**5. HCNG blends**

Table 2 compares the main physical properties for pure fuels, methane and hydrogen. In the same table, LHV represents the Lower Heating Value of the fuel, AFR is the air-to-fuel ratio and LHV*stoich*, *mix* [MJ/Nm3] is the volumetric lower heating value for a stoichiometric air-fuel mixture.


Table 2. CH4 and H2 properties (Glassman & Yetter, 2008).

Table 3 shows the main fuel characteristics of natural gas and hydrogen-natural gas blends with 10% (HCNG10), 20% (HCNG20) and 30% (HCNG30) of hydrogen in volume. The volumetric hydrogen content is calculated according to equation 14.

$$H\_2[\%vol.] = \frac{V\_{H\_2}}{V\_{NG} + V\_{H\_2}} \tag{14}$$

The volumetric Lower Heating Value is the fuel energy per unit volume, so it is a measure of the energy that can be stored in the fuel tank. It is 7% lower than NG for HCNG10, 14% for HCNG20 and 21% for HCNG30. LHV*stoich*, *mix*, which is proportional to the engine power output, is negligibly affected by hydrogen addition.


Table 3. NG and HCNG fuel properties.

#### **5.1 Combustion characteristics**

6 Will-be-set-by-IN-TECH

MJ/m<sup>3</sup> (Sørensen, 2005). The main disadvantage, however, is the weight of the storage alloys.

Table 2 compares the main physical properties for pure fuels, methane and hydrogen. In the same table, LHV represents the Lower Heating Value of the fuel, AFR is the air-to-fuel ratio and LHV*stoich*, *mix* [MJ/Nm3] is the volumetric lower heating value for a stoichiometric air-fuel

> Adiabatic flame temperature of stoichiometric mixtures [K] 2210 2400 Flammability limits in air at 25◦*C* and 1 bar [% vol.] 5.0-15 4.0-75 Minimum ignition energy in air at *φ* = 1 and 1 bar [mJ] 0.47 0.02 LHV [MJ/kg] 50.0 120.3 LHV*vol* [MJ/Nm3] 35.3 10.6 AFR*stoich* 17.2 34.3 LHV*stoich*, *mix* [MJ/Nm3] 3.351 3.143

Table 3 shows the main fuel characteristics of natural gas and hydrogen-natural gas blends with 10% (HCNG10), 20% (HCNG20) and 30% (HCNG30) of hydrogen in volume. The

*<sup>H</sup>*2[%*vol*.] = *VH*<sup>2</sup>

H2 [% vol.] - 10 20 30 H2 [% energy] - 3.2 7.0 14.4 LHV [MJ/kg] 45.3 46.2 46.7 48.5 LHV*vol* [MJ/Nm3] 36.9 34.3 31.7 29.2 AFR*stoich* 15.6 15.8 16.1 16.4 LHV*stoich*, *mix* [MJ/Nm3] 3.375 3.367 3.358 3.349

The volumetric Lower Heating Value is the fuel energy per unit volume, so it is a measure of the energy that can be stored in the fuel tank. It is 7% lower than NG for HCNG10, 14% for HCNG20 and 21% for HCNG30. LHV*stoich*, *mix*, which is proportional to the engine power

*VNG* + *VH*<sup>2</sup>

**Natural Gas HCNG10 HCNG20 HCNG30**

**CH4 H2**

(14)

Other storage options are under investigation but still at prototypal stage (Bakker, 2010).

Furthermore refuelling times are affected by absorption rates.

Table 2. CH4 and H2 properties (Glassman & Yetter, 2008).

output, is negligibly affected by hydrogen addition.

Table 3. NG and HCNG fuel properties.

volumetric hydrogen content is calculated according to equation 14.

**5. HCNG blends**

mixture.

Since hydrogen laminar combustion speed is about eight times greater than methane, it provides a reduction of combustion duration when mixed with natural gas in small concentrations. Many studies have been carried out to measure the flame speed of hydrogen-methane air mixtures at different hydrogen concentrations and equivalence ratios. Ilbas et al. (2006) performed the measurements at ambient temperatures with hydrogen-methane blends up to 100% hydrogen.

Fig. 2. Flame speed of different fuels versus equivalence ratio *φ* (Ilbas et al., 2006).

Figure 2 shows the flame speed for methane and a 50% hydrogen-methane blend plotted versus the equivalence ratio. The maximum flame speed for the blend is 0.69 m/s while the maximum for methane is 0.39 m/s for an equivalence ratio *φ* = 1.1. The flammable regions were also widened as the hydrogen content increased in the mixtures.

