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Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition
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Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition Changming Gong a, Dong Li b, Zhaohui Li b,c, Fenghua Liu a,* a
College of Electromechanical Engineering, Dalian Nationalities University, Dalian 116600, China State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130022, China c School of Vehicle Engineering, Chongqing University of Technology, Chongqing 400054, China b
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abstract
Article history:
Combustion, carbon monoxide (CO), Nitrous oxide (NO), formaldehyde (HCHO) and un-
Received 8 July 2015
burned methanol (CH3OH) emissions characteristics in a direct injection spark ignition
Accepted 10 November 2015
(DISI) methanol engine with hydrogen addition ratio from 5% to 15% have been investi-
Available online xxx
gated numerically based on the detailed methanol oxidation reaction mechanism combined with the nitrogen oxides (NOX) reaction mechanism using the PREMIX code of
Keywords:
CHEMKIN program with AVL FIRE software. Maximum cylinder pressure, maximum heat
Methanol engine
release rate and highest cylinder temperature increase and their corresponding crank
Hydrogen addition
angles advance with hydrogen addition from 0 to 15%. CO emission decreases and NO
Combustion
emission increases with increasing hydrogen addition ratio, whereas, formaldehyde and
Emission
the unburned methanol significantly reduce with increasing hydrogen addition. Thus,
Numerical simulation
hydrogen addition can provide a new way to address formaldehyde and unburned methanol unregulated emissions in a DISI methanol engine. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Oil resources are being rapidly consumed and environmental problems are becoming more serious. Consequently, many countries are stepping up the pace to seek clean alternative fuels. Methanol (CH3OH) is considered to be a promising alternative with several advantages over traditional fuels (e.g. gasoline or diesel) such as a large number of sources and low regulated emissions [1e3]. However, methanol engines have issues with high unregulated emissions and difficulties in cold start [4,5]. Gong et al. [6e8] investigated the effects of ambient temperature on firing behavior and unregulated emissions of spark-ignition (SI) methanol and liquefied petroleum gas
(LPG)/methanol engines during cold start. They found that with ambient temperatures near and below 289 K, the methanol engine cannot be started reliably without assisting measures. Furthermore, formaldehyde (HCHO) emissions increase significantly and unburned methanol decreases with rising ambient temperature. Hydrogen is another promising alternative fuel which has excellent combustion characteristics, high calorific value and environmental benefits, and may provide a mechanism to improve combustion and emissions performance of methanol engines [9,10]. Ji et al. [11e13] investigated the cold start characteristics of hydrogen-enriched methanol engines and showed that both flame development and propagation periods were reduced with hydrogen addition. Hydrocarbon (HC)
* Corresponding author. Tel.: þ86 411 87188945; fax: þ86 411 87656133. E-mail address: [email protected] (F. Liu). http://dx.doi.org/10.1016/j.ijhydene.2015.11.062 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Gong C, et al., Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.062
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and carbon monoxide (CO) emissions during cold start were also reduced, as well as the particulate matter (PM). Huang et al. [14e19] investigated in detail the effects of natural gasehydrogen blends on combustion, performance and emissions of a direct injection engine. They found that addition of hydrogen increased the burning velocity of the natural gas due to the increased H, O and OH radicals in the flame, and the flame stability at early combustion stage was enhanced after the hydrogen addition. They also showed improved engine thermal efficiency and extended engine lean burn limit. Previous studies have focused on combustion and emissions of hydrogenegasoline mixtures, hydrogen enriched natural gas, hydrogen enriched LPG, hydrogen enriched ethanol, and hydrogen SI engines [20e25], whereas less attention has been given to combustion and emissions in a direct injection methanol engines with hydrogen addition, particularly regarding unregulated emissions. This paper studies this issue computational fluid dynamics (CFD) software, AVL FIRE (AVL List GmbH, 2013), and incorporating methanol oxidation and the nitrogen oxides (NOX) reaction. Subsequently, the combustion and emissions of a direct injection methanol engine were analyzed and compared with the hydrogen addition ratios (by volume) from 0 to 15% in the intake port.
Table 1 e Engine specifications. Engine Combustion chamber type Bore (mm) Stroke (mm) Displacement (L) Compression ratio Rated power/speed (kW/rpm) Combustion system Injection pump type Plunger diameter (mm) Number of nozzle holes Diameter of nozzle hole (mm) Injector needle valve opening pressure (MPa) Ignition system Cooling system
Methanol engine u shape 130 150 1.99 14:1 18.3/2000 Direct injection 6A95 9.5 10 0.3 17.5 Spark-ignition Water-cooled
Models used Engine model The methanol engine employed for this study was modified from a single-cylinder, four-stroke, naturally aspirated, watercooled, high-compression ratio direct-injection diesel engine, and uses an ignition system to ignite the methanoleair mixture. A 10-hole 0.30 mm non-uniform spray-line distribution nozzle was used to increase orifice area to accommodate the low heating value of methanol and form stratifiedcharge methanoleair mixtures. Other important engine specifications are shown in Table 1. Based on a three-dimensional (3D) engine model built in Pro/E, the computational mesh was created using AVL Fame Engine Plus meshing tool [26]. Regardless of the intake and exhaust process, the mixture formation and combustion process in the cylinder was considered to improve the efficiency of the simulation calculation (this is feasible for a suitable initial vortex ratio is given). The simulation ranged from 560 crank angle (CA) (intake valve closed) to 850 CA (exhaust valve opened). The 3D CFD computational meshes at 560 CA, 720 CA (TDC), and 850 CA are shown in Fig. 1. The maximum number of the mesh cells was 497,615, and the maximum grid sizes were is 0.2 cm and 0.05 cm, respectively. Run time for a closed cycle simulation took 15 h using a workstation incorporating 16 Xeon processors and 36 GB RAM. To validate the mesh accuracy and independence, a finer mesh with 734,681 cells was compared to the above grid just for the 5% hydrogen addition case. The cylinder pressure curves of the two meshes are shown in Fig. 2, and show no significant difference. Therefore, the smaller mesh was adopted to save computational time, with no loss of accuracy.
