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Control of backfire and NOx emission reduction in a hydrogen fueled multi-cylinder spark ignition engine using cooled EGR and water injection strategies
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Control of backfire and NOx emission reduction in a hydrogen fueled multi-cylinder spark ignition engine using cooled EGR and water injection strategies Vipin Dhyani, K.A. Subramanian* Engines and Unconventional Fuels Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi 110016, India
article info
abstract
Article history:
The experimental study was carried out on a constant speed multi-cylinder spark ignition
Received 3 July 2018
engine fueled with hydrogen. Exhaust gas recirculation (EGR) and water injection tech-
Received in revised form
niques were adopted to control combustion anomalies (backfire and knocking) and reduce
26 December 2018
NOx emission at source level. The experimental tests were conducted on the engine with
Accepted 10 January 2019
varied EGR rate (0%e28% by volume) and water to hydrogen ratio (WHR) (0e9.25) at 15 kW
Available online 5 February 2019
load. It was observed from the experiments that both the strategies can control backfire effectively, but water injection can effectively control backfire compared to EGR. The water
Keywords:
injection and EGR reduce the probability of backfire occurrence and its propagation due to
Hydrogen fueled spark ignition en-
the increase in the requirement of minimum ignition energy (MIE) of the charge, caused
gine
mainly due to charge dilution effect, and reduction in flame speed respectively. The NOx
Backfire
emission was continuously reduced with increase in EGR rate and WHR, but at higher rates
NOx emission
(of EGR and WHR), there was an issue of stability of engine operation. It was found from the
Exhaust gas recirculation (EGR)
experimental results that at 25% EGR, there was 57% reduction in NOx emission without
Water injection
drop in brake thermal efficiency whereas, with WHR of 7.5, the NOx emission was reduced by 97% without affecting the efficiency. The salient point emerging from the study is that water injection technique can control backfire with ultra-low (near zero) NOx emission without compromising the performance of the hydrogen fueled spark ignition engine. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Among various alternative fuels, hydrogen is the only carbon free fuel available and considered as most promising clean fuel for internal combustion (IC) engines. Hydrogen can be used in compression ignition (CI) engine as well as in spark
ignition (SI) engine. However, use of neat (100%) hydrogen cannot be possible in CI engine because of its high autoignition temperature that requires some ignition source. Along with high auto-ignition temperature, hydrogen is having high octane number which makes it more suitable fuel for spark ignition (SI) engines. High flame velocity of hydrogen improves the performance of SI engine as compared to
* Corresponding author. E-mail address: [email protected] (K.A. Subramanian). https://doi.org/10.1016/j.ijhydene.2019.01.129 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature Abbreviations aBDC after bottom dead center aTDC after top dead center bBDC before bottom dead center BOER backfire occurring equivalence ratio BSFC brake specific fuel consumption bTDC before top dead center CD combustion duration CFD computational fluid dynamics CFR cooperative fuel research CI compression ignition CNG compressed natural gas CO carbon monoxide carbon dioxide CO2 COV coefficient of variation DM demineralized ECU electronic control unit EGR exhaust gas recirculation EVO exhaust valve opening FDA flame development angle FKGR flame kernel growth rate GDI gasoline direct injection water vapor H2O HC hydrocarbon IC internal combustion IMEP indicated mean effective pressure ISFC indicated specific fuel consumption IVC intake valve closing MBT maximum brake torque MIE minimum ignition energy nitrogen gas N2 NIMEP net indicated mean effective pressure NOx oxides of nitrogen
conventional gasoline fueled SI engines. However, there are some technical issues such as backfire, power drop and high NOx emission need to be resolved in a hydrogen fueled SI engine. Backfire is a pre-ignition phenomenon that takes place in the intake manifold and/or combustion chamber during intake stroke of the engine. During backfire, the flame propagates from the combustion chamber to upstream of the intake manifold. Backfire is identified by a high pitched sound and sudden rise in intake manifold pressure and in-cylinder pressure during the intake stroke of the engine. The cause of backfire includes the presence of any high temperature source in the combustion chamber of the engine such as hot exhaust valves, hot residual gas, hot spark plug, radicals and lubricating oil particles. The next combustion anomalies associated with hydrogen fueled SI engine are pre-ignition and knocking. The detailed information about these combustion anomalies (backfire, pre-ignition and knocking) including the difference between them and their relationship with each other is given in the recently published work [1]. The factor
NTP OBA SI SOC WHR
normal temperature and pressure overall burning angle spark ignition start of combustion water to hydrogen ratio
Symbols a crank radius (m) A instantaneous heat transfer surface area of combustion chamber (m2) average molar specific heat of the charge at cavg constant pressure (kJ/kmol K) molar specific heat at constant pressure (kJ/kmol cp K) d quenching distance (m) h convective heat transfer coefficient (W/m2 K) l length of connecting rod (m) p in-cylinder pressure (Pascal) Q net heat release (J) heat transfer to cylinder wall (J) Qht compression ratio rc gas constant of the mixture (J/kg K) Rmix Tb flame temperature (K) unburnt gas temperature (K) Tu mean cylinder wall temperature (K) Tw V instantaneous cylinder volume (m3) clearance volume (m3) Vc swept volume (m3) Vs g ratio of specific heats brake thermal efficiency hbth hv volumetric efficiency q crank angle density of incoming air (kg/m3) ra molar density of the charge (mol/m3) rb
which reduces the chance of one combustion anomaly also reduces the probability of another combustion anomaly. Varde and Frame [2], Liu et al. [3], Subramanian and Salvi [4], Hong et al. [5] and Dhyani and Subramanian [1] reported in their respective studies that the probability of backfire occurrence in a hydrogen fueled engine can be reduced by delaying the timing of hydrogen injection due to availability of more time for cooling of the in-cylinder mixture by fresh incoming air during intake stroke of the engine. Salvi and Subramanian [6] and Ding et al. [7] reported that increasing compression ratio reduces the chances of backfire due to the reduced amount of residual gases in the combustion chamber. Varying valve overlap period is also reported as an effective method of controlling backfire in a hydrogen fueled SI engine [8e10,20]. Postponing the ignition timing can also control backfire occurrence [1,11] due to reduction in reaction rate of the chemical reaction. A limited number of researchers reported in literature about knocking in hydrogen fueled SI engine. Li and Karim [12] reported that knocking is the most important consideration to be avoided in hydrogen fueled SI
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engines at high equivalence ratio. The visualization study on knocking suggests that auto-ignition of end gas is the cause of development of high pressure waves that lead to knocking [13]. Szwaja and Naber reported that the instable and fast combustion of hydrogen can cause knocking in a hydrogen fueled engine with low intensity as compared to that caused by auto-ignition of end gas which was of high intensity [14]. The research findings of Dhyani and Subramanian indicate that backfire and knocking are interlinked with each other at high equivalence ratio and the conditions of reducing one lead to reduction in the probability of another [1]. The delayed hydrogen injection and retarded spark timing are the strategies used for controlling these combustion anomalies. Along with backfire, hydrogen fueled SI engine also emits higher NOx emission as compared to CNG or gasoline fueled SI engine at high equivalence ratio. Therefore, it is necessary to reduce NOx emission along with backfire at high equivalence ratio. It is reported by Woolley and Henriksen [15] and Das [16] that some form of diluent such as exhaust gas recirculation (EGR) or water injection is necessary for operating hydrogen fueled SI engine above equivalence ratio of 0.6 (f 0.6). EGR is a well-established technique to reduce engine out NOx emission at the source level. The exhaust gas of hydrogen fueled SI engine mainly constituents of H2O vapor, N2, excess oxygen and unburned hydrogen. The constituents of EGR depend on the type of EGR strategy adopted namely hot EGR and cooled EGR. In hot EGR, the exhaust gas is directly recirculated without cooling whereas in cooled EGR, the exhaust gas is cooled by some cooling medium such as cold water, and thus the water vapor gets condensed and removed and therefore in cooled EGR, only gases (N2, excess oxygen, and unburned hydrogen) are introduced in the engine cylinder. Das and Mathur reported that 15% EGR is the optimum percentage under part-throttle conditions [17]. The reduction in BSFC was also observed with increasing EGR rate due to reduced pumping work and reduced heat loss to cylinder wall caused by lower in-cylinder temperatures. They also mentioned the reduction in the possibility of backfire (due to reduced flame velocity with EGR) and knocking with EGR but did not provide any data. Heat exchanger was used to reduce the temperature of EGR upto atm state. Heffel achieved near zero NOx emission (less than 1 ppm) by using EGR (exact EGR % was not specified) and 3-way catalytic convertor [18]. The EGR strategy was also compared with lean-burn strategy and suggested that EGR is more beneficial at high load and leanburn strategy is more suitable at low load because EGR results in low emission and high power output with low efficiency whereas lean-burn results in low emission and high efficiency with low power output. Verhelst et al. reported that the stoichiometric operation of the engine with hydrogen is possible with supercharging without backfire and pre-ignition while maintaining low NOx emission by using EGR strategy with 3-way catalytic convertor [19,20]. They also reported higher power output with hydrogen as compared to gasoline and methane with the strategy. Mohsen et al. carried out a CFD study to observe the effect of initial charge pressure (supercharging) and different EGR rates on performance and emissions of a spark ignition engine fueled with gasoline and various alternative fuels (methane, propane, methanol, ethanol, and hydrogen) [21]. The NOx emission drastically
