2 OverviewIdentify differences between internal and external combustion enginesUnderstand 2-stroke vs. 4-stroke enginesUnderstand subsystems of small gas enginesDiscuss procedures for assembling and disassembling small gas engines
3 Internal Combustion Engines (ICE) vs. External Combustion Engines (ECE) External combustion engines: produce heat outside of the cylinder containing the pistonOften used boilers to create steamInternal combustion engines: produce heat Inside of the cylinder containing the pistonMore reliable than ECEProduce more power than similar size ECEUsed to power MOST vehicles in the USAUsed in agriculture and construction industries
4 Continued…Cylinder ( aka. cylinder bore): is a hole in the block that directs the piston during movementThe ICE began replacing the ECE about 100 years ago.
5 Engine TheoryAll ICEs convert chemical energy into mechanical power and share common mechanical elementsTwo main types of enginesTwo StrokeFour Stroke
6 4-Stroke EngineCan be any number of cylinders (1,2,3,4,6,8,10,12) and all are coupled to a single crank-shaftCrank-shaft: converts the reciprocal motion of the pistons into rotary motion and powers the loadPiston: a cylindrical engine component that slides back and forth in the cylinder when propelled by the force of combustion.
7 4-Stroke Engine (continued) Stroke: the movement of the piston from the bottom limit of its travel to the top limit of its travel in the cylinder bore.Require 4 strokes of the piston to complete one cycleIntake StrokeCompression StrokePower Stroke (combustion)Exhaust Stroke
9 4-Stroke Engine (1-INTAKE) Intake Stroke: (downward) creates a partial vacuum drawing air into the cylinder through the carburetor where liquid fuel is atomized and mixed with the air (called a fuel-air charge).Intake valve is openExhaust valve is closed4-stroke graphic
10 4-Stroke Engine (2-Compression) Compression Stroke: (upward) Fuel-air charge is squeezed to about 1/10th of its original volumeBottom Dead Center (BDC) when the piston is at its lowest point (crankshaft is rounding the bottom of its travel)Top Dead Center (TDC) when the piston is at its highest point (crankshaft is rounding the top of its travel)Compression ratio is mathematical relationship between BDC and TDC (ie: 10:1 compression)Intake and exhaust valves are closed4-stroke graphic
11 4-Stroke Engine (3-Power) Power Stroke: (downward) With piston near TDC the compressed fuel-air charge is detonated (by the spark plug)Combusting gasses expand pushing down piston.The connecting rod pushes down on the crank shaft causing it to rotateIntake and exhaust valves are closed4-stroke graphic
12 4-Stroke Engine (4-Exhaust) Exhaust: (upward) Piston moves from BDC to TDC pushing the spent fuel-air mixture out of the cylinderPiston is moved up by momentum or by power stroke of another piston pushing on the crank shaftIntake valve is closedExhaust valves is open4-stroke graphic
15 2-Stroke Engine Every upward stroke is a compression stroke Every downward stroke is a power strokeIntake and Exhaust stroke occur during the compression and power strokesEvery revolution of the crankshaft produces powerOn a 4-stroke engine, it takes 2 revolutions
16 2-Stroke Engines (Advantages) 2-stroke engines are more powerful for their sizeGood at high RPM (revolutions per minute) applicationsSimpler design than 4-stroke (less parts)No valve trainNo cam-shaftLighter than 4-stroke engines of comprable powerNo oil reservoirNo valve train, cam, etc.Can be operated at any angle (no oil reservoir)
17 2-Stroke EnginesIntake and exhaust occur through ports on the side of the cylinder.Oil is mixed with the fuel and burned in the combustion chamber.Pressure from the moving piston pushes gas/air/oil where it needs to go.
18 2-Stroke Engine (disadvantages) Exhaust is dirtier than 4-stroke because oil is burnedThey wear more quickly than 4-stroke because every other stroke is a power strokeThey don’t last as longMixing oil with fuel is inconvenient and if forgotten it will destroy the engine
20 Engine Subsystems Many of them on all engines All must perform properly for peak performanceCooling subsystemElectrical subsystemLubrication subsystemMechanical subsystemGoverning subsystemFuel subsystem
21 Cooling Subsystem Can be cooled by air or liquid Air cooled systems Cooling fins increase surface areaFlywheel blades direct air across engine finsSheet metal shrouds direct the airLiquid cooled systemsWater jackets surround cylinder wallsWater pumps move water through jackets to radiatorRadiator expose surface area to surrounding airThermostat allows/impedes flow of water to radiator
22 Lubrication Subsystem Oil distribution mechanismOil sealsPiston ringsOil
23 Lubrication Subsystem ALL moving parts must be lubricatedSplash lubrication methodBetter for small gas engines“Oil dipper” attached to bottom of connecting rod flings oil up on bottom of pistonsPiston RingsOil ring: (bottom ring) limits the amount of oil that squeezes past the piston into the combustion chamberCompression ring(s): (upper ring(s)) contain combustion, scrape oil off of cyl. walls back into crankcase.