Fig. 1 e 3D CFD computational mesh in 560 CA, 720 CA and 850 CA.
Mathematical model The simulation calculation was based on energy and mass conservation, and there are many suitable mathematical models describing combustion and spay provided in AVL FIRE.
Chemistry model The fuel combustion process is controlled by chemical kinetics and turbulent flow in the actual engine. A reaction rate for each species was formulated considering these effects
Fig. 2 e Comparison of the cylinder pressure curves of the two meshes.
Please cite this article in press as: Gong C, et al., Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.062
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ui ¼
Yi0 Yi ; tkin;i þ f tturb
(1)
whereYi0 is the current concentration, and Yi is the equilibrium concentration for species i;tkin;i is the kinetic timescale;tturb is the turbulent timescale; andf is the fraction of combustion products in the mixture. When the fuel concentration approaches zero, the species concentration is Yinþ1 Yin ¼ ui dt ¼
tkin DYi ; tkin þ f tturb
(2)
whereDYi , is the CHEMKIN solution. If f ¼ 0, the solution is controlled by chemical kinetics. However, the turbulence affects the combustion through property transport, heat flux, and mixture preparation [27].
Spray model Spray simulations involve multi-phase flow phenomena and as such require simultaneous numerical solutions of conservation equations for gas and liquid phases [26]. For the liquid phase, practically all spray calculations are based the discrete droplet method (DDM) [28]. For evaporation, heat and mass transfer are described by a model originally derived by Dukowicz [29]. Since the growth of an initial perturbation on a liquid surface is linked to its wavelength and to other physical and dynamic parameters of the injected fuel and the domain fluid [30], a wave model was used. As particles pass through the flow, it is assumed that they interact with individual turbulent eddies, which was modeled by a dispersion model. Particle collisions were, as usual, modeled statistically [31]. In addition, the wall interaction of liquid droplets can play a major role for methanol engines and the Walljet model of Naber and Reitz [32] was applied. The simulation of the sprayline distribution is shown in Fig. 3, which shows that small dispersive droplets at tip of the spray are swept by the flow vortex.
Combustion model In combination with the spray model, the combustion module enables the calculation of spray combustion processes in direct injection engines. Under these conditions, mixture formation and combustion are simultaneous processes exhibiting a significant degree of interaction and interdependence. Since the general gas phase reaction (the chemical reaction kinetics was input to the module) and combustion modules cannot be activated simultaneously, a spherical selection was created to replace the spark plug in the computational domain and the proper temperature was set at the ignition timing to achieve reliable ignition [33]. The location of the spark plug and the injector spray-line distribution are shown in Fig. 3a.
Choice of chemical kinetics mechanisms The methanol chemical kinetics mechanisms (21-species, 84reactions) developed by Li et al. [34] were employed. Hydrogen chemical kinetics was also included, but NOx, one of the important exhaust emissions, was omitted. Hence, simple
Fig. 3 e (a) Injector spray-line distribution (1) spark plug (2) fuel spray-line (3) air swirl direction; (b) Simulation result of spray-line distribution.
NOx chemical kinetics was added, as shown in Table 2. The methanol and NOx chemical kinetics mechanisms were validated in many experimental tests [34,35].
Initial parameter settings Initial temperature and pressure were determined by combining test and relevant experience. The boundary temperature conditions were: piston ¼ 600 K, cylinder liner ¼ 415 K, and cylinder head ¼ 550 K. The time step was 0.5 CA before the spark ignition and decreased to 0.25 CA after ignition to track the combustion period. Other parameters were obtained from the 1-D simulation (see Fig. 4) with the relevant experimental databased on GT-POWER.
Validation of the numerical model To guarantee reliability of the simulation results, the model must be validated for some easily verifiable measure. The mean cylinder pressure and heat release ratio were chosen to assess the model accuracy. Fig. 5 shows the calculated and experimental cylinder pressure and heat release ratio at engine speed 1600 rpm, brake mean effective pressure 0.67 MPa and no hydrogen addition. The simulation and experimental curves match well, with average error less than 5%. Therefore, the numerical model was validated as suitable calculate other parameters.
