Effect of Reformer Gas on HCCI CombustionPart II: Low Octane Fuels Vahid Hosseini, and M David Checkel Mechanical Engineering University of Alberta, Edmonton, Canada
project supported by Auto21 National Center of Excellence SAE World Congress 2007 Detroit, MI, USA April, 16-19 (Monday April 16- Room D2-15)
2007-01-0206
Outline
Introduction Low octane HCCI combustion, Research target and methodology, Reformer gas Experimental setup Definitions Experimental Results Operating region, pressure trace characteristics, heat release analysis Modeling Heat release, temperature, intermediate species Conclusions
2007-01-0206
Outline
Introduction Low octane HCCI combustion, Research target and methodology, Reformer gas Experimental setup Definitions Experimental Results Operating region, pressure trace characteristics, heat release analysis Modeling Heat release, temperature, intermediate species Conclusions
2007-01-0206
Introduction
(1)
HCCI combustion engine fueled with n-Heptane
Low-octane HCCI is easier to achieve and more practical than high octane fuel Double-stage ignition process happens in the engine time scale Varying EGR mainly controls ignition timing, while varying λ controls combustion duration*
* Peng et al. SAE paper 2003-01-0747
2007-01-0206
Introduction
(2)
Research target, methodology
Control combustion timing of n-heptane fueled HCCI engine independent of EGR and λ. Use blended dual-fuel concept, taking advantage of double-stage combustion and reaction inhibition Experimental work on a research engine, combustion diagnosis, confirmation of results with a chemical kinetic model. Use the model to examine chemistry alteration. 2007-01-0206
Introduction
(3)
Reformer Gas (RG)
RG is a light gas dominated by hydrogen and carbon monoxide It can be produced on-board by re-forming conventional fuels Current progress on fuel reformers is mainly aimed at fuel cell applications RG can be produced by partial oxidation, steam reforming or autothermal reforming Study used 75% H2 + 25% CO as RG 2007-01-0206
Outline
Introduction Low octane HCCI combustion, Research target and methodology, Reformer gas Experimental setup Definitions Experimental Results Operating region, pressure trace characteristics, heat release analysis Modeling Heat release, temperature, intermediate species Conclusions
2007-01-0206
Experimental Setup
(4)
Engine and Instrumentation T
EGR
RG
Heater
P
Throttle
P
Fuel
Feedback to heater
Constant Tmixture
T
T
CFR Engine
Air Air Mass Flow Meter
2007-01-0206
Experimental Setup
(5)
Operating Condition
The engine was not motored Steady state constant speed of 700 RPM Wide open throttle, natural aspiration Simulated RG mixture: H2 (75%) and CO (25%) Other inert gases in RG compensated by EGR Electrical intake heater controlled to provide constant intake temperature AFTER fuel injection and EGR.
2007-01-0206
Outline
Introduction Low octane HCCI combustion, Research target and methodology, Reformer gas Experimental setup Definitions Experimental Results Operating region, pressure trace characteristics, heat release analysis Modeling Heat release, temperature, intermediate species Conclusions
2007-01-0206
Definitions
(6)
Engine operating parameters
λ was calculated based on the both fuel and reformer gas Tintake is the temperature of the mixture entering the cylinder (air, fuel, RG, EGR) & RG m RG mass fraction (%) = 100 × & RG + m & fuel m
2007-01-0206
Definitions
(7)
Example of RG replacement effect
N=700 RPM, EGR=0%, WOT, Heptane, λ=2.0 Heptane flow rate reduction 0% 23%
RG mass fraction 0% 30%
Air flow reduction 0% 3% Energy flow reduction 0% 1%
2007-01-0206
Definitions
(8)
Combustion Diagnosis Parameters Gross cumulative heat release HTRmax
Rate of heat release
90% GHRmax
D
Combustion Duration (CD)
RH R
=0 J
/C A
NTC
GHRmax
LTRmax
-35
-25 -15 CAD, aTDC LT R tim
in g
NTC
10% GHRmax (SOC)
-5 HT R
tim
in g
LTR: Low Temperature Reaction HTR: High Temperature Reaction NTC: Negative Temperature Coefficient SOC: Start of Combustion CD: Combustion Duration GHR: Gross Heat Release
-40
-30
-20 CAD, aTDC
-10
0
2007-01-0206
Outline
Introduction Low octane HCCI combustion, Research target and methodology, Reformer gas Experimental setup Definitions Experimental Results Operating region, pressure trace characteristics, heat release analysis Modeling Heat release, temperature, intermediate species Conclusions
2007-01-0206
Experimental Results
(9)
RG effect on operating region boundaries RG addition at any fixed EGR rate pushed back the knock boundary toward richer mixture
1.6
n-Heptane base fueled CR=9.5 Intake Temperature =100 oC N=700 RPM
no RG added operating region
RG
2
λ
in c rea se
2.4
RG-enriched operating region
1.2
0
10
20 30 EGR (%)
40
50
2007-01-0206
Experimental Results
(10)
RG effect on pressure characteristics 48
Increasing RG fraction decreased maximum pressure and maximum rate of pressure rise
n-Heptane fueled CR=11.5 λ=1.24 ±0.02 EGR=40%
Pmax 44 6 (dP/dθ)max 40 4 36
2
cyl. max. pressure (bar)
cyl. max. pressure rate (bar/CAD)
8
32
0
5 10 15 RG mass fraction (%)
20
2007-01-0206
Experimental Results
(11)
*NTC region is period between first-stage and second-stage (main) ignition.