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reduced between 0 and 10% EGR and significantly between 10 and 20% EGR in all cases. The maximum reduction in NOx emission due to EGR was reported with methane. The IMEP of the engine decreased with EGR as EGR replaces the incoming air and thus the availability of in-cylinder oxygen which causes to decrease the flame speed and thus the combustion rate resulting in lower in-cylinder peak pressure. The IMEP of the engine with hydrogen was the least affected by EGR because of its higher flame speed and higher calorific value compared to other fuels. The effect of supercharging was also the least on hydrogen fueled engine. Subramanian et al. evaluated the effect of various diluents such as N2, CO2 and hot EGR on performance, emission and combustion characteristics of the engine [22]. It was concluded that N2 is the best diluent followed by hot EGR and CO2 in terms of low NOx emission and high thermal efficiency. The NOx emission was reduced to 1000 ppm with EGR of 12.2% along with the sacrifice of 1.8% indicated brake thermal efficiency. It was also suggested that the use of cooled EGR could give better results than that of with hot EGR. Safari et al. carried out a simulation study for comparative evaluation of the effect of EGR (cooled and hot) and lean-burn strategies on NOx emission, thermal efficiency and power output of hydrogen fueled SI engine [23]. Based on their analytical work they reported that lean burn is only beneficial at low load while EGR is better at mid and high load. The comparison was also done between cooled and hot EGR and reported that for same NOx emission reduction, the cooled EGR is more effective as it results in high indicated thermal efficiency and high indicated power as compared to hot EGR. The reason for this is the high temperature and constituents of the hot EGR (i.e water vapor and N2 at stoichiometric mixture) which reduces the intake charge density (thus volumetric efficiency) and reduces the in-cylinder temperature and pressure due to higher dilution effect as compared to cooled EGR which contains mainly nitrogen (at stoichiometric mixture) at relatively low temperature. Kosmadakis et al. carried out experimental and simulation study on a single cylinder cooperative fuel research (CFR) engine with compression ratio of 9:1 and constant speed of 600 rpm for analyzing combustion phenomena at high EGR rates (12%e 47% by mass) [24]. The EGR strategy was used to control the engine load with reduced NOx emission while maintaining stoichiometric hydrogen-air mixture. The predictions of CFD code were highly accurate mainly for low and mid EGR rates. The load and gross IMEP of the engine decreased with EGR rates. However, the indicated efficiency first increased with EGR due to lower combustion dissociation and low engine heat transfer and then decreased with the EGR rate (42% onwards) due to low combustion efficiency caused by reduced flame speed, increased combustion duration and unburned hydrogen in the exhaust. The unburned hydrogen of up to 0.5% by volume at high EGR rate was reported in the study. Salvi and Subramanian conducted a comparative study between spark timing retardation and EGR technique in terms of NOx emission reduction from a single cylinder SI engine generator set fueled with hydrogen and concluded that EGR is the most suitable technique [6]. They reported 50% reduction in NOx emission with marginally affecting thermal efficiency and engine power output with 20% EGR (volume based). The experimental investigation on the effect of EGR on flame
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kernel growth rate (FKGR) was also carried out [25]. It was reported in the findings that FKGR was reduced from 16.7 m/s to 6.6 m/s with EGR of 0e18% (by volume) respectively. This reduction in FKGR with the use of EGR signifies the abnormal combustion free operation at high equivalence ratio. The EGR (14%) with hydrogen-SCR was used by Krishnan Unni et al. to reduce the NOx emission by 70% from a multi-cylinder turbocharged SI engine fueled with hydrogen at equivalence ratio of 0.64 [26]. Szwaja and Naber reported in their findings that EGR can be used to eliminate knock in hydrogen fueled SI engine [27]. Another technique for reducing NOx emission at source level in a hydrogen fueled SI engine is water injection. The water vapor present in the engine exhaust can be used for this purpose after condensing and storing it in a tank. It is reported that Roger Billings (in 1972) was the first researcher to use water induction for NOx emission reduction in a hydrogen fueled engine [15]. Woolley and Henriksen conducted experiments on a 1975 Dodge 440 CID, V8 engine fueled with hydrogen [15]. The engine was of carburetor based fuel supply system and having compression ratio of 12:1. The water to hydrogen fuel ratio up to 12 was tested. The variation in equivalence ratio of about 10% between cylinder to cylinder was reported as a possible cause of random backfire. It was reported that water induction eliminates backfire at all equivalence ratios along with the exponential reduction in NOx emission. Engine exhaust gas temperature was not changed significantly (remains almost same) with all water to hydrogen fuel ratios. Spark timing (which was set for least NOx) was also not affected by water induction. The brake thermal efficiency of about 33% was observed at all water flow rates and it was not affected significantly with water induction. The power output was also not affected by water induction. The amount of water that is required for backfire elimination depends mainly on the engine speed and equivalence ratio. It is reported that water to fuel ratio of 4:1 was an effective proportional to eliminate backfire without causing any ill effects [16]. Subramanian et al. reported 67.5% reduction in NOx emission (from 7670 ppm to 2490 ppm) without affecting brake thermal efficiency with 5.9 kg/h water flow rate at equivalence ratio of 0.82 [28]. The maximum water to hydrogen ratio of 7.5 was used in the work. They also concluded that water injection is more beneficial as compared to spark timing retardation in terms of NOx emission reduction. Kahraman et al. carried out experiments on a carburetor based four cylinder SI engine fueled with hydrogen by varying speed from 2600 rpm to 3800 rpm [29]. They mentioned that the water injection in intake manifold was essential to operate engine above 2600 rpm in order to get backfire free operation. The amount of water injection depends on the engine speed as water quantity needs to increase with increase in engine speed. The NOx emission was observed very low as compared to the engine operating with gasoline due to water injection along with lean operation of the engine. Based on the simulation study, Boretti reported that stoichiometric operation of hydrogen fueled turbocharged SI engine can be possible with water injection [30]. Port water injection along with direct hydrogen injection and jet ignition can suppress the combustion anomalies associated with hydrogen engine (pre-
ignition, backfire and knocking) and reduce NOx emission. At full load and maximum speed of 5000 rpm, brake thermal efficiency of about 40% was reported in the paper. The paper is fully based on simulation study and does not provide any experimental validation. There are only few studies available in the literature on water injection in hydrogen fueled SI engine whereas water injection has been extensively used with other fuels. Mingrui et al. carried out simulation study on the effect of direct water injection in a GDI (gasoline direct injection) engine [31]. The optimum water ratio of 15% (of fuel) was reported in terms of performance and emissions characteristics of the engine. The researchers reported higher peak in-cylinder pressure with water injection than that of without water injection in a turbocharged SI engine [32,33]. The water to fuel ratio of up to 0.17 was limited due to an excessive rise in in-cylinder pressure beyond this ratio [32]. The knock was effectively suppressed at this water to fuel ratio. The expansion work and thermal efficiency and thus BSFC was improved with water injection. The water to fuel ratio of up to 0.2 (mass basis) was limited by Ref. [33] due to unaccepted engine oil dilution in the crankcase. Hoppe et al. performed experimental work on a single cylinder gasoline direct injection engine with direct water injection [34]. It was reported that ISFC decreases with water injection. It was suggested that further reduction in fuel consumption can be achieved by combining water injection with cooled EGR. The simulation study of Bozza et al. on a turbocharged two cylinder SI engine fueled with gasoline shows an improvement in BSFC, knock resistance characteristics of the engine with cooled EGR and port water injection [35]. It was also reported that the flame speed was decreased faster with water injection than EGR [33]. Water injection was also used to increase the hydrogen energy share [36], to suppress knocking and thermal NOx emission [37] in dual fuel CI engines. The above introduction section indicates that the EGR and water injection techniques have been mainly used to control NOx emission in hydrogen fueled SI engines. The effect of these techniques on combustion anomalies (backfire and knocking) is mentioned in very few papers, so a detailed study is required. Along with this, a comparative evaluation of the cooled EGR and water injection in terms of backfire control, NOx emission reduction and performance of a hydrogen fueled multi-cylinder spark ignition engine is not available in the literature. So the present study is aimed to address this research gap in the literature. In this study, a comparative evaluation of cooled EGR and water injection in terms of their effects on backfire, combustion, NOx emission and performance of a hydrogen fueled multi-cylinder spark ignition engine was carried out.
Experimental details The experiment tests were conducted on a multi-cylinder SI engine fueled with hydrogen. The engine was a genset of 28 kW rated power output with CNG and modified to run with hydrogen. The technical specifications of the engine are given in Table 1. The engine was connected with a loading system comprises of heating coil and cooling fan. The schematic diagram of the experimental setup is shown in Fig. 1. The ignition and injection characteristics of the engine were
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Table 1 e Technical specifications of the engine. Description Rated power output No. of cylinders Speed Bore Stroke Compression ratio Intake valve open and close Exhaust valve open and close
Values 28 kW 4 1500 rpm 105 mm 120 mm 12:1 3.5 0bTDC and 81.5 0aBDC 73 0bBDC and 6.5 0aTDC
controlled by an electronic control unit (ECU). A piezoelectric pressure transducer and an optical crank angle encoder were mounted on cylinder head and crankshaft of the engine respectively. The output signal of the pressure transducer and crank angle encoder was given to combustion analyzer for analyzing combustion characteristics with the help of a post processing software. The engine out NOx emission was measured by AVL di-gas analyzer which is a non-dispersive infrared based gas analyzer. Flash back arrestors were placed in the hydrogen gas supply line for avoiding any hazard due to backfire. A gas mass flow meter, based on coriolis principle, and an air flow meter, based on thermal principle, were used for measuring the hydrogen gas flow rate and air flow rate respectively. Two surge tanks, one in the air supply line and another in the exhaust line, were used for ensuring the uniform supply of air/exhaust gas (for EGR) to the engine without any fluctuations. The flow rate of EGR was controlled by EGR knob. An intercooler was employed in the EGR line to reduce the temperature of the exhaust gas. The water vapor which gets condensed was removed with the help of a valve placed after the intercooler. The rotameter was used to measure the EGR flow rate. A similar approach was used by various researchers [38,39,40] to measure the flow rate of exhaust gas. The demineralized water (DM water) was selected for water
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injection purpose. The DM water was supplied to the engine via water injectors and the amount of water injected was controlled by a separate ECU and measured by a weighing machine as shown in the figure. In order to prove the accuracy of the experiments, the uncertainty analysis was performed and the details of the analysis are given in Appendix A.
