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natural gas blends on transit buses
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On road experimental tests of hydrogen/natural gas blends on transit buses Antonino Genovese a, Nicola Contrisciani a, Fernando Ortenzi b,*, Vittorio Cazzola c a
ENEA e Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Italy CTL e Centre for Transport and Logistics, Sapienza University of Rome, Italy c ATM e Public Transit Company of Ravenna, Italy b
article info
abstract
Article history:
The promise of reducing harmful and CO2 emissions by focusing on hydrogen-methane
Received 10 September 2010
blends (HCNG) have recently attracted the interest of vehicle manufacturers and transport
Received in revised form
operators. Several experiments have been conducted in laboratory facilities to assess the
27 October 2010
potential of HCNG blends in order to decrease the exhaust emissions. This paper reports
Accepted 28 October 2010
the results of experimental tests performed at the ENEA Casaccia Research Center aiming
Available online 15 December 2010
to evaluate the energy and environmental performances of a CNG vehicle when fuelled with a hydrogen-methane blend. Two buses for urban transit service were fuelled with
Keywords:
HCNG blends with different percentage of hydrogen (5%, 10%, 15%, 20% and 25% of
Hydrogen CNG blends
hydrogen by volume). A 100% methane gas was used as reference to compare the advan-
Experimental tests
tages and disadvantages that can be derived from the use of HCNG blends. Road tests have
Emissions
been carried out by running fixed tracks, which are representative of urban and suburban
Public transit bus
driving cycles. Vehicles were powered with a lean burn engine whose setup - based on ignition advance angle, has been tuned for controlling the NOx emissions. CO2 emissions have been investigated to evaluate the leverage effect based on an increased CO2 reduction resulting from an increased engine efficiency. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Natural gas (NG) is an energy source that is widely used in many fields: residential, industrial, electricity production and transport as well. The annual consumption of NG is steadily increasing in many countries in Europe and in other extraEuropean countries. The NG consumption for transport is growing due to its lower level of pollutants emissions and its very low particles emissions that make more attractive this intensive use. In public transport the NG is considered the most promising alternative to diesel fuel because of the high standards of service with no burden on the urban environment. Furthermore the lower carbon content in NG meets the
CO2 emissions reduction requirements in order to decrease the greenhouse gases (GHG) emissions in the atmosphere. Manufacturers were ready to fulfill the expectations of transit companies putting on the market vehicles designed and produced to be fuelled with NG. In NG stoichiometric engine the emissions of NOx are high, consequently vehicles are provided with a catalytic converter for the reduction of NOx emissions acting on NOx,, HC and CO. The reduction of NOx emissions can also be achieved using lean burn engines. In such engines the air-fuel ratio (AFR) is greater than the stoichiometric AFR giving air to fuel mixture lean. A value of l (l ¼ actual air-fuel ratio/stoichiometric air-fuel ratio) up to 1.6 provides a lean mixture with a very low production of CO2 and
* Corresponding author. E-mail address: [email protected] (F. Ortenzi). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.10.092
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very low emissions of NOx. The lean threshold takes place when the flame speed is very low and then this misfiring can decrease the engine performance and increase HC emissions. Furthermore, with a value closer to the lean threshold, the disturbance in mixture composition can result in poor engine stability. Improving the lean threshold can be done by adding hydrogen in natural gas in order to increase the combustion characteristics of the fuel. Hydrogen is now considered as the future fuel that can replace fossil fuels in many sectors e including transport, and several analysts agree with a larger use of hydrogen in automotive applications. Despite these promises, the transition toward fully hydrogen-powered vehicles is not immediately operational because of the considerable amount of time necessary for both the development of reliable engines and the adjustment of the fuel distribution network. The Hydrogen, used as fuel in internal combustion engines has mainly the following properties: a bigger laminar burning speed than methane (about 8 times than natural gas), no presence of Carbon in the fuel, but also a lower heating value in volume than CNG, as reported in [1e4]. The percent energy of H2, that is also the theoretical percent reduction of CO2 for each blend, is reported in Table 1, The use of HCNG blends could be considered as a more reasonable and reliable way in the transition to the intensive use of hydrogen as transport fuel. This solution is supported by the consistency of the engine technology that shows on the market the presence of engines optimized to be fuelled with NG. The introduction of HCNG blends requires nearly no changes in the vehicle layout or likely small changes in the engine setup. Mixture of hydrogen and NG is commonly known as “Hythane” although Hythane is a registered trademark for a blend of 20% hydrogen and 80% natural gas by volume. In this paper “hydromethane” will be used to indicate a generic HCNG mixture.
1.1.
Previous studies and experiments
In the past years, research has been carried out with a major concern on the use of hydromethane in internal combustion engines In [5] a GM 454 spark ignited engine (8 V cylinders, 7.4 L of displacement) was adjusted for gaseous fuel at test rig. The fuel supply system was designed to provide a variable mixture of hydrogen and NG. All measurements were made at 3800 rpm, which is the maximum power engine speed, and a comparison was achieved between NG and two blends with 10% and 20% of hydrogen by volume. Results showed that hydromethane was able to run as much leaner as higher is the
Table 1 e HCNG properties. H2% 0 5 10 15 20 25
Lower heating value kWh/kg
Lower heating value kWh/m3
% energy from H2
13.89 14.02 14.16 14.31 14.48 14.67
9.92 9.57 9.23 8.88 8.53 8.18
0 1.55 3.22 5.03 6.98 9.09
hydrogen content, increasing the lean thresholds from 1.35 to 1.55 and to 1.7 respectively. However a large engine power reduction was observed with a higher value of lean mixture. The engine efficiency increased with 10% of H2 and no extra benefits were observed at 20% of H2. The optimum ignition advance decreased with hydromethane because hydrogen has a higher flame speed. Without changes in engine tuning the emissions resulted in a NOx increase and in an HC decrease. In [6] a Volvo TD100 six-cylinder engine was modified for a single cylinder NG operation and then used for tests at 1200 rpm. Two different geometries for piston bowl configuration were adopted. Each geometry was tested with hydromethane fuel at different content of hydrogen (0%, 5%, 10%, 15% and 0%, 7%, 13%, 19% by volume respectively). The addition of hydrogen to natural gas increases the lean burn rate and extends the lean threshold, while it lowers HC emissions and increases NOx emissions for constant values of AFR and ignition timing. On the other hand, the increased lean burn speed allowed for a retarded ignition timing, which in turn decreased heat losses resulting in a higher efficiency. Furthermore the ignition timing reduction has generated a lower temperature and then lower NOx emissions. In [7] a comparison between NG fuel and hydromethane blend at 24.8% hydrogen by volume was performed using a Volvo TG103 heavy duty NG engine running at 1000 rpm. This study shows a small improvement of the engine efficiency with hydromethane fuel, probably due to the faster combustion that increases the effective expansion ratio. CO2 emissions reduction is given by the reduced carbon content per energy unit. NOx emissions are lower for hydromethane than NG ratio. The hydrogen content is high enough so that some existing natural gas engines may experience a decreased performance/efficiency due to their optimization to the existing natural gas quality. Modification of the engine setup may be necessary to take the full advantage of the change in fuel properties. In [8] a pilot study was conducted on the performance and emissions of a spark ignition engine operating with NGhydrogen blends. Tests were performed using a three-cylinder automotive engine of 0.796 L of displacement and 26.5 kW rated power. Engine was running at 2000 rpm and three hydromethane blends were used to fuel the engine: 10%, 20% and 26% of hydrogen by volume. The results confirmed the capability to extend the lean burn threshold with the addition of hydrogen into NG. lt from 1.5 for NG to 1.8 for 26% of H2. Thermal efficiency decreased for hydrogen fraction under 20% and increased for fraction of 26%. HC emissions decreased and NOx increased slightly at higher H2 fraction. In [9] a Volkswagen four-cylinder 1 L displacement gasoline engine was modified for CNG fuel with EGR. Engine efficiency, at best ignition timing, was increased with 10% of H2 and decreased with 15% of H2 in comparison with CNG. EGR affected positively the NOx and HC reduction in comparison with the methane baseline. In [10] a survey research was carried out on the use of hydromethane in ICE. This study evidenced decreased emissions of HC, CO and CO2 with the increasing of hydrogen percentage. Generally an increase of NOx emissions has been observed although adopting EGR or catalytic converter the emissions can be reduced to very low values.