4 h ea
t re
leas
e ra
tio
2
24
TC
22
20
18 0
d
3
1 st/2 n
1
CR=11.5 λ=2.03 ±0.02 EGR=10%
26
0
Increasing RG prolonged NTC* and reduced 1st to 2nd rate of heat release ratio
N
negative temp. coef., NTC (CAD)
28
5 10 15 RG mass fraction (%)
20
LTRmax to HTRmax rate of heat release ratio (%)
5
RG effect on heat release characteristics
25
2007-01-0206
Experimental Results
(12)
RG effect on combustion characteristics
5 1 2 3 4 combustion duration, CD (CAD)
CR=11.5 λ=2.03 ±0.02 EGR=10%
C
-4
-8
-12
0
Increasing RG fraction retarded the main combustion and had no significant impact on combustion duration
CD
SO
start of combustion, SOC (CAD, aTDC)
0
0
5 10 15 RG mass fraction (%)
20
25
2007-01-0206
Experimental Results
(13)
RG effect on rate of heat release
Increasing RG fraction retarded the main combustion and had no significant impact on combustion duration
CR=11.5 λ=1.24 ±0.02 EGR=40%
120
80
40
rate of heat release (J/CAD)
RG=0% RG=7% RG=10% RG=12% RG=20%
0
-40
-20
0
20
CAD, aTDC
2007-01-0206
Experimental Results
(14)
RG effect on efficiencies
CR=11.5 λ=2.03 ±0.02 EGR=10%
100 e al m er h t
cy n e ci i f f
25
98 20 combustion efficiency
96
thermal efficiency (%)
Increasing RG fraction slightly increased thermal efficiency and decreased combustion efficiency (This study does not take RG reforming efficiency and intake heater power into account )
combustion efficiency (%)
15 0
5 10 15 RG mass fraction (%)
20
2007-01-0206
Outline
Introduction Low octane HCCI combustion, Research target and methodology, Reformer gas Experimental setup Definitions Experimental Results Operating region, pressure trace characteristics, heat release analysis Modeling Heat release, temperature, intermediate species Conclusions
2007-01-0206
Modeling
(15)
Chemical kinetic model * ,**
Single Zone Semi-reduced mechanism of 290 reactions and 57 species for n-heptane *** SOC, LTR, and NTC timing is predicted precisely *Kongsereeparp et al, 2005, Fall Technical Conference of the ASME ICE Division, Ottawa, Canada
** Kongsereeparp et al ,SAE 2007-01-0205
25
20
Pressure (bar)
15 Experiment 10 ChemComb-SZM
5
0 rate of heat release -5 -60
***Mechanisms by Dr. V Golovichev,
Chalmers University of Technology, http://www.tfd.chalmers.se/~valeri/MECH/chem.inp_c7h16
-50
-40 -30 CAD, aTDC
-20
-10
2007-01-0206
Modeling
(16)
RG effect on rate of heat release RG = 0 % RG = 5 % RG = 10 % RG = 17 % RG = 23 %
With increasing RG replacement fraction, the model showed very similar effects to the engine experiments.
