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Combustion analysis of a spark ignition i. c. engine fuelled alternatively with natural gas and hydrogen-natural gas blends
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 3 8 ( 2 0 1 3 ) 1 6 1 6 e1 6 2 3
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Combustion analysis of a spark ignition i. c. engine fuelled alternatively with natural gas and hydrogen-natural gas blends Antonio Mariani a,*, Maria Vittoria Prati a, Andrea Unich b, Biagio Morrone b a b
Istituto Motori CNR, viale Marconi 8, 80125 Napoli, Italy Seconda Universita´ degli Studi di Napoli, via Roma 29, 81031 Aversa (CE), Italy
article info
abstract
Article history:
This paper describes an experimental activity performed on a passenger car powered by
Received 2 August 2012
a spark ignition engine fuelled alternatively with natural gas (CNG) and hydrogen-natural
Received in revised form
gas blends, with 15% (HCNG15) and 30% (HCNG30) of hydrogen by volume. The vehicle was
6 November 2012
tested on a chassis dynamometer over different driving cycles, allowing the investigation
Accepted 8 November 2012
of more realistic operating conditions than those examined on an engine test bed at steady
Available online 8 December 2012
state conditions. Fuel consumption was estimated using the carbon balance methodology, allowing the comparison of engine average efficiency over the driving cycles for the tested
Keywords:
fuels. Furthermore, cylinder pressure was measured and, by processing the pressure
Hydrogen
signal, a combustion analysis was performed allowing to estimate the burning rate and
Natural gas
combustion phasing. Ignition timing was the same for all the tested fuels, in order to assess
Internal combustion engine
their interchangeability on in-use vehicles. Results showed CO2 emission reduction
Combustion analysis
between 3% and 6% for HCNG15 and between 13% and 16% for HCNG30 respect to natural
Engine efficiency
gas. Fuel consumption in MJ/km did not show significant differences between CNG and
Driving cycles
HCNG15, while reductions between 3% and 7% have been observed with HCNG30. The heat release rate increased with hydrogen content in the blends, reaching values higher than those attained using CNG. The combustion duration, calculated as the angle between 10% and 90% of heat released, has been shortened, with 16% reduction for HCNG15 and 21% for HCNG30 respect to CNG at 2.5 bar imep and 2400 rpm. As a consequence, hydrogen addition resulted in a combustion phasing advance respect to CNG. Cycle-by-cycle variability decreased, particularly at low loads, due to the positive effect of hydrogen on combustion stability. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In the last ten years the number of natural gas vehicles worldwide has grown rapidly, with the biggest contribution
coming from the Asia-Pacific and Latin America regions [1]. As natural gas is the “cleanest” fossil fuel, the exhaust emissions from natural gas spark ignition vehicles are lower than those of gasoline-powered vehicles [2,3]. Moreover,
* Corresponding author. Tel.: þ39 0815010287. E-mail address: [email protected] (A. Mariani). URL: http://www.im.cnr.it 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.11.051
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natural gas is less affected by price fluctuations and its reserves are more evenly distributed over the globe than crude oil. However, natural gas combustion rate is lower than gasoline and natural gas spark ignition engines have a lower efficiency than Diesel engines. Many research activities have been focused on improving the performance of natural gas engines by means of hydrogen addition to natural gas, obtaining blends usually named as HCNG. Being hydrogen an energy carrier, it can be produced by different energy sources with costs strongly affected by the technology adopted. Gorensek et al. [4] identified six characteristics that impact the relative value of hydrogen produced by different methods in specific markets, like oxygen as a by-product, avoidance of carbon dioxide releases, hydrogen transport, hydrogen storage, availability of low-cost heat, and institutional factors. The European Parliament describes hydrogen as a clean way of powering vehicles for the future and the hydrogen mixtures as a transition fuel towards the use of pure hydrogen to facilitate the introduction of hydrogen powered vehicles in Member State where natural gas infrastructure is good [5,6]. Nagalingam et al. [7] proved that the higher burning rate of hydrogen natural gas blends compared to natural gas requires a reduced spark advance to obtain the maximum torque. This burning velocity increase can improve engine efficiency but also promote an increment of NOx emissions because of higher temperatures attained during combustion [8e10]. To reduce NOx emissions, several authors [11e15] have carried out investigations on Lean-Burn HCNG engines, obtaining very low NOx emissions. The engines were operated close to the lean limit, which was significantly extended, respect to natural gas, by hydrogen addition. NOx emissions can be controlled even on Stoichiometric-Burn engines adopting exhaust gas recirculation [10]. Besides, the excellent anti-knock qualities of natural gas are not undermined by the presence of relatively small amounts of hydrogen in the blend [16]. The effects of hydrogen addition to natural gas on cycle-by-cycle variations were studied in many papers and results showed that the coefficients of variation (COV) of maximum pressure and indicated mean effective pressure (imep) are reduced by increasing hydrogen content both with lean air-to-fuel mixtures [17,18] and with high exhaust gas recirculation rates [18]. This paper presents an experimental activity performed on a passenger car powered by a spark ignition engine. The vehicle was installed on a chassis dynamometer and fuelled alternatively with natural gas and HCNG blends. By means of fuel consumption measurement, the comparison of engine average efficiency over driving cycles for the tested fuels has been accomplished. Cylinder pressure has been measured in order to evaluate the effects of hydrogen addition on burning rate, combustion phasing and cycle-by-cycle variability. The results showed CO2 emission reduction between 3% and 6% for HCNG15 and between 13% and 16% for HCNG30 respect to natural gas. Fuel consumption in MJ/km did not show significant differences between CNG and HCNG15, while reductions between 3% and 7%, have been observed with HCNG30. Because of a faster combustion and an unchanged ignition timing, peak pressure values raised and their position moved towards Top Dead Center (TDC). The increased heat release rates of HCNG blends respect to CNG reduced the
Table 1 e Vehicle characteristics. Vehicle Engine type Displacement Compression ratio Rated power Reference mass
Fiat Panda 1.2 NP L4, spark ignition 1242 cm3 9.8:1 38 kW @ 5000 rpm 1025 kg
combustion duration, calculated as the angle between 10% and 90% of heat released. The reduction was 16% for HCNG15 and 21% for HCNG30 respect to CNG at 2.5 bar imep and 2400 rpm. Cycle-by-cycle variability decreases, particularly at low loads, due to the positive effect of hydrogen on combustion stability.