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In [11] a Ford engine four-cylinder spark ignition of 1.8 L displacement was tested with 10%, 20% and 30% of H2 fraction and compared with pure methane fuel. The ignition timing was the same for all blends and the engine revolution was 2000 rpm. These tests showed a reduction in CO,HC and CO2 emissions with the increasing of H2 percentage and at the same time an increase of NOx emissions. Also the brake thermal efficiency increased with the increasing of the H2 percentage. In [12] two buses of the local fleet were fuelled for more than two years with a blend of 8% of H2 by volume. Buses were equipped with a lean burn engine without no modifications in engine tuning. Following these tests the buses were fuelled with a blend of 20% of H2 by volume that required a modification in the engine control map for both ignition and AFR. Buses have operated on road more than 160,000 km without operational problems caused by the fuel. The fuel consumption measured by a daily journal has shown a reduction over 20% in comparison with the CNG fuel consumption. The same engine was tested at test bench at 2000 rpm and it showed an increase in brake thermal efficiency, as well as a reduction in HC emissions and an increase in NOx emissions. Further tests with a 20% H2 by volume showed a significant improvement compared with CNG. NOx emissions were reduced by using an higher air/fuel ratio and by optimizing the ignition timing. In [13] four transit buses, two of them fuelled with CNG and the other two with hydromethane were tested for 24,000 miles during in-service operations at Thousand Palms, California. With preliminary tests performed at test bench, a blend of 20% H2 by volume was selected to fuel the buses. The buses fuelled with hydromethane were optimized using a lean AFR and retarding the spark ignition. Results of the on road consumption indicated a greater energy consumption for hydromethane in comparison with CNG. Additional engine calibration is necessary to reduce the energy consumption. Emissions were measured with the mobile laboratory of West Virginia University adopting two different driving cycles: urban and suburban. Buses fuelled with hydromethane demonstrated a reduction for more than 50% of NOx emissions and no difference has been detected for HC and CO emissions.
In [14] an experimental test was carried out at the ENEA’s labs, aimed at the identification of the potential use of blends of natural gas and hydrogen in existing ICE vehicles. The tested vehicle was an IVECO Daily CNG, originally fuelled with natural gas and the tests were made on the ECE15 driving cycle by comparing the emissions levels of the CNG configuration with the results obtained with different HCNG blends and control strategies as well. The vehicle under test was a light-duty commercial vehicle Euro III with three-way catalytic converter, IVECO Daily 2.8 CNG and the main parameters investigated were the air/fuel ratio, the spark advance angles and the enrichment during transients. For stoichiometric configuration can be seen that the simple fuel substitution without engine tuning, gives bad results especially for NOx emissions, Using the optimized maps, the emissions levels were even lower than the original CNG ones, especially for NOx, while for CO and HC there are improvements caused by a better combustion quality and a less carbon presence in the fuel. For lean burn mixtures the CO emissions are always lower for blends with 10% and 15% of H2 by volume. Nevertheless, concerning the NOx emissions, the values are lower than those ones using pure CNG, but not as good as
Fig. 1 e Bus under test Bredamenarini model Vivacity CNG.
Fig. 3 e Refueling station.
Fig. 2 e Ballast for payload simulation.
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Fig. 4 e Test track profile.
Fig. 5 e Casaccia driving cycle: urban cycle.
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Table 2 e Driving cycle comparison.
Lenght Mean speed Max speed Time
Table 3 e Ignition advance setup.
Casaccia
Brunschweig
Orange County
H2 %
3800 m 20 km/h 40 km/h 730 s
10873 m 22.9 km/h 58.2 km/h 1740 s
10526 m 19.9 km/h 65.4 km/h 1909 s
0% 5% 5% 10% 15%
stoichiometric values. Furthermore, HC emissions increase for the lower combustion quality due to lean mixture. In terms of consumption moving towards leaner mixtures and higher percentages of hydrogen give better results.
1.2.
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20% 25%
No change in setup for NG As the NG mapping 1 of delay for all load 1 of delay for all load Partial load: no change with load < 50% and advance > 29 degrees 75% > load >50%: 2 degrees of delay Full load: 3 degrees of delay As 15% blend 4 of delay for all the loads
Hydromethane project
The encouraging expectations described in research frameworks concerning hydrogen-NG blends as clean fuel stimulated the Regione ‘Emilia Romagna’ to evaluate energy and environmental impacts of hydromethane blends as fuel for buses currently fuelled with NG. In order to verify the real impacts of the use of hydrogen-methane mixtures a pilot test was launched in cooperation with two local bus fleet companies: ATM and ATR, operating respectively in Ravenna and Forlı`-Cesena. ENEA, the Italian National Agency for New Technologies, Energy and Sustainable Economic Development, was charged to carry out the experimental tests aimed to compare energy consumption and emissions for NG with hydromethane blends. On road tests were performed at ENEA Casaccia Research Center using two buses fuelled with hydromethane blends at different percentage of H2. Results were used to assess the optimal percentage of H2, to decrease the energy consumption, to evaluate CO2 reduction, to characterize the HC, NOx and CO emissions and to demonstrate that no modifications are required on service directives. Two different buses were used: a long 12-m bus and a 8-m bus. Both buses were of type CNG and manufactured by BREDAMENARINIBUS. The first bus was tested with the 5% H2eCH4 blend only and a suburban driving cycle was used. On the second bus many hydromethane blends were tested and an urban driving cycle was executed. Both the long and the small buses were also fuelled with a pure methane fuel to settle the reference baseline.
2.