CR=11.5 Intake temp =373 K N=700 RPM
λ=2.02
rate of heat release, RHR (J/CAD)
250
200
150
100 RG mass fraction increase
50
0
-50 -35
-30
-25 -20 CAD,aTDC
-15
-10
2007-01-0206
Modeling
(17)
RG effect on end-of-compression and combustion temperatures 860
CR=11.5 Intake temp =373 K N=700 RPM λ=2.02
840 Temperature (K)
Increasing RG fraction raises end-of-compression temperature but lowers combustion temperature
n ctio fra ss e ma eas RG incr
820
800 RG = 0 % RG = 5 % RG = 10 % RG = 17 % RG = 23 %
780
760 -29
-28
-27 CAD, aTDC
-26
-25
2007-01-0206
Modeling
(18)
HCCI combustion of nheptane without RG
RHR (J/CAD)
O H2
C
OH appears at the main ignition stage and is present in combustion products. H2O2 and CH2O appear at the first-stage ignition and disappear at the main ignition stage.
O2 H2
RHR
OH, H2O2, and CH2O mol fraction (mol/mol)
Important intermediate species
OH
-35
-30
-25 -20 -15 CAD, aTDC
-10
-5
2007-01-0206
Modeling
0.004
(19) 0.002
H2O2 production rate
RG = 0 % RG = 5 % RG = 10 % RG = 17 % RG = 23 %
Increasing RG fraction -reduces H2O2 production rate at the 1st stage, -delays but amplifies H2O2 production at the 2nd stage of combustion
-27 d[H2O2]/dt (mole/cc-S)
0
0.08
0.06
CR=11.5 Intake temp =373 K N=700 RPM λ=2.02
-25
LTR 0.04
HTR
0.02
-26 CAD, aTDC
RG
ma
a cti r f ss
on
ase e r in c
0 -15
-14
-13
-12
CAD, aTDC
2007-01-0206
RG mass increase
0.08
re pr act od io uc n 1 tio 22 n ra t
e
0
RG = 23 % RG = 17 % RG = 10 % RG = 5 % RG = 0 % 800
ate r l o ta
1200
re de ac st tio ru n ct 12 io 3 n ra te
-0.08
t
-0.16
CR=11.5 Intake temp =373 K N=700 RPM λ=2.02
RHR
RG mass increase
d[H2O2]/dt (mole/cc-S)
H2O2 production / destruction by key reactions
(20)
0.16
Modeling
1600
2000
T (K)
#122: #123:
HO2+ HO2Ù H2O2 + O2 OH + OH (+M) Ù H2O2 (+M)
2007-01-0206
0.002
c in as re
0.001
e
RG = 0 % RG = 5 % RG = 10 % RG = 17 % RG = 23 %
0
H2O2 mole fraction (molH2O2/moltotal)
n
tio ac fr
s
CR=11.5 Intake temp =373 K N=700 RPM λ=2.02
as m
Overall, total molar concentration of H2O2 is reduced, particularly during the period between 1st and 2nd stage ignition
G
(21)
R
H2O2 mole fraction
0.003
Modeling
-28
-24
-20 CAD, aTDC
-16
-12
2007-01-0206
Outline
Introduction Low octane HCCI combustion, Research target and methodology, Reformer gas Experimental setup Definitions Experimental Results Operating region, pressure trace characteristics, heat release analysis Modeling Heat release, temperature, intermediate species Conclusions
2007-01-0206
Conclusion
(22)
Experimental study showed that RG can effectively control ignition timing independent of λ and EGR, (ie. timing control at a essentially constant load). Increasing RG mass fraction:
Did not change LTR timing (1st stage ignition). Reduced LTRmax (magnitude of 1st stage ignition) Prolonged NTC (period between 1st stage & main ignition) Delayed HTR (time of main stage ignition)
Effect of RG on engine operating parameters was:
Expanded operating range toward richer mixture, pushing back knocking limit Higher thermal efficiency and lower combustion efficiency at any constant λ and EGR.
2007-01-0206
Conclusion
(23)
A Single-zone heat release model predicted SOC precisely and showed the same effect of RG on combustion timing as experiments.
Increasing RG fraction raised end-of-compression temperature and decreased post-combustion temperature.
Increasing RG fraction accelerated both production and destruction rates of critical intermediate species but total molar concentration was lower, particularly in the period between 1st stage and main ignition.
2007-01-0206
Thank you for your attention QUESTIONS? Vahid Hosseini
[email protected]
The contribution of the Auto21 National Center of Excellence to supporting this work is gratefully acknowledged. 2007-01-0206