Methodology The equivalence ratio (4) of the engine was varied from 0.44 to 0.82 in order to observe backfire. At 4 ¼ 0.82, combustion anomalies namely knocking and backfire (of low intensity and of high intensity) were observed. This equivalence ratio (4 ¼ 0.82) can be called as backfire occurring equivalence ratio (BOER). At this 4, the EGR and water injection strategies were used to control the combustion anomalies and NOx emission. Then the optimum EGR rate (on volume basis) and optimum water to hydrogen ratio (WHR, on mass basis) were identified based on the NOx emission reduction, engine stability and performance characteristics of the engine. In each case, the observations were taken and analyzed. The in-cylinder pressure with respect to crank angle was measured and various combustion characteristics were calculated as given below. The in-cylinder temperature of the mixture, T, is calculated between intake valve closing (IVC) and exhaust valve opening (EVO) by Eq. (1) T¼
pV m_ mix Rmix
(1)
where, p is the in-cylinder pressure in Pascal, V is the instantaneous volume of the cylinder in cubic meters and calculated by Eq. (2), m_ mix is the mass flow rate of the mixture (¼ mass flow rate of air (m_ a ) þ mass flow rate of hydrogen (m_ H2 )) and Rmix is the gas constant of the mixture in Joules per grams Kelvin.
Fig. 1 e Schematic diagram of experimental setup.
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"
( V ¼ Vc
2
1 l l 1 þ ðrc 1Þ þ 1 cos q 2 sin2 q 2 a a
COVNIMEP ¼
12 #) (2)
where, Vc is the clearance volume, rc is the compression ratio, l is the connecting rod length, a is the radius of crank and q is the crank angle. The heat release rate can be calculated as given in Eq. (3) dQ g dV 1 dp dQht ¼ p þ V þ dq g 1 dq g 1 dq dq
(3)
where, Q is the heat release rate in Joules per degree crank angle, g is the ratio of specific heats, and Qht is heat transfer to the cylinder walls in Joules per degree crank angle and calculated by Eq. (4). dQht dA ¼ hðT Tw Þ dq dq
(4)
where, h is convective heat transfer coefficient which was calculated by Woschni's correlation [27], Tw is the mean cylinder wall temperature in Kelvin, A is the instantaneous heat transfer surface area of the combustion chamber in square meters. The cumulative heat release can be calculated by Eq. (5) Qcum ðqÞ ¼ Qðq 1Þ þ QðqÞ
(5)
The combustion duration (CD) can be calculated by Eq. (6) 90% heat Z release
CD ¼
dq
(6)
SOC
where, SOC (start of combustion) is the crank angle at which 5% heat is released. The volumetric efficiency and brake thermal efficiency of the engine can be calculated as given in Eq. (7) and Eq. (8) respectively m_ a hv ¼ 2 ra Vs N hbth
(7)
þ360CA Z
pdV
(9)
360CA
The coefficient of variation (COV) of maximum in-cylinder pressure (Pmax) and net indicated mean effective pressure (NIMEP) can be calculated by Eq. (10) and Eq. (11) respectively. COVPmax ¼
where, s is the standard deviation, which can be calculated by Eq. (12), and m is the mean value of the parameter (Pmax, NIMEP) vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ) u( N u 1 X ðxi mx Þ2 s¼t N i¼1
(12)
where, N is total number of cycles, i is the cycle number, and x is the parameter (Pmax, NIMEP) The minimum ignition energy (MIE) of hydrogen-air charge can be calculated by Eq. (13), which is modified from Ref. [41] by introducing a constant A for hydrogen. The calculated MIE is in good agreement with the reported value in Ref. [42]. MIE ¼
A 3 pd rb cavg ðTb Tu Þ 6
(13)
Where, A is a constant (¼0.5 for hydrogen), d is quenching distance, rb is molar density of the charge at adiabatic flame temperature (Tb ), cavg is average molar specific heat of the charge at constant pressure in the temperature range of Tu to Tb . The molar specific heats of various constituents of incylinder charge (air, hydrogen, water vapor, nitrogen etc.) as a function of temperature can be determined by Eq. (14) cp ¼ a þ bT þ cT2 þ dT3
(14)
where, cp is in kJ/kmolK, T is in K and a, b, c and d are the constants, whose values are taken from Ref. [43]. As in the present study, cooled EGR strategy was used so it is necessary to determine the saturation temperature of water vapor in the recirculated exhaust gas. Below is the procedure of determining the saturation temperature of water vapor. The generalized combustion equation of hydrogen for any equivalence ratio (4) can be written as given in Eq. (15) H2 þ
1 3:76 ðO2 þ 3:76N2 Þ/H2 O þ N2 þ 24 24
1 0:5 O2 24
(15)
(8)
where, ra is the density of incoming air, Vs is the swept volume of the engine, N is speed of the engine and CV is calorific value of hydrogen fuel. Net indicated mean effective pressure (NIMEP) can be calculated by Eq. (9) 1 Vs
(11)
The mole fraction of water vapor (H2O) in the exhaust gas mixture can be calculated by Eq. (16)
BP ¼ m_ H2 CV
NIMEP ¼
sNIMEP 100 % mNIMEP
spmax 100 % mpmax
(10)
xH2 O ¼
þ 1 þ 3:76 24
1 h 1 24
i 0:5
(16)
The partial pressure of a species in a mixture can be calculated as per Dalton's law by using Eq. (17) Pi ¼ xi P
(17)
Where, P is the total pressure and xi is the mole fraction of the species i in the mixture. As the species of interest in the exhaust gas mixture is water (H2O), the saturation temperature of H2O at known partial pressure can be calculated by using various correlations such as Antoine equation, Magnus equation, Tetens equation, Arden Buck equation and Goff-Gratch equation. Among these correlations, Arden Buck equation is relatively
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Fig. 2 e Variation of the temperatures with EGR rates. more accurate in the measured temperature range [44], thus the equation was used in the present work and is given below in Eq. (18) PH2 O ¼ 0:61121 exp 18:678
T T 234:5 257:14 þ T
(18)
The saturation temperature of water in the exhaust gas is higher than the EGR temperature at the intercooler exit for all EGR rates as shown in Fig. 2. The water vapor gets condensed and thus removed subsequently before entering into the intake manifold. Thus the main constituents of cooled EGR are nitrogen, oxygen and some traces of unburned hydrogen in the present study.
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Fig. 4 e Increase in MIE at MBT timing with EGR rate and water injection. were used to control knocking and backfire at 4 ¼ 0.82, and the details of the study can be found in the recently published work [1]. In the present work, the EGR and water injection techniques were used to control/eliminate backfire, knocking and reduce NOx emission at high equivalence ratio (4 ¼ 0.82). The EGR rate was varied from 5% to 28% (volume basis), and water to hydrogen ratio (WHR) was varied from 1.5 to 9.25. The equivalence ratio was maintained at 0.82 ± 0.01 during the experiments. The findings of the study are discussed below.
Effect of EGR and water injection on combustion anomalies
The high power output of an engine can be achieved by running the engine at high equivalence ratio, but there are issues such as backfire, knocking and high NOx emission at high equivalence ratio in a hydrogen fueled SI engine. The initial experimental tests were conducted on the engine by varying equivalence ratio till the backfire occurred. The equivalence ratio of 0.82 was identified as backfire occurrence equivalence ratio. Several techniques can be used to control backfire at high equivalence ratio such as delayed hydrogen gas injection, retarded ignition, EGR, water injection etc. The delayed hydrogen gas injection and retarded ignition timing
The combustion anomalies associated with hydrogen fueled SI engine at high equivalence ratio (4 ¼ 0.82) are knocking and backfire as shown in Fig. 3. The cycle based analysis shows that the peak in-cylinder pressure reduces to 25 bar (motoring value) due to backfire. With the use of EGR and water injection, the combustion anomalies were eliminated as shown in the figure. The EGR rate and WHR for the given trends in the figure are 10% and 1.5 respectively. The EGR and water injection in the intake manifold increase the molar specific heat of the in-cylinder mixture. It can be seen from Eq. (13) that the MIE requirement of hydrogen-air mixture is directly proportional to the molar specific heat of the mixture. Thus MIE increases with specific heat of the mixture. The MIE of hydrogen-air mixture at maximum brake torque (MBT) timing (16 0bTDC) was
Fig. 3 e Control of combustion anomalies with EGR and water injection.
Fig. 5 e Variation of peak in-cylinder pressure with EGR rate and water injection.
Results and discussion
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Fig. 6 e Variation of peak in-cylinder temperature with EGR rate and water injection.
Fig. 7 e Variation of exhaust gas temperature with EGR rate and water injection.
Fig. 9 e Variation of volumetric efficiecy of the engine with EGR rate and water injection.
The peak in-cylinder pressure was not changed up to 15% EGR but further increasing the EGR rate decreases the peak incylinder pressure marginally whereas it decreases significantly with water injection (Fig. 5). Similar kind of trends was observed for peak in-cylinder temperature also (Fig. 6). The reason for these trends is the dilution of in-cylinder mixture caused by the high specific heat of diluents (EGR and water). In this study, the main constituent of cooled EGR is nitrogen and its specific heat is 1.039 kJ/kgK (at NTP) and the specific heat of water is 4.18 kJ/kgK (at NTP) which are higher than the specific heat of air (1.005 kJ/kgK, at NTP), main constituent of the incylinder mixture. This difference between the specific heats of nitrogen and water is the main reason for the faster reduction in in-cylinder pressure and temperature with WHR as compared to EGR. Therefore, more the WHR or EGR rate (means more nitrogen) more is the dilution of in-cylinder mixture and thus reduction in in-cylinder pressure and temperature. The variation of exhaust gas temperature with EGR rate and water injection is shown in Fig. 7. It can be seen from the figure that there was a marginal change in exhaust gas temperature with EGR rates. A similar trend was also observed with water injection up to WHR of 7.5, but further increase in WHR reduced the exhaust gas temperature drastically. The variation of the various combustion events with EGR rate and water injection are shown in Fig. 8. The start of
Fig. 8 e Variation of combustion events with EGR rate and water injection.
calculated for each case (with and without diluents (EGR and water)). The percentage change in the MIE at MBT timing is shown in Fig. 4 and it indicates that the MIE of hydrogen-air mixture increases marginally with EGR rate and it increases significantly with WHR. This increase in MIE requirement reduces the probability of backfire occurrence with EGR and water injection.
