Assessment of the Influence of hydrogen share on performance, combustion, and emissions in a four-stroke gasoline engine

This study aims to develop a one-dimensional model to investigate the effect of hydrogen share in gasoline fuel on performance, combustion, and exhaust emissions of a gasoline direct-injection engine. Iso-octane was used as a reference fuel to compare performance, combustion, and emission parameters. The model was developed using commercial GT-Suite and ANSYS software. The simulation results using GT-Suite were validated with the published data and ANSYS results. The hydrogen fractions were varied from 0% to 11.09% to validate the simulation results with the published results. The investigation continued with three higher hydrogen fractions (15%, 20% and 25%) to study the performance, combustion, emissions, and sustainability parameters. Compared to neat gasoline, hydrogen-shared fuels show a maximum 2% higher exergy efficiency, 51% higher exergy and 42% energy rates while reducing carbon dioxide (CO2) emissions by 51% with a penalty of nitrogen oxide emissions (NOx) by 62% at an excess ratio of 1.3. Other novel findings, including higher sustainability indices, lower depletion potentials, and lower unitary cost indices with higher-fraction hydrogen fuels, suggest that they are environmentally and economically sustainable. In the second part of this study, the NOx formation mechanism and its associated factors, including in-cylinder temperature, heat transfer rate, cumulative heat release, and burned rate were confirmed and compared with gasoline and neat ethylene.


I. INTRODUCTION
Anthropogenic activities, such as using petroleum fuels in transportation, contribute to harmful environmental emissions, and according to the literature, the transportation and industrial sectors are the largest consumers of fossil fuels [1]. Environmental and human health consequences compel researchers to develop fossil-based fuels and resources [2]. Concerning fuel consumption and exhaust emissions, hydrogen with a specific standard appears to be the most promising, cutting conventional diesel engines' fuel consumption and hazardous emissions dramatically [3]. Compared to 1990, European emission targets a 60% greenhouse gas emission reduction by 2050 and a 20% reduction by 2030 relative to 2008 [4]. Hydrogen is considered an alternative fuel for internal combustion engines due to its higher flame velocity and higher calorific value [5]. Hydrogen was also tested in a compression ignition engine to control the CO2, and other criteria pollutants, including PM, THC and CO emissions. Nag et al. [6] conducted experiments with hydrogen fuel in a hydrogen-diesel dual-fuel engine. The experiment varied the engine load from 25% to 75% of the full load. Hydrogen energy was varied 0% to 20% (0%, 5%, 10% and 20%). They reported that sharing hydrogen energy with diesel improves the peak pressure and reduces engine knock at lower loads. Engine vibrations were increased at higher loads but were moderate at lower loads. However, the authors did not report any hydrogen-diesel dual fuel emissions or engine performance. Cernat et al. [7] conducted an experimental investigation with hydrogen as fuel in a K9K automotive diesel engine. They performed the experiments at an engine speed of 2000 rpm with 40%, 55%, 70% and 85% engine load and with different proportions of hydrogen. The authors reported a higher thermal efficiency and lower exhaust emissions, including CO2, unburned hydrocarbon (HC), NOx and smoke emissions with the use of hydrogen as a fuel for compression ignition engines. Another study [8] investigated hydrogen introduction into an intake manifold of a diesel engine. The authors conducted similar experiments at engine speeds of 750 rpm, 900 rpm, 1100 rpm, 1400 rpm, 1750 rpm, and 2100 rpm with full load conditions. The authors reported a slight increase in HC emissions with a substantial reduction in CO2 and carbon monoxide (CO) emissions with increased hydrogen fractions. The authors also reported an increased peak cylinder pressure and heat release rate with increased hydrogen fractions. Du et al. [9] did engine experiments with several hydrogen fractions (0%, 3.99%, 5.87%, 9.41%, and 11.09%) in a modified spark-ignition engine facilitated to hydrogen direct injection system. The experiments were performed at excess air ratios of 1.0, 1.1, 1.2, 1.3, 1.4 and 1.5. Their results informed improved mean effective pressure and thermal efficiency with the increase in hydrogen fractions. The authors also reported a decrease in CO, HC emissions with a penalty of NOx emissions. However, their study was limited to 11.09% hydrogen fractions. Martin et al. [10] experiment in a hybrid hydrogen-gasoline