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)