chapter1 introduction icengines
TRANSCRIPT
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Internal Combustion Engine
The internal combustion (IC) engine is a heat engine that convertschemical energy in a fuel into mechanical energy, usually made
available on a rotating output shaft.
History of IC engines:
1700s - Steam engines (external combustion engines)
1860 - Lenoir engine (h = 5%)
1867 - Otto-Langen engine (h = 11%, 90 RPM max.)
1876 - Otto four stroke “spark ignition” engine (h = 14%, 160 RPM max.)
1880s - Two stroke engine1892 - Diesel four stroke “compression ignition” engine
1957 - Wenkel “rotary” engine
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Atmospheric Engine
Process 1-2: Fuel air mixture introduced into cylinder at
atmospheric pressureProcess 2-3: Constant pressure combustion (cylinder open
to atmosphere)
Process 3-4: Constant volume cooling (produces vacuum)
Process 4-5: Isentropic compression (atmosphere pushes piston)
Process 5-1: Exhaust process
31
2Po
4
5
P
V
VALVEPatm
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Historical IC Engines
FLYWHEEL
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Two-stroke Lenoir Engine
Process 1-2: Fuel air mixture introduced into cylinder at
atmospheric pressure
Process 2-3: At half-stroke inlet valve closed and combustion
initiated constant volume due to heavy piston
producing high pressure products
Process 3-4: Products expand producing work
Process 4-5: At the end of the first stroke exhaust valve opens and
blowdown occursProcess 5-1: Exhaust stroke
3
1 2Po
4
5
P
V
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Two-stroke Otto-Langen Engine
Process 1-2: Fuel air mixture introduced into cylinder at
atmospheric pressure
Process 2-3: Early in the stroke inlet valve closed and combustion
initiated constant volume due to heavy piston
producing high pressure products
Process 3-4: Products expand accelerating a free piston
momentum generates a vacuum in the tube
Process 4-5: Atmospheric pressure pushes piston back, pistonrack engaged through clutch to output shaft
Process 5-1: Valve opens gas exhausted
Disengagedoutput shaft
Engaged
output shaft
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Four stroke Spark Ignition (SI) Engine
Stroke 1: Fuel-air mixture introduced into cylinder through intake
valveStroke 2: Fuel-air mixture compressed
Stroke 3: Combustion (roughly constant volume) occurs and
product gases expand doing work
Stroke 4: Product gases pushed out of the cylinder through the
exhaust valve
Compression
Stroke
Power
StrokeExhaust
Stroke
A
I
R
Combustion
Products
Ignition
Intake
Stroke
FUEL
Fuel/Air
Mixture
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Crank shaft
90o
180o
BC
TC
0o
270o
q
Engine Operating Cycle
Spark plug for SI engineFuel injector for CI engine
Top
Center
(TC)
Bottom
Center
(BC)
Valves
Clearance
volume
Cylinderwall
Piston
Stroke
CA
rev
rev
sCA
360
1
speedcrank
anglescranktime
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Pressure-Volume Graph 4-stroke SI engine
One power stroke for every two crank shaft revolutions
1 atm
Spark
TC
Cylinder volume
BC
Pressure
Exhaust valve
opens
Intake valve
closes
Exhaust
valve
closes
Intake
valve
opens
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IVO - intake valve opens, IVC – intake valve closes
EVO – exhaust valve opens, EVC – exhaust valve opens
Xb – burned gas mole fraction
Motored Four-Stroke Engine
10
Pressure (bar)
100
Intake Exhaust
TC
BC
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IVO - intake valve opens, IVC – intake valve closes
EVO – exhaust valve opens, EVC – exhaust valve opens
Xb – burned gas mole fraction
Four-Stroke SI Engine
Valve overlap
Exhaust gas
residual
10
Pressure (bar)
100
Intake Exhaust
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Compression
Stroke
Power
Stroke
Exhaust
Stroke
A
I
R
Combustion
Products
Intake
Stroke
Air
Fuel Injector
Four stroke Compression Ignition (CI) Engine
Stroke 1: Air is introduced into cylinder through intake valveStroke 2: Air is compressed
Stroke 3: Combustion (roughly constant pressure) occurs and
product gases expand doing work
Stroke 4: Product gases pushed out of the cylinder through the
exhaust valve
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SOI – start of injection
EOI – end of injection
SOC – start of combustionEOC – end of combustion
Four-Stroke CI Engine
Fuel mass
flow rate
Fuel mass
burn rate
Cylinder
volume
Cylinder
pressure
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Camshaft
Intake valve
Rocker arm
Piston
Connecting rod
Crankshaft
Oil pump
Exhaust valve
Carburetor
Crank sprocket Oil pickup
Timing belt
Cam sprocket
Air cleaner
