Transcript Document

Internal Combustion Engine Group
The effect of compression ratio on exhaust
emissions from a PCCI Diesel engine
ECOS 2006
12-14 July 2006
Laguitton, Crua, Cowell, Heikal, Gold
Content
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Introduction
Experimental set-up
Validation of single cylinder design
Strategy for low NOx, soot and FC
Conclusions
Highly pre-mixed and cool combustion
REDUCE OXYGEN
CONCENTRATION
+
INCREASED EGR
RATES AND
TEMPERATURE
MANAGEMENT
IMPROVED AIR
SYSTEM EFFICIENCY
IMPROVED AIR/FUEL
MIXING
INCREASE
EFFICIENCY
ADVANCED AIR/EGR
SYSTEMS
REDUCED
COMPRESSION
RATIO COMBUSTION
SYSTEM DESIGN
COLD START
TECHNOLOGY
INCREASED IGNITION
DELAY
ADVANCED FIE
TECHNOLOGY
ROBUSTNESS
CONTROL
ADVANCED
COMBUSTION & AIR
PATH CONTROL
Oxygen concentration
Euro 4 – O2 Concentration Map
Approach is to reduce the oxygen concentration
characteristics over the engine speed and load
operating area:
– Oxygen concentration in the flame is reduced
– Less NOx are formed
10
Level 3 – O2 Concentration Map
Local Equivalence Ratio
9
Soot formation area
8
7
6
5
4
3
2
1
1000
1400
Combustion trend to
more pre-mixed and
lower temperature
1800
2200
2600
3000
Temperature /(K)
NOx formation area
Source: MTZ 11/2002: Toyota
Low NOx strategy
Level of premixed fuel
66 %
50 %
Euro 4
8.00
SOC - real SOI (°CA)
Drive current
Rate of injection
Injection pulse
Start of combustion
100 %
10.00
Improved air/fuel mixing to achieve low
soot and good combustion efficiency
Level 2
6.00
Level 3
4.00
2.00
Increasing Load
0.00
0.00
5.00
10.00
15.00
20.00
25.00
30.00
Injection period (°CA)
Time
SOC – Real SOI
Injection
period
Trend is clear:
- Injection durations reduced by increased
injection pressure and nozzle flow
- Ignition delay increased by changes to
air/fuel, CR, intake temperature and EGR
Single cylinder engine facility
Single cylinder – Ricardo HYDRA:
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500cc swept volume (86mmx86mm)
2.0L high-flow head
Variable swirl (1.0-3.5 Rs)
Compression Ratio 18.4:1 and 16.0:1
Off-engine HP pump + common rail
Delphi injector
Delphi nozzle library
EmTronix FIE controller
Reference ultra low sulphur diesel fuel
Test bed:
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Horiba gas analyser MEXA 7100DEGR
AVL733 dynamic fuel meter
AVL415 variable sampling smoke meter
High speed data logger
Custom-built low speed data logger
TDM post processing
Piston-bowl cross-sections
Validation of single cylinder design
Full Load Results
Net IMEP [bar]
25
20
3.5
Single
Multi
Smoke [FSN]
15
3.0
10
5
0
1500 rpm 2000 rpm 2000 rpm 1000 rpm 2000 rpm 4000 rpm
2 bar
6 bar
16 bar
Full Load Full Load Full Load
Single
2.5
2.0
Multi
1.5
1.0
0.5
35
30
1000
1500
AFR
25
0.0
Single
Multi
20
2000
2500
3000
Engine Speed [rev/min]
3500
4000
15
Part Load Results
10
3.0
5
0
2.5
EGR Rate [%]
50
40
Single
Multi
30
20
Smoke [FSN]
1500 rpm 2000 rpm 2000 rpm 1000 rpm 2000 rpm 4000 rpm
2 bar
6 bar
16 bar
Full Load Full Load Full Load
2000 rpm,
6 bar
2.0
2000 rpm,
10 bar
2000 rpm,
16 bar
1.5
1.0
1500 rpm,
2 bar
0.5
Single
Multi
10
0.0
0
1500 rpm 2000 rpm 2000 rpm 1000 rpm 2000 rpm 4000 rpm
2 bar
6 bar
16 bar
Full Load Full Load Full Load
0
5
10
15
NOx [g/h]
20
25
30
Effect of compression ratio on NOx emissions
12.00
Fixed calibration