Experimental setup and test procedure
In this paper results of the small bus only are reported because it has been fuelled with a wide set of blends : 5%, 10%, 15%, 20%, and 25% of H2 by volume respectively. The vehicle under test was a Vivacity CNG (see Fig. 1) equipped with a Mercedes CNG turbocharged engine with 6880 cc displacement, six cylinders, and 170 kW rated power. The engine is of type ”lean burn” and the bus is classified as EEV (Enhanced Environmentally friendly Vehicle). Four-cylinder fuel tanks are positioned on the roof with a total capacity of 1263 L. To reduce the refueling time only one cylinder has been used as fuel tank while the other three cylinders have been shut out. To simulate the real operative conditions a ballast was used as payload (see Fig. 2). A small refueling station (see Fig. 3) was employed to refill this vehicle tank at maximum charging pressure of 190 bar. The refueling station is based on a compressor unit with three-pressure stages to optimize the tank refilling process. Hydromethane mixtures were premixed in factory and transferred to the refueling station tanks. Methane G20 (CH4 99% pure) was adopted as reference fuel to operate the benchmark for comparison. Hydromethane blends have been prepared using the same methane G20 with the addition of H2 in percentages as mentioned above. Between each refueling process with different hydromethane blends the vehicle tank has been completely emptied.
Fig. 6 e On-board emission instrument: gas analyzer (a) and exhaust gas flowmeter (b).
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Table 4 e Mass fuel consumption. Mass fuel consumption
CH4 Hy 5% Hy 5% 1 deg Hy 10% Hy 15% Hy 20% Hy 25%
g/km
D%
309.18 293.56 289.49 274.43 259.73 256.82 248.76
0 5.1 6.4 11.2 16.0 16.9 19.5
Table 5 e Energy consumption. Energy consumption
CH4 Hy 5% Hy 5% 1 deg Hy 10% Hy 15% Hy 20% Hy 25%
kWh/km
g/km CH4 equiv.
D%
4.29 4.12 4.06 3.89 3.72 3.72 3.65
309.18 296.31 292.20 279.76 267.58 267.73 262.73
0 4.2 5.5 9.5 13.5 13.4 15.0
On road tests were performed at the ENEA Casaccia Research Center by defining an urban driving cycle using a well identified test track (see Fig. 4). The road profile is not as flat due to the presence of small variances in altitude. The Casaccia driving cycle includes a 3.8 km total route with the bus stops spaced at 250 m each other.. The bus has covered this circuit with a mean speed of 20 km/h and a maximum speed of 40 km/h (see Fig. 5). Table 2 shows the comparison between this cycle and other driving cycles used in other experimental activities. The outcomes indicate a high relationship between the Casaccia driving cycle and the Braunschweig cycle. The Casaccia cycle is considered to be very close to a real-life cycle and then different from the European Transient Cycle (ETC) adopted as the official cycle. In [15] and [16] investigations were made about the influence of the driving cycle test on energy consumption and
emissions, and results stressed the difference between the real driving cycle and the ETC. Because of this disparity the measured vehicle emissions can come out higher than EURO limits as indicated by the ETC cycle. Measurements of both fuel consumption and exhaust gas emissions were carried out using an on-board Horiba OBS1300 gas analyzer system [17] (see Fig. 6). The OBS-1300 system measures concentration of CO, CO2, HC, NOx, as well as the exhaust flow and the air-fuel ratio in real time. Then it calculates mass emissions of each pollutant and fuel consumption. The fuel consumption can be calculated in two ways, either using carbon mass balance or the AFR ratio: in the present work the first was used. However the two calculations were in accordance. Sixteen driving cycles for 60 km of distance, were carried out for each blend. The same driver has operated the bus for all the experiments. To take confidence with the track test a lot of trips were executed before starting the experimental campaign. The driver was selected with a previous personal experience in driving bus for public service and he attempted to adopt an optimal driving style in all tests. The measurements were performed modifying the ignition timing with different values of timing delay in ignition advance. These changes were made in order to decrease NOx emissions. The ignition timing was changed by modifying the engine maps with a dedicated hardware module as used also in [4]. Changes in ignition advance were operated as described in Table 3. The changes made on the ignition advance were made according with the previous experience made in Casaccia in 2006 [4,10]; increasing the percentage of hydrogen the advance was reduced. However the advance value for a real engine is a compromise of several phenomena and for a lean burn engine, as the engine of the present work, bigger variations of the ignition could reach problems of incomplete combustion with production of HC and also too much power losses.
3.
Experimental results
3.1.
Energy consumption
Experimental results show that all hydromethane blends produced better mass fuel savings compared to the use of
Fig. 7 e Comparison in energy consumption.
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Table 6 e Measured CO2 emission.
CH4 Hy 5% Hy 5% 1 deg Hy 10% Hy 15% Hy 20% Hy 25%
CO2 g/km
DCO2 %
kgCO2/kgfuel
833.32 782.06 769.68 734.44 691.75 671.00 640.86
0 6.2 7.6 11.9 17.0 19.5 23.1
2.71 2.67 2.69 2.65 2.65 2.62 2.60
Table 7 e Theoretical CO2 emissions taking account only for the Carbon reduction in the fuel. H2% 0 5 10 15 20 25
CO2 g/Whfuel
DCO2%
0.198 0.195 0.192 0.188 0.184 0.180
0 1.55 3.22 5.03 6.98 9.09
methane, as reported in Table 4. Mass fuel consumption decreased of 5% when using blends with lower content of H2 and of 20% with the 25% of H2. However, it is more significant to compare the values of energy consumption since each hydromethane blend has different energy content per unit of weight. The energy consumption as reported in Table 5 shows the equivalent methane consumption defined as the methane mass consumption producing the same amount of energy consumption of the hydromethane blend consumption. In Fig. 7 the bar histogram shows the percentage of energy consumption as referred to the methane energy consumption normalized to 100. Hydromethane blends with 15% and 20% of H2 have produced the same amount of energy consumption despite of their different energy content; it has been also evidenced that a further increase in H2 percentage produces a moderate reduction in energy consumption. The energy consumption reduction depends on the increased engine efficiency derived from an improved combustion caused by
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the higher flame speed available at higher H2 percentages. The engine power, increasing the percentage of hydrogen decreased (due to a lower heating values in volume), so to run the same driving cycle the engine had to be set to higher loads, that have higher engine efficiency, as reported also by the driver. This is a benefic effect in terms of consumption, but not in terms of driveability. For 5% of H2 a small increase in energy consumption can be also obtained with a small change in ignition advance of 1 only. This reinforces the role of the higher flame speed of hydromethane blend and then the necessity to delay the ignition time.
3.2.
CO2 emissions
By definition, a complete combustion of 1 M weight of methane yields 1 M weight of CO2 corresponding to a 2.75 kg of CO2 for each kilogram of methane. Experimental data confirmed that this ratio comes out employing 2.71 kg of CO2 for each kg of CH4. The CO2 emissions when using hydromethane blends are lower because the carbon atoms in fuel are decreased by their substitution with hydrogen atoms. The emissions of CO2 e in g/km, decreased of 23% with higher percentages of H2 and of 6% with lower contents of H2, as shown in Table 6. This improvement in CO2 emissions is not entirely due to the carbon content reduction in fuel because it also depends on both the increased engine efficiency and the related fuel consumption reduction. The evaluation of theoretical reduction of CO2 emissions for carbon substitution only must be carried out supposing an invariance in engine efficiency with all types of fuel. In such conditions, keeping the same value of the output power, the engine consumption is lower because hydromethane blends have a higher energy content. Table 7 shows the CO2 emissions in theoretical conditions when the same energy input is produced with different blends. Difference between theoretic and experimental outcomes is due to the improved engine efficiency that results in a lower fuel consumption and, consequently, in a further decrease in CO2 emissions. This occurrence can be defined as the leverage effect that amplifies the expectations of CO2 reduction by carbon atoms substitution from three to five times. Fig. 8 shows the
Fig. 8 e Comparison in CO2 emission.