engine. The authors compared engine performance, and exhaust emission results between reformed exhaust gas recirculation (rEGR) and traditional exhaust gas recirculation (EGR). The findings show that rEGR has the potential to improve thermal efficiency while lowering gaseous emissions and lowering PM generation all at the same time. However, their results did not include exergy, and sustainability related parameters. Like gasoline, ethylene could be used as a substitute fuel for a spark-ignition engine. Wan et al. [11] used a stainless steel shock tube to examine Ethylene-Air ignition behaviours at different temperatures. They found that the ignition lag decreases at high temperatures. They also reported that the increased pressure shortens the ignition lag for fuel rich and stoichiometric mixtures. Another study conducted by Grigorean et al. [12], presented the simulation results of combustion of ethylene. They reported minimum CO emissions at 700K with an excess air ratio of 1.5, the pressure of 1 bar, while the maximum CO was observed at an 800K, I bar and excess air ratio of 1.5. Similarly, the maximum CO2 was found to be at 700K, 3 bar, and excess air ratio of 3, while the lowest CO2 was observed to be at 700K, 1 bar pressure, and 1.5 excess air ratio. This study reports on the effect of utilising hydrogen fuel direct injection into a gasoline engine on performance, combustion, emissions and fundamental exergy and energy parameters. A four-cycle gasoline engine with a variable compression ratio of 9.5-10 was used for the numerical modelling. Special attention was given to the utilisation of a higher percentage of hydrogen fuels shared with iso-octane.
One of the key novelties of this study is to investigate why the exergy and energy parameters are higher for higher percentage hydrogen fuels than traditional gasoline (isooctane) fuel. Secondly, a comprehensive examination was conducted of why hydrogen shared fuels with gasoline are sustainable in terms of depletion factor, sustainability index, and greenhouse gas (CO2) emissions. The third vital novelties of this investigation are to find out the dominant factors for higher NOx emissions with hydrogen shared fuels. Finally, this investigation confirmed the principal causes for NOx formation by introducing another fuel ethylene in a gasoline engine. The main operating parameters investigated for NOx formation were excess air ratio, incylinder temperature, heat transfer rate, heat release rate and cumulative heat release.
To the best of the authors' knowledge, a comprehensive literature review revealed no articles were found that discussed the exergy, energy, exergy efficiency, sustainability index, depletion factor, unitary cost index, and CO2 emissions for hydrogen fuels shared with iso-octane (neat gasoline). Additionally, no studies were found that discussed the associated factors for the causes of NOx formation.

II. METHODOLOGY
1-D modelling with GT-Suite and ANSYS was conducted for a four-stroke gasoline engine, where gasoline is injected directly, while hydrogen is injected into the intake manifold for an engine speed of 1500 rpm. Furthermore, the modelling was carried out for excess air ratios of 1.1 to 1.5 with an increment of 0.1 for neat gasoline (reference fuel) and different hydrogen fuels. A snapshot of the methodology is shown in Figure 1.

II (i). MODELLING WITH GT-SUITE
As mentioned previously, some of the published experimental data were validated using GT-Suite and ANSYS software. The GT-Suite model was used to predict the in-cylinder pressure, heat release rate, cumulative heat release, and in-cylinder temperature. To check the accuracy of the model by GT-Suite, the in-cylinder pressure and temperature data were also compared with those obtained from ANSYS. The data variations between experiments and GT-Suite and ANSYS were within 5-10%. For any fuel, the following fuel property parameters shown in Table 1 are needed [13] to input into the software.    1-D modelling with GT-Suite involves the solution of the conservation of momentum, energy, and continuity equations. In the model, the engine system was discretised into volume numbers. The continuity, energy and momentum equations are shown in equations (i), (ii) and (iii), respectively [13]. For different notations, Greek symbols, and abbreviations in equations (i-vi), the readers are referred to [13]. The specifications and schematic diagram for the 1-D modelling engine are shown in Table 2 and The fuel flow rate was computed by equation (iv) Woschini's equation for the convective heat transfer shown in equation (v), was used for the in-cylinder combustion model [13].
The burn rate was computed with Wiebe's function shown in equation (vi) [13].

