Small Gas EnginesChapter 14
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
Overview
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
Internal Combustion Engines (ICE) vs.External Combustion Engines (ECE)
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.
Continued…
All ICEs convert chemical energy into mechanical power and share common mechanical elements
Two main types of engines◦ Two Stroke ◦ Four Stroke
Engine Theory
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.
4-Stroke Engine
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
4-Stroke Engine (continued)
4-Stroke Engines
Intake Compression Power Exhaust
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 Engine (1-INTAKE)
4-stroke graphic
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 Engine (2-Compression)
4-stroke graphic
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 Engine (3-Power)
4-stroke graphic
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 Engine (4-Exhaust)
4-stroke graphic
http://upload.wikimedia.org/wikipedia/commons/a/a6/4-Stroke-Engine.gif
4-Stroke Engine Animation
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
2-Stroke Engine
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)
2-Stroke Engines (Advantages)
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.
2-Stroke Engines
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
2-Stroke Engine (disadvantages)
http://www.animatedengines.com/twostroke.html
http://fr.wikipedia.org/wiki/Fichier:2-Stroke_Engine_ani.gif
2-Stroke Engine Animations
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
Engine Subsystems
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
Cooling Subsystem
Oil distribution mechanism Oil seals Piston rings Oil
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.
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)
Lubrication Subsystem
Oil Viscosity Chart
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.
Mechanical Subsystem
Mechanical Subsystem
Opens and closes valves by pushing on rods called lifters (some are adjustable for cam wear)
Mechanical Subsystem: Camshaft
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
Mechanical Subsystem: Flywheel
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)
Mechanical Subsystem
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
Electrical Subsystem 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
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
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
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.
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
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
Governing System Definitions
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.
Measuring, Testing and Troubleshooting
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
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
Efficiency
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)
Horsepower
Dynamometer
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
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)