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Fig. 9 e NOx emission reduction.
comparison of CO2 emissions reduction in relation to CO2 emissions of CH4 normalized to 100.
3.3.
NOx emissions
The flame temperature of a combustion with hydrogen is higher than that obtainable only with natural gas; consequently NOx emissions result higher than methane combustion. A potential solution for reducing the extra NOx emissions could be given by decreasing the ignition advance angle. Results coming out from on road tests showed a reduction of 40% in NOx emission for 5%, 10% and 15% of H2 when the ignition advance timing was reduced by 1e3 . Specifically, NOx emissions with 5% of H2 without modifications in ignition advance showed no significant difference in comparison with the NOx emissions produced by the methane fuel (see Fig. 9). For higher percentages of H2 e such as >20%, it has been evidenced that the only tuning of the ignition advance is not sufficient to recover the increased NOx emissions caused by the higher temperatures in combustion chamber. Consequently, higher emissions levels have been produced compared to the methane fuel. In order to improve the NOx emissions, a leaner AFR is feasible with hydromethane fuel at higher H2 percentages. An actual value of the AFR of 1.2 can be pushed until 1.4 with a lower NOx production, however during on road tests it was not possible to perform an engine tuning in order to gain a leaner combustion.
Table 8 e CO and HC emission results. Emissions g/km
CH4 Hy 5% Hy 5% 1 deg Hy 10% Hy 15% Hy 20% Hy 25%
CO
HC
0.07 0.09 0.10 0.18 0.20 0.11 0.12
3.40 3.17 3.93 3.37 3.34 3.35 2.79
3.4.
CO and HC emissions
Measurements confirmed the very low level of CO emissions for all blends with results close to the low detection limit of the gas analyzer, as shown in Table 8.. For HC data collected during on road, tests showed an invariance in HC emissions defined as the total hydrocarbon (methane and no-methane). With all fuels the HC emissions resulted higher than the EURO limits recalculated for the average energy consumed in Casaccia cycle.
4.
Conclusions
The results of these experimental tests can be summarized as follows: In urban driving the bus when fuelled with hydromethane blends showed a trend towards improved energy performance as the result of a higher engine efficiency due to hydrogen contents. The performance is positively influenced already at low values of hydrogen content. With a content of 5% of H2 has been evidenced an improvement of about 4% of energy consumption. This value is confirmed also by the decrease of 1 C degree in ignition advance. The reduction seems to settle around 15% for blends between 15% and 25% of hydrogen content. It should be stressed however that this comparison points out the improved performance when applying an increase of the percentage of hydrogen in the mixture; on the other hand these differences could be partly different when operating an optimization in ignition time. The reduction of CO2 emissions is one of the goal of hydromethane fuels. Tests carried out evidenced an amount of reduction beyond theoretical values as expected when reducing carbon atoms. A leverage effect occurs to amplify the reduction in CO2 emissions. The decreasing in fuel consumption is attributed to the improved fuel efficiency that reduces energy consumption, and therefore the emissions of combustion products. The leverage effect as observed is estimated between 3 and 5 times the expected theoretical value, calculated with the assumption of
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invariance of engine performance. The mixture of 25% H2 produces a 25% reduction in CO2 emissions compared with the 9% theoretical expected value, and this is the largest reduction detected so far. CO emissions are very low at the instrument sensitivity limit. The emissions of unburned hydrocarbons are generally opposite of NOx. With an increased percentage of hydrogen HC emissions present a tendency to decrease as a result of better combustion conditions. Meanwhile, an increase of H2 percentage increments NOx emissions for an increased combustion temperature. The output measurements have shown that the levels of emitted HC using Casaccia test cycles are basically constant even though above the limits EEV. With Casaccia cycles the NOx emissions were up to 47% less than when using the 5% blend with 1 C of delay in the ignition advance. Similar values have also been reached with mixtures of 10% and 15% - with delays in ignition advance of 1 C and 2 C respectively. With an increase of H2 content up 25% the NOx emissions are subject to increase also with an advanced ignition delay of 4 . The strategy for further reductions of NOx, along with an increase of the advance, is based on the leaning AFR. Conversely, the addition of hydrogen allows for a better and faster combustion, which enables the use of a mixture poorer of fuel showing an further reduction of NOx emissions. Consequently hydromethane blends indicated that HC and CO decrease with the increase of H2 percentages although a fine tuning in both ignition advance timing and leaning AFR is necessary to decrease the NOx emissions CO2 can also be reduced thanks to the lower carbon content in hydromethane blends and to the higher engine efficiency allowed from an increased combustion speed.
[2]
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Acknowledgements
[13]
The authors wish to acknowledge the support for this project from the Region Emilia Romagna, ATM Ravenna and ATR Forli-Cesena. Special thanks to Mr. Bernardini of ENEA for assistance on management and operation of the project.