Figure 3, where the flame speed is plotted versus hydrogen content, shows the non-linear dependence of this property on hydrogen percentage.

Mandilas et al. (2007) performed experiments in a spherical stainless steel vessel at initial temperatures up to 600 K and initial pressures up to 1.5 MPa to study the effects of hydrogen addition on laminar and turbulent premixed methane-air flames. The burning velocity, u*l*, was found using equation 9. Methane can be ignited for 0.6 ≤ *φ* ≤ 1.3, with the peak burning velocity occurring at *φ* = 1.0. The addition of H2 extends the ignition limits to the range 0.5 ≤ *φ* ≤ 1.4 and increases the values of u*<sup>l</sup>* at lean equivalence ratios, while u*<sup>l</sup>* does not increase for rich equivalence ratios. The authors also compared the turbulent velocity u*tr* for methane and a blend with 30% of hydrogen. As in the laminar case, the addition of hydrogen

Fig. 4. Unstretched laminar burning velocity *ul* versus the equivalence ratio *φ* for HCNG20

A Review of Hydrogen-Natural Gas Blend Fuels in Internal Combustion Engines 25

found that 10% hydrogen increases engine efficiency moderatly whereas 20% hydrogen gives

Recently, Ma et al. (2010) investigated the effect of high hydrogen volumetric content, up to 55%, on the performance of a turbocharged lean burn natural gas engine. The authors found that the addition of hydrogen significantly extends the lean limit, decreases burn duration and yields higher thermal efficiency. The plot of the engine efficiency versus *λ*, Figure 7, shows a negative trend in engine efficiency for natural gas for *λ* values greater than 1.3, while the

The increased hydrogen/carbon ratio and engine efficiency bring a reduction of CO2 emissions. By the way, as a consequence of a faster combustion, higher temperature are attained in the combustion chamber, increasing NOx emissions in HCNG fuelled engines compared to natural gas, for a given equivalence ratio *φ*. NOx can be kept down and engine efficiency further improved if the engine is run with lean mixtures or adopting EGR at

Sierens & Rosseel (2000) found the maximum NOx emissions at a relative air-fuel ratio *λ* = 1.1. For higher *λ* values, the reduction in heat of combustion available for the charge mixture reduces the temperature and NOx as a consequence, as shown in Figure 8. However, such

Hoekstra et al. (1995) obtained very low NOx emissions operating with HCNG blends close the lean limit, significantly extended compared with natural gas. Besides, the excellent anti

blend with the higher hydrogen content shows positive trend up to *λ* = 1.6.

conditions cause an increase in THC emissions, as shown in Figure 9.

(Miao et al., 2009).

stoichiometric air-fuel ratio.

negligible extra benefit, as shown in Figure 6.

Fig. 3. Flame speeds versus hydrogen content in methane-hydrogen blends at *φ* = 1 (Ilbas et al., 2006).

extends the ignition limits and higher u*tr* values, in particular at lean air-fuel mixtures, are attained compared to methane.

A comparison of results obtained by several authors for the unstretched laminar burning velocity versus the equivalence ratio, for HCNG20, is shown in Figure 4 (Miao et al., 2009). It is observed in any case that the maximum flame speed is attained at *φ* ∼= 1.1 with values around 0.5 m/s.

#### **5.2 The impact of HCNG blends on engine efficiency and exhaust emissions**

The reduction of combustion duration promoted by hydrogen addition results in increased engine efficiency respect to natural gas and enhances combustion stability, reducing cycle-by-cycle variation. Nagalingam et al. (1983) proved that the high burning rate of HCNG blends requires an ignition timing lower than natural gas to obtain the Maximum Brake Torque (MBT).

The MBT spark advance versus the hydrogen content, shown in Figure 5 (Karim et al., 1996), is noticeably affected by hydrogen addition, in particular for very lean air-fuel mixtures. The plot shows that for blends containing significant amount of hydrogen, small adjustments to the ignition timing are needed when the equivalence ratio is changed.

