Lecture 10.
Turbulent Combustion in Piston Engines
part 2
X.S. Bai
TC in piston engines
Content
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Spark-ignition engines
Auto-ignition of fuels, engine knock
Fuel injection and spray flame structures in Diesel engines
Fuel consumption and NOx emission in Diesel engines
HCCI engines
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TC in piston engines
Spark ignition engines
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TC in piston engines
Combustion stages
• Ignition stage: spark ignition needs to last a short time to initiate
the flame propagation. The flame kernel initially is very small. The
small kernel is not highly wrinkled by turbulence. The propagation
speed of the small flame kernel is low.
• Turbulent flame propagation stage: As the flame kernel grows the
flame surface area becomes also more wrinkled. The flame
propagation speed is much higher than the laminar flame speed.
The higher the turbulence level the faster the turbulent flame
speed. This makes the combustion duration in terms of crank
angle degrees roughly the same at different engine speed.
• Burning in the post flame zones: If the engine runs at very high
speed, the combustion intermediates such as CO may not fully
burned at the flame front. Since the engine is at very high
pressure and temperature, these intermediate can be further
oxidized later before the exhaust gas is released.
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TC in piston engines
Combustion duration in SI engine
Turbulent flame propagation
Initial flame
development
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TC in piston engines
Emissions
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TC in piston engines
Compression ratio in SI engines
• Engine knock at high compression ratio
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TC in piston engines
Compression ratio in SI engines
• Engine knock at high compression ratio
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TC in piston engines
Octane number, cetane number
• Octane number indicates the tendency of fuels to knock. The
higher the octane number the more difficult the auto-ignition.
– n-Heptane (C7H16) has a octane number 0,
– iso-octane (C8H18) has a octane number 100.
– Gasoline has a octane number 93 – 97.
• cetane number denotes the ignition delay time (the start of the
injection of diesel fuel to the onset of the auto-ignition). The
cetane number ranks the fuels; the higher the cetane number the
faster the auto-ignition.
– Isooctane has a cetane of 15
– diesel has a cetane number about 37 – 56.
– Cetane (C16H34) has a cetane number 100.
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TC in piston engines
Compression ignition engines
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TC in piston engines
Compression ignited engine (”Diesel combustion”)
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Combustion during injection
Diffusion flame
Yellow flame (=soot)
Local lambda ~1
Overall lambda ~1,5 (no three-way cat)
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•Diffusion flame
Mixing controlled
TC in piston engines
Compression ignited engine (”Diesel combustion”)
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Combustion during injection
Diffusion flame
Yellow flame (=soot)
Local lambda ~1
Overall lambda ~1,5 (no three-way cat)
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•
High efficiency, ~45%, high also at part
load
– Load control through fuel amount
– No throttling (smoke…)
– Compression ignition => high
compression ratio
TC in piston engines
Hydrogen/air auto-ignition
Initial temperature
Ignition delay time
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TC in piston engines
Hydrogen/air auto-ignition
Stoichiometry H2/O2 auto-ignition
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TC in piston engines
n-heptane-air auto-ignition
Heptyl-radicals (R)
C7H15
n-heptane
C7H16
Low
temperature
chemistry
RO2
KET
e.g. C7H16 + O2 = C7H15 + HO2
C7H16 + HO2 = C7H15 + H2O
High
temperature
chemistry
C7H15 + O2 = C7H15O2
First ignition
e.g. HO2 + O2 = H2O2 + O2
H2O2 = OH + OH
C7H14O3 = C7H13O2 + OH
Second
ignition
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TC in piston engines
n-heptane-air auto-igntion
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TC in piston engines
Octane number, cetane number
• Octane number indicates the tendency of fuels to knock. The
higher the octane number the more difficult the auto-ignition.
– n-Heptane (C7H16) has a octane number 0,
– iso-octane (C8H18) has a octane number 100.
– Gasoline has a octane number 93 – 97.
• cetane number denotes the ignition delay time (the start of the
injection of diesel fuel to the onset of the auto-ignition). The
cetane number ranks the fuels; the higher the cetane number the
faster the auto-ignition.
– Isooctane has a cetane of 15
– diesel has a cetane number about 37 – 56.
– Cetane (C16H34) has a cetane number 100.
