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8.21 The Physics of Energy
Fall 2009
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
8.21 Lecture 11
Internal Combustion Engines
October 2, 2009
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
Internal Combustion Engines
Much energy used for transport–cars, trucks, planes, boats, . . .
• U.S.: 25% Energy use, 33% CO2
• Globally 15%/20%, growing
Presents unique challenge: need portable fuel, light engine
Engine type
Spark ignition
Compression ignition
Gas turbines
Use
Cycle
cars, light trucks
trucks, heavy vehicles
airplanes
Otto (Const. V)
Diesel (Const. P)
Brayton
Most cars ∼ 20%-25% efficient.
Increase 5%: save 15 EJ/year globally! (200M people’s total E use)
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
4-stroke Spark Ignition (SI) engine: stages of operation
4-stroke SI engine image removed due to copyright restrictions.
[Milton]
(a) Air/fuel intake: piston goes down
(b) Compression: piston goes up
(c + d) Power: ignition, combustion, piston goes down
(e) Exhaust: piston goes up
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
Reciprocating SI engine
Reciprocating SI engine image removed due to copyright restrictions.
• First IC engines modeled on cannon!
• Reciprocating engine: linear piston motion
• Connecting rod → rotates crankshaft
• Crankshaft → camshaft → valve control
• Piston up: top dead center (TDC/TC)
• Single cycle: crankshaft rotates twice
• Max/min volume: V1 /V2 (Vt /Vc )
Displaced volume V1 − V2 (Vt − Vc = Vd )
• Compression Ratio r = V1 : V2 (typical 9.5:1)
[Heywood]
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
Idealize as thermodynamic Otto cycle (constant volume combustion)
Otto cycle images removed due to copyright restrictions.
[Milton]
Otto + intake/exhaust [Milton]
1 → 2. Adiabatic compression: good approx. for compression (b)
2 → 3. Isometric heating: OK approximation for combustion (c)
3 → 4. Adiabatic expansion: OK approximation for power stroke (d)
4 → 1. Isometric cooling: OK approx. for exhaust + intake (e + a)
Isometric cooling really just “blowdown”;
Better: intake + exhaust as separate constant p (isobaric) processes
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
“Ideal gas” Otto cycle analysis
• Approximate combustion at constant
volume
3
p3
• Qin from combustion of fuel
η=
p
-
T4 −T1
T3 −T2
⇒ T2 = T1 rγ−1 (r = V1 /V2 )
and similarly T3 = T4 rγ−1
2
Qout = CV (T4 − T1 )
4
p4
1
p1
V2 = V3
=1−
p1 V1γ = p2 V2γ ⇒ T1 V1γ−1 = T2 V2γ−1
Qin = CV (T3 − T2 )
p2
Qin −Qout
Qin
V
-
V1 = V4
So
η =1−
1
T2 − T1
=
rγ−1
T2
Critical feature: Compression ratio r
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
Hydrocarbons: molecules composed of hydrogen + carbon atoms
• Crude oil: mix of many hydrocarbons
• Refineries: separate by properties
• Gasoline: ∼ 500 molecules, 5-12 C’s
Mixed for: High compression/efficiency,
Minimize pollution
Image removed due to copyright restrictions.
iso-octane
H
H
H C HH C H
H
H
H C
C C
(2-2-4-trimethylpentane)
H H H
H
C
C H
H
H C H
H
[Heywood]
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
Combustion
SI: Spark Ignites fuel-air mixture, HC’s + O2 → H2 O + CO2
e.g. iso-octane: 2 C8 H18 + 25 O2 → 18 H2 O + 16 CO2 + 10.94 MJ
where ∆H combustion ∼
= 5.47 MJ/mol ∼
= 47.9 MJ/kg (hhc)
iso−octane
Air/fuel ratio nomenclature:
Stoichiometric: just enough O2 to burn all fuel (gasoline: ∼
= 14.7 : 1)
Lean: excess oxygen (higher efficiency)
Rich: excess fuel (lower efficiency, slightly rich → slight power increase)
Exhaust: CO2 , H2 O + unburned HC’s, CO, NOx , . . .
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
Combustion process + “knock”
• Combustion process: ∼ 1/6 rotation
centered into expansion phase
Image removed
removed due to
copyright restrictions.
[Milton]
Image removed
removed due to
copyright restrictions.
[Heywood]
• Heat + pressure ⇒
premature combustion: Knock
• Straight paraffins knock most easily
Measured by “critical compression ratio”,
CCRiso−octane ∼ 2 − 3× CCRn−heptane
• “Octane rating” N: Knocks as
N% iso-octane, 100 − N% n-heptane
• 93 octane gasoline: knocks r ∼ 10.5
• Additives: aromatics, lead,. . . improve
• Bottom line: rmax ≈ 10
with current gasoline mixtures
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
Real SI engines
Maximum compression ratio: 10:1
At 1500-2500 K,
γhot air ≈ γair+spent fuel ≈ 1.3
ηOtto =
1
∼
= 0.50
1 − rγ−1
Theoretical “Ideal gas” thermo analysis: 50% efficiency
Real SI engines: max 35-40%. e.g. 4-cylinder Camry: ηmax ≈ 35%
SI engine losses
Image removed
removed due to
copyright restrictions.
• Combustion not instantaneous
• Heat loss during expansion
• Blowdown: not constant volume
• Exhaust/intake: “pumping losses”
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
Throttle: reduces efficiency
Image removed removed due to
copyright restrictions.
• Let up on gas: throttle reduces air/fuel flow
(typical driving: intake 0.5 Atm)
⇒ reduced pressure on intake (increased pumping loss)
Combining all these inefficiencies, power to systems:
∼ 25% delivered mechanical energy, as in transport example
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
Atkinson cycle
3
p3
p
• Keep valve open at start of compression
⇒ can have exp. ratio > comp. ratio
p2
• Extract more energy w/ expansion
2
p4
p1
1b
V2 = V3
V
V1b
4
1
V1 = V4
• Reduces pumping losses
[DOE report]
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
Atkinson cycle in real engines
Used in 2007 Toyota Camry hybrid
• Expansion ratio 12.5:1 (compression ratio 9.5:1)
• Maximum efficiency 35% → 38% [DOE report]
• Reduces pumping losses
• Maximum power 160hp → 147hp, made up by battery assist
• Superior acceleration
[DOE report]
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
Diesel (constant pressure combustion) cycle — Compression Ignition (CI)
Image removed removed due to
copyright restrictions.
• Inject fuel after compression
⇒ can have r = V1 /V2 ∼ 15 − 20 : 1
• Efficiency → 70% theoretical, ∼ 45% realized (ideal cycle inaccurate)
• Need heavy piston/cylinder head ⇒ trucks, buses, boats, . . .
• More flexible fuel options (∼ biodiesel)
8.21 Lecture 11: Internal Combustion Engines
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Otto cycle
Combustion and fuels
Real engines
Other engine cycles
SUMMARY
• SI engines modeled by “ideal gas” Otto (constant V combustion) cycle
• ηOtto =
1
1−rγ−1
• r = V1 /V2 limited to ∼ 10 by “knock”
• Real engines: combustion process, heat loss, pumping
reduce η →∼ 35% max, 25% average delivered.
• Atkinson cycle: valve timing → more expansion, efficiency
• Diesel (constant P combustion): CI → increased compression, efficiency
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