Experimental Tests of Blends of Hydrogen and Natural Gas in Light Duty Vehicles. Fernando Ortenzi

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1 Experimental Tests of Blends of Hydrogen and Natural Gas in Light Duty Vehicles Fernando Ortenzi Centre for Transport and Logistics, University La Sapienza of Rome Rome, Italy Maria Chiesa Environmental Physics Department, Catholic University of the Sacred Heart Brescia, Italy Francesco Conigli ENEA, Italian National Agency for New Technologies and Environment Rome, Italy ABSTRACT Main drivers for the introduction of hydrogen as a fuel for vehicles are the reduction of both greenhouse gases emissions and local pollutants, above all in congested urban environments. While the use of hydrogen in fuel cell vehicles will be the ultimate target for sustainable mobility, much attention has to be devoted to a suitable bridge technology, to accelerate the process of the introduction of these new technologies and in the meantime to build an infrastructure suitable for the utilization of hydrogen. In the short-medium term, the atmospheric emissions could be significantly reduced by using different percentages of hydrogen mixed with natural gas in internal combustion engines. The paper is based on the results of an experimental test campaign carried on in ENEA labs, aimed at identifying the prospective of the use of blends of natural gas and hydrogen (HCNG) in existing ICE vehicles. The tested vehicle is a Iveco Daily CNG, originally fuelled with natural gas and the tests had been made on the ECE15 driving cycle comparing the emission levels of the original configuration (CNG) with the results obtained with different blends (percentage of hydrogen in the fuel) and control strategies (stoichiometric or lean-burn). Also the steady results, that were used for tuning the engine from CNG to HCNG are reported. Key words Hydrogen and Natural gas blends, HCNG, Engine tuning, experimental test 1. INTRODUCTION In 23, according to British Petroleum (BP), the worldwide consumption of primary energy had reached 1 billions TEP. Fossil sources had contributed to this value for the 85%, with CO 2 emissions equal to 6.1 GtC. Antropic activities are responsible for the increase of atmospheric CO 2. Actually, during the last 15 years the CO 2 atmospheric concentration had risen from 28 ppm to 37 ppm (see Figure 1); consequently, it is necessary to foresee structural changes of the energetic system. HYSYDAYS Turin 27 1

2 1.1. State of the art Figure 1: CO 2 increase from the pre industrial era The Intergovernmental Panel on Climate Change (IPCC) foresees that, without taking any effective measure, the CO2 emissions will grow up to three times their present values in this century, reaching values around 2 GtC in 21. In order to reduce CO2 emissions different technological solutions can be taken over: among others, an increase of the global efficiency of the energetic cycles or a reduction of the carbon content in the fuels. In this way, from the beginning of the industrial era onwards, the trends related to the consumption of different fuels had always underlined that the market moves towards the more cleaner ones, with decreasing C/H ratios. If we think about CH4, the fossil fuel with the lowest C/H ratio, the next step could be represented by zero carbon fuels such as hydrogen. In this sense, the use of pure hydrogen could represent a cultural shift towards a great abatement of local pollutants and CO2. Nevertheless, a widely diffused use of hydrogen must foresee the existence of infrastructures that are not yet available; a possible bridge technology between the use of CH4 and H2 could be represented by the use of blends of hydrogen and natural gas in internal combustion engines. The use of blends doesn t need dedicated infrastructures apart from a little hydrogen storage and a column for the mixing of hydrogen with the natural gas coming from the grid via pipeline. Experiences of application of blends of hydrogen and natural gas in internal combustion engines had started in 1991, in the framework of a research programme financed by DOE and NREL, called the Denver Hythane Project. The results are shown in the Table below: Table 1: Denver Hythane Project NMHC CO NOx (g/mile) (g/mile) (g/mile) Gasoline ULEV Natural Gas Hythane In the same years, the University of Pisa and ENEA had carried on some activities with the following interesting results:.93 Equivalent ratio % 2% 36% H2 by volume NOx,gr/HPh Figure 2: NOx emissions in function of both the equivalent ratio and the H 2 percentage by volume During the last 15 years many experiences had been conducted all over the world. All the experiments had mainly examined the reduction of the emissions of NOx with respect to different air/fuel ratios (λ values) and percentages of hydrogen by volume. All the experiments had shown that the blends of hydrogen and natural gas reduce the exhaust emissions of both regulated pollutants and CO 2 and increases the efficiency of a spark ignition engine. During the last years, a number of fleet tests had been carried on. The recent Hythane (24.8% vol. Hydrogen, Frank Lynch, Hydrogen Components, Inc., HCI), bus demonstration project at Sunline transit in California used a 7% hydrogen by HYSYDAYS Turin 27 2