Fig. 10 e Variation of brake thermal efficiency of the engine with EGR rate and water injection.
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Fig. 11 e Variation of COV with EGR rate and water injection.
Fig. 12 e Variation of NOx emission with EGR rate and water injection.
combustion (SOC) was delayed with EGR rate and water injection due to the dilution caused by recirculated exhaust gas and water of high heat capacity which increases the requirement of minimum ignition energy. There was increase in flame development angle (FDA) and overall burning angle (OBA) with EGR rate and water injection due to reduction in flame velocity caused by dilution of in-cylinder mixture and therefore the combustion duration was increased with EGR rate and water injection. The cumulative effect of all these combustion events reduces the probability of backfire occurrence. The change in these combustion events was higher with water injection than EGR rate. This shows that the backfire can be controlled faster with water injection as compared to that of with EGR.
Effect of EGR and water injection on performance The volumetric efficiency (hv) of the engine was marginally affected with EGR (Fig. 9). The change in the volumetric
efficiency is under uncertainty up to 25% of EGR, and it improved marginally at EGR rate of 28%. The reason for this marginal improvement in volumetric efficiency could be the improved density of incoming charge due to the cooling effect of high flow rate of cooled EGR. A similar trend of volumetric efficiency was observed with water injection, and it was under uncertainty limit up to WHR of 7.5. Beyond WHR of 7.5, the volumetric efficiency reduced marginally due to the high flow rate of water which restricted the flow of incoming charge to engine's cylinder. The brake thermal efficiency of the engine was not affected up to EGR rate of 25% but further increase in EGR rate, thermal efficiency was reduced significantly (Fig. 10). The reason for this reduction in thermal efficiency of the engine at 28% EGR could be the high dilution caused by the high flow rate of EGR. The variation of brake thermal efficiency with WHR is under uncertainty limit in all cases. The reason for this trend could be the nature of water which acts as a working fluid, steam, under high in-cylinder temperatures (above 2000K) and pressure (above 30 bar) conditions. An another reason of these trends of volumetric efficiency and brake thermal efficiency with cooled EGR and water injection could be the equivalence ratio of the engine, which was maintained around 0.82 (±0.01) by changing the throttle position. Another important parameter is the coefficient of variation (COV) of maximum in-cylinder pressure (Pmax) and indicated mean effective pressure (IMEP) that indicates the stability of engine operation. It can be observed from Fig. 11 that stable operation of the engine can be achieved till the WHR of 7.5 and EGR rate of 25%. Further increase in WHR and EGR rate, increases the cyclic variation drastically. In the present study, the COV was considered as one of the parameters, along with performance and NOx emission reduction, to decide the optimum flow rate of EGR and water injection.
Effect of EGR and water injection on NOx emission Hydrogen fueled spark ignition engine emits mainly NOx emission as it is a carbon-free fuel. However, some traces of HC and CO were sometimes observed in the exhaust due to evaporation of engine lubricating oil. High in-cylinder temperature, availability of oxygen, number of chemical species present in the combustion chamber are the main culprits of higher NOx emission with hydrogen fueled SI engine as compared to gasoline or CNG fueled SI engine. This high NOx emission is one of the main technical challenges in hydrogen fueled engine. In the present study, the NOx emission was reduced by employing EGR and water injection in the intake manifold of the engine, and the result is shown in Fig. 12. The NOx was reduced significantly with EGR rate from 10.52 g/ kWh to 4.10 g/kWh at 0% EGR to 28% EGR respectively. The reduction in NOx emission was exponential with water injection. It was reduced to 0.1 g/kWh at WHR of 9.25. This WHR
Table 2 e Summary of the optimum operating conditions for the engine. Strategy Cooled EGR Water injection
Hydrogen consumption (g/kWh)
Water consumption (g/kWh)
EGR (%)
WHR
NOx emission (g/kWh)
104.45 104.8
e 785.4
25 e
e 7.5
4.55 0.34
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Water injection is more effective strategy for controlling backfire with near zero (ultra-low) NOx emission as compared to cooled EGR
Cooled EGR can control backfire with significant reduction in NOx emission
Water injection 2
The NOx emission reduced significantly due to decrease in in-cylinder temperature caused by (i) charge dilution and (ii) increased number of chemical species in the combustion reaction The NOx emission reduced exponentially due to a drastic decrease in in-cylinder temperature caused by (i) high charge dilution and (ii) increased number of chemical species in the combustion reaction The probability of backfire occurrence reduced due to increase in molar specific heat of in-cylinder mixture which increases the requirement of MIE and thus delays the combustion events The increase in molar specific heat of incylinder mixture and the requirement of MIE were higher with water injection as compared to EGR, so this strategy is more effective for controlling backfire Cooled EGR 1
Effect on NOx emission Effect on backfire Strategy S.No.
Table 3 e Summary of the effects of cooled EGR and water injection on backfire and NOx emission.
Concluding Remark
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 4 4 ( 2 0 1 9 ) 6 2 8 7 e6 2 9 8
corresponds to hydrogen consumption rate of 103.63 g/kWh and water consumption rate of 958.8 g/kWh. The main reasons for this drastic reduction in NOx emission are the lower in-cylinder temperature caused by the dilution of the incylinder mixture by water and increase in the number of chemical species in the combustion reaction. It can be clearly seen in the figure that near zero (ultra-low) NOx emission is possible with water injection. The summary of the optimum operating conditions for the engine with both the strategies is given in Table 2 and the results of cooled EGR and water injection on backfire and NOx emission are given in Table 3.
Conclusions The experimental work was carried out on a multi-cylinder hydrogen fueled SI engine at a constant speed of 1500 rpm. The equivalence ratio of the engine was maintained around 0.82 (±0.01). The cooled EGR and water injection strategies were used to control backfire and reduce NOx emission. From the observations made in the study, following conclusions are emerged. Both the techniques (EGR and water injection) are very effective for controlling backfire. But backfire can effectively be controlled with water injection than that of with EGR due to higher MIE requirement and higher flame speed reduction with water injection as compared to EGR. The NOx emission was exponentially reduced with water injection, and it was reduced significantly with EGR rate. With 25% EGR and WHR of 7.5, the NOx emission was reduced by 57% and 97% respectively without affecting the performance of the engine. The optimum WHR of 7.5 and cooled EGR rate of 25% were observed in terms of performance, emission and stability of the engine operation. The salient point which is emerged from this study is that backfire-free engine operation, and near zero (ultra-low) NOx emission can be achieved with water injection without affecting the performance of the engine. EGR can also reduce the NOx emission up to certain level without affecting the performance of the engine. Therefore, it can be concluded from the present study that water injection is a better strategy as compared to cooled EGR for achieving backfire-free engine operation with near zero (ultra-low) NOx emission.
Appendix A. Uncertainty analysis The uncertainty in the measurement of several parameters such as mass flow rate of hydrogen, air flow rate, etc. was determined by using standard deviation (SD) and number of measurements (n) (standard uncertainty or standard error of SD ffiffi) with 95% confidence level (1.96 SE). mean ðSEÞ ¼ p n Whereas, the uncertainty in calculated parameters such as brake thermal efficiency, volumetric efficiency, etc. was determined by using the method given by Taylor [45] for finding the uncertainty in a function of several independent variables and is explained below.
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Table A.1 e Uncertainty in various parameters S.No 1 2 3 4 5 6 7 8 9 10
Parameter
Measured/calculated
Uncertainty (%)
Air flow rate Fuel flow rate Exhaust gas temperature Engine speed Equivalence ratio Brake thermal efficiency Volumetric efficiency NOx emission EGR percentage Water to hydrogen ratio (WHR)
Measured Measured Measured Measured Calculated Calculated Calculated Calculated Calculated Calculated
±0.48 ±0.70 ±0.11 ±1.00 ±0.85 ±0.22 ±0.41 ±0.84 ±1.98 ±1.78
If q is a function of x1 x2 , …, xn ðq ¼ f ðx1 ; x2 ; …; xn ÞÞ ), and the variables are measured independently with the uncertainty of dx1 ; dx2 ; …; dxn respectively, then the uncertainty in q is calculated by Eq. (A.1)
[2]
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi u( 2 2 2 ) u vq vq vq t dx1 þ dx2 þ … þ dxn dq ¼ ± vx1 vx2 vxn
[3]
(A.1) The uncertainty in the calculation of equivalence ratio is given below as a sample application of Eq. (A.1); The equivalence ratio (4) of hydrogen fueled SI engine can be calculated from Eq. (A.2) 4 ¼ 34:3
m_ f m_ a
[4]
[5]
(A.2)
The uncertainty in the measurement of air flow rate (dm_ a ) and hydrogen flow rate (dm_ f ) is ±0.1041 g/s and ±0.0036 g/s respectively. The uncertainty in the equivalence ratio (d4) can be calculated as shown in Eq. (A.3)e(A.6) vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u( 2 2 ) u v4 v4 t _ _ dmf þ dma d4 ¼ ± vm_ f vm_ a ffi vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u8 !2 9 2 u< = 34:3 m_ f 34:3 u d4 ¼ ±t dm_ f þ dm_ a 2 : m_ a ; m_ a
(A.3)
[8]
(A.5) d4 ¼ ±6:9915 10 z±0:007z±0:85%
[7]
(A.4)
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u( 2 2 ) u 34:3 34:3 0:5133 t 0:0036 þ 0:1041 d4 ¼ ± 21:4709 21:47092
3
[6]
[9]
[10]
(A.6)
In the same manner, the uncertainty in several calculated parameters was determined. The uncertainty in various measured and calculated parameters is given in Table A1.