[14]
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references [16] [17] [1] Ma Fanhua, Liu Haiquan, Wang Yu, Li Yong, Wang Junjun, Zhao Shuli. Combustion and emission characteristics of
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a port-injection HCNG engine under various ignition timings. Int J Hydrogen Energy 2008;33:816e22. Xu Jian, Zhang Xin, Liu Jianhua, Fan Longfei. Experimental study of a single-cylinder engine fueled with natural gasehydrogen mixtures. Int J Hydrogen Energy 2010;35: 2909e14. Morrone B, Unich A. Numerical investigation on the effects of natural gas and hydrogen blends on engine combustion. Int J Hydrogen Energy 2009;34:4626e34. F.Ortenzi, M.Chiesa, F.Conigli, Esperimental Tests of Blends of Hydrogen and Natural Gas in Light Duty Vehicles, HYSYDays - Turin 2007, 2th World Congress of Young Scientist on Hydrogen Energy Systems Sierens R, Rossell E. Variable composition hydrogen/ natural gas mixtures for increased engine efficiency and decreased emissions. J Eng Gas Turbines Power 2000;122: 135e40. P. Tunesta˚l, M. Christensen, P. Einewall, T. Andersson, B. Johansson, O. Jo¨nsson, Hydrogen Addition For Improved Lean Burn Capability of Slow and Fast Burning Natural Gas Combustion Chambers, SAE Paper N. 2002-01-2686. Tunesta˚l P, Einewall P, Stenla˚a˚s O, Johansson B. Possible short-term introduction of hydrogen as vehicle fuel/fuel additive. In: Which fuel for low CO2 engines ? Paris: Technip editions; 2004. Zuohua H, Bing L, Ke Z, Yinyu H, Deming J, Xibin W, et al. Experimental study on engine performance and emission for an engine fueled with natural gas-hydrogen mixtures. Energy Fuels 2006;20:131e2136. Dimopoulos P, Bach C, Soltic P, Boulouchos K. Hydrogennatural gas blends fuelling passenger car engines: combustion, emissions and well-to-wheels assessments. Int J Hydrogen Energy 2008;33:7224e36. lu N. Internal Akansu O, Dulger Z, Kahraman N, Vezirog Combustion Engines Fuelled by natural gas-hydrogen mixtures. Int J Hydrogen Energy 2004;29:1527e39. Akansu O, Kahraman N, C¸eper B. Experimental study on a spark ignition engine fuelled by methane-hydrogen mixtures. Int J Hydrogen Energy 2007;32:4279e84. B. Riddel, C. Bro, Malmo¨ Hydrogen and CNG/Hydrogen filling station and Hythane bus project. In: 16 WHEC, Lyon France, (2006). A. Del Toro, M. Frailey, F. Lynch, S. Munshi, S. Wayne, Development and Demonstration of hydrogen and Compressed Natural Gas Blend Transit Buses, Technical report NREL/TP-540-38707, (2005). Ortenzi F, Chiesa M, Scarcelli R, Pede G. Experimental tests of blends of hydrogen and natural gas in light-duty vehicles. Int J Hydrogen Energy 2008;33:3225e9. L. Pelkmans, D. De Keukeleere, H. Bruneel, G. Lanaers, Influence of Vehicle Test Cycle Characteristics on Fuel Consumption and Emissions of City Buses, SAE Paper 2001 01FL-308. AB Svensk Bilprovning Motortestcenter. Report 9707 (1997). N. Kihara, The OBS-1000 Series On-board Engine Emission Measurement System, Horiba technical report, Readout 0911-064.
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On road experimental tests of hydrogen/natural gas blends on transit buses Antonino Genovese a, Nicola Contrisciani a, Fernando Ortenzi b,*, Vittorio Cazzola c a
ENEA e Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Italy CTL e Centre for Transport and Logistics, Sapienza University of Rome, Italy c ATM e Public Transit Company of Ravenna, Italy b
article info
abstract
Article history:
The promise of reducing harmful and CO2 emissions by focusing on hydrogen-methane
Received 10 September 2010
blends (HCNG) have recently attracted the interest of vehicle manufacturers and transport
Received in revised form
operators. Several experiments have been conducted in laboratory facilities to assess the
27 October 2010
potential of HCNG blends in order to decrease the exhaust emissions. This paper reports
Accepted 28 October 2010
the results of experimental tests performed at the ENEA Casaccia Research Center aiming
Available online 15 December 2010
to evaluate the energy and environmental performances of a CNG vehicle when fuelled with a hydrogen-methane blend. Two buses for urban transit service were fuelled with
Keywords:
HCNG blends with different percentage of hydrogen (5%, 10%, 15%, 20% and 25% of
Hydrogen CNG blends
hydrogen by volume). A 100% methane gas was used as reference to compare the advan-
Experimental tests
tages and disadvantages that can be derived from the use of HCNG blends. Road tests have
Emissions
been carried out by running fixed tracks, which are representative of urban and suburban
Public transit bus
driving cycles. Vehicles were powered with a lean burn engine whose setup - based on ignition advance angle, has been tuned for controlling the NOx emissions. CO2 emissions have been investigated to evaluate the leverage effect based on an increased CO2 reduction resulting from an increased engine efficiency. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Natural gas (NG) is an energy source that is widely used in many fields: residential, industrial, electricity production and transport as well. The annual consumption of NG is steadily increasing in many countries in Europe and in other extraEuropean countries. The NG consumption for transport is growing due to its lower level of pollutants emissions and its very low particles emissions that make more attractive this intensive use. In public transport the NG is considered the most promising alternative to diesel fuel because of the high standards of service with no burden on the urban environment. Furthermore the lower carbon content in NG meets the
CO2 emissions reduction requirements in order to decrease the greenhouse gases (GHG) emissions in the atmosphere. Manufacturers were ready to fulfill the expectations of transit companies putting on the market vehicles designed and produced to be fuelled with NG. In NG stoichiometric engine the emissions of NOx are high, consequently vehicles are provided with a catalytic converter for the reduction of NOx emissions acting on NOx,, HC and CO. The reduction of NOx emissions can also be achieved using lean burn engines. In such engines the air-fuel ratio (AFR) is greater than the stoichiometric AFR giving air to fuel mixture lean. A value of l (l ¼ actual air-fuel ratio/stoichiometric air-fuel ratio) up to 1.6 provides a lean mixture with a very low production of CO2 and
* Corresponding author. E-mail address: [email protected] (F. Ortenzi). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.10.092
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very low emissions of NOx. The lean threshold takes place when the flame speed is very low and then this misfiring can decrease the engine performance and increase HC emissions. Furthermore, with a value closer to the lean threshold, the disturbance in mixture composition can result in poor engine stability. Improving the lean threshold can be done by adding hydrogen in natural gas in order to increase the combustion characteristics of the fuel. Hydrogen is now considered as the future fuel that can replace fossil fuels in many sectors e including transport, and several analysts agree with a larger use of hydrogen in automotive applications. Despite these promises, the transition toward fully hydrogen-powered vehicles is not immediately operational because of the considerable amount of time necessary for both the development of reliable engines and the adjustment of the fuel distribution network. The Hydrogen, used as fuel in internal combustion engines has mainly the following properties: a bigger laminar burning speed than methane (about 8 times than natural gas), no presence of Carbon in the fuel, but also a lower heating value in volume than CNG, as reported in [1e4]. The percent energy of H2, that is also the theoretical percent reduction of CO2 for each blend, is reported in Table 1, The use of HCNG blends could be considered as a more reasonable and reliable way in the transition to the intensive use of hydrogen as transport fuel. This solution is supported by the consistency of the engine technology that shows on the market the presence of engines optimized to be fuelled with NG. The introduction of HCNG blends requires nearly no changes in the vehicle layout or likely small changes in the engine setup. Mixture of hydrogen and NG is commonly known as “Hythane” although Hythane is a registered trademark for a blend of 20% hydrogen and 80% natural gas by volume. In this paper “hydromethane” will be used to indicate a generic HCNG mixture.
1.1.