Table 2 e NOx chemical kinetics mechanisms. k ¼ ATb expðE=RTÞ
Chemical reaction A
b
N þ O2 ¼ NO þ O 6.40E þ 09 1 3.27E þ 12 0.3 N þ NO ¼ N2þO N þ OH ¼ NO þ H 3.80E þ 14 0 1.00E þ 13 0 N þ HO2 ¼ NO þ OH NO þ M ¼ N þ O þ M 9.64E þ 14 0 H2/2.0/O2/6.0/H2O/6.0/CH4/2.0/CO/1.5/CO2/3.5/C2H6/3.0/ N2O þ M ¼ N2þO þ M 1.60E þ 14 0 7.60E þ 13 0 N2O þ H ¼ N2 þ OH 5.00E þ 15 0 N2O þ O ¼ 2NO 2.00E þ 12 0 N2O þ OH ¼ N2 þ HO2
E 6280 0 0 2000 148,300 51,600 15,200 28,200 10,000
Please cite this article in press as: Gong C, et al., Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.062
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Fig. 4 e 1-D simulation model.
Calculation As discussed above, the hydrogen addition ratio was defined as the volume of hydrogen (H2) divided by the total volume of the mixture in the intake port. Ignoring the intake process,
hydrogen is treated as a component proportionally added to the air [36]. The mass fraction can be calculated from the volume ratio. It was assumed that the air consisted of O2 and N2 with volume fractions 20.9% and 79.1%, respectively, and the corresponding mass fractions were calculated in the same way for H2 [37,38]. The mass fractions of H2, O2 and N2 for different hydrogen addition ratios are listed in Table 3. The excess air ratio of the hydrogenemethanoleair mixture is [39] l¼
mair ; mH2 AFstH2 þ mmet AFstmet
(3)
where mair , mH2 , and mmet are the masses of air, hydrogen, and methanol, respectively; and AFstH2 and AFstmet are the stoichiometric air-to-fuel ratios of hydrogen and methanol, respectively.
Table 3 e Mass fraction of H2, O2 and N2 in the air for different hydrogen addition ratios. Hydrogen addition ratio (%) 0 5 10 12.5 15
Mass fraction of H2
Mass fraction of O2
Mass fraction of N2
0 0.003637 0.007647 0.00981 0.012091
0.232626 0.23178 0.230847 0.230344 0.229813
0.767374 0.764583 0.761506 0.759846 0.758096
Table 4 e Injection quantity of methanol for different hydrogen addition ratios when l ¼ 2. Parameter
Hydrogen addition ratio 0%
Fig. 5 e Comparison of the calculated mean cylinder pressure and heat release rate and the experimental date.
5%
10%
12.5%
15%
injection quantity 184.6154 138.6909 88.05389 60.73555 31.93977 of methanol (mg/cycle)
Please cite this article in press as: Gong C, et al., Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.062
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e9
Fig. 6 e Cylinder pressure profiles for different hydrogen addition ratios.
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Fig. 8 e Heat release rate profiles with hydrogen addition ratios from 0 to 15%.
Once the operation mode was determined, the intake mass flow remained unchanged. To keep the same overall excess air ratio, the methanol injection quantity in a cycle was adjusted with the different hydrogen addition ratios, as shown in Table 4 for the case where l ¼ 2. The calculation was conducted for engine speed 1600 rpm and brake mean effective pressure 0.67 MPa. The spark timing was constant at 14 crank angle before top dead center (CABTDC), the injection timing was constant at 45 CABTDC, and the overall excess air ratio was fixed at 2. The combustion and emissions of the direct injection methanol engine were analyzed and compared with hydrogen addition ratios varying from 0 to 15% in the intake port.
Results and discussion Combustion characteristics Cylinder pressure Fig. 6 shows cylinder pressure profiles for different hydrogen addition ratios. The maximum cylinder pressure increases, and its corresponding crank angle advances, with increasing hydrogen. This is a consequence of the ignition energy of
Fig. 7 e Cylinder temperature profiles for different hydrogen addition ratios.
Fig. 9 e CO mass fraction profiles with hydrogen addition ratios from 0 to 15%.
hydrogen being low, which means it can be ignited quickly, hence the flame propagation velocity increases, and the burning velocity increases compared to a methanol-only engine. Thus, the hydrogen addition accelerates the reaction rate and shortens the ignition delay and burning period. The
Fig. 10 e NO mass fraction profiles with hydrogen addition ratios from 0 to 15%.
Please cite this article in press as: Gong C, et al., Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.062
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maximum cylinder pressure increases by 28% and 50%, and the corresponding position advanced 0 CA and 7 CA for hydrogen addition of 5% and 15%, respectively, compared to no hydrogen addition.
Cylinder temperature Fig. 7 shows cylinder temperature profiles for different hydrogen addition ratios. The highest cylinder temperature increases rapidly with increasing hydrogen, with cylinder
Table 5 e Comparison of NO formation and cylinder temperature field distributions in combustion chamber at 730 CA Hydrogen building ratio
Temperature field distribution at 730 CA
No field distribution 730 CA
0%
5%
10%
12.5%
15%
Please cite this article in press as: Gong C, et al., Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.062
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temperature at 5%, 10%, and 15% hydrogen addition being 15%, 30%, and 36% higher, with corresponding positions advanced 5 CA, 10 CA, and 15 CA, compare with no hydrogen addition, respectively. The flame temperature of hydrogen significantly higher than that of methanol, having a higher calorific value, and the mixture burns more completely, hence increasing the cylinder temperature.