2.
Methods
2.1.
Experimental apparatus
The experimental activity has been performed on a passenger car equipped with a spark ignition engine, which main characteristics are reported in Table 1. Ignition timing was the same for all the tested fuels. The vehicle was installed on a Schenck Single-Axle Large Roll chassis dynamometer (with a 1.59 m roll diameter). The test bench allows a maximum test speed of 200 km/h and a maximum power absorption of 184 kW. The car was driven by a Horiba 7000 Automatic Driving System (ADS) to minimize the difference between the actual and the reference speed, reducing tests variability caused by driver inaccuracy. Exhaust gases were diluted with purified ambient air and drawn through a dilution tunnel. The total diluted flow was held constant by a constant volume sampling positive displacement pump (CVS-PDP). A Horiba Mexa 7200 H multi-range gas analyser was employed to measure carbon monoxide (CO), total unburnt hydrocarbons (THC), methane (CH4) and carbon dioxide (CO2). Nondispersive infrared (NDIR) analysers were used for CO and CO2 measurements, a flame ionization detector (FID) analyser for hydrocarbons and methane. Their accuracy is 0.5% of the measuring range. Combustion analysis was performed by means of an AVL Indimicro system, with 1 MHz sampling rate and 18 bits ADC resolution for each channel. Cylinder pressure was measured by means of a spark plug with integrated pressure measurement function. The piezoelectric sensor is placed into the spark
Table 2 e Fuel properties. Natural gas HCNG15 HCNG30 H2 [% vol.] H2 [% energy] LHV [MJ/kg] LHV vol. [MJ/Nm3] AFR stoic. [kg/kg] LHV vol. stoic. mix. [MJ/Nm3]
e e 45.3 36.9 15.6 3.37
14.0 4.61 46.6 33.2 15.9 3.36
29.3 11.4 48.5 29.2 16.4 3.35
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Table 3 e Characteristics of the driving cycles.
NEDC Artemis urban Artemis road Artemis motorway
Average speed km/h
Max speed km/h
MPA m/s2
Idle %
Cruising %
Acceleration %
33.6 17.7 57.5 96.9
120 57.3 111.1 131.4
0.528 0.531 0.359 0.271
20.4 20.7 1.5 0.7
38.8 9.6 21.6 26.0
23.6 36.0 39.7 40.6
plug shell, allowing pressure measurements without engine modifications. The transducer measuring range is 0e200 bar with a sensitivity of 8 pC/bar. Data have been acquired with a resolution of 1 crank angle (CA) degree. The car has been tested with natural gas as well as HCNG15 and HCNG30 blends. The main characteristics of the tested fuels are reported in Table 2.
2.2.
Driving cycles
The vehicle was tested on the type-approval New European Driving Cycle (NEDC) and on real-world Common Artemis Driving Cycles (CADC) composed by Urban, Road and Motorway phases. The NEDC cycle includes the Urban Driving Cycle (UDC) consisting of four repetitions of the same module, and an Extra-Urban segment (EUDC). It was driven in cold
start condition as prescribed by the European legislation. Before the cold start test, the vehicle was allowed to soak for at least 8 h at a temperature of about 24 C. The Artemis driving cycles were originally designed by INRETS (Institut National de Recherche sur les Transports et leur Se´curite´, France) with the aim of obtaining a common set of reference real-world driving cycles for European cars [19]. The main kinematic characteristics of the driving cycles are reported in Table 3 [20]. It can be noticed that the Artemis Urban cycle spends more time in acceleration and deceleration and less in cruising than NEDC. Fuel consumption has been estimated by means of a carbon balance in exhaust gases [21]. Tests have been repeated 3 times for each fuel. Cylinder pressure data, obtained averaging 300 cycles for each test, were collected at constant speed test conditions, i.e. 15, 30, 50, 70, 100 and 120 km/h.
Fig. 1 e Engine operating conditions in the imep-engine speed plane over different driving cycles.
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 3 8 ( 2 0 1 3 ) 1 6 1 6 e1 6 2 3
CO2
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CNG HCNG15 HCNG30
250 200
g/km
150 100 50
0 NEDC
Artemis Urban
Artemis Road
Artemis Motorway
Fig. 2 e CO2 emissions over NEDC and Artemis driving cycles.
3.
Results
Fig. 4 e In-cylinder pressure versus crank angle.
Each driving cycle performed during the experimental activity resulted in a particular distribution of actual engine operating conditions in terms of imep and engine speed (n). Such distributions, shown in Fig. 1, are obtained placing a grid with a constant resolution on the imep-n plane and attributing measured imep-n values to the closest point of the grid. The bubble diameter is proportional to the frequency of the imepn value. As the impact of hydrogen addition on combustion speed varies with the engine load, the frequency distribution of the engine operating condition over the driving cycles gives helpful information for data analysis. NEDC and Artemis Urban cycles show high occurrence of idle and low loads whereas the Artemis Road and Motorway are characterized by high engine load and speed.