II (ii). MODELLING WITH ANSYS
A numerical simulation of an engine combustion model was developed with the help of ANSYS. For solving the turbulent model, Reynold's Average Navier-Stokes (RANS) method was used in this study. An Instantaneous Analog Analogy was adopted for disintegrating the variations and timedependent average quantities. A rebound/Sliding model was created to resolve the interaction between the spray-wall. Another approach of using Re-Normalisation Group (RNG) K-epsilon was used to investigate the turbulence characteristics inside the cylinder. According to the previous study, more accurate and reliable results can be found using this approach [14][15][16][17]. Compressible turbulence can also be determined numerically by using this method which is established by the following equations.
Equations (vii) and (viii) are the dissipation rate equation ε and kinetic energy equation k. S denotes the turbulent energy generation, U represents the velocity vector, Dk and Dε represent the turbulent diffusion, C is constant, and ρ is the fluid density (kg/m 3 ). The following parameters in Tables 3,  4, and 5 were considered for conducting the simulation [14,17].      Figure 4 is a comparison of brake mean effective pressure (BMEP) between simulation results with the experiments conducted by Du et al. [9] for the neat gasoline (H2_0%) and five hydrogen fractions for three excess air ratios (1.1, 1,2, 1.5). As seen in the Figure, the BMEP increases as hydrogen fraction increases for all three excess air ratios. As also seen from the Figure that higher excess air ratios show lower BMEPs, while the lowest excess air ratio, 1.1, offers the highest BMEP for both simulation and experimental data.

III. RESULTS AND DISCUSSIONS
The higher BMEP at a lower excess air ratio is due to a larger amount of fuel burned into the combustion chamber. BMEPs for both experimental and simulation data are close enough for all excess air ratios and all hydrogen fractions. The highest variations of BMEP between experimental and simulated data are 5.88% for an excess air ratio of 1.1. The maximum variations between experimental and simulation results for excess air ratios of 1.2 and 1.5 are 1.88% and 3.02%. Based on the above discussion and comparison, it can be concluded that the predicted simulation results are in good agreement with those of the experimental results.  in-cylinder temperature results using ANSYS and GT-Suite. The comparison was made for an excess air ratio of 1.5 and engine speed of 1500 rpm. It is widely accepted that excess air ratio is the ratio between the actual air to fuel ratio and the stoichiometric air to fuel ratio. The reason for choosing excess air ratio (Lamda, shortened as Lam in Figure 4) is to compare the simulation results with published data, as the published data (discussed in Figure 4) of brake mean effective pressure (BMEP) versus hydrogen fractions were plotted for different excess air factors (1.1, 1.2, 1.5). The other reason for choosing excess air ratio is that the excess air ratio is an extensively used parameter to quantify whether the mixture of air and fuel in the combustion chamber is lean or rich. Both in-cylinder pressure and in-cylinder temperature results are almost identical for ANSYS and GT-Suite. The peak pressure was found to be 4.91 MPa for ANSYS and 4.75 MPa for GT-Suite. The percentage variations of peak cylinder pressure with ANSYS and GT-Suite are 3.35%. Figure 5(b) displays the in-cylinder temperature for the same excess air ratio of 1.5 using the same two software (ANSYS, GT-Suite). Insignificant variations in in-cylinder temperature are observed for the ANSYS and GT-Suite. The peak cylinder temperature with ANSYS is found to be 2035.47K, while for GT-Suite, the peak cylinder temperature was observed to be 2016.35Kthe variations are less than 1%.
The exergy rates were estimated with the following equation: Where, ϕ is the exergy factor; m f is the mass flow rate of fuel. Figure 5: Comparison of (a) in-cylinder pressure and (b) incylinder temperature for ANSYS and GT-Suite for an excess air ratio of 1.5 and an engine speed of 1500 rpm.
The energy rate was computed using equation (x).
Exergy efficiency calculation was based on equation (xi).

Exergy efficiency=
The energy flow accompanying work The combustion efficiency was computed by using equation (xii).