3 energy formula and the NOx emissions were reduced by 5%. Based on success with Hythane buses, and the cost-effectiveness of Hythane compared to available fuel cell technology, a number of projects are currently carried on all around the world, like the Beijing Hythane Bus Project, whose demonstration phase will be to adapt 3 natural gas engines for Hythane operation before 21. At a European level, the most significative example of application of blends of hydrogen and natural gas in internal combustion engines is given by the tests still ongoing in Malmo (Sweden) on urban buses. The experimental results are available for blends with a hydrogen content of 8% by volume for their use on real driving cycles, while the data relative to a blend of 25% by volume are available just for the engine tests, in specific functioning points. The available data show an increase of the engine performance with the increase of the hydrogen quantity in the blend. Furthermore, depending on the λ values associated to the combustion of the blends, an overall environmental benefit can be noticed for λ values higher than 1 (see Figure 3 and Figure 4). 35 % Funzionamento non stabile Figure 4 Emissions of an ICE fuelled with a HCNG8 blend From Figure 3 an efficiency increase from 31,2% till 35% for a λ value of 1.61 is illustrated. For inferior values of λ the efficiency is higher; nevertheless, since the first target related to the use of blends is the reduction of atmospheric emissions of urban pollutants and CO2 the tests have to optimise the combustion trying to obtain the maximum reduction of total hydrocarbons (HC), nitric oxides (NO X ) and carbon monoxide (CO). From Figure 3b it emerges that for λ = 1.61 there is a reduction of all the pollutants emissions with the exception of nitric oxides whose emissions don t decrease with respect to the use of pure NG. Nevertheless, for the blends of hydrogen and natural gas the combustion remains stable even for air/fuel ratios values that exceed those ones relative to natural gas, equal to 1.6, overwhich the combustion remains unstable. 31.2% Figure 3 Efficiency of an ICE fuelled with a HCNG8 blend 2. METHANE-HYDROGEN BLENDS As had been seen in the previous paragraphs, a good opportunity in the short term can be represented by the utilization of blends of hydrogen with other fuels, first of all with natural gas (HCNG). When used in an Internal Combustion Engine (ICE), even the addition of a small amount of hydrogen to natural gas (5-3% by volume, that means ~1.5-1% by energy) leads to many advantages, because of some particular physical and chemical properties of the two fuels. Methane has a slow flame speed while hydrogen has a flame speed about eight times HYSYDAYS Turin 27 3

4 higher (Figure 5); therefore, when the equivalence ratio (lambda) is much higher than for the stoichiometric condition, the combustion of methane is not as stable as with HCNG. Laminar Burning Speed [m/s] Methane Hydrogen φ Figure 5 Flame laminar speed for methane and HCNG15 [11] As a consequence of the addition of hydrogen to natural gas an overall better combustion had been verified, even in a wide range of operating conditions (lambda, compression ratio, etc.), finding the following main benefits: a higher efficiency lower CO2 production and emissions Because of hydrogen chemical and physical properties, HCNG, despite its higher LHV per kg, has a lower LHV per Nm 3 (Figure 6), depending on the hydrogen content. Therefore, a natural gas engine, when fuelled with HCNG, shows a lower power output, while maintaining its better efficiency. LHV LHV (MJ/kg) LHV (MJ/m3) % volume H2 Figure 6 LHV for different percentage of Hydrogen To restore a good value of power output, especially for lean burn mixtures (for lambda=1.4 the engine loses 5% of its power), a good solution could be represented by a turbocharged engine with an higher charging pressure. Additionally, CO2 emissions had been reduced not only as a result of the substitution of CNG by hydrogen. The special properties of hydrogen as a combustion stimulant can produce leverage factors much greater than 1 by improving fossil fuels and not just displacing them. Hydrogen leverage is defined as the following ratio : (% Emissions Reduction)/(% Energy Supplied as Hydrogen). The increased efficiency makes this value higher than 1. An obvious benefit of the leverage effect is that a CO 2 reduction is possible even if the hydrogen used is produced by natural gas without any sequestration of CO ENEA EXPERIMENTAL TESTS The light duty commercial vehicle under test had been an Euro III (with 3way catalytic converter) Daily 2.8 CNG (belonging to the fleet of ASM SpA of Brescia) and it had been mainly modified in the control system (ECU) for the tests. Its original engine was a 2.8L CNG fuelled, manufactured by IVECO (see Table 2 for specifications). Table 2 Engine specifications Displaced volume 28 cm 3 Compression ratio 12.2 Bore 94.4 mm Stroke 1 mm Rated power Rpm Max Torque 22 Rpm Emission Standard Euro III The roller bench comes from APICOM; its control system had been recently upgraded by Assing. The cycle adopted for the characterisation had been the urban part of NEDC (repeated at least 2 times) and the value for the vehicle mass had been set to 35 kg; therefore, the tests had been more severe than the homologation one to be sure that the vehicle, under this condition, would have had HYSYDAYS Turin 27 4