3.1.
CO2 emissions and fuel consumption
HCNG30 over the NEDC. Fuelling the engine with CNG, CO2 emissions were 206 1 g/km over the Artemis Urban, 119 1 over the Road and 112 1 over the Motorway. The corresponding values with HCNG15 were 198 1 g/km, 115 1 and 108 1, whereas with HCNG30 174 1, 104 1, 94 1. The lower CO2 emissions with HCNG blends are caused both by the lower carbon content of the fuel and by the reduced fuel consumption. Also shown in the same figure, the standard deviation of the measurements obtained for each driving cycle and each fuel. As the lower heating value of the tested fuels is different, the effect of the fuel on engine efficiency could be estimated considering the fuel consumption in MJ/km, as shown in Fig. 3. Results did not show significant differences between CNG and HCNG15 while a reduction has been observed
Fig. 2 summarizes CO2 emissions measured at the exhaust of the test vehicle fuelled with CNG and HCNG blends over NEDC and Artemis cycles. CO2 emissions were 139 2 g/km when the engine was fuelled with CNG, 131 1 with HCNG15 and 118 1 with
FUEL CONSUMPTION 4.0
CNG
3.5
HCNG15
MJ/km
3.0
HCNG30
2.5 2.0 1.5 1.0 0.5 0.0 NEDC
Artemis Urban
Artemis Road
Artemis Motorway
Fig. 3 e Fuel consumption in MJ/km over NEDC and ARTEMIS driving cycles.
Fig. 5 e Heat release rate versus crank angle.
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Fig. 6 e Combustion duration (q90L10) versus indicated mean effective pressure.
Fig. 8 e 50% Mass fraction burned (MFB) crank angle versus imep.
fuelling the engine with HCNG30. Fuel consumption obtained with this blend was 2.33 MJ/km for the NEDC, 6% lower than CNG. On the Artemis cycles, fuel consumption was 3.41, 2.04 and 1.86 MJ/km for the Urban, Road and Motorway, respectively 6%, 3% and 7% lower than natural gas. The increase of the engine average efficiency for HCNG30 respect to CNG can be ascribed to a faster combustion promoted by hydrogen addition. Exhaust gas analysis allowed the estimation of air-fuel ratio, which resulted close to the stoichiometric value for all the operating conditions.
same ignition timing has been adopted for all the fuels, the increase of peak pressure value and the shift of its position towards TDC for the blends prove that combustion speed increases with hydrogen content. Similar trends have been observed for all the investigated operating conditions. The effects of hydrogen on combustion speed are also shown in Fig. 5, where the heat release rate plotted versus crank angle shows higher values for HCNG blends. It can be observed that the maximum value of heat release rate increases, the combustion duration reduces and the angular position of the maximum heat release rate advances as the hydrogen content in the blend increases. Fig. 6 shows combustion duration (q9010), calculated as the angle between 10% and 90% of heat released, for CNG and HCNG blends. The burning rate increase promoted by
3.2.
Combustion analysis
The analysis of cylinder pressure data allowed evaluating the effects of fuel on the combustion process. Fig. 4 shows incylinder pressure versus crank angle at 6 bar imep and 3900 rpm. The plots are obtained averaging 300 engine cycles in such operating conditions for all the tested fuels. Since the
100%
10% HEAT RELEASED CRANK ANGLE
90% 80%
CNG
70%
HCNG15
60%
HCNG30
50% 40% 30% 20% 10% 0% 1.2 bar
5.0 bar
Fig. 7 e Effect of HCNG blends on 10% heat released crank angle at different loads.
Fig. 9 e Coefficient of variation in indicated mean effective pressure (COVimep) versus imep.
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Fig. 10 e Maximum cylinder pressure ( pmax) versus maximum pressure crank angle ðqpmax Þ.
hydrogen addition reduces the combustion duration respect to CNG. Fig. 7 shows the effect of hydrogen addition on the early stages of combustion. The crank angle interval between the ignition and the time when 10% of heat has been released, indicated with q10, is compared for the tested fuels. The comparison is carried out using, as a reference, the value q10 obtained from CNG and the percent values are evaluated as the ratio between the actual q10 crank angle for any investigated fuels and that for CNG. The effect of HCNG blends in reducing q10 is evident, particularly at low loads. In details, the HCNG15 blend shows a 3% reduction of q10, while a larger reduction is observed for HCNG30, equal to 14%, when the imep is 1.2 bar, i.e. low load. When higher loads are considered, i.e. imep ¼ 5.0 bar, the observed reduction for HCNG30 is smaller and equal to 10%. Since the ignition timing was the same for all the tested fuels, the 50% Mass Fraction Burned (MFB) CA versus imep has been reported in Fig. 8 in order to verify a proper combustion phasing. It has to be noted that during CNG operation the combustion phasing is retarded, particularly at low loads, being this crank angle larger than the optimal value, which is between 8 and 10 ATDC [22]. The combustion phasing for the HCNG blends is closer to the optimal values than for CNG,
being slightly retarded for HCNG15 at part loads and advanced for HCNG30 at high loads. The coefficient of variation in indicated mean effective pressure, COVimep, which is a measure of cyclic variability [23], is reported in Fig. 9 as a function of the imep. It can be observed that COVimep reduces with hydrogen addition mainly at low loads due to the positive effect of hydrogen on combustion stability. In fact, a 35% and 55% lowering are observed for HCNG15 and HCNG30 respectively, compared with CNG, up to about 1.2 bar imep. The observed COVimep reductions are smaller when higher loads are considered. Fig. 10 illustrates maximum cylinder pressure pmax and the relative crank angle qpmax of each engine cycle at 1.2 bar imep and 2200 rpm. As in Fig. 1, the bubble diameter varies in size providing information about the frequency of the pmax qpmax values. Indeed, high bubble diameter in the plot indicates high frequency occurrence of engine cycles with such characteristics. The fastest burning cycles, located in the top left part of the figure, show a small dispersion. As qpmax increases, the dispersion becomes larger since the burning rate variations have a larger impact on the cycle. In fact, the presence of slow combustion cycles determines a bimodal distribution, which occurs when the pressure increase due to combustion is more than offset by the effect of volume variation [23]. Such
Fig. 11 e Maximum cylinder pressure ( pmax) versus maximum pressure crank angle ðqpmax Þ.
