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Small Gas Engines Chapter 14.

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Presentation on theme: "Small Gas Engines Chapter 14."— Presentation transcript:

1 Small Gas Engines Chapter 14

2 Overview Identify differences between internal and external combustion engines Understand 2-stroke vs. 4-stroke engines Understand subsystems of small gas engines Discuss 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 piston Often used boilers to create steam Internal combustion engines: produce heat Inside of the cylinder containing the piston More reliable than ECE Produce more power than similar size ECE Used to power MOST vehicles in the USA Used in agriculture and construction industries

4 Continued… Cylinder ( aka. cylinder bore): is a hole in the block that directs the piston during movement The ICE began replacing the ECE about 100 years ago.

5 Engine Theory All ICEs convert chemical energy into mechanical power and share common mechanical elements Two main types of engines Two Stroke Four Stroke

6 4-Stroke Engine Can be any number of cylinders (1,2,3,4,6,8,10,12) and all are coupled to a single crank-shaft Crank-shaft: converts the reciprocal motion of the pistons into rotary motion and powers the load Piston: 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 cycle Intake Stroke Compression Stroke Power Stroke (combustion) Exhaust Stroke

8 4-Stroke Engines Intake Compression Power Exhaust

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 open Exhaust valve is closed 4-stroke graphic

10 4-Stroke Engine (2-Compression)
Compression Stroke: (upward) Fuel-air charge is squeezed to about 1/10th of its original volume Bottom 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 closed 4-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 rotate Intake and exhaust valves are closed 4-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 cylinder Piston is moved up by momentum or by power stroke of another piston pushing on the crank shaft Intake valve is closed Exhaust valves is open 4-stroke graphic

13 4-Stroke Engine Animation

14 2-Stroke engine

15 2-Stroke Engine Every upward stroke is a compression stroke
Every downward stroke is a power stroke Intake and Exhaust stroke occur during the compression and power strokes Every revolution of the crankshaft produces power On a 4-stroke engine, it takes 2 revolutions

16 2-Stroke Engines (Advantages)
2-stroke engines are more powerful for their size Good at high RPM (revolutions per minute) applications Simpler design than 4-stroke (less parts) No valve train No cam-shaft Lighter than 4-stroke engines of comprable power No oil reservoir No valve train, cam, etc. Can be operated at any angle (no oil reservoir)

17 2-Stroke Engines Intake 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 burned They wear more quickly than 4-stroke because every other stroke is a power stroke They don’t last as long Mixing oil with fuel is inconvenient and if forgotten it will destroy the engine

19 2-Stroke Engine Animations
html Stroke_Engine_ani.gif

20 Engine Subsystems Many of them on all engines
All must perform properly for peak performance Cooling subsystem Electrical subsystem Lubrication subsystem Mechanical subsystem Governing subsystem Fuel subsystem

21 Cooling Subsystem Can be cooled by air or liquid Air cooled systems
Cooling fins increase surface area Flywheel blades direct air across engine fins Sheet metal shrouds direct the air Liquid cooled systems Water jackets surround cylinder walls Water pumps move water through jackets to radiator Radiator expose surface area to surrounding air Thermostat allows/impedes flow of water to radiator

22 Lubrication Subsystem
Oil distribution mechanism Oil seals Piston rings Oil

23 Lubrication Subsystem
ALL moving parts must be lubricated Splash lubrication method Better for small gas engines “Oil dipper” attached to bottom of connecting rod flings oil up on bottom of pistons Piston Rings Oil ring: (bottom ring) limits the amount of oil that squeezes past the piston into the combustion chamber Compression ring(s): (upper ring(s)) contain combustion, scrape oil off of cyl. walls back into crankcase.

24 Lubrication Subsystem
Oil Protects internal parts from corrosion Cleans 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 stroke Improve fuel economy by reducing friction Viscosity: measures resistance to flow (thickness) Developed by the Society of Automotive Engineers (SAE)

25 Oil Viscosity Chart

26 Mechanical Subsystem Converts the force of the expanding gasses during combustion into mechanical power and delivers it to the crankshaft Engine block (housing for all components) Piston Piston pin (aka: Wrist pin) Connecting rod Crankshaft (crankpin journal) In a 4-stroke engine the crankshaft also powers the camshaft and valvetrain.

27 Mechanical Subsystem

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 Crankshaft Inertia of the rotating engine created by power stroke helps the engine coast through the exhaust, intake and compression stroke Smoothes out the power produced by the engine so it does not continually speed up and slow down

30 Mechanical Subsystem This system takes the most wear (usually not visible) Measurements are made in critical areas for wear and for warpage Micrometers Feeler gauges (AKA: thickness Gauge)

31 Electrical Subsystem Produces the current that fires the sparkplug
Permanent magnet in the flywheel Magnet passes the armature as flywheel spins creating low voltage Converted to high voltage in the ignition coil Spark 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 TDC Spacing of armature Too close will rub on flywheel Too far produces weak spark Sparkplug Must be “gapped” properly using feeler guage


34 Governing and Fuel Subsystem
Work in conjunction with one another Governing system is designed to keep the engine running at the desired speed regardless of load Fuel subsystem is responsible for creating the fuel/air mix used to power the engine and deliver it to the combustion chamber Carburetor Fuel injectors

35 Fuel Injected System Fuel is pressurized and sprayed into the cylinder before TDC Very common on cars and trucks with gas or diesel engines Regulated 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 Cylinder Choke: 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 work Choke 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 load Choke is open Throttle is closed Idle 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 best Emissions testing Temperature regulation Tune-ups Air filter changes Oil changes Etc.

40 Efficiency Volumetric 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 Efficiency Thermal 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 supply If used for motive power it is measured in MPG Takes into account all losses of efficiency friction Drag Thermal loss, etc

42 Horsepower Developed 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 second Horsepower capability is affected by Bore: diameter of the piston Stroke: Distance from TDC to BDC Frictional loss: within the engine (frictional vs non- frictional bearings)

43 Dynamometer

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 high Indicated 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 engine ihp-bhp=fhp Rated 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)

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