Heat release rate Fig. 8 shows heat release rate profiles with hydrogen addition ratios from 0 to 15%. Overall, the maximum heat release rate increases, and its corresponding crank angle advances, with increasing hydrogen. The heat release rate first decreases then increases when adding 5% hydrogen, indicating that the latent heat of vaporization of the methanol at that ratio exceeds the heat release of hydrogen, whereas when adding more than 5% hydrogen, the heat release always exceeds the total latent heat of vaporization of the methanol. The maximum heat release rate of the hydrogen addition at 5%, 10%, and 15% is 120%, 156%, and 269% higher, and the corresponding positions advanced 6 CA, 9 CA, and 21 CA, compared to the no hydrogen case, respectively. Increased hydrogen addition, produces increased flame propagation speed, and enhanced combustion quality, with consequential higher maximum heat release.
Fig. 11 e Effects of adding hydrogen on the production of formaldehyde for different hydrogen addition ratios.
falls below 1800 K, the NO formation rate is slow, producing stable NO yield. The NO formation and cylinder temperature field distributions are compared in Table 5. NO yields increase with increasing flame propagation velocity, and NO forms in the flame front and the post-flame gases. With increasing hydrogen addition, the cylinder temperature and NO rapidly increase.
Emissions Carbon monoxide emission Fig. 9 shows the CO mass fraction for hydrogen addition from 0 to 15%. CO emission significantly reduces with increasing hydrogen, and the corresponding crank angle of the peak advances. The maximum CO mass fraction at hydrogen addition 5%, 10%, and 15% are 16%, 22%, and 91% lower, and the corresponding positions advanced 5 CA, 6 CA, and 17 CA, compared to no hydrogen, respectively. Hydrogen assists the relevant reaction for a more complete oxidation, which causes the mass fraction of CO to effectively decline. Consequently, hydrogen addition has a cleansing effect on CO emission which is an important pollutant.
Nitrous oxide emission
Fig. 12 e Mass fraction of residual formaldehyde when the exhaust valve is opening.
Fig. 10 shows the NO mass fraction for hydrogen addition from 0 to 15%.The extended Zeldovich mechanism [40] argues that temperature, oxygen concentration, and the duration of high temperature are the main factors affecting the formation of NO emissions, with temperature being the major effect. NO formation rates increase rapidly when the temperature exceeds 1800 K, and the maximum temperature in cylinder increases with increasing hydrogen addition. Consequently, NO emissions, which have a positive correlation with temperature, increase with increasing hydrogen. However, our results show that NO yields increase then decrease and stabilize with increasing hydrogen. The Zeldovich mechanism assumes the reaction is reversible. When the engine reaches 10 after top dead center (ATDC), the cylinder temperature cylinder is high and the reaction is rapid, which uses the oxygen supply. Hence, the reverse reaction rate intensifies, and NO begins to be consumed, decreasing NO yields. When the temperature
Fig. 13 e Mass fraction of residual unburned methanol when the exhaust valve is opening.
Please cite this article in press as: Gong C, et al., Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.062
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Fig. 14 e Unburned methanol in combustion chamber when the exhaust valve is opening.
Formaldehyde emission Formaldehyde is an important unregulated emission from methanol engines [41], and its formation is significantly affected by the temperature and the concentration of oxygen. Fig. 11 shows formaldehyde emissions for different hydrogen addition ratios. The mass fraction of formaldehyde decreases rapidly with increasing hydrogen. When hydrogen addition is less than 10%, the decline becomes more significant, whereas for greater than 10%, the trend slows. Formaldehyde oxidation quickens as active radicals, such as H, O, and OH are introduced. The maximum formaldehyde mass fraction for 5%, 10%, and 15% hydrogen addition are 20%, 72%, and 82% lower than with no hydrogen, and the corresponding position advances 6 CA, 11 CA, and 14 CA, respectively. Fig. 12 shows the mass fraction of residual formaldehyde when the exhaust valve is opening. The mass fraction of residual formaldehyde is almost zero for hydrogen addition levels of 12.5% and 15%, which argues an exponential decay of emissions. The mass fraction of residual formaldehyde for 10% hydrogen addition is more than 99% lower than with no hydrogen.
2) For regulated emissions, the mass fraction of CO decreases and NO yields, which are determined by temperature, also decrease for increasing hydrogen addition. 3) Increased hydrogen reduces unregulated emissions in the methanol engine. Formaldehyde and unburned methanol significantly reduce with increasing hydrogen addition. The mass fraction of residual formaldehyde and unburned methanol for hydrogen addition of 10% is more than 99% lower than with no hydrogen.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No.51176063) and the Fundamental Research Funds for the Central Universities (Grant No.DC 201502010203).
references
Unburned methanol emission Fig. 13 shows the mass fraction of residual unburned methanol when the exhaust valve is opening. The mass fraction of unburned methanol decreases with increasing hydrogen addition. When hydrogen addition is less than 10%, the decline very sharp, but slows for more than 10% addition. The mass fraction of residual unburned methanol for hydrogen addition of 10% at the exhaust valve opening is approximately 99% lower than with no hydrogen. Unburned methanol is largely distributed onto the cylinder wall (see Fig. 14), which results from the quenching effects. Hydrogen accelerates the reaction rate of the unburned methanol thinning the quenching layer. Hence, hydrogen addition can reduce unburned methanol emission in methanol engines.
Conclusions The combustion and emission characteristics in a direct injection methanol engine with hydrogen addition were investigated numerically based on a developed simulation. The main results can be summarized as follows: 1) With increasing hydrogen addition, maximum cylinder pressure and maximum heat release rate increase and their corresponding crank angles advance.