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behaviour is more evident using CNG, Fig. 10 (a), than HCNGs, 10 (b) and (c). HCNG blends showed a distribution centered at higher values of pmax compared to natural gas. At higher loads, Fig. 11, the dispersion reduces, no bimodal distributions are observed and, using HCNG blends, the center of gravity of the distribution moves to higher pmax values and lower qpmax then CNG, due to the effect of HCNG blends on combustion phasing.
4.
New European driving cycle Nitrogen oxides Engine speed, rpm Top dead center Total unburned hydrocarbon
Greek symbols Combustion duration, q9010 q50%MFB Combustion phasing angle,
Conclusion
A passenger car equipped with a spark ignition engine, fuelled with natural gas (CNG) and hydrogen-natural gas blends with 15% (HCNG15) and 30% (HCNG30) of hydrogen by volume, has been tested on a chassis dynamometer over different driving cycles with the same ignition timing. CO2 emissions were 139 2 g/km over the NEDC when the engine was fuelled with CNG, while values of 131 1 and 118 1 g/km were obtained with HCNG15 and HCNG30 respectively, with a decrease of 6% and 15% respect to CNG. Over the Artemis driving cycles CO2 emissions were 206 1 g/ km on the Urban part, 119 1 on the Road and 112 1 on the Motorway when fuelling the engine with CNG. The corresponding values with HCNG15 were 198 1, 115 1 and 108 1 g/km, whereas with HCNG30 174 1, 104 1, 94 1 g/ km, with reductions ranging between 3% and 4% for HCNG15 and between 13% and 16% for the HCNG30 blend respect to natural gas. Fuel consumption in MJ/km did not show significant differences between CNG and HCNG15 while reductions between 3% and 7% have been observed fuelling the engine by HCNG30. The heat release rate increased with hydrogen content in the blends, reaching values higher than CNG. The combustion duration has been reduced for all the operating conditions investigated. Cycle-by-cycle variability decreased, particularly at low loads, due to the positive effect of hydrogen on combustion stability.
Acknowledgements The authors acknowledge Regione Lombardia for providing the car, in the framework of a project with Fiat Research Center and Sapio. This work was also partially supported by Seconda Universita´ degli studi di Napoli.
Appendix A. Nomenclature
AFR ATDC CA CNG COV HCNG imep LHV MFB MPA
NEDC NOx n TDC THC
Air-fuel ratio, kgair/kgfuel After top dead center Crank angle, Compressed Natural Gas Coefficient of variation Hydrogen-natural gas blend Indicated mean effective pressure, bar Lower heating value, MJ/kg or MJ/Nm3 Mass fraction burned Mean positive acceleration, m/s2
Subscripts mix Mixture stoich Stoichiometric vol Volumetric
references
[1] IANGV. Natural gas vehicle statistics, www.iangv.org; 2011. [2] Ristovski ZD, Morawska L, Hitchins J, Thomas S, Greenaway C, Gilbert D. Particle emissions from compressed natural gas engines. J Aerosol Sci 2000;31:403e13. [3] Prati MV, Mariani A, Torbati R, Unich A, Costagliola MA, Morrone B. Emissions and combustion behavior of a bi-fuel gasoline and natural gas spark ignition engine. SAE Int J Fuels Lubricants 2011;4:328e38. [4] Gorensek MB, Forsberg CW. Relative economic incentives for hydrogen from nuclear, renewable, and fossil energy sources. Int J Hydrog Energy 2009;34:4237e42. [5] Regulation (ec) no 79/2009 of the european parliament and of the council of 14 january 2009 on type-approval of hydrogenpowered motor vehicles, and amending directive 2007/46/ec. Tech. rep.; Europian Parliament; 2009. [6] Klell M, Eichlseder H, Sartory M. Mixtures of hydrogen and methane in the internal combustion engine - synergies, potential and regulations. Int J Hydrog Energy 2012;37: 11531e40. [7] Nagalingam B, Duebel F, Schmillen K. Performance study using natural gas, hydrogen-supplemented natural gas and hydrogen in AVL research engine. Int J Hydrog Energy 1983;8: 715e20. [8] Ma F, Wang Y, Liu H, Li Y, Wang J, Zhao S. Experimental study on thermal efficiency and emission characteristics of a lean burn hydrogen enriched natural gas engine. Int J Hydrog Energy 2007;32:5067e75. [9] Genovese A, Contrisciani N, Ortenzi F, Cazzola V. On road experimental tests of hydrogen/natural gas blends on transit buses. Int J Hydrog Energy 2011;36:1775e83. [10] Mariani A, Morrone B, Unich A. Numerical evaluation of internal combustion spark ignition engines performance fuelled with hydrogen - natural gas blends. Int J Hydrog Energy 2012;37: 2644e54. [11] Hoekstra R, Collier K, Mulligan N, Chew L. Experimental study of a clean burning vehicle fuel. Int J Hydrog Energy 1995;20:737e45. [12] Larsen JF, Wallace JS. Comparison of emissions and efficiency of a turbocharged lean-burn natural gas and hythane-fuelled engine. J Eng Gas Turbines Power 1997;119:218e26. [13] Bysveen M. Engine characteristics of emissions and performance using mixtures of natural gas and hydrogen. Energy 2007;32:482e9. [14] Huang Z, Wang J, Liu B, Zeng K, Yu K, Jiang D. Combustion characteristics of a direct-injection engine fuelled with natural gas-hydrogen blends under different ignition timings. Fuel 2007;86:381e7.