Where h P � and h R ��� are enthalpies of product and reactant, respectively. The readers are referred to the references of Nabi, et al. [18] and Odibi et al. [19] for details to estimate exergy and energy parameters, including exergy rates, energy rates, thermal efficiency and combustion efficiency. H₂_0% ANSYS H₂_0% GT-Suite Figure 6(a) depicts the variations of exergy rates against excess air ratios for neat octane (H2_0%) and three hydrogen fuels (H2_15%, H2_20%, H2_25%). A general trend of decreasing exergy rate with the increase in excess air ratio is observed irrespective of the fuels or the percentage of hydrogen-shared with octane. An R-squared value (linear fit) of higher than 0.99 indicates a strong correlation between exergy rate and excess air ratio for all fuels. All hydrogen fuels show a higher exergy rate than that of neat gasoline (H2_0%). As seen from the Figure, there is higher exergy at the leaner side (comparatively lower amount of air than the richer side). Relative to H2_0%, a maximum of 28.98% increase in exergy rate is observed with an H2_15% fuel at an excess air ratio of 1.1. For the same excess air ratio (1.1), the rise in exergy rate with the other two fuels (H2_20% and H2_25%) increase a maximum of 39.55% and 50.52%, respectively.
Similarly, for an excess air ratio of 1.5, the increment of the exergy rates with H2_15%, H2_20%, and H2_25% are observed to be 28.1%, 39.1%, and 49.5%, respectively. The higher exergy rate with three hydrogen fuels is associated with the higher heating value of hydrogen than octane. Nabi et al. [20] reported higher exergy rates for fuels with higher heating values. The current investigation is aligned with the work of Nabi et al. [20]. The changes in energy rates to excess air ratios for neat octane (H2_0%) and three hydrogen fuels (H2_15%, H2_20%, H2_25%) are illustrated in Figure 6 (b). The decreasing trends of energy rates with the increase in excess air ratio is also noticed in Figure 6 (b) regardless of the types of fuels or the percentage of hydrogen-shared with octane. Like exergy rates, all three hydrogen fuels show a higher energy rate than neat octane (H2_0%). Like exergy rate and excess air ratio, an R-squared value (linear fit) higher than 0.99 indicates a strong correlation between energy rate and excess air ratio for all fuels. The increases in energy rates at an excess air ratio of 1.1 using the three hydrogen fuels compared to H2_0% are noted to be 24.1%, 33.2% and 41.6%, respectively. At an excess air ratio of 1.5, the associated increases are 23.8%, 32.9% and 40.9%. The present study is in good agreement with the investigation of [20]. The linear fit R-squared value indicates that both energy and exergy rates are dependent on excess air ratios. The changes in exergy efficiency with excess air ratio are shown in Figure 7(a) for four fuels. For all fuels, as seen in the Figure, the exergy efficiencies are maximum at an excess air ratio of 1.1, where the fuel-rich region exists compared to other excess air ratios at the fuel-lean regions. The exergy efficiency for fuel with 0% hydrogen share shows the lowest excess air ratios, while the 25% shared hydrogen fuel exhibit the highest exergy efficiency at all excess air ratios. The two other hydrogen fuels also show higher exergy efficiencies than baseline gasoline (H2_0%) for the same operating conditions. At an excess air ratio of 1.5, H2_0% fuel shows an exergy efficiency of 27.13%, while the three hydrogen fuels (H2_15%, H2_20% and H2_25%) show 27.3%, 27.9% and 28.3%, respectively. Relative to H2_0% fuel, at an excess air ratio of 1.5, the three hydrogen fuels increase in exergy efficiencies by 0.65%, 2.9% and 4.30%, respectively. The lowest increases in exergy efficiencies at an excess air ratio of 1.1 with H2_15%, H2_20%, and H2_25% fuels are 0.54%, 2.7% and 4.1%, respectively. The results of Figure  7(a) indicate the advantage and suitability of using a higher percentage of hydrogen fuel concerning exergy efficiency. Relative to H2_0% fuel, a 4.3% increase in exergy efficiency with H2_25% fuel is significantly high. Figure 7(b) illustrates combustion efficiencies to excess air ratios for the same four fuels as shown in the previous Figures. It is well-known that higher combustion efficiencies of a fuel indicate better combustion, leading to a lower level of combustion products. Figure 7(b) shows higher combustion efficiencies at higher excess air ratios for all fuels investigated in this