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Please cite this article in press as: Gong C, et al., Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.062
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition Changming Gong a, Dong Li b, Zhaohui Li b,c, Fenghua Liu a,* a
College of Electromechanical Engineering, Dalian Nationalities University, Dalian 116600, China State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130022, China c School of Vehicle Engineering, Chongqing University of Technology, Chongqing 400054, China b
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abstract
Article history:
Combustion, carbon monoxide (CO), Nitrous oxide (NO), formaldehyde (HCHO) and un-
Received 8 July 2015
burned methanol (CH3OH) emissions characteristics in a direct injection spark ignition
Accepted 10 November 2015
(DISI) methanol engine with hydrogen addition ratio from 5% to 15% have been investi-
Available online xxx
gated numerically based on the detailed methanol oxidation reaction mechanism combined with the nitrogen oxides (NOX) reaction mechanism using the PREMIX code of
Keywords:
CHEMKIN program with AVL FIRE software. Maximum cylinder pressure, maximum heat
Methanol engine
release rate and highest cylinder temperature increase and their corresponding crank
Hydrogen addition
angles advance with hydrogen addition from 0 to 15%. CO emission decreases and NO
Combustion
emission increases with increasing hydrogen addition ratio, whereas, formaldehyde and
Emission
the unburned methanol significantly reduce with increasing hydrogen addition. Thus,
Numerical simulation
hydrogen addition can provide a new way to address formaldehyde and unburned methanol unregulated emissions in a DISI methanol engine. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Oil resources are being rapidly consumed and environmental problems are becoming more serious. Consequently, many countries are stepping up the pace to seek clean alternative fuels. Methanol (CH3OH) is considered to be a promising alternative with several advantages over traditional fuels (e.g. gasoline or diesel) such as a large number of sources and low regulated emissions [1e3]. However, methanol engines have issues with high unregulated emissions and difficulties in cold start [4,5]. Gong et al. [6e8] investigated the effects of ambient temperature on firing behavior and unregulated emissions of spark-ignition (SI) methanol and liquefied petroleum gas
(LPG)/methanol engines during cold start. They found that with ambient temperatures near and below 289 K, the methanol engine cannot be started reliably without assisting measures. Furthermore, formaldehyde (HCHO) emissions increase significantly and unburned methanol decreases with rising ambient temperature. Hydrogen is another promising alternative fuel which has excellent combustion characteristics, high calorific value and environmental benefits, and may provide a mechanism to improve combustion and emissions performance of methanol engines [9,10]. Ji et al. [11e13] investigated the cold start characteristics of hydrogen-enriched methanol engines and showed that both flame development and propagation periods were reduced with hydrogen addition. Hydrocarbon (HC)
* Corresponding author. Tel.: þ86 411 87188945; fax: þ86 411 87656133. E-mail address: [email protected] (F. Liu). http://dx.doi.org/10.1016/j.ijhydene.2015.11.062 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Gong C, et al., Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.062
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and carbon monoxide (CO) emissions during cold start were also reduced, as well as the particulate matter (PM). Huang et al. [14e19] investigated in detail the effects of natural gasehydrogen blends on combustion, performance and emissions of a direct injection engine. They found that addition of hydrogen increased the burning velocity of the natural gas due to the increased H, O and OH radicals in the flame, and the flame stability at early combustion stage was enhanced after the hydrogen addition. They also showed improved engine thermal efficiency and extended engine lean burn limit. Previous studies have focused on combustion and emissions of hydrogenegasoline mixtures, hydrogen enriched natural gas, hydrogen enriched LPG, hydrogen enriched ethanol, and hydrogen SI engines [20e25], whereas less attention has been given to combustion and emissions in a direct injection methanol engines with hydrogen addition, particularly regarding unregulated emissions. This paper studies this issue computational fluid dynamics (CFD) software, AVL FIRE (AVL List GmbH, 2013), and incorporating methanol oxidation and the nitrogen oxides (NOX) reaction. Subsequently, the combustion and emissions of a direct injection methanol engine were analyzed and compared with the hydrogen addition ratios (by volume) from 0 to 15% in the intake port.
Table 1 e Engine specifications. Engine Combustion chamber type Bore (mm) Stroke (mm) Displacement (L) Compression ratio Rated power/speed (kW/rpm) Combustion system Injection pump type Plunger diameter (mm) Number of nozzle holes Diameter of nozzle hole (mm) Injector needle valve opening pressure (MPa) Ignition system Cooling system
Methanol engine u shape 130 150 1.99 14:1 18.3/2000 Direct injection 6A95 9.5 10 0.3 17.5 Spark-ignition Water-cooled
Models used Engine model The methanol engine employed for this study was modified from a single-cylinder, four-stroke, naturally aspirated, watercooled, high-compression ratio direct-injection diesel engine, and uses an ignition system to ignite the methanoleair mixture. A 10-hole 0.30 mm non-uniform spray-line distribution nozzle was used to increase orifice area to accommodate the low heating value of methanol and form stratifiedcharge methanoleair mixtures. Other important engine specifications are shown in Table 1. Based on a three-dimensional (3D) engine model built in Pro/E, the computational mesh was created using AVL Fame Engine Plus meshing tool [26]. Regardless of the intake and exhaust process, the mixture formation and combustion process in the cylinder was considered to improve the efficiency of the simulation calculation (this is feasible for a suitable initial vortex ratio is given). The simulation ranged from 560 crank angle (CA) (intake valve closed) to 850 CA (exhaust valve opened). The 3D CFD computational meshes at 560 CA, 720 CA (TDC), and 850 CA are shown in Fig. 1. The maximum number of the mesh cells was 497,615, and the maximum grid sizes were is 0.2 cm and 0.05 cm, respectively. Run time for a closed cycle simulation took 15 h using a workstation incorporating 16 Xeon processors and 36 GB RAM. To validate the mesh accuracy and independence, a finer mesh with 734,681 cells was compared to the above grid just for the 5% hydrogen addition case. The cylinder pressure curves of the two meshes are shown in Fig. 2, and show no significant difference. Therefore, the smaller mesh was adopted to save computational time, with no loss of accuracy.