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[15] Ma F, Wang M. Performance and emission characteristics of a turbocharged spark-ignition hydrogen-enriched compressed natural gas engine under wide open throttle operating conditions. Int J Hydrog Energy 2010;35:12502e9. [16] Karim GA, Wierzba I, Al-Alousi Y. Methane-hydrogen mixtures as fuels. Int J Hydrog Energy 1996;21:625e31. [17] Ma F, Wang Y, Liu H, Li Y, Wang J, Ding S. Effects of hydrogen addition on cycle-by-cycle variations in a lean burn natural gas spark-ignition engine. Int J Hydrog Energy 2008;33:823e31. [18] Wang J, Chen H, Liu B, Huang Z. Study of cycle-by-cycle variations of a spark ignition engine fuelled with natural gashydrogen blends. Int J Hydrog Energy 2008;33:4876e83. [19] Andre´ M. The artemis european driving cycles for measuring car pollutant emissions. Sci Total Environ 2004;334-335:73e84.
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[20] Barlow T, Latham S, McCrae I, Boulter P. A reference book of driving cycles for use in the measurement of road vehicle emissions, http://www.dft.gov.uk/pgr/roads/environment/ emissions/ppr-354.pdf; 2009. [21] Consolidated text of council directive of 20 march 1970 on the approximation of the laws of the member states relating to measures to be taken against air pollution by gases from positive-ignition engines of motor vehicles. Tech. rep. 70/220/ EEC; 2007. [22] Klimstra J. The optimal combustion phasing angle a convenient engine tuning criterion. SAE Technical Paper 852090; 1985. [23] Heywood JB. Internal combustion engines fundamentals. New York: Mc Graw-Hill; 1989.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Combustion analysis of a spark ignition i. c. engine fuelled alternatively with natural gas and hydrogen-natural gas blends Antonio Mariani a,*, Maria Vittoria Prati a, Andrea Unich b, Biagio Morrone b a b
Istituto Motori CNR, viale Marconi 8, 80125 Napoli, Italy Seconda Universita´ degli Studi di Napoli, via Roma 29, 81031 Aversa (CE), Italy
article info
abstract
Article history:
This paper describes an experimental activity performed on a passenger car powered by
Received 2 August 2012
a spark ignition engine fuelled alternatively with natural gas (CNG) and hydrogen-natural
Received in revised form
gas blends, with 15% (HCNG15) and 30% (HCNG30) of hydrogen by volume. The vehicle was
6 November 2012
tested on a chassis dynamometer over different driving cycles, allowing the investigation
Accepted 8 November 2012
of more realistic operating conditions than those examined on an engine test bed at steady
Available online 8 December 2012
state conditions. Fuel consumption was estimated using the carbon balance methodology, allowing the comparison of engine average efficiency over the driving cycles for the tested
Keywords:
fuels. Furthermore, cylinder pressure was measured and, by processing the pressure
Hydrogen
signal, a combustion analysis was performed allowing to estimate the burning rate and
Natural gas
combustion phasing. Ignition timing was the same for all the tested fuels, in order to assess
Internal combustion engine
their interchangeability on in-use vehicles. Results showed CO2 emission reduction
Combustion analysis
between 3% and 6% for HCNG15 and between 13% and 16% for HCNG30 respect to natural
Engine efficiency
gas. Fuel consumption in MJ/km did not show significant differences between CNG and
Driving cycles
HCNG15, while reductions between 3% and 7% have been observed with HCNG30. The heat release rate increased with hydrogen content in the blends, reaching values higher than those attained using CNG. The combustion duration, calculated as the angle between 10% and 90% of heat released, has been shortened, with 16% reduction for HCNG15 and 21% for HCNG30 respect to CNG at 2.5 bar imep and 2400 rpm. As a consequence, hydrogen addition resulted in a combustion phasing advance respect to CNG. Cycle-by-cycle variability decreased, particularly at low loads, due to the positive effect of hydrogen on combustion stability. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In the last ten years the number of natural gas vehicles worldwide has grown rapidly, with the biggest contribution
coming from the Asia-Pacific and Latin America regions [1]. As natural gas is the “cleanest” fossil fuel, the exhaust emissions from natural gas spark ignition vehicles are lower than those of gasoline-powered vehicles [2,3]. Moreover,
* Corresponding author. Tel.: þ39 0815010287. E-mail address: [email protected] (A. Mariani). URL: http://www.im.cnr.it 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.11.051
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natural gas is less affected by price fluctuations and its reserves are more evenly distributed over the globe than crude oil. However, natural gas combustion rate is lower than gasoline and natural gas spark ignition engines have a lower efficiency than Diesel engines. Many research activities have been focused on improving the performance of natural gas engines by means of hydrogen addition to natural gas, obtaining blends usually named as HCNG. Being hydrogen an energy carrier, it can be produced by different energy sources with costs strongly affected by the technology adopted. Gorensek et al. [4] identified six characteristics that impact the relative value of hydrogen produced by different methods in specific markets, like oxygen as a by-product, avoidance of carbon dioxide releases, hydrogen transport, hydrogen storage, availability of low-cost heat, and institutional