[11]
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Control of backfire and NOx emission reduction in a hydrogen fueled multi-cylinder spark ignition engine using cooled EGR and water injection strategies Vipin Dhyani, K.A. Subramanian* Engines and Unconventional Fuels Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi 110016, India
article info
abstract
Article history:
The experimental study was carried out on a constant speed multi-cylinder spark ignition
Received 3 July 2018
engine fueled with hydrogen. Exhaust gas recirculation (EGR) and water injection tech-
Received in revised form
niques were adopted to control combustion anomalies (backfire and knocking) and reduce
26 December 2018
NOx emission at source level. The experimental tests were conducted on the engine with
Accepted 10 January 2019
varied EGR rate (0%e28% by volume) and water to hydrogen ratio (WHR) (0e9.25) at 15 kW
Available online 5 February 2019
load. It was observed from the experiments that both the strategies can control backfire effectively, but water injection can effectively control backfire compared to EGR. The water
Keywords:
injection and EGR reduce the probability of backfire occurrence and its propagation due to
Hydrogen fueled spark ignition en-
the increase in the requirement of minimum ignition energy (MIE) of the charge, caused
gine
mainly due to charge dilution effect, and reduction in flame speed respectively. The NOx
Backfire
emission was continuously reduced with increase in EGR rate and WHR, but at higher rates
NOx emission
(of EGR and WHR), there was an issue of stability of engine operation. It was found from the
Exhaust gas recirculation (EGR)
experimental results that at 25% EGR, there was 57% reduction in NOx emission without
Water injection
drop in brake thermal efficiency whereas, with WHR of 7.5, the NOx emission was reduced by 97% without affecting the efficiency. The salient point emerging from the study is that water injection technique can control backfire with ultra-low (near zero) NOx emission without compromising the performance of the hydrogen fueled spark ignition engine. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Among various alternative fuels, hydrogen is the only carbon free fuel available and considered as most promising clean fuel for internal combustion (IC) engines. Hydrogen can be used in compression ignition (CI) engine as well as in spark
ignition (SI) engine. However, use of neat (100%) hydrogen cannot be possible in CI engine because of its high autoignition temperature that requires some ignition source. Along with high auto-ignition temperature, hydrogen is having high octane number which makes it more suitable fuel for spark ignition (SI) engines. High flame velocity of hydrogen improves the performance of SI engine as compared to
* Corresponding author. E-mail address: [email protected] (K.A. Subramanian). https://doi.org/10.1016/j.ijhydene.2019.01.129 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature Abbreviations aBDC after bottom dead center aTDC after top dead center bBDC before bottom dead center BOER backfire occurring equivalence ratio BSFC brake specific fuel consumption bTDC before top dead center CD combustion duration CFD computational fluid dynamics CFR cooperative fuel research CI compression ignition CNG compressed natural gas CO carbon monoxide carbon dioxide CO2 COV coefficient of variation DM demineralized ECU electronic control unit EGR exhaust gas recirculation EVO exhaust valve opening FDA flame development angle FKGR flame kernel growth rate GDI gasoline direct injection water vapor H2O HC hydrocarbon IC internal combustion IMEP indicated mean effective pressure ISFC indicated specific fuel consumption IVC intake valve closing MBT maximum brake torque MIE minimum ignition energy nitrogen gas N2 NIMEP net indicated mean effective pressure NOx oxides of nitrogen
conventional gasoline fueled SI engines. However, there are some technical issues such as backfire, power drop and high NOx emission need to be resolved in a hydrogen fueled SI engine. Backfire is a pre-ignition phenomenon that takes place in the intake manifold and/or combustion chamber during intake stroke of the engine. During backfire, the flame propagates from the combustion chamber to upstream of the intake manifold. Backfire is identified by a high pitched sound and sudden rise in intake manifold pressure and in-cylinder pressure during the intake stroke of the engine. The cause of backfire includes the presence of any high temperature source in the combustion chamber of the engine such as hot exhaust valves, hot residual gas, hot spark plug, radicals and lubricating oil particles. The next combustion anomalies associated with hydrogen fueled SI engine are pre-ignition and knocking. The detailed information about these combustion anomalies (backfire, pre-ignition and knocking) including the difference between them and their relationship with each other is given in the recently published work [1]. The factor
NTP OBA SI SOC WHR
normal temperature and pressure overall burning angle spark ignition start of combustion water to hydrogen ratio
Symbols a crank radius (m) A instantaneous heat transfer surface area of combustion chamber (m2) average molar specific heat of the charge at cavg constant pressure (kJ/kmol K) molar specific heat at constant pressure (kJ/kmol cp K) d quenching distance (m) h convective heat transfer coefficient (W/m2 K) l length of connecting rod (m) p in-cylinder pressure (Pascal) Q net heat release (J) heat transfer to cylinder wall (J) Qht compression ratio rc gas constant of the mixture (J/kg K) Rmix Tb flame temperature (K) unburnt gas temperature (K) Tu mean cylinder wall temperature (K) Tw V instantaneous cylinder volume (m3) clearance volume (m3) Vc swept volume (m3) Vs g ratio of specific heats brake thermal efficiency hbth hv volumetric efficiency q crank angle density of incoming air (kg/m3) ra molar density of the charge (mol/m3) rb
which reduces the chance of one combustion anomaly also reduces the probability of another combustion anomaly. Varde and Frame [2], Liu et al. [3], Subramanian and Salvi [4], Hong et al. [5] and Dhyani and Subramanian [1] reported in their respective studies that the probability of backfire occurrence in a hydrogen fueled engine can be reduced by delaying the timing of hydrogen injection due to availability of more time for cooling of the in-cylinder mixture by fresh incoming air during intake stroke of the engine. Salvi and Subramanian [6] and Ding et al. [7] reported that increasing compression ratio reduces the chances of backfire due to the reduced amount of residual gases in the combustion chamber. Varying valve overlap period is also reported as an effective method of controlling backfire in a hydrogen fueled SI engine [8e10,20]. Postponing the ignition timing can also control backfire occurrence [1,11] due to reduction in reaction rate of the chemical reaction. A limited number of researchers reported in literature about knocking in hydrogen fueled SI engine. Li and Karim [12] reported that knocking is the most important consideration to be avoided in hydrogen fueled SI
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engines at high equivalence ratio. The visualization study on knocking suggests that auto-ignition of end gas is the cause of development of high pressure waves that lead to knocking [13]. Szwaja and Naber reported that the instable and fast combustion of hydrogen can cause knocking in a hydrogen fueled engine with low intensity as compared to that caused by auto-ignition of end gas which was of high intensity [14]. The research findings of Dhyani and Subramanian indicate that backfire and knocking are interlinked with each other at high equivalence ratio and the conditions of reducing one lead to reduction in the probability of another [1]. The delayed hydrogen injection and retarded spark timing are the strategies used for controlling these combustion anomalies. Along with backfire, hydrogen fueled SI engine also emits higher NOx emission as compared to CNG or gasoline fueled SI engine at high equivalence ratio. Therefore, it is necessary to reduce NOx emission along with backfire at high equivalence ratio. It is reported by Woolley and Henriksen [15] and Das [16] that some form of diluent such as exhaust gas recirculation (EGR) or water injection is necessary for operating hydrogen fueled SI engine above equivalence ratio of 0.6 (f 0.6). EGR is a well-established technique to reduce engine out NOx emission at the source level. The exhaust gas of hydrogen fueled SI engine mainly constituents of H2O vapor, N2, excess oxygen and unburned hydrogen. The constituents of EGR depend on the type of EGR strategy adopted namely hot EGR and cooled EGR. In hot EGR, the exhaust gas is directly recirculated without cooling whereas in cooled EGR, the exhaust gas is cooled by some cooling medium such as cold water, and thus the water vapor gets condensed and removed and therefore in cooled EGR, only gases (N2, excess oxygen, and unburned hydrogen) are introduced in the engine cylinder. Das and Mathur reported that 15% EGR is the optimum percentage under part-throttle conditions [17]. The reduction in BSFC was also observed with increasing EGR rate due to reduced pumping work and reduced heat loss to cylinder wall caused by lower in-cylinder temperatures. They also mentioned the reduction in the possibility of backfire (due to reduced flame velocity with EGR) and knocking with EGR but did not provide any data. Heat exchanger was used to reduce the temperature of EGR upto atm state. Heffel achieved near zero NOx emission (less than 1 ppm) by using EGR (exact EGR % was not specified) and 3-way catalytic convertor [18]. The EGR strategy was also compared with lean-burn strategy and suggested that EGR is more beneficial at high load and leanburn strategy is more suitable at low load because EGR results in low emission and high power output with low efficiency whereas lean-burn results in low emission and high efficiency with low power output. Verhelst et al. reported that the stoichiometric operation of the engine with hydrogen is possible with supercharging without backfire and pre-ignition while maintaining low NOx emission by using EGR strategy with 3-way catalytic convertor [19,20]. They also reported higher power output with hydrogen as compared to gasoline and methane with the strategy. Mohsen et al. carried out a CFD study to observe the effect of initial charge pressure (supercharging) and different EGR rates on performance and emissions of a spark ignition engine fueled with gasoline and various alternative fuels (methane, propane, methanol, ethanol, and hydrogen) [21]. The NOx emission drastically