Fig. 1 e 3D CFD computational mesh in 560 CA, 720 CA and 850 CA.
Mathematical model The simulation calculation was based on energy and mass conservation, and there are many suitable mathematical models describing combustion and spay provided in AVL FIRE.
Chemistry model The fuel combustion process is controlled by chemical kinetics and turbulent flow in the actual engine. A reaction rate for each species was formulated considering these effects
Fig. 2 e Comparison of the cylinder pressure curves of the two meshes.
Please cite this article in press as: Gong C, et al., Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.062
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ui ¼
Yi0 Yi ; tkin;i þ f tturb
(1)
whereYi0 is the current concentration, and Yi is the equilibrium concentration for species i;tkin;i is the kinetic timescale;tturb is the turbulent timescale; andf is the fraction of combustion products in the mixture. When the fuel concentration approaches zero, the species concentration is Yinþ1 Yin ¼ ui dt ¼
tkin DYi ; tkin þ f tturb
(2)
whereDYi , is the CHEMKIN solution. If f ¼ 0, the solution is controlled by chemical kinetics. However, the turbulence affects the combustion through property transport, heat flux, and mixture preparation [27].
Spray model Spray simulations involve multi-phase flow phenomena and as such require simultaneous numerical solutions of conservation equations for gas and liquid phases [26]. For the liquid phase, practically all spray calculations are based the discrete droplet method (DDM) [28]. For evaporation, heat and mass transfer are described by a model originally derived by Dukowicz [29]. Since the growth of an initial perturbation on a liquid surface is linked to its wavelength and to other physical and dynamic parameters of the injected fuel and the domain fluid [30], a wave model was used. As particles pass through the flow, it is assumed that they interact with individual turbulent eddies, which was modeled by a dispersion model. Particle collisions were, as usual, modeled statistically [31]. In addition, the wall interaction of liquid droplets can play a major role for methanol engines and the Walljet model of Naber and Reitz [32] was applied. The simulation of the sprayline distribution is shown in Fig. 3, which shows that small dispersive droplets at tip of the spray are swept by the flow vortex.
Combustion model In combination with the spray model, the combustion module enables the calculation of spray combustion processes in direct injection engines. Under these conditions, mixture formation and combustion are simultaneous processes exhibiting a significant degree of interaction and interdependence. Since the general gas phase reaction (the chemical reaction kinetics was input to the module) and combustion modules cannot be activated simultaneously, a spherical selection was created to replace the spark plug in the computational domain and the proper temperature was set at the ignition timing to achieve reliable ignition [33]. The location of the spark plug and the injector spray-line distribution are shown in Fig. 3a.
Choice of chemical kinetics mechanisms The methanol chemical kinetics mechanisms (21-species, 84reactions) developed by Li et al. [34] were employed. Hydrogen chemical kinetics was also included, but NOx, one of the important exhaust emissions, was omitted. Hence, simple
Fig. 3 e (a) Injector spray-line distribution (1) spark plug (2) fuel spray-line (3) air swirl direction; (b) Simulation result of spray-line distribution.
NOx chemical kinetics was added, as shown in Table 2. The methanol and NOx chemical kinetics mechanisms were validated in many experimental tests [34,35].
Initial parameter settings Initial temperature and pressure were determined by combining test and relevant experience. The boundary temperature conditions were: piston ¼ 600 K, cylinder liner ¼ 415 K, and cylinder head ¼ 550 K. The time step was 0.5 CA before the spark ignition and decreased to 0.25 CA after ignition to track the combustion period. Other parameters were obtained from the 1-D simulation (see Fig. 4) with the relevant experimental databased on GT-POWER.
Validation of the numerical model To guarantee reliability of the simulation results, the model must be validated for some easily verifiable measure. The mean cylinder pressure and heat release ratio were chosen to assess the model accuracy. Fig. 5 shows the calculated and experimental cylinder pressure and heat release ratio at engine speed 1600 rpm, brake mean effective pressure 0.67 MPa and no hydrogen addition. The simulation and experimental curves match well, with average error less than 5%. Therefore, the numerical model was validated as suitable calculate other parameters.