factors. The European Parliament describes hydrogen as a clean way of powering vehicles for the future and the hydrogen mixtures as a transition fuel towards the use of pure hydrogen to facilitate the introduction of hydrogen powered vehicles in Member State where natural gas infrastructure is good [5,6]. Nagalingam et al. [7] proved that the higher burning rate of hydrogen natural gas blends compared to natural gas requires a reduced spark advance to obtain the maximum torque. This burning velocity increase can improve engine efficiency but also promote an increment of NOx emissions because of higher temperatures attained during combustion [8e10]. To reduce NOx emissions, several authors [11e15] have carried out investigations on Lean-Burn HCNG engines, obtaining very low NOx emissions. The engines were operated close to the lean limit, which was significantly extended, respect to natural gas, by hydrogen addition. NOx emissions can be controlled even on Stoichiometric-Burn engines adopting exhaust gas recirculation [10]. Besides, the excellent anti-knock qualities of natural gas are not undermined by the presence of relatively small amounts of hydrogen in the blend [16]. The effects of hydrogen addition to natural gas on cycle-by-cycle variations were studied in many papers and results showed that the coefficients of variation (COV) of maximum pressure and indicated mean effective pressure (imep) are reduced by increasing hydrogen content both with lean air-to-fuel mixtures [17,18] and with high exhaust gas recirculation rates [18]. This paper presents an experimental activity performed on a passenger car powered by a spark ignition engine. The vehicle was installed on a chassis dynamometer and fuelled alternatively with natural gas and HCNG blends. By means of fuel consumption measurement, the comparison of engine average efficiency over driving cycles for the tested fuels has been accomplished. Cylinder pressure has been measured in order to evaluate the effects of hydrogen addition on burning rate, combustion phasing and cycle-by-cycle variability. The results showed CO2 emission reduction between 3% and 6% for HCNG15 and between 13% and 16% for HCNG30 respect to natural gas. Fuel consumption in MJ/km did not show significant differences between CNG and HCNG15, while reductions between 3% and 7%, have been observed with HCNG30. Because of a faster combustion and an unchanged ignition timing, peak pressure values raised and their position moved towards Top Dead Center (TDC). The increased heat release rates of HCNG blends respect to CNG reduced the
Table 1 e Vehicle characteristics. Vehicle Engine type Displacement Compression ratio Rated power Reference mass
Fiat Panda 1.2 NP L4, spark ignition 1242 cm3 9.8:1 38 kW @ 5000 rpm 1025 kg
combustion duration, calculated as the angle between 10% and 90% of heat released. The reduction was 16% for HCNG15 and 21% for HCNG30 respect to CNG at 2.5 bar imep and 2400 rpm. Cycle-by-cycle variability decreases, particularly at low loads, due to the positive effect of hydrogen on combustion stability.
2.
Methods
2.1.
Experimental apparatus
The experimental activity has been performed on a passenger car equipped with a spark ignition engine, which main characteristics are reported in Table 1. Ignition timing was the same for all the tested fuels. The vehicle was installed on a Schenck Single-Axle Large Roll chassis dynamometer (with a 1.59 m roll diameter). The test bench allows a maximum test speed of 200 km/h and a maximum power absorption of 184 kW. The car was driven by a Horiba 7000 Automatic Driving System (ADS) to minimize the difference between the actual and the reference speed, reducing tests variability caused by driver inaccuracy. Exhaust gases were diluted with purified ambient air and drawn through a dilution tunnel. The total diluted flow was held constant by a constant volume sampling positive displacement pump (CVS-PDP). A Horiba Mexa 7200 H multi-range gas analyser was employed to measure carbon monoxide (CO), total unburnt hydrocarbons (THC), methane (CH4) and carbon dioxide (CO2). Nondispersive infrared (NDIR) analysers were used for CO and CO2 measurements, a flame ionization detector (FID) analyser for hydrocarbons and methane. Their accuracy is 0.5% of the measuring range. Combustion analysis was performed by means of an AVL Indimicro system, with 1 MHz sampling rate and 18 bits ADC resolution for each channel. Cylinder pressure was measured by means of a spark plug with integrated pressure measurement function. The piezoelectric sensor is placed into the spark
Table 2 e Fuel properties. Natural gas HCNG15 HCNG30 H2 [% vol.] H2 [% energy] LHV [MJ/kg] LHV vol. [MJ/Nm3] AFR stoic. [kg/kg] LHV vol. stoic. mix. [MJ/Nm3]
e e 45.3 36.9 15.6 3.37
14.0 4.61 46.6 33.2 15.9 3.36
29.3 11.4 48.5 29.2 16.4 3.35
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Table 3 e Characteristics of the driving cycles.
NEDC Artemis urban Artemis road Artemis motorway
Average speed km/h
Max speed km/h
MPA m/s2
Idle %
Cruising %
Acceleration %
33.6 17.7 57.5 96.9
120 57.3 111.1 131.4
0.528 0.531 0.359 0.271
20.4 20.7 1.5 0.7
38.8 9.6 21.6 26.0
23.6 36.0 39.7 40.6
plug shell, allowing pressure measurements without engine modifications. The transducer measuring range is 0e200 bar with a sensitivity of 8 pC/bar. Data have been acquired with a resolution of 1 crank angle (CA) degree. The car has been tested with natural gas as well as HCNG15 and HCNG30 blends. The main characteristics of the tested fuels are reported in Table 2.
2.2.