study. H2_0% shows combustion efficiencies of 93.7%, 93.81%, 93.93%, 94% and 94.1% at excess air ratios of 1.1, 1.2, 1.3, 1.4, and 1.5, respectively. H2_15% indicates slightly higher combustion efficiencies than H2_0% fuel at the same five excess air ratios. H2_20% reveals better than H2_15% and H2_0% fuels. Fuel H2_25% shows the highest combustion efficiencies among four fuels at all excess air ratios. Figures 8(a) and 8(b) are the support data of instantaneous indicated mean effective pressure (IMEP), indicating why hydrogen fuel blends have higher thermal efficiencies compared to neat gasoline fuel. Figure 8(a) shows the instantaneous IMEP for an excess air ratio of 1.5, while Figure 8(b) for an excess air ratio of 1.3 for neat gasoline and three higher-fraction hydrogen blends. It is generally accepted that the indicated mean effective pressure is generated into the cylinder, and it is an indication of the capability of doing work. As can be seen from the Figure, the instantaneous IMEP for neat gasoline is the lowest for all four fuels, while the higher-fraction hydrogen fuels (H2_25%) shows the highest. For an excess air ratio of 1.5, neat gasoline shows a maximum IMEP of 0.88 MPa at a crank angle of 188 degrees. The three higher-fraction hydrogen blends (H2_15%, H2_20% and H2_25%) show the IMEP values of 1.045 MPa, 1.098 MPa and 1.15 MPa at crank angles of 188°, 180.05°, 179.68° and 179.48°, respectively. Interesting to note that the peak of the IMEP for hydrogen blends shift from right to left. The shifting is prominent for the higher-fraction hydrogen blend -H2_25% in this case. This indicates the faster combustion with hydrogen fuel blends compared to neat gasoline fuels. Similar trends are observed for the same four fuel blends at an excess ratio of 1.3. At an excess air ratio of 1.3, the IMEP for neat gasoline is observed to be 0.973 MPa at a crank angle of 180.64°. The same three hydrogen blends show the IMEP values of 1.155 MPa, 1.213 MPa and 1.26 MPa at crank angles of 180.64°, 179.51°, 179.45° and 179.30°, respectively. Also, it can be noted that a lower excess air ratio (1.3) shows higher IMEP for all fuels compared to a higher excess air ratio of 1.5. Compared to neat gasoline, at an excess air ratio of 1.5, a maximum of 19% increase in IMEP was observed with H2_15% fuel. The H2_20% and H2_25% increase IMEPs by 25% and 31%. Almost similar percentages (18.5%, 24.98% and 30%) increase in IMEPs were realised for an excess air ratio of 1.3 for the same three hydrogen blends relative to neat gasoline fuel. Based on the discussion for Figure 8, it can be concluded that higherfraction hydrogen fuels realised much higher IMEPs which in turn suggest the higher thermal efficiency.
The energy balance for three hydrogen fuels and gasoline is displayed in Figure 9. The results are plotted for excess air ratios of 1.5 and 1.3. The energy balance results for the other excess air ratios (not shown) also reveal similar trends. As seen in Figure 9, the net brake and heat transfer for all three hydrogen fuels are higher compared to gasoline at both excess air ratios, while the friction and exhaust losses are lower at both excess air ratios. The higher net brake and heat transfer and lower friction and exhaust losses with the three hydrogen fuels are the additional causes of higher thermal efficiencies than gasoline.   10 is a contour map interpretation for the three hydrogen fuels and reference gasoline as indicated. For mapping these contours, the excess air ratio was kept constant to 1.1. More specifically, the engine speed was varied from 1400 rpm to 2400 rpm. The hydrogen fractions are kept at 0%, 15%, 20% and 30%, while the excess air ratio was fixed at 1.1. The contours for other excess air ratios show (results not shown) similar trends. As seen in Figure 10(a), the lowest indicated torque was observed at a hydrogen fraction of 0% with an engine rotational speed of 1400 rpm.   