Table 2 e NOx chemical kinetics mechanisms. k ¼ ATb expðE=RTÞ
Chemical reaction A
b
N þ O2 ¼ NO þ O 6.40E þ 09 1 3.27E þ 12 0.3 N þ NO ¼ N2þO N þ OH ¼ NO þ H 3.80E þ 14 0 1.00E þ 13 0 N þ HO2 ¼ NO þ OH NO þ M ¼ N þ O þ M 9.64E þ 14 0 H2/2.0/O2/6.0/H2O/6.0/CH4/2.0/CO/1.5/CO2/3.5/C2H6/3.0/ N2O þ M ¼ N2þO þ M 1.60E þ 14 0 7.60E þ 13 0 N2O þ H ¼ N2 þ OH 5.00E þ 15 0 N2O þ O ¼ 2NO 2.00E þ 12 0 N2O þ OH ¼ N2 þ HO2
E 6280 0 0 2000 148,300 51,600 15,200 28,200 10,000
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Fig. 4 e 1-D simulation model.
Calculation As discussed above, the hydrogen addition ratio was defined as the volume of hydrogen (H2) divided by the total volume of the mixture in the intake port. Ignoring the intake process,
hydrogen is treated as a component proportionally added to the air [36]. The mass fraction can be calculated from the volume ratio. It was assumed that the air consisted of O2 and N2 with volume fractions 20.9% and 79.1%, respectively, and the corresponding mass fractions were calculated in the same way for H2 [37,38]. The mass fractions of H2, O2 and N2 for different hydrogen addition ratios are listed in Table 3. The excess air ratio of the hydrogenemethanoleair mixture is [39] l¼
mair ; mH2 AFstH2 þ mmet AFstmet
(3)
where mair , mH2 , and mmet are the masses of air, hydrogen, and methanol, respectively; and AFstH2 and AFstmet are the stoichiometric air-to-fuel ratios of hydrogen and methanol, respectively.
Table 3 e Mass fraction of H2, O2 and N2 in the air for different hydrogen addition ratios. Hydrogen addition ratio (%) 0 5 10 12.5 15
Mass fraction of H2
Mass fraction of O2
Mass fraction of N2
0 0.003637 0.007647 0.00981 0.012091
0.232626 0.23178 0.230847 0.230344 0.229813
0.767374 0.764583 0.761506 0.759846 0.758096
Table 4 e Injection quantity of methanol for different hydrogen addition ratios when l ¼ 2. Parameter
Hydrogen addition ratio 0%
Fig. 5 e Comparison of the calculated mean cylinder pressure and heat release rate and the experimental date.
5%
10%
12.5%
15%
injection quantity 184.6154 138.6909 88.05389 60.73555 31.93977 of methanol (mg/cycle)
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Fig. 6 e Cylinder pressure profiles for different hydrogen addition ratios.
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Fig. 8 e Heat release rate profiles with hydrogen addition ratios from 0 to 15%.
Once the operation mode was determined, the intake mass flow remained unchanged. To keep the same overall excess air ratio, the methanol injection quantity in a cycle was adjusted with the different hydrogen addition ratios, as shown in Table 4 for the case where l ¼ 2. The calculation was conducted for engine speed 1600 rpm and brake mean effective pressure 0.67 MPa. The spark timing was constant at 14 crank angle before top dead center (CABTDC), the injection timing was constant at 45 CABTDC, and the overall excess air ratio was fixed at 2. The combustion and emissions of the direct injection methanol engine were analyzed and compared with hydrogen addition ratios varying from 0 to 15% in the intake port.
Results and discussion Combustion characteristics Cylinder pressure Fig. 6 shows cylinder pressure profiles for different hydrogen addition ratios. The maximum cylinder pressure increases, and its corresponding crank angle advances, with increasing hydrogen. This is a consequence of the ignition energy of
Fig. 7 e Cylinder temperature profiles for different hydrogen addition ratios.
Fig. 9 e CO mass fraction profiles with hydrogen addition ratios from 0 to 15%.
hydrogen being low, which means it can be ignited quickly, hence the flame propagation velocity increases, and the burning velocity increases compared to a methanol-only engine. Thus, the hydrogen addition accelerates the reaction rate and shortens the ignition delay and burning period. The
Fig. 10 e NO mass fraction profiles with hydrogen addition ratios from 0 to 15%.
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maximum cylinder pressure increases by 28% and 50%, and the corresponding position advanced 0 CA and 7 CA for hydrogen addition of 5% and 15%, respectively, compared to no hydrogen addition.
Cylinder temperature Fig. 7 shows cylinder temperature profiles for different hydrogen addition ratios. The highest cylinder temperature increases rapidly with increasing hydrogen, with cylinder
Table 5 e Comparison of NO formation and cylinder temperature field distributions in combustion chamber at 730 CA Hydrogen building ratio
Temperature field distribution at 730 CA
No field distribution 730 CA
0%
5%
10%
12.5%
15%
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temperature at 5%, 10%, and 15% hydrogen addition being 15%, 30%, and 36% higher, with corresponding positions advanced 5 CA, 10 CA, and 15 CA, compare with no hydrogen addition, respectively. The flame temperature of hydrogen significantly higher than that of methanol, having a higher calorific value, and the mixture burns more completely, hence increasing the cylinder temperature.
