylinder walls.CompressionRingWiper RingOil RingWiper ringMeters oil film on cylinder wallsMust be installed correctly.Oil ringConstructed of two thin rails with holes or slots cut in-between.Has the highest pressure against the cylinder wall of the three rings.
11 Crankcase BreatherMaintains 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?No
12 Compression ReleaseCompression 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
14 Introduction The function of the fuel system includes: storing meteringatomizingvaporizingmixing fuel and airdelivering fuel air mixture to intake manifoldWhat symptom(s) will an engine exhibit if the fuel system has failed?Failure in any part of the system results in an engine that is hard to start and/or will not produce rated power.
15 Engines Fuels Common Small Engine Fuels include: Gasoline Diesel LPG LNGCNGWhy are so many different types of fuel available?Each type has advantages and disadvantages
16 Fuel system—cont. Fuel system parts: Supply (tank) Lines Shut off valvesFilterPumpCarburetorWhat is the function of each part?
17 Fuel system—cont. Additional facts about the fuel system: Fuel must be clean and good qualityAir fuel mixture must be richer for startingExcess 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
18 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 operationPoor hot weather operationSpark plug deposit buildupVapor lockCombustion chamber deposit buildupPoor fuel economyPoor cold startingExcessive fuel evaporationFuel bubbles develop in system.What is vapor lock?
19 What is required to vaporize a liquid fuel? VaporizationVaporization 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?Heat
21 Identify the parts on the carburetor and explain their function(s)? “Carburetor: the engine component that provides the required air-fuel mixture to the combustion chamber based on engine speed and load.”Newer carburetorIdentify the parts on the carburetor and explain their function(s)?ABCWhat is different?A. Crankcase breather tube attachment Attach crankcase breather to carburetorB. Low speed adjustment screw Adjust low speed air-fuel mixtureC. Throttle Set engine speedD. Idle speed adjustment screw Set idle speedE. Float bowel Fuel reservoir for carburetorF. High speed adjustment screw High speed fuel adjustmentG. Fuel inlet Fuel line attaches to carburetorH. Choke Enrich fuel for startingDHEGF
22 Four Fluid PrinciplesFluids flow from areas of high pressure to areas of low pressure.When there is no pressure difference--there is no fluid flow.Fluids exert pressure of the same value throughout a system.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.Airplane wingName another application of Bernoulli’s principle.
23 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 psiLow 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?Clogged air filterObstruction in carburetorStuck choke valve
24 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?
25 Carburetor--VenturiA fluid will not flow without a pressure differenceA 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?Very few.
26 Carburetor-ThrottleTo 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 bowelless pressure difference = less fuel flowless fuel flow = less speed.Engine speed increasesOpen throttle = more air flow = greater pressure difference = greater fuel flowWhat happens when the throttle is opened?Why?
27 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 bowelMaximum pressure difference = maximum fuel flowMaximum fuel flow = maximum speed & power.A carburetor with this design would function well under varying loads and speeds,What controls the position of the throttle?Governor and governor spring.Older carburetors used a needle valve to adjust the fuel flow into the emulsion tube.Newer carburetors use a fixed orifice.
28 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.
29 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.
30 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.NoMany new walk behind mowers are single speed.They are either running at full governed speed or off?Will these carburetors have an idle circuit.
31 Carburetor-FloatTo 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.
32 Carburetor-Additional Features Several additional features have been tried/added to improve carburetor performance.Air bleedsFixed jetsTransition portsPilot jets
33 Three Types of Briggs & Stratton Carburetors Vacu-jetCarburetor 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.
34 Three Types of Briggs & Stratton Carburetors--cont. Pulsa-jetCarburetor 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.
36 Three Types of Briggs & Stratton Carburetors--cont. Flow-jetDifferent 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.
38 IntroductionThe 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.
39 Mechanical GovernorThe 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?Engine goes to maximum rpm and something breaks.
40 Mechanical GovernorThe governor spring is constantly trying to open the throttle and the governor is always trying to close the throttle.
42 IntroductionElectricity 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.
43 Electrical TermsBefore attempting to understand small engine electrical systems, it is important to know the terms and parts associated with electricity and the electrical systems.ElectricityConductorElectronFree ElectronVoltageLoadCurrentDirect CurrentAlternating CurrentPolarityAmperesResistanceShort circuitSeries CircuitsParallel CircuitsOhm’s LawMagnetismInductionSolenoidDiodesVoltage RegulatorBatteryPrimary windingSecondary windingCondenser
44 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 ResistanceAWG NumberDiameter/1000 ft (68 oF)1280.81.61464.12.51650.84.01840.36.4203210.22225.3516.2
45 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.PermanentTemporary
46 InductionInduction: 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.
47 Small Gas Engine Electrical Systems Small engines may have one or more of five (5) electrical systems.ChargingIgnitionStartingAccessoriesSafety
48 1. Charging SystemCharging systems produces electrical to operate accessories and the replace electrical energy taken from a battery.Two different systems can be used.GeneratorAlternatorGenerator 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.
49 Charging System--cont. The components of a charging system may include:CoilMagnetsVoltage regulatorRectifierSwitchesConductors
50 2. Ignition SystemThe ignition system provides a high voltage spark in the combustion chamber at the proper time.Two types of ignition systemsBatteryMagnetoBattery systems transforms the battery voltage and fires the spark plug at the correct time.Magneto systems must produce the current, transform the voltage and time the spark plug.Most small engines use the magneto systemTwo types of magneto systems:Breaker point ignitionSolid state (electronic) ignition
51 2. Ignition System-cont. Breaker point ignition Solid state ignition Older system. Most manufacturers have replaced them with solid state.Uses a set of points to break the primary circuit.Solid state ignitionUses a transistor to break the primary circuit.
52 Magneto Ignition System Parts MagnetsPoints (Breaker point only)Trigger coilConductorsSpark plugCondenser (Breaker point only)Lamination stackPrimary windingSecondary winding
53 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.
54 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.
55 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.
56 3. Starting SystemThe purpose of the starting system is to use energy to turn the engine until it starts.System components may include:Electrical sourceStarting motorConductorsIgnition switchSolenoid switchTwo primary electrical systems.Single switchSolenoid
57 Staring Systems--cont. Single SwitchFor 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.SolenoidIn 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.
58 4. AccessoriesSmall engines are used on machines that may require electricity to operate accessories.Accessories may include:LightsElectrical clutchesElectrical lift systemsRadio, etc.The conductors must be sized for the electrical load.Each separate circuit should have overload protection.
59 5. SafetyIt 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 switchSeat switchAnti after fire solenoidDeck switchTransmission switchTilt switch