Previous studies and experiments
In the past years, research has been carried out with a major concern on the use of hydromethane in internal combustion engines In [5] a GM 454 spark ignited engine (8 V cylinders, 7.4 L of displacement) was adjusted for gaseous fuel at test rig. The fuel supply system was designed to provide a variable mixture of hydrogen and NG. All measurements were made at 3800 rpm, which is the maximum power engine speed, and a comparison was achieved between NG and two blends with 10% and 20% of hydrogen by volume. Results showed that hydromethane was able to run as much leaner as higher is the
Table 1 e HCNG properties. H2% 0 5 10 15 20 25
Lower heating value kWh/kg
Lower heating value kWh/m3
% energy from H2
13.89 14.02 14.16 14.31 14.48 14.67
9.92 9.57 9.23 8.88 8.53 8.18
0 1.55 3.22 5.03 6.98 9.09
hydrogen content, increasing the lean thresholds from 1.35 to 1.55 and to 1.7 respectively. However a large engine power reduction was observed with a higher value of lean mixture. The engine efficiency increased with 10% of H2 and no extra benefits were observed at 20% of H2. The optimum ignition advance decreased with hydromethane because hydrogen has a higher flame speed. Without changes in engine tuning the emissions resulted in a NOx increase and in an HC decrease. In [6] a Volvo TD100 six-cylinder engine was modified for a single cylinder NG operation and then used for tests at 1200 rpm. Two different geometries for piston bowl configuration were adopted. Each geometry was tested with hydromethane fuel at different content of hydrogen (0%, 5%, 10%, 15% and 0%, 7%, 13%, 19% by volume respectively). The addition of hydrogen to natural gas increases the lean burn rate and extends the lean threshold, while it lowers HC emissions and increases NOx emissions for constant values of AFR and ignition timing. On the other hand, the increased lean burn speed allowed for a retarded ignition timing, which in turn decreased heat losses resulting in a higher efficiency. Furthermore the ignition timing reduction has generated a lower temperature and then lower NOx emissions. In [7] a comparison between NG fuel and hydromethane blend at 24.8% hydrogen by volume was performed using a Volvo TG103 heavy duty NG engine running at 1000 rpm. This study shows a small improvement of the engine efficiency with hydromethane fuel, probably due to the faster combustion that increases the effective expansion ratio. CO2 emissions reduction is given by the reduced carbon content per energy unit. NOx emissions are lower for hydromethane than NG ratio. The hydrogen content is high enough so that some existing natural gas engines may experience a decreased performance/efficiency due to their optimization to the existing natural gas quality. Modification of the engine setup may be necessary to take the full advantage of the change in fuel properties. In [8] a pilot study was conducted on the performance and emissions of a spark ignition engine operating with NGhydrogen blends. Tests were performed using a three-cylinder automotive engine of 0.796 L of displacement and 26.5 kW rated power. Engine was running at 2000 rpm and three hydromethane blends were used to fuel the engine: 10%, 20% and 26% of hydrogen by volume. The results confirmed the capability to extend the lean burn threshold with the addition of hydrogen into NG. lt from 1.5 for NG to 1.8 for 26% of H2. Thermal efficiency decreased for hydrogen fraction under 20% and increased for fraction of 26%. HC emissions decreased and NOx increased slightly at higher H2 fraction. In [9] a Volkswagen four-cylinder 1 L displacement gasoline engine was modified for CNG fuel with EGR. Engine efficiency, at best ignition timing, was increased with 10% of H2 and decreased with 15% of H2 in comparison with CNG. EGR affected positively the NOx and HC reduction in comparison with the methane baseline. In [10] a survey research was carried out on the use of hydromethane in ICE. This study evidenced decreased emissions of HC, CO and CO2 with the increasing of hydrogen percentage. Generally an increase of NOx emissions has been observed although adopting EGR or catalytic converter the emissions can be reduced to very low values.
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In [11] a Ford engine four-cylinder spark ignition of 1.8 L displacement was tested with 10%, 20% and 30% of H2 fraction and compared with pure methane fuel. The ignition timing was the same for all blends and the engine revolution was 2000 rpm. These tests showed a reduction in CO,HC and CO2 emissions with the increasing of H2 percentage and at the same time an increase of NOx emissions. Also the brake thermal efficiency increased with the increasing of the H2 percentage. In [12] two buses of the local fleet were fuelled for more than two years with a blend of 8% of H2 by volume. Buses were equipped with a lean burn engine without no modifications in engine tuning. Following these tests the buses were fuelled with a blend of 20% of H2 by volume that required a modification in the engine control map for both ignition and AFR. Buses have operated on road more than 160,000 km without operational problems caused by the fuel. The fuel consumption measured by a daily journal has shown a reduction over 20% in comparison with the CNG fuel consumption. The same engine was tested at test bench at 2000 rpm and it showed an increase in brake thermal efficiency, as well as a reduction in HC emissions and an increase in NOx emissions. Further tests with a 20% H2 by volume showed a significant improvement compared with CNG. NOx emissions were reduced by using an higher air/fuel ratio and by optimizing the ignition timing. In [13] four transit buses, two of them fuelled with CNG and the other two with hydromethane were tested for 24,000 miles during in-service operations at Thousand Palms, California. With preliminary tests performed at test bench, a blend of 20% H2 by volume was selected to fuel the buses. The buses fuelled with hydromethane were optimized using a lean AFR and retarding the spark ignition. Results of the on road consumption indicated a greater energy consumption for hydromethane in comparison with CNG. Additional engine calibration is necessary to reduce the energy consumption. Emissions were measured with the mobile laboratory of West Virginia University adopting two different driving cycles: urban and suburban. Buses fuelled with hydromethane demonstrated a reduction for more than 50% of NOx emissions and no difference has been detected for HC and CO emissions.
In [14] an experimental test was carried out at the ENEA’s labs, aimed at the identification of the potential use of blends of natural gas and hydrogen in existing ICE vehicles. The tested vehicle was an IVECO Daily CNG, originally fuelled with natural gas and the tests were made on the ECE15 driving cycle by comparing the emissions levels of the CNG configuration with the results obtained with different HCNG blends and control strategies as well. The vehicle under test was a light-duty commercial vehicle Euro III with three-way catalytic converter, IVECO Daily 2.8 CNG and the main parameters investigated were the air/fuel ratio, the spark advance angles and the enrichment during transients. For stoichiometric configuration can be seen that the simple fuel substitution without engine tuning, gives bad results especially for NOx emissions, Using the optimized maps, the emissions levels were even lower than the original CNG ones, especially for NOx, while for CO and HC there are improvements caused by a better combustion quality and a less carbon presence in the fuel. For lean burn mixtures the CO emissions are always lower for blends with 10% and 15% of H2 by volume. Nevertheless, concerning the NOx emissions, the values are lower than those ones using pure CNG, but not as good as
Fig. 1 e Bus under test Bredamenarini model Vivacity CNG.
Fig. 3 e Refueling station.
Fig. 2 e Ballast for payload simulation.
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Fig. 4 e Test track profile.
Fig. 5 e Casaccia driving cycle: urban cycle.
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Table 2 e Driving cycle comparison.
Lenght Mean speed Max speed Time
Table 3 e Ignition advance setup.
Casaccia
Brunschweig
Orange County
H2 %
3800 m 20 km/h 40 km/h 730 s
10873 m 22.9 km/h 58.2 km/h 1740 s
10526 m 19.9 km/h 65.4 km/h 1909 s
0% 5% 5% 10% 15%
stoichiometric values. Furthermore, HC emissions increase for the lower combustion quality due to lean mixture. In terms of consumption moving towards leaner mixtures and higher percentages of hydrogen give better results.
1.2.