Heat release rate Fig. 8 shows heat release rate profiles with hydrogen addition ratios from 0 to 15%. Overall, the maximum heat release rate increases, and its corresponding crank angle advances, with increasing hydrogen. The heat release rate first decreases then increases when adding 5% hydrogen, indicating that the latent heat of vaporization of the methanol at that ratio exceeds the heat release of hydrogen, whereas when adding more than 5% hydrogen, the heat release always exceeds the total latent heat of vaporization of the methanol. The maximum heat release rate of the hydrogen addition at 5%, 10%, and 15% is 120%, 156%, and 269% higher, and the corresponding positions advanced 6 CA, 9 CA, and 21 CA, compared to the no hydrogen case, respectively. Increased hydrogen addition, produces increased flame propagation speed, and enhanced combustion quality, with consequential higher maximum heat release.
Fig. 11 e Effects of adding hydrogen on the production of formaldehyde for different hydrogen addition ratios.
falls below 1800 K, the NO formation rate is slow, producing stable NO yield. The NO formation and cylinder temperature field distributions are compared in Table 5. NO yields increase with increasing flame propagation velocity, and NO forms in the flame front and the post-flame gases. With increasing hydrogen addition, the cylinder temperature and NO rapidly increase.
Emissions Carbon monoxide emission Fig. 9 shows the CO mass fraction for hydrogen addition from 0 to 15%. CO emission significantly reduces with increasing hydrogen, and the corresponding crank angle of the peak advances. The maximum CO mass fraction at hydrogen addition 5%, 10%, and 15% are 16%, 22%, and 91% lower, and the corresponding positions advanced 5 CA, 6 CA, and 17 CA, compared to no hydrogen, respectively. Hydrogen assists the relevant reaction for a more complete oxidation, which causes the mass fraction of CO to effectively decline. Consequently, hydrogen addition has a cleansing effect on CO emission which is an important pollutant.
Nitrous oxide emission
Fig. 12 e Mass fraction of residual formaldehyde when the exhaust valve is opening.
Fig. 10 shows the NO mass fraction for hydrogen addition from 0 to 15%.The extended Zeldovich mechanism [40] argues that temperature, oxygen concentration, and the duration of high temperature are the main factors affecting the formation of NO emissions, with temperature being the major effect. NO formation rates increase rapidly when the temperature exceeds 1800 K, and the maximum temperature in cylinder increases with increasing hydrogen addition. Consequently, NO emissions, which have a positive correlation with temperature, increase with increasing hydrogen. However, our results show that NO yields increase then decrease and stabilize with increasing hydrogen. The Zeldovich mechanism assumes the reaction is reversible. When the engine reaches 10 after top dead center (ATDC), the cylinder temperature cylinder is high and the reaction is rapid, which uses the oxygen supply. Hence, the reverse reaction rate intensifies, and NO begins to be consumed, decreasing NO yields. When the temperature
Fig. 13 e Mass fraction of residual unburned methanol when the exhaust valve is opening.
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Fig. 14 e Unburned methanol in combustion chamber when the exhaust valve is opening.
Formaldehyde emission Formaldehyde is an important unregulated emission from methanol engines [41], and its formation is significantly affected by the temperature and the concentration of oxygen. Fig. 11 shows formaldehyde emissions for different hydrogen addition ratios. The mass fraction of formaldehyde decreases rapidly with increasing hydrogen. When hydrogen addition is less than 10%, the decline becomes more significant, whereas for greater than 10%, the trend slows. Formaldehyde oxidation quickens as active radicals, such as H, O, and OH are introduced. The maximum formaldehyde mass fraction for 5%, 10%, and 15% hydrogen addition are 20%, 72%, and 82% lower than with no hydrogen, and the corresponding position advances 6 CA, 11 CA, and 14 CA, respectively. Fig. 12 shows the mass fraction of residual formaldehyde when the exhaust valve is opening. The mass fraction of residual formaldehyde is almost zero for hydrogen addition levels of 12.5% and 15%, which argues an exponential decay of emissions. The mass fraction of residual formaldehyde for 10% hydrogen addition is more than 99% lower than with no hydrogen.
2) For regulated emissions, the mass fraction of CO decreases and NO yields, which are determined by temperature, also decrease for increasing hydrogen addition. 3) Increased hydrogen reduces unregulated emissions in the methanol engine. Formaldehyde and unburned methanol significantly reduce with increasing hydrogen addition. The mass fraction of residual formaldehyde and unburned methanol for hydrogen addition of 10% is more than 99% lower than with no hydrogen.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No.51176063) and the Fundamental Research Funds for the Central Universities (Grant No.DC 201502010203).
references
Unburned methanol emission Fig. 13 shows the mass fraction of residual unburned methanol when the exhaust valve is opening. The mass fraction of unburned methanol decreases with increasing hydrogen addition. When hydrogen addition is less than 10%, the decline very sharp, but slows for more than 10% addition. The mass fraction of residual unburned methanol for hydrogen addition of 10% at the exhaust valve opening is approximately 99% lower than with no hydrogen. Unburned methanol is largely distributed onto the cylinder wall (see Fig. 14), which results from the quenching effects. Hydrogen accelerates the reaction rate of the unburned methanol thinning the quenching layer. Hence, hydrogen addition can reduce unburned methanol emission in methanol engines.
Conclusions The combustion and emission characteristics in a direct injection methanol engine with hydrogen addition were investigated numerically based on a developed simulation. The main results can be summarized as follows: 1) With increasing hydrogen addition, maximum cylinder pressure and maximum heat release rate increase and their corresponding crank angles advance.
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Please cite this article in press as: Gong C, et al., Numerical study on combustion and emission in a DISI methanol engine with hydrogen addition, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.11.062