Higher than 0% hydrogen fraction, the indicated torque increases significantly at all engine speeds. It is also noticed that the higher than 0% hydrogen fraction and at higher engine rotational speed, the indicated torque becomes higher. From the Figure, the highest indicated torque is observed to be at an engine speed of 2400 rpm with a hydrogen fraction of 30%. Also, as per the contour map, the H2_10% could be the optimum hydrogen fraction in terms of indicated torque. From Figure 10(b), similar results can be observed for the same three hydrogen fuels and reference gasoline for indicated mean effective pressure (IMEP). Figure 10(c), maps the predicted specific fuel consumption (ISFC) for engine rotational speed and hydrogen fractions. Interestingly, at the lower engine speed with a zero percent hydrogen fraction (H2_0%, reference gasoline), the ISFC is maximum. The opposite can be found for H2_30% fuel from the same plot. 30% hydrogen fraction (H2_30%) at an engine speed of 2400 rpm indicates the lowest ISFC. An intermediate value of ISFC is observed for hydrogen fractions of 0% -30%. Maximum in-cylinder temperature mapping for engine rotational speed and hydrogen fractions is illustrated in Figure 10(d). Like IMEP and ISFC, the incylinder heat transfer map realises (results not shown) the highest at 2400 rpm with 30% hydrogen fraction and the lowest with 0% hydrogen fraction at an engine rotational speed of 1400 rpm. Maximum in-cylinder pressure is plotted against hydrogen fractions and engine rotational speed (Figure 10(e)) for an excess air ratio of 1.1 like Figures 10(a) -10(d), maximum pressure in Fig 10(e) shows the lowest for H2_0%, while the maximum for H2_25%. The higher maximum pressure with hydrogen fuels leads to higher thermal efficiency. Figure 10(f) is the contour plot of the maximum pressure rise rate with respect to hydrogen fractions and engine rotational speed. It is clear from the Figure that the higher the hydrogen fractions, the higher the maximum pressure rise rate. Also, the maximum pressure rise rate is higher at higher engine speeds for all fuels. The plots in Figure 11 present the changes in NOx emissions for excess ratios of 1.5 (Fig. 11a) and 1.3 (Fig. 11b) for the three hydrogen fuels (with hydrogen) and gasoline (without hydrogen). It is generally accepted that the formation of NOx mainly depends on high flame temperature, oxygen content, injection parameters and properties of fuel [21]. As evident from the Figure, the NOx emissions for hydrogen fuels are higher compared to gasoline. A notable increase in NOx emissions is observed for both excess ratios with three hydrogen fuels. The increase is significant for a higher percentage of hydrogen fractions. The increase in NOx emissions with H2_15% is 1.40 fold higher compared to gasoline at an excess air ratio of 1.5. H2_20% and H2_25% fuels increase NOx emissions by 2.8 and 3.6 fold at an excess ratio of 1.5. The corresponding figures for the three hydrogen fuels for an excess air ratio of 1.3 are 1.42, 1.50 and 1.60 fold, respectively. The increase in NOx emissions with hydrogen fuels is due to higher in-cylinder temperature, higher heat transfer rate, and higher cumulative heat release relative to reference gasoline fuel. The higher in-cylinder temperature with hydrogen fuels for the two excess air ratios (1.3 and 1.5) are displayed in Figure 12. As anticipated, the higher in-cylinder temperature is realised at an excess air ratio of 1.3 compared to 1.5 for all fuels investigated in this study. This is associated with the higher amount of fuel is burned at a lower excess air ratio; 1.3, in this case. According to the Zeldovich NOx formation phenomenon, the higher incylinder temperature is associated with higher NOx formation using three hydrogen fuels. The other factors for higher NOx emissions are heat transfer rate and cumulative heat release, as seen in Figures 13 and 14. The heat transfer rate in Figure 13 for an excess air ratio of 1.5 with the three hydrogen fuels are observed to be increased by 1.35, 1.47, and 1.60 times, while for the excess air ratio of 1.3, the corresponding values are 1.33, 1.45, and 1.54, respectively. Similarly, the cumulative heat release in Figure 14 for the same three hydrogen fuels are noted to be 1.23, 1.32, and 1.40 times higher than gasoline at an excess air ratio of 1.5.
Almost similar values (1.24, 1.33, and 1.39 times higher cumulative heat release) are observed at an excess air ratio of 1.3 for the H2_15%, H2_20% and H2_25% hydrogen fuels, respectively. In reality, regardless of fuels or fuel types, the key contributions to NOx generation are the temperature of the gas flame, the time of fuel injection, residence time, and the characteristics of the fuel [22]. The current study reveals the same about NOx formation and supports this phenomenon. The NOx emissions with hydrogen shared fuels are higher due to higher in-cylinder temperature and other associated factors described above compared to those without hydrogen fuel.