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20% 25%
No change in setup for NG As the NG mapping 1 of delay for all load 1 of delay for all load Partial load: no change with load < 50% and advance > 29 degrees 75% > load >50%: 2 degrees of delay Full load: 3 degrees of delay As 15% blend 4 of delay for all the loads
Hydromethane project
The encouraging expectations described in research frameworks concerning hydrogen-NG blends as clean fuel stimulated the Regione ‘Emilia Romagna’ to evaluate energy and environmental impacts of hydromethane blends as fuel for buses currently fuelled with NG. In order to verify the real impacts of the use of hydrogen-methane mixtures a pilot test was launched in cooperation with two local bus fleet companies: ATM and ATR, operating respectively in Ravenna and Forlı`-Cesena. ENEA, the Italian National Agency for New Technologies, Energy and Sustainable Economic Development, was charged to carry out the experimental tests aimed to compare energy consumption and emissions for NG with hydromethane blends. On road tests were performed at ENEA Casaccia Research Center using two buses fuelled with hydromethane blends at different percentage of H2. Results were used to assess the optimal percentage of H2, to decrease the energy consumption, to evaluate CO2 reduction, to characterize the HC, NOx and CO emissions and to demonstrate that no modifications are required on service directives. Two different buses were used: a long 12-m bus and a 8-m bus. Both buses were of type CNG and manufactured by BREDAMENARINIBUS. The first bus was tested with the 5% H2eCH4 blend only and a suburban driving cycle was used. On the second bus many hydromethane blends were tested and an urban driving cycle was executed. Both the long and the small buses were also fuelled with a pure methane fuel to settle the reference baseline.
2.
Experimental setup and test procedure
In this paper results of the small bus only are reported because it has been fuelled with a wide set of blends : 5%, 10%, 15%, 20%, and 25% of H2 by volume respectively. The vehicle under test was a Vivacity CNG (see Fig. 1) equipped with a Mercedes CNG turbocharged engine with 6880 cc displacement, six cylinders, and 170 kW rated power. The engine is of type ”lean burn” and the bus is classified as EEV (Enhanced Environmentally friendly Vehicle). Four-cylinder fuel tanks are positioned on the roof with a total capacity of 1263 L. To reduce the refueling time only one cylinder has been used as fuel tank while the other three cylinders have been shut out. To simulate the real operative conditions a ballast was used as payload (see Fig. 2). A small refueling station (see Fig. 3) was employed to refill this vehicle tank at maximum charging pressure of 190 bar. The refueling station is based on a compressor unit with three-pressure stages to optimize the tank refilling process. Hydromethane mixtures were premixed in factory and transferred to the refueling station tanks. Methane G20 (CH4 99% pure) was adopted as reference fuel to operate the benchmark for comparison. Hydromethane blends have been prepared using the same methane G20 with the addition of H2 in percentages as mentioned above. Between each refueling process with different hydromethane blends the vehicle tank has been completely emptied.
Fig. 6 e On-board emission instrument: gas analyzer (a) and exhaust gas flowmeter (b).
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Table 4 e Mass fuel consumption. Mass fuel consumption
CH4 Hy 5% Hy 5% 1 deg Hy 10% Hy 15% Hy 20% Hy 25%
g/km
D%
309.18 293.56 289.49 274.43 259.73 256.82 248.76
0 5.1 6.4 11.2 16.0 16.9 19.5
Table 5 e Energy consumption. Energy consumption
CH4 Hy 5% Hy 5% 1 deg Hy 10% Hy 15% Hy 20% Hy 25%
kWh/km
g/km CH4 equiv.
D%
4.29 4.12 4.06 3.89 3.72 3.72 3.65
309.18 296.31 292.20 279.76 267.58 267.73 262.73
0 4.2 5.5 9.5 13.5 13.4 15.0
On road tests were performed at the ENEA Casaccia Research Center by defining an urban driving cycle using a well identified test track (see Fig. 4). The road profile is not as flat due to the presence of small variances in altitude. The Casaccia driving cycle includes a 3.8 km total route with the bus stops spaced at 250 m each other.. The bus has covered this circuit with a mean speed of 20 km/h and a maximum speed of 40 km/h (see Fig. 5). Table 2 shows the comparison between this cycle and other driving cycles used in other experimental activities. The outcomes indicate a high relationship between the Casaccia driving cycle and the Braunschweig cycle. The Casaccia cycle is considered to be very close to a real-life cycle and then different from the European Transient Cycle (ETC) adopted as the official cycle. In [15] and [16] investigations were made about the influence of the driving cycle test on energy consumption and
emissions, and results stressed the difference between the real driving cycle and the ETC. Because of this disparity the measured vehicle emissions can come out higher than EURO limits as indicated by the ETC cycle. Measurements of both fuel consumption and exhaust gas emissions were carried out using an on-board Horiba OBS1300 gas analyzer system [17] (see Fig. 6). The OBS-1300 system measures concentration of CO, CO2, HC, NOx, as well as the exhaust flow and the air-fuel ratio in real time. Then it calculates mass emissions of each pollutant and fuel consumption. The fuel consumption can be calculated in two ways, either using carbon mass balance or the AFR ratio: in the present work the first was used. However the two calculations were in accordance. Sixteen driving cycles for 60 km of distance, were carried out for each blend. The same driver has operated the bus for all the experiments. To take confidence with the track test a lot of trips were executed before starting the experimental campaign. The driver was selected with a previous personal experience in driving bus for public service and he attempted to adopt an optimal driving style in all tests. The measurements were performed modifying the ignition timing with different values of timing delay in ignition advance. These changes were made in order to decrease NOx emissions. The ignition timing was changed by modifying the engine maps with a dedicated hardware module as used also in [4]. Changes in ignition advance were operated as described in Table 3. The changes made on the ignition advance were made according with the previous experience made in Casaccia in 2006 [4,10]; increasing the percentage of hydrogen the advance was reduced. However the advance value for a real engine is a compromise of several phenomena and for a lean burn engine, as the engine of the present work, bigger variations of the ignition could reach problems of incomplete combustion with production of HC and also too much power losses.
3.
Experimental results
3.1.
Energy consumption
Experimental results show that all hydromethane blends produced better mass fuel savings compared to the use of
Fig. 7 e Comparison in energy consumption.
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Table 6 e Measured CO2 emission.
CH4 Hy 5% Hy 5% 1 deg Hy 10% Hy 15% Hy 20% Hy 25%
CO2 g/km
DCO2 %
kgCO2/kgfuel
833.32 782.06 769.68 734.44 691.75 671.00 640.86
0 6.2 7.6 11.9 17.0 19.5 23.1
2.71 2.67 2.69 2.65 2.65 2.62 2.60
Table 7 e Theoretical CO2 emissions taking account only for the Carbon reduction in the fuel. H2% 0 5 10 15 20 25
CO2 g/Whfuel
DCO2%
0.198 0.195 0.192 0.188 0.184 0.180
0 1.55 3.22 5.03 6.98 9.09
methane, as reported in Table 4. Mass fuel consumption decreased of 5% when using blends with lower content of H2 and of 20% with the 25% of H2. However, it is more significant to compare the values of energy consumption since each hydromethane blend has different energy content per unit of weight. The energy consumption as reported in Table 5 shows the equivalent methane consumption defined as the methane mass consumption producing the same amount of energy consumption of the hydromethane blend consumption. In Fig. 7 the bar histogram shows the percentage of energy consumption as referred to the methane energy consumption normalized to 100. Hydromethane blends with 15% and 20% of H2 have produced the same amount of energy consumption despite of their different energy content; it has been also evidenced that a further increase in H2 percentage produces a moderate reduction in energy consumption. The energy consumption reduction depends on the increased engine efficiency derived from an improved combustion caused by
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the higher flame speed available at higher H2 percentages. The engine power, increasing the percentage of hydrogen decreased (due to a lower heating values in volume), so to run the same driving cycle the engine had to be set to higher loads, that have higher engine efficiency, as reported also by the driver. This is a benefic effect in terms of consumption, but not in terms of driveability. For 5% of H2 a small increase in energy consumption can be also obtained with a small change in ignition advance of 1 only. This reinforces the role of the higher flame speed of hydromethane blend and then the necessity to delay the ignition time.