The engine efficiency can be increased fuelling the engine by HCNG blends. Sierens & Rosseel (2000) developed a fuel system which supplies hydrogen-natural gas mixtures in variable proportion to the engine. For low brake mean effective pressures high efficiency can be achieved by increasing the hydrogen content reducing throttling losses. The authors 8 Will-be-set-by-IN-TECH

Fig. 3. Flame speeds versus hydrogen content in methane-hydrogen blends at *φ* = 1 (Ilbas

extends the ignition limits and higher u*tr* values, in particular at lean air-fuel mixtures, are

A comparison of results obtained by several authors for the unstretched laminar burning velocity versus the equivalence ratio, for HCNG20, is shown in Figure 4 (Miao et al., 2009). It is observed in any case that the maximum flame speed is attained at *φ* ∼= 1.1 with values

The reduction of combustion duration promoted by hydrogen addition results in increased engine efficiency respect to natural gas and enhances combustion stability, reducing cycle-by-cycle variation. Nagalingam et al. (1983) proved that the high burning rate of HCNG blends requires an ignition timing lower than natural gas to obtain the Maximum Brake

The MBT spark advance versus the hydrogen content, shown in Figure 5 (Karim et al., 1996), is noticeably affected by hydrogen addition, in particular for very lean air-fuel mixtures. The plot shows that for blends containing significant amount of hydrogen, small adjustments to

The engine efficiency can be increased fuelling the engine by HCNG blends. Sierens & Rosseel (2000) developed a fuel system which supplies hydrogen-natural gas mixtures in variable proportion to the engine. For low brake mean effective pressures high efficiency can be achieved by increasing the hydrogen content reducing throttling losses. The authors

**5.2 The impact of HCNG blends on engine efficiency and exhaust emissions**

the ignition timing are needed when the equivalence ratio is changed.

et al., 2006).

around 0.5 m/s.

Torque (MBT).

attained compared to methane.

Fig. 4. Unstretched laminar burning velocity *ul* versus the equivalence ratio *φ* for HCNG20 (Miao et al., 2009).

found that 10% hydrogen increases engine efficiency moderatly whereas 20% hydrogen gives negligible extra benefit, as shown in Figure 6.

Recently, Ma et al. (2010) investigated the effect of high hydrogen volumetric content, up to 55%, on the performance of a turbocharged lean burn natural gas engine. The authors found that the addition of hydrogen significantly extends the lean limit, decreases burn duration and yields higher thermal efficiency. The plot of the engine efficiency versus *λ*, Figure 7, shows a negative trend in engine efficiency for natural gas for *λ* values greater than 1.3, while the blend with the higher hydrogen content shows positive trend up to *λ* = 1.6.

The increased hydrogen/carbon ratio and engine efficiency bring a reduction of CO2 emissions. By the way, as a consequence of a faster combustion, higher temperature are attained in the combustion chamber, increasing NOx emissions in HCNG fuelled engines compared to natural gas, for a given equivalence ratio *φ*. NOx can be kept down and engine efficiency further improved if the engine is run with lean mixtures or adopting EGR at stoichiometric air-fuel ratio.

Sierens & Rosseel (2000) found the maximum NOx emissions at a relative air-fuel ratio *λ* = 1.1. For higher *λ* values, the reduction in heat of combustion available for the charge mixture reduces the temperature and NOx as a consequence, as shown in Figure 8. However, such conditions cause an increase in THC emissions, as shown in Figure 9.

Hoekstra et al. (1995) obtained very low NOx emissions operating with HCNG blends close the lean limit, significantly extended compared with natural gas. Besides, the excellent anti

Fig. 6. Engine efficiency versus relative air-fuel ratio *λ* for different fuels (Sierens & Rosseel,

A Review of Hydrogen-Natural Gas Blend Fuels in Internal Combustion Engines 27

MBT ignition timing has been adopted for all fuels and operating conditions investigated. Fuel consumption is reduced as the hydrogen content increases due to the positive effect on average engine efficiency over the driving cycle, with values 2.5%, 4.7% and 5.7% lower than NG for HCNG10, 20 and 30 respectively. Fuel consumption is further reduced adopting 10% EGR for HCNG blends, with values 5.4%, 6.6% and 7.7% lower than NG for HCNG10, 20 and 30 respectively. NOx emissions, expressed in g/km over the driving cycles, are reported in Figure 14. Adding hydrogen higher in-cylinder temperatures are attained as a consequence of a faster combustion, resulting in increased NOx emissions with values 3.6%, 10.7% and 19.7% higher than NG for HCNG10, HCNG20 and HCNG30 respectively. The use of EGR results in lower NOx emissions with respect to the case without EGR, with values about 85% lower

HCNG blends can be distributed by the present natural gas refuelling stations, providing them with a mixing equipment in order to obtain blends with the selected hydrogen content. The system must operate to assure a high accuracy of hydrogen percentage because the fuel composition influences engine performances hence requiring customized engine calibration. In particular, the increased combustion velocity requires a reduction of the ignition advance as the hydrogen concentration increases to obtain the maximum engine torque. Furthermore, the fuel supply system should be calibrated to compensate the variation of fuel properties caused by hydrogen addition. In fact, present natural gas vehicles requires stoichiometric air-fuel ratio to obtain a high conversion efficiency of HC, CO and NOx emissions in the three-way

2000).

than CNG for each HCNG fuel.