III (i). SUSTAINABILITY OF HYDROGEN SHARED FUELS
The depletion factor and sustainability index are related to each other are estimated by the equations (xiii) and (xiv), respectively [23] whereas the unitary cost index was computed by equation (xv) [18].
Depletion factor (number) = 1 -exergy efficiency --(xiii) Sustainability index = 1/depletion factor (number) --(xiv) Unitary cost index = 1/exergy efficiency --(xv) As seen in Figure 15(a), the sustainability index decreases with excess air ratio for all fuels. While showing the highest sustainability index at the lowest excess air ratio, the minimum sustainability index is observed at the highest excess air ratio for the same fuels. Compared to neat gasoline (H2_0%), all hydrogen fuels show the higher sustainability index at all excess air ratios. Also, compared among hydrogen fuels, a higher sustainability index is observed with the higher hydrogen percentage in the blends. The higher sustainability index with all hydrogen fuels indicates that they are sustainable fuels for the internal combustion engine -gasoline engine in this investigation. Figure 15(b) displays the depletion factor, which was estimated with equation (xiii), for neat gasoline and three hydrogen fuels at excess air ratios of 1.1, 1.2, 1.3, 1.4, and 1.5. A close look at equation (xiv), the depletion factor has a reciprocal relation to the sustainability index. This suggests, the higher the depletion number, the lower the sustainability index of a particular fuel. The gasoline fuel (iso-octane, H2_0%) shows the highest depletion factor at all excess air ratios, while H2_25% fuel shows the lowest. The higher depletion factor with gasoline fuel results in being environmentally unsustainable. Figure 15(c) displays the unitary cost index of the four fuels for five different excess air ratios. The unitary cost index implies the minimum amount of exergy required by an internal combustion engine to produce one exergy unit of product [18]. The unitary cost index for all fuels was estimated by using equation (xv). Compared to neat gasoline, all three hydrogen fuels show a lower unitary cost index at all excess air ratios. At an excess air ratio of 1.5, the unitary cost index for the three hydrogen fuels is 3.66, 3.58, 3.53, while for neat gasoline is 3.69. The lower-cost index with the hydrogen fuels compared to gasoline confirms the suitability of using hydrogen fuels as internal combustion engine fuels again.
The variations in greenhouse gas (CO2) emissions at different excess air ratios for the four fuels are illustrated in Figure 16. Compared to neat gasoline, all three hydrogen fuels reduced CO2 emissions significantly. Interestingly, the higher hydrogen fraction fuel (H2_25%) reduced the highest      study. The current investigation is in good agreement with BoroumandJazi et al. [24].
The plots of unitary cost index and depletion factor/potential (DF) versus exergy efficiency are illustrated in Figure 17(b). The unitary cost index, also known as the unitary exergy cost index (CI), is the amount of exergy required by an engine (in this case, an internal combustion engine) to generate one exergy unit of product [18]. The depletion factor and the unitary exergy cost index were estimated using equations (xiii) and (xv), respectively. As can be seen in Figure 17(b), both unitary cost indices and depletion factors decrease with the increase in exergy efficiencies. Also, the magnitude of reductions in exergy cost index and depletion factor is higher for the higher-fraction hydrogen fuels than neat gasoline. A close look at equation (xv) discloses that the cost index has a reciprocal relationship with exergy efficiency. This indicates that the higher the exergy efficiency, the lower the exergy cost index is. As the hydrogen fuels exhibit higher exergy efficiencies, they show lower exergy cost indices compared to neat gasoline fuel. The reason for lower depletion factors with hydrogen fuels is due to their higher exergy efficiencies.
In regards to sustainability analysis and discussion in Figures  17(a-b), all hydrogen fuels show higher sustainability indices, significantly lower greenhouse gas (CO2) emissions, higher exergy efficiencies, lower depletion potentials (factors), lower exergy cost indices, which are some of the key contributions of this study that could help the vehicle manufactures and fuel researchers.

III (ii). INVESTIGATION FOR NOx FORMATION
In this section, the NOx formation mechanism and its associated factors are confirmed again with gasoline (isooctane, H2_0%) and neat ethylene (C2H4).