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TC in piston engines
The main component of gasoline and diesel
Primary reference fuel
PRFx = x%n-heptane +
(100-x)%iso-octane
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TC in piston engines
Oil refinery
Diesel
C10 – C15
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TC in piston engines
Compression ignited engine (”Diesel combustion”)
Conceptual model of diffusion part of Diesel flame (from
Dec):
Flame lift-off
Diffusion flame
stabilization
Air
entrainment
Spray
Flame
Diffusion flame in Scania 14-liters engine.
Foto: Anders Larsson, Scania
[From Johan Dec]
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TC in piston engines
Engine head and fuel injector
From Joseph C Oefelein, Sandia National Lab
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TC in piston engines
Fuel injectors
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TC in piston engines
Fuel injection
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TC in piston engines
Fuel injection and flames
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TC in piston engines
Jet model
air
air
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TC in piston engines
Flame liftoff structures
Liftoff height
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TC in piston engines
Liftoff height
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TC in piston engines
Liftoff height
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TC in piston engines
Liftoff height
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TC in piston engines
Flame length
Conceptual model of diffusion part of Diesel flame (from
Dec):
Flame lift-off
Characteristic behaviour of stationary flame when
increasing fuel flow (from Glassman):
Diffusion flame
stabilization
Air
entrainment
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•
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Flame lift-off caused by intense turbulence in the early stage of the flame,
chemistry is decreased
At high enough turbulence intensity chemistry is quenched. blow-off.
TC in piston engines
Combustion in diesel engines
p
C
D
B
Normal
engine run
A
Mortored
run
TDC
A: start of injection of fuel (SOI); B: start of combustion (SOC);
C: end of fuel injection (EOI); D: end of combustion (EOC);
AB: ignition delay; AC: duration of fuel injection;
BD: duration of combustion
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TC in piston engines
θ
NTC and cool flame, n-heptane flame
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TC in piston engines
Emissions
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TC in piston engines
Emissions legislations
Level
ECE R49
ECE R49-20%
EuroI
EuroII
EuroIII
EuroIV
EuroV
USA EPA07
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From year
1982
1990
1993
1996
2001
2005
2008
2010
NOx
18
14,4
8
7
5
3.5
2.0
0,27
(g/kWh)
(NOx+HC)
PM
0.25
0.15
0.10
0.02
0.02
0,013
TC in piston engines
(g/kWh)
Fuel consumption & CO2
Challenge for the diesel engine
NOx
Nitrous oxides
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= NO + NO2
TC in piston engines
Injection timing
12
5.2
10
660
640
620
5.1
5
4.9
4.8
8
600
580
6
560
4
540
2
4.7
4.6
520
500
0
40
45
50
55
60
-5
-4
-3
-2
T inlet (after cooler)
-1
0
1
2
3
Injection timing
Exhaust Gas Recirculation
8
NOx (g/kWh)
7
6
5
4
3
Require appropriate EGR-cooling
2
1
0
0
5
10
15
20
25
30
EGR rate %
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TC in piston engines
4
5
CO2 [g/kWh]
5.3
NOx (g/kWh)
NOx (g/kWh)
Intake air temperature
Ways to decrease emissions and fuel consumption
Fuel consumption
Base engine
Spray angle
Intercooling
Increased
inj.pressure
New engine
concepts
After treatment
4 valves
EGR
fuel
Turbo compound
NOx-emissions
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TC in piston engines
Homogeneous charge compression
ignition (HCCI) engines
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TC in piston engines
Homogeneous Charge Compression Ignition
(HCCI)
+
+
Premixed
“homogeneous combustion”
“Zero” NOx and soot
High efficiency ~45%
also at partload
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- Controll
- Charging/EGR
- MIT
- Noise
- Cold start
TC in piston engines
Experimental setup
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TC in piston engines
Local structure and burning rate
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CAD 3
CAD 3.5
CAD 4
CAD 4.5
CAD 5
TC in piston engines
Area burning rate
CAD 2
CAD 2.5
CAD 4
CAD 4.5
Expansion speed
HCCI: 70-90 m/s
SI: 5-15 m/s
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CAD 3
CAD 5
Global burning rate
HCCI: 3.5 m2/s
SI: 0.65 m2/s
TC in piston engines
CAD 3.5
CAD 5.5
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TC in piston engines
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TC in piston engines
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TC in piston engines
Operating characteristics -Timing/Emissions
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TC in piston engines