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reduced between 0 and 10% EGR and significantly between 10 and 20% EGR in all cases. The maximum reduction in NOx emission due to EGR was reported with methane. The IMEP of the engine decreased with EGR as EGR replaces the incoming air and thus the availability of in-cylinder oxygen which causes to decrease the flame speed and thus the combustion rate resulting in lower in-cylinder peak pressure. The IMEP of the engine with hydrogen was the least affected by EGR because of its higher flame speed and higher calorific value compared to other fuels. The effect of supercharging was also the least on hydrogen fueled engine. Subramanian et al. evaluated the effect of various diluents such as N2, CO2 and hot EGR on performance, emission and combustion characteristics of the engine [22]. It was concluded that N2 is the best diluent followed by hot EGR and CO2 in terms of low NOx emission and high thermal efficiency. The NOx emission was reduced to 1000 ppm with EGR of 12.2% along with the sacrifice of 1.8% indicated brake thermal efficiency. It was also suggested that the use of cooled EGR could give better results than that of with hot EGR. Safari et al. carried out a simulation study for comparative evaluation of the effect of EGR (cooled and hot) and lean-burn strategies on NOx emission, thermal efficiency and power output of hydrogen fueled SI engine [23]. Based on their analytical work they reported that lean burn is only beneficial at low load while EGR is better at mid and high load. The comparison was also done between cooled and hot EGR and reported that for same NOx emission reduction, the cooled EGR is more effective as it results in high indicated thermal efficiency and high indicated power as compared to hot EGR. The reason for this is the high temperature and constituents of the hot EGR (i.e water vapor and N2 at stoichiometric mixture) which reduces the intake charge density (thus volumetric efficiency) and reduces the in-cylinder temperature and pressure due to higher dilution effect as compared to cooled EGR which contains mainly nitrogen (at stoichiometric mixture) at relatively low temperature. Kosmadakis et al. carried out experimental and simulation study on a single cylinder cooperative fuel research (CFR) engine with compression ratio of 9:1 and constant speed of 600 rpm for analyzing combustion phenomena at high EGR rates (12%e 47% by mass) [24]. The EGR strategy was used to control the engine load with reduced NOx emission while maintaining stoichiometric hydrogen-air mixture. The predictions of CFD code were highly accurate mainly for low and mid EGR rates. The load and gross IMEP of the engine decreased with EGR rates. However, the indicated efficiency first increased with EGR due to lower combustion dissociation and low engine heat transfer and then decreased with the EGR rate (42% onwards) due to low combustion efficiency caused by reduced flame speed, increased combustion duration and unburned hydrogen in the exhaust. The unburned hydrogen of up to 0.5% by volume at high EGR rate was reported in the study. Salvi and Subramanian conducted a comparative study between spark timing retardation and EGR technique in terms of NOx emission reduction from a single cylinder SI engine generator set fueled with hydrogen and concluded that EGR is the most suitable technique [6]. They reported 50% reduction in NOx emission with marginally affecting thermal efficiency and engine power output with 20% EGR (volume based). The experimental investigation on the effect of EGR on flame
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kernel growth rate (FKGR) was also carried out [25]. It was reported in the findings that FKGR was reduced from 16.7 m/s to 6.6 m/s with EGR of 0e18% (by volume) respectively. This reduction in FKGR with the use of EGR signifies the abnormal combustion free operation at high equivalence ratio. The EGR (14%) with hydrogen-SCR was used by Krishnan Unni et al. to reduce the NOx emission by 70% from a multi-cylinder turbocharged SI engine fueled with hydrogen at equivalence ratio of 0.64 [26]. Szwaja and Naber reported in their findings that EGR can be used to eliminate knock in hydrogen fueled SI engine [27]. Another technique for reducing NOx emission at source level in a hydrogen fueled SI engine is water injection. The water vapor present in the engine exhaust can be used for this purpose after condensing and storing it in a tank. It is reported that Roger Billings (in 1972) was the first researcher to use water induction for NOx emission reduction in a hydrogen fueled engine [15]. Woolley and Henriksen conducted experiments on a 1975 Dodge 440 CID, V8 engine fueled with hydrogen [15]. The engine was of carburetor based fuel supply system and having compression ratio of 12:1. The water to hydrogen fuel ratio up to 12 was tested. The variation in equivalence ratio of about 10% between cylinder to cylinder was reported as a possible cause of random backfire. It was reported that water induction eliminates backfire at all equivalence ratios along with the exponential reduction in NOx emission. Engine exhaust gas temperature was not changed significantly (remains almost same) with all water to hydrogen fuel ratios. Spark timing (which was set for least NOx) was also not affected by water induction. The brake thermal efficiency of about 33% was observed at all water flow rates and it was not affected significantly with water induction. The power output was also not affected by water induction. The amount of water that is required for backfire elimination depends mainly on the engine speed and equivalence ratio. It is reported that water to fuel ratio of 4:1 was an effective proportional to eliminate backfire without causing any ill effects [16]. Subramanian et al. reported 67.5% reduction in NOx emission (from 7670 ppm to 2490 ppm) without affecting brake thermal efficiency with 5.9 kg/h water flow rate at equivalence ratio of 0.82 [28]. The maximum water to hydrogen ratio of 7.5 was used in the work. They also concluded that water injection is more beneficial as compared to spark timing retardation in terms of NOx emission reduction. Kahraman et al. carried out experiments on a carburetor based four cylinder SI engine fueled with hydrogen by varying speed from 2600 rpm to 3800 rpm [29]. They mentioned that the water injection in intake manifold was essential to operate engine above 2600 rpm in order to get backfire free operation. The amount of water injection depends on the engine speed as water quantity needs to increase with increase in engine speed. The NOx emission was observed very low as compared to the engine operating with gasoline due to water injection along with lean operation of the engine. Based on the simulation study, Boretti reported that stoichiometric operation of hydrogen fueled turbocharged SI engine can be possible with water injection [30]. Port water injection along with direct hydrogen injection and jet ignition can suppress the combustion anomalies associated with hydrogen engine (pre-
ignition, backfire and knocking) and reduce NOx emission. At full load and maximum speed of 5000 rpm, brake thermal efficiency of about 40% was reported in the paper. The paper is fully based on simulation study and does not provide any experimental validation. There are only few studies available in the literature on water injection in hydrogen fueled SI engine whereas water injection has been extensively used with other fuels. Mingrui et al. carried out simulation study on the effect of direct water injection in a GDI (gasoline direct injection) engine [31]. The optimum water ratio of 15% (of fuel) was reported in terms of performance and emissions characteristics of the engine. The researchers reported higher peak in-cylinder pressure with water injection than that of without water injection in a turbocharged SI engine [32,33]. The water to fuel ratio of up to 0.17 was limited due to an excessive rise in in-cylinder pressure beyond this ratio [32]. The knock was effectively suppressed at this water to fuel ratio. The expansion work and thermal efficiency and thus BSFC was improved with water injection. The water to fuel ratio of up to 0.2 (mass basis) was limited by Ref. [33] due to unaccepted engine oil dilution in the crankcase. Hoppe et al. performed experimental work on a single cylinder gasoline direct injection engine with direct water injection [34]. It was reported that ISFC decreases with water injection. It was suggested that further reduction in fuel consumption can be achieved by combining water injection with cooled EGR. The simulation study of Bozza et al. on a turbocharged two cylinder SI engine fueled with gasoline shows an improvement in BSFC, knock resistance characteristics of the engine with cooled EGR and port water injection [35]. It was also reported that the flame speed was decreased faster with water injection than EGR [33]. Water injection was also used to increase the hydrogen energy share [36], to suppress knocking and thermal NOx emission [37] in dual fuel CI engines. The above introduction section indicates that the EGR and water injection techniques have been mainly used to control NOx emission in hydrogen fueled SI engines. The effect of these techniques on combustion anomalies (backfire and knocking) is mentioned in very few papers, so a detailed study is required. Along with this, a comparative evaluation of the cooled EGR and water injection in terms of backfire control, NOx emission reduction and performance of a hydrogen fueled multi-cylinder spark ignition engine is not available in the literature. So the present study is aimed to address this research gap in the literature. In this study, a comparative evaluation of cooled EGR and water injection in terms of their effects on backfire, combustion, NOx emission and performance of a hydrogen fueled multi-cylinder spark ignition engine was carried out.
Experimental details The experiment tests were conducted on a multi-cylinder SI engine fueled with hydrogen. The engine was a genset of 28 kW rated power output with CNG and modified to run with hydrogen. The technical specifications of the engine are given in Table 1. The engine was connected with a loading system comprises of heating coil and cooling fan. The schematic diagram of the experimental setup is shown in Fig. 1. The ignition and injection characteristics of the engine were
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Table 1 e Technical specifications of the engine. Description Rated power output No. of cylinders Speed Bore Stroke Compression ratio Intake valve open and close Exhaust valve open and close
Values 28 kW 4 1500 rpm 105 mm 120 mm 12:1 3.5 0bTDC and 81.5 0aBDC 73 0bBDC and 6.5 0aTDC
controlled by an electronic control unit (ECU). A piezoelectric pressure transducer and an optical crank angle encoder were mounted on cylinder head and crankshaft of the engine respectively. The output signal of the pressure transducer and crank angle encoder was given to combustion analyzer for analyzing combustion characteristics with the help of a post processing software. The engine out NOx emission was measured by AVL di-gas analyzer which is a non-dispersive infrared based gas analyzer. Flash back arrestors were placed in the hydrogen gas supply line for avoiding any hazard due to backfire. A gas mass flow meter, based on coriolis principle, and an air flow meter, based on thermal principle, were used for measuring the hydrogen gas flow rate and air flow rate respectively. Two surge tanks, one in the air supply line and another in the exhaust line, were used for ensuring the uniform supply of air/exhaust gas (for EGR) to the engine without any fluctuations. The flow rate of EGR was controlled by EGR knob. An intercooler was employed in the EGR line to reduce the temperature of the exhaust gas. The water vapor which gets condensed was removed with the help of a valve placed after the intercooler. The rotameter was used to measure the EGR flow rate. A similar approach was used by various researchers [38,39,40] to measure the flow rate of exhaust gas. The demineralized water (DM water) was selected for water
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injection purpose. The DM water was supplied to the engine via water injectors and the amount of water injected was controlled by a separate ECU and measured by a weighing machine as shown in the figure. In order to prove the accuracy of the experiments, the uncertainty analysis was performed and the details of the analysis are given in Appendix A.