5 enough power to make the ECE15 cycle if fuelled with very lean blends. Figure 7 APICOM roller bench, the emissions analyzer and the Iveco Daily tested Therefore, the results are not directly comparable with OEM data, but surely are more significative considering the guide cycle of the ASM SpA vehicles that could use these blends in the future (for example, the waste collecting vehicles) Engine ECU Tuning tool The tool used to modify the engine maps is called RACE2, elaborated by Dimensione Sport. This software is able to transform data contained in the original EPROM in easily modifiable electronic mapping. In order to modify the engine maps in real time during engine tuning phase, an EPROM emulator (MET16) had been used. The final flashing of the EPROM had been obtained thanks to the EMP21, an EPROM programmer produced by Needham s Electronics Measurement system Two sets of instruments had been used: the first one for the measurement of the in-cylinder pressure and the other one for the measurement of both the emissions of CO, CO2, HC, NOx and fuel consumption. However, in order to be sure to have an accurate measurement of fuel consumption the cylinders had been weighed before and after every test Cylinder pressure A single cylinder head had been equipped with a piezoelectric pressure transducer hosted on a special spark plug and signals had been processed by an amplifier while the angular position had been measured with a inductive crank-angle calculator module for on-line indicating measurements. The equipment had been manufactured by AVL and all data had been collected by a Yokogawa DL 716 digital scope. In the picture below, an example of preliminary testing with pure methane (15 rpm at low loads) is given, for the engine model validation. A comparison between measured data with the engine simulation model[16] is reported in Figure 8. Pressure [bar] Measured Simulation model Crank angle Figure 8 Comparison between measured cylinder pressure and model data Emissions Emissions had been measured using a HORIBA OBS-13 integrated system. This is composed by a MEXA-117 HNDIR (Non Dispersive Infra-red Detectors) that measures CO, CO2 and HC in real time and by a MESA-72NOx (ZrO2 technology), for the evaluation of nitric oxides concentrations and air-fuel ratio (AFR); furthermore, a flowmeter (pitot type) mounted on the sampling probe permits to calculate the exhaust flow rate and exhaust pressure and temperature (see Figure 7 and Figure 9). This instrumentation also measures the atmospheric conditions (pressure, temperature and humidity) and, thanks to a GPS receiver, even the latitude, longitude, altitude along with the vehicle speed. HYSYDAYS Turin 27 5

6 Figure 9 Screenshot of Horiba collection software Fuel Consumption Two blends had been tested, characterised by hydrogen percentages of 1% and 15% (HCNG1 and HCNG15) by volume and used as a fuel for the urban part of the ECE-15 driving cycle. Pure methane of certified composition and certified mixtures had been used for the tests Engine tuning The main objective of the engine tuning phase had been the reduction of the CO2 production without increasing the emissions. Two different blends had been tested: the HCNG1 and the HCNG15, giving more attention to the last one in the tuning phase. The main parameters that had been investigated are lambda (with values of 1 and 1.4), different spark advance angles and different values for the enrichment of the blends during transients. Advance Rpm Load Figure 11 Iveco Daily CNG Advance map Figure 1 HCNG and methane cylinders used for the experiments For characterization tests, single cylinders had been used. To assure the requested precision with regard to energy consumption measures, the cylinders had been weighed before and after drive tests (ECE 15) on our roller bench after a consistent time ( about 2 ECE15 cycles). As main exhaust parameter which had been considered as a constraint that had not to be overcome in case of stoichiometric set up had been the NOx emissions. Actually, hydrogen addition implies a higher laminar combustion speed and this causes an increase of combustion temperature and therefore higher NOx emissions. On the contrary, CO and HC emissions are always lower, thanks both to the lower quantity of carbon and to the improved combustion process. For lean-burn mixtures, not only NOx, but also HC monitoring had been a decisive parameter that had been taken into consideration. Actually, also HC emissions can raise due to the fact that laminar combustion speed decreases remarkably. This produces a not complete oxidation of HC. Moreover, higher gas cooling during expansion delays the fuel oxidation, in particularly near cylinder s walls and in the most hidden parts of the combustion chamber. HYSYDAYS Turin 27 6