Timing belt
tensor
Engine Anatomy
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Poppet Valve Actuation with Overhead Camshaft
Camshaft
Spring
Air manifold
Stem
Guide
Valve head
Valve seat
Piston
Sparkplug
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Cylinder Head Design
Honda VTEC (variable intake valve timing)
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Modern Two-Stroke Spark Ignition Engine
Stroke 1: Fuel-air mixture is introduced into the cylinder andis then compressed, combustion initiated at the end of
the stroke
Stroke 2: Combustion products expand doing work and then
exhausted
* Power delivered to the crankshaft on every revolution
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Two Stroke Spark Ignition Engine
Intake (“Scavenging”)
Compression Ignition
ExhaustExpansion
Fuel-air-oil
mixture
Fuel-air-oil
mixture
Crank
shaft
Reed
valve
Exhaust
Port*
Transfer
Port*
*No valves andthus no camshaft
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EPO – exhaust port openEPC – exhaust port closed
IPO – intake port open
IPC – intake port closed
Two-Stroke CI Engine
scavenging
Ai
Ae
Intake area (Ai)
Exhaust area (Ae)
P i
P e
Exhaust Press (Pe)
Intake Press (Pi)
Cylinder Press (P)
110 CA
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Cross Loop Uniflow
Scavenging in Two-Stroke Engine
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Advantages of the two stroke engine:
• Power to weight ratio is higher than the four stroke engine since thereis one power stroke per crank shaft revolution.
• No valves or camshaft, just ports
Most often used for low cost, small engine applications such as lawn
mowers, marine outboard engines, motorcycles….
Disadvantages of the two-stroke engine:
• Incomplete scavenging or to much scavenging
• Burns oil mixed in with the fuel
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Single Cylinder Engine
Single-cylinder engine gives one power stroke per crank revolution
(360 CA) for 2 stroke, or every two revolutions for 4 stroke.
The torque pulses on the crank shaft are widely spaced, and engine
vibration and smoothness are significant problems.
Used in small engine applications where engine size is more important
180 CA0 CA(TC)
720 CA(TC)
540 CA360 CA(TC)
180 CA
4-stroke
2-stroke
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Multi-cylinder Engines
Multi-cylinder engines spread out the displacement volume amongst
multiple smaller cylinders. Increased frequency of power strokes
produces smoother torque characteristics.
Most common cylinder arrangements are in-line 4 and V-6:
Engine balance (inertia forces associated with accelerating and
decelerating piston) better for in-line versus V configuration.
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V-6 Engine
Air intake
manifold
Inlet
runner
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• In spark ignition engines the air and fuel are usually mixed prior to entry
into the cylinder.
• Initially a purely mechanical device known as a carburetor was used to
mix the fuel and the air
• Most modern cars use electronic fuel-injection systems:
- 1980s single injector used to spray fuel continuously into the air manifold
- 1990s one injector per cylinder used to spray fuel intermittently into the
intake port
Fuel-Air Mixing
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Gasoline Direct Injection (GDI) Engine
• Fuel is injected directly into the cylinder during the intake stroke or the
compression stroke
• High pressure injector required, 5-10 MPa
• Need bowl in piston design to direct the fuel spray towards the spark plug
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Direct-Injection Stratified-Charge Engines
• Create easily ignitable fuel-air mixture at the spark plug and a leaner
fuel-air mixture in the rest of the cylinder.
• Lean burn results in lower emissions and higher energy efficiency
Example:
Mitsubishi GDI engine achieves complete combustion with an air-fuel
ratio of 40:1 compared to 15:1 for conventional engines
This results in a 20% improvement in overall fuel efficiency and CO2
production, and reduces NOx emissions by 95% with special catalyst
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Stratified Charge Engine
During intake stroke air enters the cylinder
Near the end of the compression stroke fuel is injected and directed
by the piston head bowl towards the spark plug
The mixture at the spark plug is “rich” in fuel thus easy to ignite but
the amount of fuel injected results in an overall “lean” fuel-air mixture
Lowers heat transfer to the walls but increases thermal cyclic load on
the spark plug, and standard catalytic converter doesn’t work