Methodology The equivalence ratio (4) of the engine was varied from 0.44 to 0.82 in order to observe backfire. At 4 ¼ 0.82, combustion anomalies namely knocking and backfire (of low intensity and of high intensity) were observed. This equivalence ratio (4 ¼ 0.82) can be called as backfire occurring equivalence ratio (BOER). At this 4, the EGR and water injection strategies were used to control the combustion anomalies and NOx emission. Then the optimum EGR rate (on volume basis) and optimum water to hydrogen ratio (WHR, on mass basis) were identified based on the NOx emission reduction, engine stability and performance characteristics of the engine. In each case, the observations were taken and analyzed. The in-cylinder pressure with respect to crank angle was measured and various combustion characteristics were calculated as given below. The in-cylinder temperature of the mixture, T, is calculated between intake valve closing (IVC) and exhaust valve opening (EVO) by Eq. (1) T¼
pV m_ mix Rmix
(1)
where, p is the in-cylinder pressure in Pascal, V is the instantaneous volume of the cylinder in cubic meters and calculated by Eq. (2), m_ mix is the mass flow rate of the mixture (¼ mass flow rate of air (m_ a ) þ mass flow rate of hydrogen (m_ H2 )) and Rmix is the gas constant of the mixture in Joules per grams Kelvin.
Fig. 1 e Schematic diagram of experimental setup.
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"
( V ¼ Vc
2
1 l l 1 þ ðrc 1Þ þ 1 cos q 2 sin2 q 2 a a
COVNIMEP ¼
12 #) (2)
where, Vc is the clearance volume, rc is the compression ratio, l is the connecting rod length, a is the radius of crank and q is the crank angle. The heat release rate can be calculated as given in Eq. (3) dQ g dV 1 dp dQht ¼ p þ V þ dq g 1 dq g 1 dq dq
(3)
where, Q is the heat release rate in Joules per degree crank angle, g is the ratio of specific heats, and Qht is heat transfer to the cylinder walls in Joules per degree crank angle and calculated by Eq. (4). dQht dA ¼ hðT Tw Þ dq dq
(4)
where, h is convective heat transfer coefficient which was calculated by Woschni's correlation [27], Tw is the mean cylinder wall temperature in Kelvin, A is the instantaneous heat transfer surface area of the combustion chamber in square meters. The cumulative heat release can be calculated by Eq. (5) Qcum ðqÞ ¼ Qðq 1Þ þ QðqÞ
(5)
The combustion duration (CD) can be calculated by Eq. (6) 90% heat Z release
CD ¼
dq
(6)
SOC
where, SOC (start of combustion) is the crank angle at which 5% heat is released. The volumetric efficiency and brake thermal efficiency of the engine can be calculated as given in Eq. (7) and Eq. (8) respectively m_ a hv ¼ 2 ra Vs N hbth
(7)
þ360CA Z
pdV
(9)
360CA
The coefficient of variation (COV) of maximum in-cylinder pressure (Pmax) and net indicated mean effective pressure (NIMEP) can be calculated by Eq. (10) and Eq. (11) respectively. COVPmax ¼
where, s is the standard deviation, which can be calculated by Eq. (12), and m is the mean value of the parameter (Pmax, NIMEP) vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ) u( N u 1 X ðxi mx Þ2 s¼t N i¼1
(12)
where, N is total number of cycles, i is the cycle number, and x is the parameter (Pmax, NIMEP) The minimum ignition energy (MIE) of hydrogen-air charge can be calculated by Eq. (13), which is modified from Ref. [41] by introducing a constant A for hydrogen. The calculated MIE is in good agreement with the reported value in Ref. [42]. MIE ¼
A 3 pd rb cavg ðTb Tu Þ 6
(13)
Where, A is a constant (¼0.5 for hydrogen), d is quenching distance, rb is molar density of the charge at adiabatic flame temperature (Tb ), cavg is average molar specific heat of the charge at constant pressure in the temperature range of Tu to Tb . The molar specific heats of various constituents of incylinder charge (air, hydrogen, water vapor, nitrogen etc.) as a function of temperature can be determined by Eq. (14) cp ¼ a þ bT þ cT2 þ dT3
(14)
where, cp is in kJ/kmolK, T is in K and a, b, c and d are the constants, whose values are taken from Ref. [43]. As in the present study, cooled EGR strategy was used so it is necessary to determine the saturation temperature of water vapor in the recirculated exhaust gas. Below is the procedure of determining the saturation temperature of water vapor. The generalized combustion equation of hydrogen for any equivalence ratio (4) can be written as given in Eq. (15) H2 þ
1 3:76 ðO2 þ 3:76N2 Þ/H2 O þ N2 þ 24 24
1 0:5 O2 24
(15)
(8)
where, ra is the density of incoming air, Vs is the swept volume of the engine, N is speed of the engine and CV is calorific value of hydrogen fuel. Net indicated mean effective pressure (NIMEP) can be calculated by Eq. (9) 1 Vs
(11)
The mole fraction of water vapor (H2O) in the exhaust gas mixture can be calculated by Eq. (16)
BP ¼ m_ H2 CV
NIMEP ¼
sNIMEP 100 % mNIMEP
spmax 100 % mpmax
(10)
xH2 O ¼
þ 1 þ 3:76 24
1 h 1 24
i 0:5
(16)
The partial pressure of a species in a mixture can be calculated as per Dalton's law by using Eq. (17) Pi ¼ xi P
(17)
Where, P is the total pressure and xi is the mole fraction of the species i in the mixture. As the species of interest in the exhaust gas mixture is water (H2O), the saturation temperature of H2O at known partial pressure can be calculated by using various correlations such as Antoine equation, Magnus equation, Tetens equation, Arden Buck equation and Goff-Gratch equation. Among these correlations, Arden Buck equation is relatively
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Fig. 2 e Variation of the temperatures with EGR rates. more accurate in the measured temperature range [44], thus the equation was used in the present work and is given below in Eq. (18) PH2 O ¼ 0:61121 exp 18:678
T T 234:5 257:14 þ T
(18)
The saturation temperature of water in the exhaust gas is higher than the EGR temperature at the intercooler exit for all EGR rates as shown in Fig. 2. The water vapor gets condensed and thus removed subsequently before entering into the intake manifold. Thus the main constituents of cooled EGR are nitrogen, oxygen and some traces of unburned hydrogen in the present study.
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Fig. 4 e Increase in MIE at MBT timing with EGR rate and water injection. were used to control knocking and backfire at 4 ¼ 0.82, and the details of the study can be found in the recently published work [1]. In the present work, the EGR and water injection techniques were used to control/eliminate backfire, knocking and reduce NOx emission at high equivalence ratio (4 ¼ 0.82). The EGR rate was varied from 5% to 28% (volume basis), and water to hydrogen ratio (WHR) was varied from 1.5 to 9.25. The equivalence ratio was maintained at 0.82 ± 0.01 during the experiments. The findings of the study are discussed below.
Effect of EGR and water injection on combustion anomalies
The high power output of an engine can be achieved by running the engine at high equivalence ratio, but there are issues such as backfire, knocking and high NOx emission at high equivalence ratio in a hydrogen fueled SI engine. The initial experimental tests were conducted on the engine by varying equivalence ratio till the backfire occurred. The equivalence ratio of 0.82 was identified as backfire occurrence equivalence ratio. Several techniques can be used to control backfire at high equivalence ratio such as delayed hydrogen gas injection, retarded ignition, EGR, water injection etc. The delayed hydrogen gas injection and retarded ignition timing
The combustion anomalies associated with hydrogen fueled SI engine at high equivalence ratio (4 ¼ 0.82) are knocking and backfire as shown in Fig. 3. The cycle based analysis shows that the peak in-cylinder pressure reduces to 25 bar (motoring value) due to backfire. With the use of EGR and water injection, the combustion anomalies were eliminated as shown in the figure. The EGR rate and WHR for the given trends in the figure are 10% and 1.5 respectively. The EGR and water injection in the intake manifold increase the molar specific heat of the in-cylinder mixture. It can be seen from Eq. (13) that the MIE requirement of hydrogen-air mixture is directly proportional to the molar specific heat of the mixture. Thus MIE increases with specific heat of the mixture. The MIE of hydrogen-air mixture at maximum brake torque (MBT) timing (16 0bTDC) was
Fig. 3 e Control of combustion anomalies with EGR and water injection.
Fig. 5 e Variation of peak in-cylinder pressure with EGR rate and water injection.
Results and discussion
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Fig. 6 e Variation of peak in-cylinder temperature with EGR rate and water injection.
Fig. 7 e Variation of exhaust gas temperature with EGR rate and water injection.
Fig. 9 e Variation of volumetric efficiecy of the engine with EGR rate and water injection.
The peak in-cylinder pressure was not changed up to 15% EGR but further increasing the EGR rate decreases the peak incylinder pressure marginally whereas it decreases significantly with water injection (Fig. 5). Similar kind of trends was observed for peak in-cylinder temperature also (Fig. 6). The reason for these trends is the dilution of in-cylinder mixture caused by the high specific heat of diluents (EGR and water). In this study, the main constituent of cooled EGR is nitrogen and its specific heat is 1.039 kJ/kgK (at NTP) and the specific heat of water is 4.18 kJ/kgK (at NTP) which are higher than the specific heat of air (1.005 kJ/kgK, at NTP), main constituent of the incylinder mixture. This difference between the specific heats of nitrogen and water is the main reason for the faster reduction in in-cylinder pressure and temperature with WHR as compared to EGR. Therefore, more the WHR or EGR rate (means more nitrogen) more is the dilution of in-cylinder mixture and thus reduction in in-cylinder pressure and temperature. The variation of exhaust gas temperature with EGR rate and water injection is shown in Fig. 7. It can be seen from the figure that there was a marginal change in exhaust gas temperature with EGR rates. A similar trend was also observed with water injection up to WHR of 7.5, but further increase in WHR reduced the exhaust gas temperature drastically. The variation of the various combustion events with EGR rate and water injection are shown in Fig. 8. The start of
Fig. 8 e Variation of combustion events with EGR rate and water injection.
calculated for each case (with and without diluents (EGR and water)). The percentage change in the MIE at MBT timing is shown in Fig. 4 and it indicates that the MIE of hydrogen-air mixture increases marginally with EGR rate and it increases significantly with WHR. This increase in MIE requirement reduces the probability of backfire occurrence with EGR and water injection.