3.2.
CO2 emissions
By definition, a complete combustion of 1 M weight of methane yields 1 M weight of CO2 corresponding to a 2.75 kg of CO2 for each kilogram of methane. Experimental data confirmed that this ratio comes out employing 2.71 kg of CO2 for each kg of CH4. The CO2 emissions when using hydromethane blends are lower because the carbon atoms in fuel are decreased by their substitution with hydrogen atoms. The emissions of CO2 e in g/km, decreased of 23% with higher percentages of H2 and of 6% with lower contents of H2, as shown in Table 6. This improvement in CO2 emissions is not entirely due to the carbon content reduction in fuel because it also depends on both the increased engine efficiency and the related fuel consumption reduction. The evaluation of theoretical reduction of CO2 emissions for carbon substitution only must be carried out supposing an invariance in engine efficiency with all types of fuel. In such conditions, keeping the same value of the output power, the engine consumption is lower because hydromethane blends have a higher energy content. Table 7 shows the CO2 emissions in theoretical conditions when the same energy input is produced with different blends. Difference between theoretic and experimental outcomes is due to the improved engine efficiency that results in a lower fuel consumption and, consequently, in a further decrease in CO2 emissions. This occurrence can be defined as the leverage effect that amplifies the expectations of CO2 reduction by carbon atoms substitution from three to five times. Fig. 8 shows the
Fig. 8 e Comparison in CO2 emission.
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Fig. 9 e NOx emission reduction.
comparison of CO2 emissions reduction in relation to CO2 emissions of CH4 normalized to 100.
3.3.
NOx emissions
The flame temperature of a combustion with hydrogen is higher than that obtainable only with natural gas; consequently NOx emissions result higher than methane combustion. A potential solution for reducing the extra NOx emissions could be given by decreasing the ignition advance angle. Results coming out from on road tests showed a reduction of 40% in NOx emission for 5%, 10% and 15% of H2 when the ignition advance timing was reduced by 1e3 . Specifically, NOx emissions with 5% of H2 without modifications in ignition advance showed no significant difference in comparison with the NOx emissions produced by the methane fuel (see Fig. 9). For higher percentages of H2 e such as >20%, it has been evidenced that the only tuning of the ignition advance is not sufficient to recover the increased NOx emissions caused by the higher temperatures in combustion chamber. Consequently, higher emissions levels have been produced compared to the methane fuel. In order to improve the NOx emissions, a leaner AFR is feasible with hydromethane fuel at higher H2 percentages. An actual value of the AFR of 1.2 can be pushed until 1.4 with a lower NOx production, however during on road tests it was not possible to perform an engine tuning in order to gain a leaner combustion.
Table 8 e CO and HC emission results. Emissions g/km
CH4 Hy 5% Hy 5% 1 deg Hy 10% Hy 15% Hy 20% Hy 25%
CO
HC
0.07 0.09 0.10 0.18 0.20 0.11 0.12
3.40 3.17 3.93 3.37 3.34 3.35 2.79
3.4.
CO and HC emissions
Measurements confirmed the very low level of CO emissions for all blends with results close to the low detection limit of the gas analyzer, as shown in Table 8.. For HC data collected during on road, tests showed an invariance in HC emissions defined as the total hydrocarbon (methane and no-methane). With all fuels the HC emissions resulted higher than the EURO limits recalculated for the average energy consumed in Casaccia cycle.
4.
Conclusions
The results of these experimental tests can be summarized as follows: In urban driving the bus when fuelled with hydromethane blends showed a trend towards improved energy performance as the result of a higher engine efficiency due to hydrogen contents. The performance is positively influenced already at low values of hydrogen content. With a content of 5% of H2 has been evidenced an improvement of about 4% of energy consumption. This value is confirmed also by the decrease of 1 C degree in ignition advance. The reduction seems to settle around 15% for blends between 15% and 25% of hydrogen content. It should be stressed however that this comparison points out the improved performance when applying an increase of the percentage of hydrogen in the mixture; on the other hand these differences could be partly different when operating an optimization in ignition time. The reduction of CO2 emissions is one of the goal of hydromethane fuels. Tests carried out evidenced an amount of reduction beyond theoretical values as expected when reducing carbon atoms. A leverage effect occurs to amplify the reduction in CO2 emissions. The decreasing in fuel consumption is attributed to the improved fuel efficiency that reduces energy consumption, and therefore the emissions of combustion products. The leverage effect as observed is estimated between 3 and 5 times the expected theoretical value, calculated with the assumption of
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invariance of engine performance. The mixture of 25% H2 produces a 25% reduction in CO2 emissions compared with the 9% theoretical expected value, and this is the largest reduction detected so far. CO emissions are very low at the instrument sensitivity limit. The emissions of unburned hydrocarbons are generally opposite of NOx. With an increased percentage of hydrogen HC emissions present a tendency to decrease as a result of better combustion conditions. Meanwhile, an increase of H2 percentage increments NOx emissions for an increased combustion temperature. The output measurements have shown that the levels of emitted HC using Casaccia test cycles are basically constant even though above the limits EEV. With Casaccia cycles the NOx emissions were up to 47% less than when using the 5% blend with 1 C of delay in the ignition advance. Similar values have also been reached with mixtures of 10% and 15% - with delays in ignition advance of 1 C and 2 C respectively. With an increase of H2 content up 25% the NOx emissions are subject to increase also with an advanced ignition delay of 4 . The strategy for further reductions of NOx, along with an increase of the advance, is based on the leaning AFR. Conversely, the addition of hydrogen allows for a better and faster combustion, which enables the use of a mixture poorer of fuel showing an further reduction of NOx emissions. Consequently hydromethane blends indicated that HC and CO decrease with the increase of H2 percentages although a fine tuning in both ignition advance timing and leaning AFR is necessary to decrease the NOx emissions CO2 can also be reduced thanks to the lower carbon content in hydromethane blends and to the higher engine efficiency allowed from an increased combustion speed.
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Acknowledgements
[13]
The authors wish to acknowledge the support for this project from the Region Emilia Romagna, ATM Ravenna and ATR Forli-Cesena. Special thanks to Mr. Bernardini of ENEA for assistance on management and operation of the project.
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[15]
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