**6. Real-life cases of HCNG use**

Fig. 5. Spark timing for maximum indicated power output versus hydrogen content (Karim et al., 1996).

knock qualities of natural gas are not undermined by the presence of relatively small amounts of hydrogen in the blend (Karim et al., 1996).

The effect of hydrogen on the lean limit, here defined as the *λ* value at which the COV*imep* attains 10%, is shown in Figure 10 (Ma et al., 2010), with values of 1.2 for NG, 2.1 for HCNG30 and 2.5 for HCNG55.

The impact of hydrogen addition to natural gas on cycle-by-cycle variations have been investigated in many scientific activities and the results showed that the coefficient of variation in maximum pressure and in indicated mean effective pressure are reduced with increasing hydrogen content, both with lean air-to-fuel ratio as well described by Ma et al. (2008) in Figure 11 and Wang et al. (2008) and with large exhaust gas recirculation ratio values, Figure 12 (Huang et al., 2009).

Numerical simulations have also been used to predict performance and emissions of internal combustion engines fuelled by HCNG blends.

Figure 13 shows the predicted fuel consumption in terms of energy per kilometer [MJ/km] over the NEDC versus the hydrogen content (Mariani et al., 2011). Stoichiometric air-to-fuel ratio was considered for each fuel in order to assure an efficient exhaust after-treatment adopting a three-way catalyst. Exhaust gas recirculation was investigated (instead of ultra lean mixture) with the aim at improving engine efficiency and reducing NOx emissions respect to undiluted charge. In fact, HCNG blends combustion properties are particularly suitable for EGR, assuring a stable combustion even if the charge is diluted (Hu et al., 2009). 10 Will-be-set-by-IN-TECH

Fig. 5. Spark timing for maximum indicated power output versus hydrogen content (Karim

knock qualities of natural gas are not undermined by the presence of relatively small amounts

The effect of hydrogen on the lean limit, here defined as the *λ* value at which the COV*imep* attains 10%, is shown in Figure 10 (Ma et al., 2010), with values of 1.2 for NG, 2.1 for HCNG30

The impact of hydrogen addition to natural gas on cycle-by-cycle variations have been investigated in many scientific activities and the results showed that the coefficient of variation in maximum pressure and in indicated mean effective pressure are reduced with increasing hydrogen content, both with lean air-to-fuel ratio as well described by Ma et al. (2008) in Figure 11 and Wang et al. (2008) and with large exhaust gas recirculation ratio values,

Numerical simulations have also been used to predict performance and emissions of internal

Figure 13 shows the predicted fuel consumption in terms of energy per kilometer [MJ/km] over the NEDC versus the hydrogen content (Mariani et al., 2011). Stoichiometric air-to-fuel ratio was considered for each fuel in order to assure an efficient exhaust after-treatment adopting a three-way catalyst. Exhaust gas recirculation was investigated (instead of ultra lean mixture) with the aim at improving engine efficiency and reducing NOx emissions respect to undiluted charge. In fact, HCNG blends combustion properties are particularly suitable for EGR, assuring a stable combustion even if the charge is diluted (Hu et al., 2009).

et al., 1996).

and 2.5 for HCNG55.

Figure 12 (Huang et al., 2009).

of hydrogen in the blend (Karim et al., 1996).

combustion engines fuelled by HCNG blends.

Fig. 6. Engine efficiency versus relative air-fuel ratio *λ* for different fuels (Sierens & Rosseel, 2000).