Fig. 10 e Variation of brake thermal efficiency of the engine with EGR rate and water injection.
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Fig. 11 e Variation of COV with EGR rate and water injection.
Fig. 12 e Variation of NOx emission with EGR rate and water injection.
combustion (SOC) was delayed with EGR rate and water injection due to the dilution caused by recirculated exhaust gas and water of high heat capacity which increases the requirement of minimum ignition energy. There was increase in flame development angle (FDA) and overall burning angle (OBA) with EGR rate and water injection due to reduction in flame velocity caused by dilution of in-cylinder mixture and therefore the combustion duration was increased with EGR rate and water injection. The cumulative effect of all these combustion events reduces the probability of backfire occurrence. The change in these combustion events was higher with water injection than EGR rate. This shows that the backfire can be controlled faster with water injection as compared to that of with EGR.
Effect of EGR and water injection on performance The volumetric efficiency (hv) of the engine was marginally affected with EGR (Fig. 9). The change in the volumetric
efficiency is under uncertainty up to 25% of EGR, and it improved marginally at EGR rate of 28%. The reason for this marginal improvement in volumetric efficiency could be the improved density of incoming charge due to the cooling effect of high flow rate of cooled EGR. A similar trend of volumetric efficiency was observed with water injection, and it was under uncertainty limit up to WHR of 7.5. Beyond WHR of 7.5, the volumetric efficiency reduced marginally due to the high flow rate of water which restricted the flow of incoming charge to engine's cylinder. The brake thermal efficiency of the engine was not affected up to EGR rate of 25% but further increase in EGR rate, thermal efficiency was reduced significantly (Fig. 10). The reason for this reduction in thermal efficiency of the engine at 28% EGR could be the high dilution caused by the high flow rate of EGR. The variation of brake thermal efficiency with WHR is under uncertainty limit in all cases. The reason for this trend could be the nature of water which acts as a working fluid, steam, under high in-cylinder temperatures (above 2000K) and pressure (above 30 bar) conditions. An another reason of these trends of volumetric efficiency and brake thermal efficiency with cooled EGR and water injection could be the equivalence ratio of the engine, which was maintained around 0.82 (±0.01) by changing the throttle position. Another important parameter is the coefficient of variation (COV) of maximum in-cylinder pressure (Pmax) and indicated mean effective pressure (IMEP) that indicates the stability of engine operation. It can be observed from Fig. 11 that stable operation of the engine can be achieved till the WHR of 7.5 and EGR rate of 25%. Further increase in WHR and EGR rate, increases the cyclic variation drastically. In the present study, the COV was considered as one of the parameters, along with performance and NOx emission reduction, to decide the optimum flow rate of EGR and water injection.
Effect of EGR and water injection on NOx emission Hydrogen fueled spark ignition engine emits mainly NOx emission as it is a carbon-free fuel. However, some traces of HC and CO were sometimes observed in the exhaust due to evaporation of engine lubricating oil. High in-cylinder temperature, availability of oxygen, number of chemical species present in the combustion chamber are the main culprits of higher NOx emission with hydrogen fueled SI engine as compared to gasoline or CNG fueled SI engine. This high NOx emission is one of the main technical challenges in hydrogen fueled engine. In the present study, the NOx emission was reduced by employing EGR and water injection in the intake manifold of the engine, and the result is shown in Fig. 12. The NOx was reduced significantly with EGR rate from 10.52 g/ kWh to 4.10 g/kWh at 0% EGR to 28% EGR respectively. The reduction in NOx emission was exponential with water injection. It was reduced to 0.1 g/kWh at WHR of 9.25. This WHR
Table 2 e Summary of the optimum operating conditions for the engine. Strategy Cooled EGR Water injection
Hydrogen consumption (g/kWh)
Water consumption (g/kWh)
EGR (%)
WHR
NOx emission (g/kWh)
104.45 104.8
e 785.4
25 e
e 7.5
4.55 0.34
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Water injection is more effective strategy for controlling backfire with near zero (ultra-low) NOx emission as compared to cooled EGR
Cooled EGR can control backfire with significant reduction in NOx emission
Water injection 2
The NOx emission reduced significantly due to decrease in in-cylinder temperature caused by (i) charge dilution and (ii) increased number of chemical species in the combustion reaction The NOx emission reduced exponentially due to a drastic decrease in in-cylinder temperature caused by (i) high charge dilution and (ii) increased number of chemical species in the combustion reaction The probability of backfire occurrence reduced due to increase in molar specific heat of in-cylinder mixture which increases the requirement of MIE and thus delays the combustion events The increase in molar specific heat of incylinder mixture and the requirement of MIE were higher with water injection as compared to EGR, so this strategy is more effective for controlling backfire Cooled EGR 1
Effect on NOx emission Effect on backfire Strategy S.No.
Table 3 e Summary of the effects of cooled EGR and water injection on backfire and NOx emission.
Concluding Remark
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 4 4 ( 2 0 1 9 ) 6 2 8 7 e6 2 9 8
corresponds to hydrogen consumption rate of 103.63 g/kWh and water consumption rate of 958.8 g/kWh. The main reasons for this drastic reduction in NOx emission are the lower in-cylinder temperature caused by the dilution of the incylinder mixture by water and increase in the number of chemical species in the combustion reaction. It can be clearly seen in the figure that near zero (ultra-low) NOx emission is possible with water injection. The summary of the optimum operating conditions for the engine with both the strategies is given in Table 2 and the results of cooled EGR and water injection on backfire and NOx emission are given in Table 3.
Conclusions The experimental work was carried out on a multi-cylinder hydrogen fueled SI engine at a constant speed of 1500 rpm. The equivalence ratio of the engine was maintained around 0.82 (±0.01). The cooled EGR and water injection strategies were used to control backfire and reduce NOx emission. From the observations made in the study, following conclusions are emerged. Both the techniques (EGR and water injection) are very effective for controlling backfire. But backfire can effectively be controlled with water injection than that of with EGR due to higher MIE requirement and higher flame speed reduction with water injection as compared to EGR. The NOx emission was exponentially reduced with water injection, and it was reduced significantly with EGR rate. With 25% EGR and WHR of 7.5, the NOx emission was reduced by 57% and 97% respectively without affecting the performance of the engine. The optimum WHR of 7.5 and cooled EGR rate of 25% were observed in terms of performance, emission and stability of the engine operation. The salient point which is emerged from this study is that backfire-free engine operation, and near zero (ultra-low) NOx emission can be achieved with water injection without affecting the performance of the engine. EGR can also reduce the NOx emission up to certain level without affecting the performance of the engine. Therefore, it can be concluded from the present study that water injection is a better strategy as compared to cooled EGR for achieving backfire-free engine operation with near zero (ultra-low) NOx emission.
Appendix A. Uncertainty analysis The uncertainty in the measurement of several parameters such as mass flow rate of hydrogen, air flow rate, etc. was determined by using standard deviation (SD) and number of measurements (n) (standard uncertainty or standard error of SD ffiffi) with 95% confidence level (1.96 SE). mean ðSEÞ ¼ p n Whereas, the uncertainty in calculated parameters such as brake thermal efficiency, volumetric efficiency, etc. was determined by using the method given by Taylor [45] for finding the uncertainty in a function of several independent variables and is explained below.
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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 4 4 ( 2 0 1 9 ) 6 2 8 7 e6 2 9 8
Table A.1 e Uncertainty in various parameters S.No 1 2 3 4 5 6 7 8 9 10
Parameter
Measured/calculated
Uncertainty (%)
Air flow rate Fuel flow rate Exhaust gas temperature Engine speed Equivalence ratio Brake thermal efficiency Volumetric efficiency NOx emission EGR percentage Water to hydrogen ratio (WHR)
Measured Measured Measured Measured Calculated Calculated Calculated Calculated Calculated Calculated
±0.48 ±0.70 ±0.11 ±1.00 ±0.85 ±0.22 ±0.41 ±0.84 ±1.98 ±1.78
If q is a function of x1 x2 , …, xn ðq ¼ f ðx1 ; x2 ; …; xn ÞÞ ), and the variables are measured independently with the uncertainty of dx1 ; dx2 ; …; dxn respectively, then the uncertainty in q is calculated by Eq. (A.1)
[2]
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi u( 2 2 2 ) u vq vq vq t dx1 þ dx2 þ … þ dxn dq ¼ ± vx1 vx2 vxn
[3]
(A.1) The uncertainty in the calculation of equivalence ratio is given below as a sample application of Eq. (A.1); The equivalence ratio (4) of hydrogen fueled SI engine can be calculated from Eq. (A.2) 4 ¼ 34:3
m_ f m_ a
[4]
[5]
(A.2)
The uncertainty in the measurement of air flow rate (dm_ a ) and hydrogen flow rate (dm_ f ) is ±0.1041 g/s and ±0.0036 g/s respectively. The uncertainty in the equivalence ratio (d4) can be calculated as shown in Eq. (A.3)e(A.6) vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u( 2 2 ) u v4 v4 t _ _ dmf þ dma d4 ¼ ± vm_ f vm_ a ffi vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u8 !2 9 2 u< = 34:3 m_ f 34:3 u d4 ¼ ±t dm_ f þ dm_ a 2 : m_ a ; m_ a
(A.3)
[8]
(A.5) d4 ¼ ±6:9915 10 z±0:007z±0:85%
[7]
(A.4)
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u( 2 2 ) u 34:3 34:3 0:5133 t 0:0036 þ 0:1041 d4 ¼ ± 21:4709 21:47092
3
[6]
[9]
[10]
(A.6)
In the same manner, the uncertainty in several calculated parameters was determined. The uncertainty in various measured and calculated parameters is given in Table A1.
[11]
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