24 Lubrication Subsystem OilProtects internal parts from corrosionCleans engine for foreign matter and allowing it to settle into the oil reservoir (crankcase or oil pan)Seal the engine by filling small spaces between moving parts (ie: piston rings and moving parts)Cushion moving parts from the power strokeImprove fuel economy by reducing frictionViscosity: measures resistance to flow (thickness)Developed by theSociety of Automotive Engineers (SAE)
26 Mechanical SubsystemConverts the force of the expanding gasses during combustion into mechanical power and delivers it to the crankshaftEngine block (housing for all components)PistonPiston pin (aka: Wrist pin)Connecting rodCrankshaft (crankpin journal)In a 4-stroke engine the crankshaft also powers the camshaft and valvetrain.
28 Mechanical Subsystem: Camshaft Opens and closes valves by pushing on rods called lifters (some are adjustable for cam wear)
29 Mechanical Subsystem: Flywheel Heavy metal disk attached to the CrankshaftInertia of the rotating engine created by power stroke helps the engine coast through the exhaust, intake and compression strokeSmoothes out the power produced by the engine so it does not continually speed up and slow down
30 Mechanical SubsystemThis system takes the most wear (usually not visible)Measurements are made in critical areas for wear and for warpageMicrometersFeeler gauges (AKA: thickness Gauge)
31 Electrical Subsystem Produces the current that fires the sparkplug Permanent magnet in the flywheelMagnet passes the armature as flywheel spins creating low voltageConverted to high voltage in the ignition coilSpark jumps the gap in the spark plug to ignite fuel/air charge
32 Electrical Subsystem Timing Spacing of armature Sparkplug Shear pin (key) keeps flywheel aligned on the crankshaft so spark is produced before TDCSpacing of armatureToo close will rub on flywheelToo far produces weak sparkSparkplugMust be “gapped” properly using feeler guage
34 Governing and Fuel Subsystem Work in conjunction with one anotherGoverning system is designed to keep the engine running at the desired speed regardless of loadFuel subsystem is responsible for creating the fuel/air mix used to power the engine and deliver it to the combustion chamberCarburetorFuel injectors
35 Fuel Injected SystemFuel is pressurized and sprayed into the cylinder before TDCVery common on cars and trucks with gas or diesel enginesRegulated by computers in modern cars to achieve maximum performance with minimum emissions
36 Carbureted System Very common on small engines and older cars Fuel vapor is drawn through the carb by the air that rushes past it (by the intake stroke)This occurs in the venturi.Venturi Effect states that pressure decreases as velocity increases.
37 Governing System Definitions Venturi: Narrow restricting section of carburetor where air speeds up and drafts the fuel vapor along with it into CylinderChoke: Plate-like device (usually) that varies the amount of air that can enter the carb.Throttle: plate-like device located in back of venturi that regulates amount of fuel air mix entering the cylinders.Load: condition under which an engine runs when it does workChoke plate and Throttle are open
38 Governing System Definitions Idle: the condition an engine will run under when it is warmed up to temperature and NOT under loadChoke is openThrottle is closedIdle Bypass Circuit: small passageway that allows some air/fuel mix to escape around the throttle plate to keep engine running
39 Measuring, Testing and Troubleshooting All complex machines need maintenance, periodic testing and troubleshooting to run their bestEmissions testingTemperature regulationTune-upsAir filter changesOil changesEtc.
40 EfficiencyVolumetric Efficiency: measures how well the engine “breathes.” Measure of how much fuel air mixture is drawn into cylinders with the amount that could be drawn in.Mechanical efficiency: Percentage of power developed in the cylinder compared to the power that is actually delivered to the crankshaft
41 EfficiencyThermal Efficiency: (aka heat efficiency) measure of how much heat is actually used to drive the pistons downward.Only about 25% is used to drive the piston downward, the rest is lost.Practical Efficiency: simple measure of how efficiently an engine uses its fuel supplyIf used for motive power it is measured in MPGTakes into account all losses of efficiencyfrictionDragThermal loss, etc
42 HorsepowerDeveloped as a means of comparing the power produced by James Watt’s steam engine to the amount of work a horse could do.550 foot-pounds per secondHorsepower capability is affected byBore: diameter of the pistonStroke: Distance from TDC to BDCFrictional loss: within the engine (frictional vs non- frictional bearings)
44 Horsepower Terminology Brake Horsepower (bhp): the hp available for use at the crankshaft. Increases with engine rpm then decrease when engine is revved to highIndicated horsepower (ihp): Theoretical term. Measure of the power developed by the fuel air charge upon ignition
45 Horsepower Terminology Frictional Horsepower (fhp): represents the part of the potential hp lost due to friction within the engineihp-bhp=fhpRated horsepower (rhp): usually represents about 80% of the engines bhp because engines should not be run at full capability all the time (the sticker rating)