It is well-known that NOx formation mechanisms are three types, including thermal NOx, prompt NOx and fuel NOx [25]. Among the three types of NOx formation, the thermal NOx is dominant for NOx formation inside the combustion chamber. In this phenomenon, the air nitrogen and oxygen combine and form NOx at high temperatures in the combustion chamber. The thermal NOx forms in the combustion chamber as per Zeldovich mechanism as follows [25]: ↔ H + NO.
In the above three equations, k1, k2, and k3 represent the forward reaction rate constants.
The variations in NOx emissions with ethylene fuel and gasoline are depicted in Figure 18(a). NOx emissions for ethylene fuel are higher at all excess air ratios compared to gasoline. Also, it is evident from the Figure that at a lower excess air ratio, the NOx formation is also higher. This trend is valid for both fuels. As per the Zeldovich NOx formation mechanism [25], the higher NOx emissions with ethylene fuel are due to the higher in-cylinder temperature ( Figure  18b) than gasoline. The current investigation agrees with the investigation conducted by Nabi et al. [14]. The other factors for higher NOx emissions with ethylene are the higher heat transfer rate, cumulative heat release and burned rate (Figures 18c, 18d, 18e). As explained before, like in-cylinder pressure and NOx formation, the in-cylinder temperature, heat transfer rate, cumulative heat release and burned rate for all fuels are higher at an excess air ratio of 1.3 than an excess air ratio of 1.5. This is due to fuel-rich conditions at lower excess ratios than fuel-lean conditions at higher excess air ratios. Again, these results indicate the different factors (incylinder temperature, heat transfer rate, cumulative heat release) that cause NOx formation.  Figure 18: Comparison of (a) NOx emissions using neat gasoline and neat ethylene for five excess air ratios; and comparison of (b) cylinder temperature, (c) heat transfer rate, (d) cumulative heat release, and (e) burned rate with neat gasoline and neat ethylene for two excess air ratios (1.5 and 1.3).

IV. CONCLUSIONS
One-dimensional modelling for engine performance, NOx and CO2 emissions for reference gasoline and three hydrogen fuels was conducted using GT-Suite and ANSYS software. The simulation results with GT-Suite were validated with those of ANSYS and published experimental results. The share of higher-fraction hydrogen fuels gives some novel findings. The addition of the fundamental exergy, energy parameters and sustainability analysis, including sustainability index, depletion number, unitary cost index, were additional novelties of this study. In the second part of this study, in-cylinder pressure, NOx emission, and its associated factors of formation were further investigated with neat ethylene and reference gasoline. The results are summarised as follows: − The lower-fraction hydrogen fuels (H2_3.99%, H2_5.87%, H2_7.68%, H2_9.41%, and H2_11.09%) have a good agreement in BMEPs between simulation and experimental results. In-cylinder pressure and temperature data also have a good alignment between GT-Suite and ANSYS. − The exergy efficiency, rate of exergy and energy, and combustion efficiencies for the three hydrogen fuels (H2_15%, H2_20% and H2_25%) are significantly higher than those of the reference gasoline (H2_0%). − The instantaneous indicated mean effective pressure was found to be substantially higher compared to neat gasoline, which resulted in higher thermal efficiencies with higher-fraction hydrogen fuels. − The CO2 emissions with the same three hydrogen fuels are considerably lower, but the NOx emissions are notably higher than the reference gasoline for the same three hydrogen fuels. A maximum of 51% reduction in CO2 emissions was observed with hydrogen fuel. The higher NOx emissions were associated with the higher in-cylinder temperature, higher heat transfer rate, higher cumulative heat release. On the other hand, the lower CO2 emissions with the hydrogen fuels were due to lower carbon content in the hydrogen fuels. − The sustainability indices are higher, depletion factor and the unitary cost indices are lower for all hydrogen-shared fuels than gasoline. Besides lower CO2 emissions, these sustainability indices further proved that hydrogen fuels are both economically and environmentally sustainable. − To confirm the NOx formation factors, the endeavour further evident that the higher NOx emissions with neat ethylene fuel were associated with the higher incylinder temperature, higher heat transfer rate, higher cumulative heat release and higher burned rate.