MBT ignition timing has been adopted for all fuels and operating conditions investigated. Fuel consumption is reduced as the hydrogen content increases due to the positive effect on average engine efficiency over the driving cycle, with values 2.5%, 4.7% and 5.7% lower than NG for HCNG10, 20 and 30 respectively. Fuel consumption is further reduced adopting 10% EGR for HCNG blends, with values 5.4%, 6.6% and 7.7% lower than NG for HCNG10, 20 and 30 respectively. NOx emissions, expressed in g/km over the driving cycles, are reported in Figure 14. Adding hydrogen higher in-cylinder temperatures are attained as a consequence of a faster combustion, resulting in increased NOx emissions with values 3.6%, 10.7% and 19.7% higher than NG for HCNG10, HCNG20 and HCNG30 respectively. The use of EGR results in lower NOx emissions with respect to the case without EGR, with values about 85% lower than CNG for each HCNG fuel.

#### **6. Real-life cases of HCNG use**

HCNG blends can be distributed by the present natural gas refuelling stations, providing them with a mixing equipment in order to obtain blends with the selected hydrogen content. The system must operate to assure a high accuracy of hydrogen percentage because the fuel composition influences engine performances hence requiring customized engine calibration. In particular, the increased combustion velocity requires a reduction of the ignition advance as the hydrogen concentration increases to obtain the maximum engine torque. Furthermore, the fuel supply system should be calibrated to compensate the variation of fuel properties caused by hydrogen addition. In fact, present natural gas vehicles requires stoichiometric air-fuel ratio to obtain a high conversion efficiency of HC, CO and NOx emissions in the three-way

Fig. 9. Hydrocarbon emission versus relative air-fuel ratio *λ* (Sierens & Rosseel, 2000).

A Review of Hydrogen-Natural Gas Blend Fuels in Internal Combustion Engines 29

Fig. 10. Lean limit versus hydrogen content in the blend (Ma et al., 2010).

Fig. 7. Engine efficiency versus relative air-fuel ratio *λ* for different fuels (Ma et al., 2010).

Fig. 8. NOx emissions versus relative air-fuel ratio *λ* (Sierens & Rosseel, 2000).

12 Will-be-set-by-IN-TECH

Fig. 7. Engine efficiency versus relative air-fuel ratio *λ* for different fuels (Ma et al., 2010).

Fig. 8. NOx emissions versus relative air-fuel ratio *λ* (Sierens & Rosseel, 2000).

Fig. 9. Hydrocarbon emission versus relative air-fuel ratio *λ* (Sierens & Rosseel, 2000).

Fig. 10. Lean limit versus hydrogen content in the blend (Ma et al., 2010).

Fig. 13. Predicted fuel consumption versus hydrogen content over the NEDC (Mariani et al.,

A Review of Hydrogen-Natural Gas Blend Fuels in Internal Combustion Engines 31

catalytic converter. HCNG fuels can be used in lean burn engines or with high EGR rates at stoichiometric conditions, exploiting their excellent combustion properties, with positive

Finally, the use of HCNG fuel can stimulate the development of the hydrogen technologies and market which are, nowadays, the main practical problems preventing it to be

Many research projects have been performed in the past and others are still going on to assess the potential benefits coming by using HCNG fuels in real-life applications. The U.S. Department of Energy Advanced Vehicle Testing Activity (AVTA) teamed with Electric Transportation Applications (ETA) and Arizona Public Service (APS) to develop a hydrogen pilot plant, where hydrogen is produced by means of PEM electrolyzer and is dispensed to vehicles that operate with different HCNG blends with hydrogen ranging from 0% to 100%. The project demonstrated the safety of operating vehicles on hydrogen and the reduction of exhaust emissions attainable with hydrogen and HCNG fuelled vehicles compared to gasoline

A hydrogen production plant with HCNG dispenser have been built in Malmö, Sweden, for

In Italy, public transportation companies of Regione Emilia Romagna and the ENEA research center are involved in experimental tests to evaluate fuel consumption and exhaust emissions

project to improve engine efficiency and reduce emissions of a bus fleet (Ridell, 2006).

of buses for urban transport service, Figure 15 (Genovese et al., 2011).

impact on engine efficiency and low exhaust emissions.

2011).

implemented.

(Francfort & Karner, 2006).

Fig. 11. *COVimep* versus relative air-fuel ratio *λ* for NG and HCNG blends (Wang et al., 2008).

Fig. 12. *COVimep* versus EGR for NG and HCNG blends (Huang et al., 2009).

14 Will-be-set-by-IN-TECH

Fig. 11. *COVimep* versus relative air-fuel ratio *λ* for NG and HCNG blends (Wang et al., 2008).

Fig. 12. *COVimep* versus EGR for NG and HCNG blends (Huang et al., 2009).