7 3.3. Stoichiometric mixture The first tests using blends to feed the engine, carried without any modification of the injection control map, had confirmed the foreseen NOx emissions increase related to the increasing combustion speed. This fact had required a modification of the spark ignition time. In Figure 12 it is possible to see that a spark advance reduction of only 3 degrees (which means a little retard compared to the case of pure methane) brings to a large decrease of NOx emissions, without torque reduction. NOx ppm NOx After NOx before Speed After Speed Before Time s. Figure 13 Comparison between standard and modified enrichment acceleration parameters (HCNG15) Vehicle Speed Km/h 9 65 NOx ppm NOx Torque Advance variation Figure 12 NOx and Torque outputs for λ=1 at different ignition advance at 15 rpm (15% H2) Notwithstanding the above reported spark ignition time correction, engine performances with methane-hydrogen blends had remained not acceptable during ECE driving cycle in terms of emissions, due to the too high NOx emission values, compared to pure methane. A more detailed examination of the engine behaviour during transients shows that fuel enrichment values (as mapped in ECU) had been too low, therefore actual λ reaches values comprised between (corresponding to the maximum production of NOx). As a result, NOx emissions had increased too much. For this reason, a map correction had been adopted concerning the acceleration phases. This variation had been possible thanks to a dedicated function of the electronic injection unit. In this way it had been possible to decrease NOx emissions to the desired values, lower than figures obtained with methane (Figure 13) Torque Nm 3.4. Lean mixtures For a lean burn blend, research of better λ (we wanted to reach a value of 1 ppm, the same of pure methane) had been limited from λ=1 to λ= Furthermore, the increase of λ values had caused very important power losses, as shown in Figure 14. NOx ppm Nox Torque Lambda Figure 14 NOx and Torque in function of λ for lean mixtures at 15 rpm (15% H2) Turbocharged engines, having the possibility to achieve more power output increasing the charging pressure, can run with leaner mixtures without great power losses. Therefore, λ=1,45 had been the maximum value initially fixed (this value substantially reduces NOx) and a series of tests had been produced changing the spark ignition advance to optimize the control strategy in order to increase the performances. Unfortunately, NOx had grown in an exponential way, while power gain hadn t been significative (Figure 15Figure 15) Torque Nm HYSYDAYS Turin 27 7

8 NOx [ppm] NOX (ppm) Coppia (Nm) Wheel Torque [Nm] the reasons explained in section 4.1.Using the optimized maps, the emissions levels are even lower than the original CNG ones, especially for NOx, while for CO and HC there are improvements caused by a better combustion quality (for HC) and a less carbon presence in the fuel (for CO). However, with the HCNG1 the vehicle presents the lowest emissions levels Advance variation Figure 15 NOx and Torque varying advance for λ=1.4 at 15 rpm (15% H2) It had also been decided that it is more convenient to adopt a lower λ value without changing the spark advance instead of setting a high λ together with optimal advance timing. Consumption g/kwh Lambda Figure 16 Specific consumption in function of λ at 15 rpm (15% H2) However, as a future development, it could be interesting to analyze λ values greater than 1.45 for their better efficiency and consumption (Figure 16) [17], but the engine will need substantial modifications such as a different compression ratio, the addition of a turbocharger, etc 4. RESULTS In the following pictures the obtained values for emissions produced on the ECE15 driving cycle for the configurations analysed are represented and compared with the original CNG values. For the stoichiometric configuration (Figure 17), it can be seen that the simple fuel substitution (without engine tuning) can give bad results especially for NOx emissions, for 29 g/km NOx CO HC NG λ=1 (HCNG15) λ=1 Opt (HCNG15) λ=1 Opt (HCNG1) Figure 17 Emissions produced on the ECE15 driving cycle with stoichiometric mixtures for HCNG1 and HCNG15 For lean burn mixtures (Figure 18Figure 17) the emissions present two different behaviours: the CO emissions are always lower for blends with 1% 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 stoichiometric values. Furthermore, HC emissions increase for the lower combustion quality due to lean mixture. g/km NOx CO HC.8.57 NG λ=1.4 Opt (HCNG15) λ=1.4 Opt (HCNG1) Figure 18 Emissions produced on the ECE15 driving cycle with lean burn mixtures for HCNG1 and HCNG15 It s interesting to underline that, operating the vehicle with the O2 sensor disabled, the fuel system hadn t be able to adjust the blend to an HYSYDAYS Turin 27 8

