The effect of EGR on HCCI combustion – impact of diluting, thermal and chemical aspects:
experimental and numerical approaches
H. Machrafi, P. Guibert, S. Cavadias
Institut Jean Le Rond d’Alembert – CNRS UMR 7190 Université Pierre et Marie Curie (Paris 6)
2, place de la Gare de Ceinture 78210 Saint-Cyr-l’Ecole, France
machrafi@ccr.jussieu.fr
Objectives :
Objectives :
Investigate the influence of EGR on the HCCI combustion: hereby the impact of several aspects of EGR on the auto-ignition process is examined experimentally:
•Impact of dilution by N2and CO2
•Impact of the EGR temperature
•Impact of chemical actives species such as CO, NO, CH2O and CH3CHO
A previously reduced and validated PRF mechanism is used for further interpretation
The HCCI (Homogeneous Charge Combustion Ignition) mode presents much advantages: Reduction of emissions of NOx,, particulate matters
Increase of combustion efficiency Reduction of CO2emissions
However, when conditions are not optimal, the emissions of hydrocarbons and CO could be too elevated PRINCIPAL PROBLEM: CONTROLLING THE AUTO-IGNITION PROCESS
PROMISING SOLUTION: CONTROLLING BY EXHAUST GAS RECIRCULATION
Auto
Auto--ignition in internal combustion engines by HCCI mode:
ignition in internal combustion engines by HCCI mode:
Experimental set
Experimental set--up
up
P
Air inlet
Fuel injection
Control of Tinlet, Pinlet, φ
T °C regulation H ea t e x ch a n g er Flow meter P E x h a u st c o n d en so r
Inlet mixture tank Pressure stabiliser vessel N2, CO2 CO, CH2O, CH3CHO, NO EGR mixture tank
Flow meter Flow
meter
Heat exchanger AUTO-IGNITION PROCESS is investigated by AUTO-IGNITION DELAYS
0.0 E+00 5.0 E+05 1.0 E+06 1.5 E+06 2.0 E+06 2.5 E+06 3.0 E+06 3.5 E+06 4.0 E+06 4.5 E+06 5.0 E+06 14 0 1 501 60170180 190200210 22 0 C AD C ylin de r p re s sure [P a] Pressure θ γ θ γ γ θ θ d dP V d dV P d dQ d dQrelease wall 1 1 1 + − − + = 0 10 20 30 40 50 60 70 140 150 160 170 180190 200 210 220 CAD H ea t re le as e [ J/C AD ] Heat release
Cool flame Final ignition
•Generally, the auto-ignition process occurs in two stages, interceded by an NTC •The auto-ignition delays are a summary of the whole process
•Studying the influence of the EGR on the auto-ignition delays allows an overall study of the influence of EGR on the auto-ignition process 180 185 190 Ig n it io n d ela y s [ C A D ]
n-heptane - cool flame delay n-heptane - final ignition delay PRF40 - cool flame delay PRF40 - final ignition delay
169 171 173 175 Ig n it io n d e la ys [C A D ]
Cool flame delay Final ignition delay
174 176 178 180 Ig n it io n d e la y s [ C A D
] •Dilution makes the ignition delays increase, meaning
a decrease in the overall reactivity – CO has a
CFR engine Engine speed: 600 rpm Displacement: 611 cm3 Compression ratio: 10 Inlet temperature: 70 °C
Conclusions :
Conclusions :
Influence of simulated EGR on auto-ignition delays in an internal combusiton engine in HCCI mode has been studied:
Dilution: reduces the overall reactivity and increases the ignition delays, CO2having a stronger effect
due to its higher heat capacity. A fuel having a lower burn rate is more sensitive to dilution EGR temperature: the effect is clear, increasing the overall kinetics and decreasing the ignition delays Chemically active species:
CO: no significant effect has been observed in the investigated range
NO: has the ability to decrease and increase the ignition delay following its reactivity. The effect is more clear for the fuel having a lower burn rate, PRF40.