10.00
80.0
8.00
60.0
4.00
Pressure (bar)
6.00
2000 rev/min
7.7 bar GIMEP
20.0
2.00
0.00
-10.0
40.0
1500 rev/min
3.0 bar GIMEP
-5.0
0.0
5.0
10.0
Main injection timing (deg CA ATDC)
0.0
0.0008
-60.0
-20.0
20.0
60.0
100.0
60.0
100.0
Crankangle (deg CA)
Instantaneous heat release
NOx mass flow (g/h)
2000 rev/min 7.7 bar GIMEP
LEVEL 2: CR 18.4 and CR 16.0:1
2000 rev/min
10.8 bar GIMEP
0.0006
0.0004
Reduced CR decreases NOx emissions
especially at high loads. At low loads
(1500 rev/min 3.0 bar GIMEP), slight
improvements but combustion is already
fully premixed, hence reduced benefits
0.0002
0.0000
-0.0002
-60.0
-20.0
20.0
Crankangle (deg CA)
Effect of compression ratio on auto-ignition delay
120.0
90.0
70.0
2000 rev/min
7.7 bar GIMEP
60.0
50.0
1500 rev/min
3.0 bar GIMEP
40.0
30.0
-10.0
Maximum dP/dT (bar/ms)
100.0
2000 rev/min
10.8 bar GIMEP
80.0
60.0
40.0
20.0
0.0
16.0-10.0
-5.0
0.0
5.0
10.0
Main injection timing (deg CA ATDC)
-5.0
0.0
5.0
10.0
Main injection timing (deg CA ATDC)
Reduced CR decreases in-cylinder
pressures. Combustions occur later,
increasing the level of premixed leading
to higher maximum pressure variations
but lower NOx
Angle of SOC (deg CA ATDC)
Pressure at SOC (bar)
80.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
-10.0
-5.0
0.0
5.0
Main injection timing (deg CA ATDC)
10.0
Late injection strategy for low NOx and soot
Summary of single injection timing responses at 1500 rev/min 6.6 bar GIMEP
(43% EGR rate, 1000 bar rail pressure)
17.0:1 AFR
19.0:1 AFR
High FC penalty with very
retarded single injections
DOE modelling
DOE model: NOx (g/h)
 NOx reduced by high EGR and low AFR
 Low soot and good fuel consumption is achieved
through improved air/fuel mixing
- Low CR, swirl and rail pressure enhancement is
critical
 Good fuel consumption is achieved by optimising
50% burn after TDC. Late combustion is avoided by
shortening combustion duration
DOE Model: Soot (g/h)
Test data for FC (kg/h)
Combustion phasing for optimum fuel consumption
Test data for FC (kg/h)
 Good combustion efficiency:
- Rapid combustion
- Centred between 0 and 10 °CA ATDC
 This is a conflicting requirement with
low NOx combustion strategies, which
require slow and late combustion
Instantaneous heat release
100.0
Pressure (bar)
80.0
60.0
40.0
20.0
0.0
-60.0
-30.0
0.0
Crankangle (°CA ATDC)
30.0
60.0
 A compromise to minimise impact on
combustion efficiency is to operate:
- Slow combustion
- Well phased combustion
Conclusions
 A good comparison with multi cylinder baseline results was achieved
 Ultra low NOx has been achieved through highly pre-mixed and cool combustion
 At 1500 rev/min, 3.0 bar GIMEP - a twin early injection strategy achieved improved HC
and CO results compared to a pilot + “late” main strategy
 At 1500 rev/min, 6.6 bar GIMEP - testing showed that a late injection strategy was
essential for low NOx. A single late injection with high EGR achieved the best overall
results
 With 16:1 CR, an early injection strategy was only beneficial below 3.0 bar GIMEP.
Late, high pressure injection combined with EGR is recommended
 With the combustion bowl geometry tested, 10 and 12 hole nozzles did not offer an
advantage at rated power. Reduced spray penetration, bowl interaction and air
utilisation was detrimental at the higher loads and speed