Driving cycles
The vehicle was tested on the type-approval New European Driving Cycle (NEDC) and on real-world Common Artemis Driving Cycles (CADC) composed by Urban, Road and Motorway phases. The NEDC cycle includes the Urban Driving Cycle (UDC) consisting of four repetitions of the same module, and an Extra-Urban segment (EUDC). It was driven in cold
start condition as prescribed by the European legislation. Before the cold start test, the vehicle was allowed to soak for at least 8 h at a temperature of about 24 C. The Artemis driving cycles were originally designed by INRETS (Institut National de Recherche sur les Transports et leur Se´curite´, France) with the aim of obtaining a common set of reference real-world driving cycles for European cars [19]. The main kinematic characteristics of the driving cycles are reported in Table 3 [20]. It can be noticed that the Artemis Urban cycle spends more time in acceleration and deceleration and less in cruising than NEDC. Fuel consumption has been estimated by means of a carbon balance in exhaust gases [21]. Tests have been repeated 3 times for each fuel. Cylinder pressure data, obtained averaging 300 cycles for each test, were collected at constant speed test conditions, i.e. 15, 30, 50, 70, 100 and 120 km/h.
Fig. 1 e Engine operating conditions in the imep-engine speed plane over different driving cycles.
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CO2
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CNG HCNG15 HCNG30
250 200
g/km
150 100 50
0 NEDC
Artemis Urban
Artemis Road
Artemis Motorway
Fig. 2 e CO2 emissions over NEDC and Artemis driving cycles.
3.
Results
Fig. 4 e In-cylinder pressure versus crank angle.
Each driving cycle performed during the experimental activity resulted in a particular distribution of actual engine operating conditions in terms of imep and engine speed (n). Such distributions, shown in Fig. 1, are obtained placing a grid with a constant resolution on the imep-n plane and attributing measured imep-n values to the closest point of the grid. The bubble diameter is proportional to the frequency of the imepn value. As the impact of hydrogen addition on combustion speed varies with the engine load, the frequency distribution of the engine operating condition over the driving cycles gives helpful information for data analysis. NEDC and Artemis Urban cycles show high occurrence of idle and low loads whereas the Artemis Road and Motorway are characterized by high engine load and speed.
3.1.
CO2 emissions and fuel consumption
HCNG30 over the NEDC. Fuelling the engine with CNG, CO2 emissions were 206 1 g/km over the Artemis Urban, 119 1 over the Road and 112 1 over the Motorway. The corresponding values with HCNG15 were 198 1 g/km, 115 1 and 108 1, whereas with HCNG30 174 1, 104 1, 94 1. The lower CO2 emissions with HCNG blends are caused both by the lower carbon content of the fuel and by the reduced fuel consumption. Also shown in the same figure, the standard deviation of the measurements obtained for each driving cycle and each fuel. As the lower heating value of the tested fuels is different, the effect of the fuel on engine efficiency could be estimated considering the fuel consumption in MJ/km, as shown in Fig. 3. Results did not show significant differences between CNG and HCNG15 while a reduction has been observed
Fig. 2 summarizes CO2 emissions measured at the exhaust of the test vehicle fuelled with CNG and HCNG blends over NEDC and Artemis cycles. CO2 emissions were 139 2 g/km when the engine was fuelled with CNG, 131 1 with HCNG15 and 118 1 with
FUEL CONSUMPTION 4.0
CNG
3.5
HCNG15
MJ/km
3.0
HCNG30
2.5 2.0 1.5 1.0 0.5 0.0 NEDC
Artemis Urban
Artemis Road
Artemis Motorway
Fig. 3 e Fuel consumption in MJ/km over NEDC and ARTEMIS driving cycles.
Fig. 5 e Heat release rate versus crank angle.
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Fig. 6 e Combustion duration (q90L10) versus indicated mean effective pressure.
Fig. 8 e 50% Mass fraction burned (MFB) crank angle versus imep.
fuelling the engine with HCNG30. Fuel consumption obtained with this blend was 2.33 MJ/km for the NEDC, 6% lower than CNG. On the Artemis cycles, fuel consumption was 3.41, 2.04 and 1.86 MJ/km for the Urban, Road and Motorway, respectively 6%, 3% and 7% lower than natural gas. The increase of the engine average efficiency for HCNG30 respect to CNG can be ascribed to a faster combustion promoted by hydrogen addition. Exhaust gas analysis allowed the estimation of air-fuel ratio, which resulted close to the stoichiometric value for all the operating conditions.
same ignition timing has been adopted for all the fuels, the increase of peak pressure value and the shift of its position towards TDC for the blends prove that combustion speed increases with hydrogen content. Similar trends have been observed for all the investigated operating conditions. The effects of hydrogen on combustion speed are also shown in Fig. 5, where the heat release rate plotted versus crank angle shows higher values for HCNG blends. It can be observed that the maximum value of heat release rate increases, the combustion duration reduces and the angular position of the maximum heat release rate advances as the hydrogen content in the blend increases. Fig. 6 shows combustion duration (q9010), calculated as the angle between 10% and 90% of heat released, for CNG and HCNG blends. The burning rate increase promoted by
3.2.
Combustion analysis
The analysis of cylinder pressure data allowed evaluating the effects of fuel on the combustion process. Fig. 4 shows incylinder pressure versus crank angle at 6 bar imep and 3900 rpm. The plots are obtained averaging 300 engine cycles in such operating conditions for all the tested fuels. Since the
100%
10% HEAT RELEASED CRANK ANGLE
90% 80%
CNG
70%
HCNG15
60%
HCNG30
50% 40% 30% 20% 10% 0% 1.2 bar
5.0 bar
Fig. 7 e Effect of HCNG blends on 10% heat released crank angle at different loads.