CH2O and CH3CHO : decreases the overall reactivity by sharing OH radicals with the fuel,
increasing thereby the ignition delays
A reduced PRF mechanism has been validated experimentally for some EGR parameters The validation is quite satisfactory with respect to the ignition delays
The PRF mechanism can be used to interpret the behaviour of the auto-ignition process influenced by EGR parameters. 155 160 165 170 175 0 5 10 15 20 25 30 35 40 45 50 N2 ra tio [vol%] Ig n it io n d ela y s [ C A D ] 155 157 159 161 163 165 167 169 25 35 45 55 65 75 85 95 105 115 EGR temperature [°C] Ig n it io n d e la ys [C A D ] 160 162 164 166 168 170 172 174 0 50 100 150 200 CO addition [ppmv] Ig n it io n d e la y s [ C A D ]
Cool flame delay Final ignition delay
155 160 165 170 175 180 185 0 200 400 600 800 1000 1200 1400 1600
Addition of formalde hyde and ace talde hyde [ppmv ]
Ig n it io n d e la y s [C A D ]
CH2O - cool flame delay CH2O - final ignition delay CH3CHO - cool flame delay CH3CHO - final ignition delay
155 160 165 170 175 180 185 190 195 0 20 40 60 80 100 120 140 160 180 Addition of NO [ppmv] Ig n itio n d ela ys [ C A D ]
n-heptane - cool flame delay n-heptane - final ignition delay PRF40 - cool flame delay PRF40 - final ignition delay
156 158 160 162 164 166 168 170 172 174 176 0 200 400 600 800 1000 1200 1400 1600 CH2O addition [ppmv] Ig nit io n d e la y s [ C A D ]
Experiment equivalence ratio = 0,32 Experiment equivalence ratio = 0,41 Mechanism equivalence ratio = 0,32 Mechanism equivalence ratio = 0,41 Final ignition Cool flame 155 160 165 170 175 180 185 190 0 10 20 30 40 50
Percentage N2 in inlet mixture [vol%]
Ig n it io n d ela ys [C A D ]
Mechanism equivalence ratio = 0,32 Experiment equivalence ratio = 0,32 Mechanism equivalence ratio = 0,41 Experiment equivalence ratio = 0,41
Cool flame Final ignition
φ = 0,32 with n-heptane and PRF40 as the fuels φ = 0,32 with PRF40 as the fuel
φ = 0,32 with n-heptane and PRF40 as the fuels
155 160 165 170 175 180 185 190 0 5 10 15 20 25 30 35 40 45 50 N2 ratio [vol%] Ig n it io n d ela y s [ C A D ]
n-heptane - cool flame delay n-heptane - final ignition delay PRF40 - cool flame delay PRF40 - final ignition delay
CO2
φ = 0,41 with PRF40 as the fuel and 23 vol% EGR
φ = 0,32 with PRF40 as the fuel φ = 0,32 with n-heptane and PRF40 as the fuels
Comparison model/experiments:
φ = 0,32/0,41 with PRF40 as the fuel in 23 vol% EGR
Comparison model/experiments: φ = 0,32/0,41 with PRF40 as the fuel
Mapping: ignition delay as function of φ and CH2O addition 23 vol% EGR, Tinlet= 70°C, ε = 10,2 and PRF40 as the fuel
Perspectives :
Perspectives :
Numerical interpretation of the auto-ignition phenomena, using the PRF mechanism, incorporating more EGR chemical species
Wider experimental validation Proposition for the control of the auto-ignition process, using EGR
a decrease in the overall reactivity – CO2has a
greater effect than N2
•The EGR temperature increases the overall kinetics •No significant influence for CO
•The species NO has two effects:
•NO + HO2= OH + NO2 increasing reactivity •NO + OH + M = HONO + M decreasing reactivity •Formaldehyde seems to increase the ignition delays apparently by:
•CH2O + OH => H2O + HO2+ CO
•Acetaldehyde seems to have a similar influence. However, the effect is not clear and should be investigated furthermore
•The fuel PRF40 is more sensitive to changes in the EGR composition than n-heptane