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Chapter 14.  Identify differences between internal and external combustion engines  Understand 2-stroke vs. 4-stroke engines  Understand subsystems.

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Presentation on theme: "Chapter 14.  Identify differences between internal and external combustion engines  Understand 2-stroke vs. 4-stroke engines  Understand subsystems."— Presentation transcript:

1 Chapter 14

2  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  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  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  All ICEs convert chemical energy into mechanical power and share common mechanical elements  Two main types of engines ◦ Two Stroke ◦ Four Stroke

6  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  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 Intake Compression PowerExhaust

9  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  Compression Stroke: (upward) Fuel-air charge is squeezed to about 1/10 th 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  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  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 ns/a/a6/4-Stroke-Engine.gif


15  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 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  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  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  html html  Stroke_Engine_ani.gif Stroke_Engine_ani.gif

20  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  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  Oil distribution mechanism  Oil seals  Piston rings  Oil

23  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  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)


26  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.


28  Opens and closes valves by pushing on rods called lifters (some are adjustable for cam wear)

29  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  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  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  Timing ◦ 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  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 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  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  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  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  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  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  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  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)


44  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  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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