Fig. 9 e Coefficient of variation in indicated mean effective pressure (COVimep) versus imep.
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Fig. 10 e Maximum cylinder pressure ( pmax) versus maximum pressure crank angle ðqpmax Þ.
hydrogen addition reduces the combustion duration respect to CNG. Fig. 7 shows the effect of hydrogen addition on the early stages of combustion. The crank angle interval between the ignition and the time when 10% of heat has been released, indicated with q10, is compared for the tested fuels. The comparison is carried out using, as a reference, the value q10 obtained from CNG and the percent values are evaluated as the ratio between the actual q10 crank angle for any investigated fuels and that for CNG. The effect of HCNG blends in reducing q10 is evident, particularly at low loads. In details, the HCNG15 blend shows a 3% reduction of q10, while a larger reduction is observed for HCNG30, equal to 14%, when the imep is 1.2 bar, i.e. low load. When higher loads are considered, i.e. imep ¼ 5.0 bar, the observed reduction for HCNG30 is smaller and equal to 10%. Since the ignition timing was the same for all the tested fuels, the 50% Mass Fraction Burned (MFB) CA versus imep has been reported in Fig. 8 in order to verify a proper combustion phasing. It has to be noted that during CNG operation the combustion phasing is retarded, particularly at low loads, being this crank angle larger than the optimal value, which is between 8 and 10 ATDC [22]. The combustion phasing for the HCNG blends is closer to the optimal values than for CNG,
being slightly retarded for HCNG15 at part loads and advanced for HCNG30 at high loads. The coefficient of variation in indicated mean effective pressure, COVimep, which is a measure of cyclic variability [23], is reported in Fig. 9 as a function of the imep. It can be observed that COVimep reduces with hydrogen addition mainly at low loads due to the positive effect of hydrogen on combustion stability. In fact, a 35% and 55% lowering are observed for HCNG15 and HCNG30 respectively, compared with CNG, up to about 1.2 bar imep. The observed COVimep reductions are smaller when higher loads are considered. Fig. 10 illustrates maximum cylinder pressure pmax and the relative crank angle qpmax of each engine cycle at 1.2 bar imep and 2200 rpm. As in Fig. 1, the bubble diameter varies in size providing information about the frequency of the pmax qpmax values. Indeed, high bubble diameter in the plot indicates high frequency occurrence of engine cycles with such characteristics. The fastest burning cycles, located in the top left part of the figure, show a small dispersion. As qpmax increases, the dispersion becomes larger since the burning rate variations have a larger impact on the cycle. In fact, the presence of slow combustion cycles determines a bimodal distribution, which occurs when the pressure increase due to combustion is more than offset by the effect of volume variation [23]. Such
Fig. 11 e Maximum cylinder pressure ( pmax) versus maximum pressure crank angle ðqpmax Þ.
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behaviour is more evident using CNG, Fig. 10 (a), than HCNGs, 10 (b) and (c). HCNG blends showed a distribution centered at higher values of pmax compared to natural gas. At higher loads, Fig. 11, the dispersion reduces, no bimodal distributions are observed and, using HCNG blends, the center of gravity of the distribution moves to higher pmax values and lower qpmax then CNG, due to the effect of HCNG blends on combustion phasing.
4.
New European driving cycle Nitrogen oxides Engine speed, rpm Top dead center Total unburned hydrocarbon
Greek symbols Combustion duration, q9010 q50%MFB Combustion phasing angle,
Conclusion
A passenger car equipped with a spark ignition engine, fuelled with natural gas (CNG) and hydrogen-natural gas blends with 15% (HCNG15) and 30% (HCNG30) of hydrogen by volume, has been tested on a chassis dynamometer over different driving cycles with the same ignition timing. CO2 emissions were 139 2 g/km over the NEDC when the engine was fuelled with CNG, while values of 131 1 and 118 1 g/km were obtained with HCNG15 and HCNG30 respectively, with a decrease of 6% and 15% respect to CNG. Over the Artemis driving cycles CO2 emissions were 206 1 g/ km on the Urban part, 119 1 on the Road and 112 1 on the Motorway when fuelling the engine with CNG. The corresponding values with HCNG15 were 198 1, 115 1 and 108 1 g/km, whereas with HCNG30 174 1, 104 1, 94 1 g/ km, with reductions ranging between 3% and 4% for HCNG15 and between 13% and 16% for the HCNG30 blend respect to natural gas. Fuel consumption in MJ/km did not show significant differences between CNG and HCNG15 while reductions between 3% and 7% have been observed fuelling the engine by HCNG30. The heat release rate increased with hydrogen content in the blends, reaching values higher than CNG. The combustion duration has been reduced for all the operating conditions investigated. Cycle-by-cycle variability decreased, particularly at low loads, due to the positive effect of hydrogen on combustion stability.
Acknowledgements The authors acknowledge Regione Lombardia for providing the car, in the framework of a project with Fiat Research Center and Sapio. This work was also partially supported by Seconda Universita´ degli studi di Napoli.
Appendix A. Nomenclature
AFR ATDC CA CNG COV HCNG imep LHV MFB MPA
NEDC NOx n TDC THC
Air-fuel ratio, kgair/kgfuel After top dead center Crank angle, Compressed Natural Gas Coefficient of variation Hydrogen-natural gas blend Indicated mean effective pressure, bar Lower heating value, MJ/kg or MJ/Nm3 Mass fraction burned Mean positive acceleration, m/s2
Subscripts mix Mixture stoich Stoichiometric vol Volumetric
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