9 objective value: in all the unsteady conditions (especially in acceleration phases) any variation towards leaner mixtures had led to higher HC emissions and richer blends had increased NOx emissions. In order to close the loop this fact has to be taken into consideration as a future development % % Fuel reduction 8.96 CO2 reduction HCNG1 λ=1 HCNG1 λ=1,4 HCNG15 λ=1 HCNG15 λ=1,4 Figure 19 Fuel and CO2 reduction for different blends and different lambda The advantages in terms of consumption are reported in Figure 19: leaner mixtures and moving towards higher percentages of hydrogen give better results. This is not the same for the emissions. The improvements of HCNG1 in stoichiometric conditions are about 3% for consumption and 4.3% for CO2 emissions reduction, while for HCNG15 and λ=1.5 are about 12.7% and 14.91%. The values for HCNG1 with λ=1.4 and HCNG15 with λ=1 are similar. In order to take into account the well-to-tank consumption (Figure 2), related to the hydrogen production by fossil fuels (we don t consider the more favourable case of hydrogen production by renewables) let we consider the minimal (2.16%) hydrogen content, in mass, for a HCNG15 mixture. HCNG1 λ=1 HCNG1 λ=1,4 HCNG15 λ=1 HCNG15 λ=1,4 Figure 2 Tank to wheel analysis: energy saving We know, from the Well-to-Wheel analysis of future automotive fuels and power-trains in the European context, EUCAR / JRC / CONCAWE, 26, that the energy consumption related to hydrogen production from natural gas (EU mix) is about 16% (efficiency: 84%). Therefore, HCNG15 production is a mere.5% less efficient, from an energetic point of view, than natural gas. If the average engine efficiency is about 2% in urban use, this value implies a 2.5% efficiency loss from Well-towheel, that is more that counterbalanced by the above reported values for energy saving measured during tests, (from 5% with a stoichiometric blend up to 1% for lean mixtures). 5. CONCLUSIONS The analysis of the results of the lab tests can be considered encouraging for the continuation of the activities, even if the tests had just covered a short period of time (a few months). It s well known that the reduction of fuel consumption brings to the enhancement of the pollutants and greenhouse gases emissions; therefore, the optimum condition can be found dealing with the problem using different approaches, i.e. with the use of lean blends in the first case (for the fuel consumption reduction) and with the use of stoichiometric blends in the second case (emissions reduction). Actually, these two approaches correspond to the different conceptual ideas adopted by VOLVO (whose engines are mounted on the urban buses of the Malmo Hythane project ) HYSYDAYS Turin 27 9