Fig. 13. Predicted fuel consumption versus hydrogen content over the NEDC (Mariani et al., 2011).

catalytic converter. HCNG fuels can be used in lean burn engines or with high EGR rates at stoichiometric conditions, exploiting their excellent combustion properties, with positive impact on engine efficiency and low exhaust emissions.

Finally, the use of HCNG fuel can stimulate the development of the hydrogen technologies and market which are, nowadays, the main practical problems preventing it to be implemented.

Many research projects have been performed in the past and others are still going on to assess the potential benefits coming by using HCNG fuels in real-life applications. The U.S. Department of Energy Advanced Vehicle Testing Activity (AVTA) teamed with Electric Transportation Applications (ETA) and Arizona Public Service (APS) to develop a hydrogen pilot plant, where hydrogen is produced by means of PEM electrolyzer and is dispensed to vehicles that operate with different HCNG blends with hydrogen ranging from 0% to 100%. The project demonstrated the safety of operating vehicles on hydrogen and the reduction of exhaust emissions attainable with hydrogen and HCNG fuelled vehicles compared to gasoline (Francfort & Karner, 2006).

A hydrogen production plant with HCNG dispenser have been built in Malmö, Sweden, for project to improve engine efficiency and reduce emissions of a bus fleet (Ridell, 2006).

In Italy, public transportation companies of Regione Emilia Romagna and the ENEA research center are involved in experimental tests to evaluate fuel consumption and exhaust emissions of buses for urban transport service, Figure 15 (Genovese et al., 2011).

The authors of this review have designed and built an high accuracy mixing equipment to produce HCNG blends with imposed hydrogen content. The device is developed on the occasion of a project which involves the research group of the Seconda Universitá degli Studi di Napoli, the Neapolitan Transportation Company (CTP), NA-MET, the company managing

A Review of Hydrogen-Natural Gas Blend Fuels in Internal Combustion Engines 33

Fig. 16. Fiat Panda HCNG tested in the laboratory of Istituto Motori-CNR (Prati, Costagliola,

Natural gas is employed as fuel since it is the cleanest fossil fuel with exhaust emissions from natural gas vehicles lower than those of gasoline-powered vehicles. Some of its drawbacks can be mitigated by enriching it with hydrogen to produce the so called hydrogen-natural gas

The laminar flame speed of methane is lower than the gasoline one and the addition of hydrogen, which presents a laminar flame speed about eight times that of methane,

In the past years, many authors have proved both experimentally and numerically that the HCNG blends improve engine efficiency and reduce CO2 emissions because of the reduced C/H ratio and fuel consumption. NOx emissions are, instead, larger than NG because of the higher in-cylinder temperature attained, for a given equivalence ratio. Anyway, the use of lean AFR or the EGR definitely reduces NOx emissions and bring about an extra increase in engine efficiency. The good combustion patterns of HCNG blends help to keep low HC

This work has been supported by a PRIST 2008 grant by the Seconda Universitá degli studi di Napoli, together with a 2011 research grant funded by the Seconda Universitá degli studi di

Torbati, Unich, Mariani, Morrone & Gerini, 2011).

significantly improves this main combustion property.

**7. Conclusion**

blends.

emissions.

Napoli.

**8. Acknowledgements**

the NG bus fleet and ECOS srl, an enterprise which develops CNG fuelling stations.

Fig. 14. Predicted NOx emissions versus hydrogen content over the NEDC (Mariani et al., 2011).

Fig. 15. Urban bus tested with HCNG blends (Genovese et al., 2011).

Regione Lombardia, Fiat Research Center, Sapio, CNR-Istituto Motori and Seconda Universitá degli studi di Napoli are involved in a project to test a passenger car fuelled by HCNG blends, varying the hydrogen content, in order to assess the impact of hydrogen addition to natural gas on combustion, exhaust emissions and fuel consumption, over different driving cycles, Figure 16 (Prati, Costagliola, Torbati, Unich, Mariani, Morrone & Gerini, 2011).

The authors of this review have designed and built an high accuracy mixing equipment to produce HCNG blends with imposed hydrogen content. The device is developed on the occasion of a project which involves the research group of the Seconda Universitá degli Studi di Napoli, the Neapolitan Transportation Company (CTP), NA-MET, the company managing the NG bus fleet and ECOS srl, an enterprise which develops CNG fuelling stations.

Fig. 16. Fiat Panda HCNG tested in the laboratory of Istituto Motori-CNR (Prati, Costagliola, Torbati, Unich, Mariani, Morrone & Gerini, 2011).