10 and by IVECO (the manufacturer of the DAILY vehicle used for our experimentations) for the realisation of their natural gas motorisation. In our case, dealing with an IVECO engine, that had been designed in order to work in stoichiometric conditions, the adoption of a lean combustion strategy had brought us to unsatisfactory results: actually, since no changements in the engine hardware had been made, as for example the compression ratio enhancement and/or the engine turbocharging, with respect to the vehicle s behaviour, a reduction of the engine specific power had been verified, along with the reduction of the power due to the inferior energy content in volume (11% for the blend with a 15% hydrogen content by volume). Nevertheless, for both the combustion strategies adopted, the vehicle had succeeded in doing the urban cycle and the engine had worked regularly obtaining encouraging results in terms of the reduction of fuel consumption and pollutants and CO 2 emissions as demonstrated even by the foreign lab and road tests, even with hydrogen produced by fossil fuels! The pollutants emissions reduction had been very promising mainly working in stoichiometric conditions, without a significative difference between the use of blends with a 1% or 15% hydrogen content by volume (see Figure 17 and Figure 18). Finally, even considering the Swedish works (that don t present problems of power losses with the use of blends) we can say that, starting from the existent motorisations, the strategies that have to be adopted for the modifications must be coherent with the base constructor philosophy; in our case, we had taken into consideration the IVECO philosophy. Consequently, we consider the obtained results with stoichiometric conditions as the base results of a possible development of the project that foresees the field experimentation of vehicles with an IVECO motorisation (for example, industrial vehicles for the transports of goods or waste collecting vehicles). Future experimental developments would foresee the optimisation of the emissions of both urban pollutants and CO 2 along with a reduction of fuel consumption for the specific vehicle driving cycle finding the best compromise after investigating the following much wider set of engine variables: A wider spectrum of λ values; Spark advance; Percentage of H2 in the blends; Compression ratio; Intake charging pressure. REFERENCES [1]. The Sustainable Mobility Project, Mobility 23: "Meeting the challenges to sustainability, World Business Council for Sustainable Development, Switzerland, 25 [2]. Riddell, B., Malmo Hydrogen and CNG/Hydrogen filling station and Hythane bus project, Carl Bro Energiekonsult AB, Sweden, 24 [3]. Tunestal P., Einewall P., Stenlaas O., Johansson B., Possible short term introduction of hydrogen as vehicle fuel/fuel additive, P. Duret and Editions Technip, Paris, 24, [4]. Karner D., Francfort J., Freedom car & vehicle technologies program Advanced vehicle testing activity- Arizona public service- Alternative fuel (Hydrogen) pilot plant, US DOE, 23 [5]. F. Linch, Hythane: a bridge to an ultraclean renewable hydrogen energy system, Atti del Workshop IEA in Denver, 1991 [6]. James Cannon, Paving the way to Natural Gas Vehicles, INFORM, Inc., 1993 [7]. R. Sierens, E. Rosseel, Variable Composition Hydrogen/Natural Gas mixtures for increased engine efficiency and decreased emissions, Journal of Engineering for Gas turbines and Power, 2, [8]. Hoekstra R. L., Collier, K and Mulligan N., Demonstration of Hydrogen Mixed Gas Vehicles, Proceedings, 1 th World Hydrogen Energy Conference, Cocoa Beach, Vol. 3, 1994 HYSYDAYS Turin 27 1

11 [9]. Hoekstra R. L., Van Blarigan P., and Mulligan N., NOx emissions and efficiency of Hydrogen, Natural Gas and Hydrogen/Natural Gas blended fuels, SAE Paper 96113, 1996 [1]. Swain M. R., Yusuf M., Dulger Z., and Swain M. N., The effects of hydrogen addition on natural gas engine operation, SAE Paper , 23 [11]. Larsen J. F., and Wallace J. S., Comparison of emissions and efficiency of a turbocharged lean-burn natural gas and hythane-fuelled engine, ASME Journal of Engineering for gas turbines and power, Vol. 119, 1997 [12]. Yusuf M., Swain M. R., Swain M. N., and Dulge Z., An approach to lean burn natural gas fuelled engine through hydrogen addition, Proceedings, 3 th ISATA Conference, Florence, paper 97EL81, 1997 [13]. Raman V., Hansel J., Fulton J., Lynch F., and Bruderly D., Hythane an ultraclean transportation fuel, Proceedings, 1 th World Hydrogen Energy Conference, Cocoa Beach Vol. 3, pp , 1994 [14]. Bell S. R., and Gupta M., Extension of the lean operating limit for natural gas fuelling of a spark ignited engine using hydrogen blending, Combustion Science and Technology, Vol. 123, pp , 1997 [15]. Toshio Iijima, Tadao Takeno, Effects of Temperature and Pressure on Burning Velocity, Combustion and Flame 65: (1986) [16]. F.Ortenzi Un modello di simulazione del processo di combustione per motori ad accensione comandata, Thesys 23 [17]. Per Tunestål, Magnus Christensen, Patrik Einewall, Tobias Andersson, Bengt Johansson Owe Jönsson Hydrogen Addition For Improved Lean Burn Capability of Slow and Fast Burning Natural Gas Combustion Chambers, SAE HYSYDAYS Turin 27 11