## 2 Hydrogen Properties and Their Implications on Use in Internal Combustion Engine  
Hydrogen has unique physical and chemical properties, compared to the conventional fossil fuels widely used in the transportation sector, namely compressed natural gas (CNG), gasoline and diesel, as shown in Table 1[4; 27; 40; 41; 42]. Engine performance with these fuels in different engine modes is commonly compared with hydrogen and will be discussed throughout the study. One of the many advantages of using hydrogen in ICE as a clean alternative fuel is its zero carbon content. This means that carbon-based emissions, mainly CO, CO\({}_{2}\) and soot, can be eliminated, leaving NOx as the only harmful combustion byproduct. With a high specific energy density, hydrogen can provide nearly three times as much energy by mass compared with other fossil fuels, reflected in its lower heating value. There are, nonetheless, a number of drawbacks related to the very low density of hydrogen (i.e., low volumetric energy content (MJ/m\({}^{3}\))). At atmospheric pressure and 273 K, the density of hydrogen is an order of magnitude less than that of natural gas, due to the very low molecular weight of hydrogen. The low boiling point suggests that compressed hydrogen will be the most prominent storage option. This presents a significant challenge for the implementation of hydrogen ICE in on-road applications due to the limited vehicle space. Increasing storage pressure is required to increase hydrogen density and hence the volumetric energy content. For instance, hydrogen at 350 bar (i.e., a current standard supply pressure for hydrogen refuelling) and 273 K can increase the gas density to \(\sim\)31 kg/m\({}^{3}\) or the volumetric energy content to \(\sim\)3700 MJ/m\({}^{3}\). With the highest auto-ignition temperature and Research Octane Number (RON \(\geq\) 130) [43] relative to the common fuels, the resistance of hydrogen to knocking is expected to be high. However, its minimum ignition energy in air at stoichiometry is an order of magnitude less than that of hydrocarbon \begin{table}
\begin{tabular}{l c c c c} \hline
**Property** & **Hydrogen** & **CNG** & **Gasoline** & **Diesel** \\ \hline Carbon content (mass\%) & **0** & 75 e & 84 & 86 \\ Lower heating value (MJ/kg) & **119.7** & 45.8 & 44.8 & 42.5 \\ Density \({}^{\mathrm{a,b}}\) (kg/m\({}^{3}\)) & **0.089** & 0.72 & 730–780 & 830 \\ Volumetric energy content \({}^{\mathrm{a,b}}\) (MJ/m\({}^{3}\)) & **10.7** & 33.0 & \(33\times 10^{3}\) & \(35\times 10^{3}\) \\ Molecular weight & 2.016 & 16.043 e & \(\sim\)110 & \(\sim\)170 \\ Boiling point \({}^{\mathrm{a}}\) (K) & 20 & 111 e & 298–488 & 453–633 \\ Auto-ignition temperature (K) & **858** & 813 e & \(\sim\)623 & \(\sim\)523 \\ Minimum ignition energy in air \({}^{\mathrm{a,d}}\) (mJ) & **0.02** & 0.29 & 0.24 & 0.24 \\ Stoichiometric air/fuel mass ratio & 34.5 & 17.2 e & 14.7 & 14.5 \\ Stoichiometric volume fraction in air (\%) & 29.53 & 9.48 & \(\sim\)2 f & - \\ Quenching distance \({}^{\mathrm{a,c,d}}\) (mm) & **0.64** & 2.1 e & \(\sim\)2 & - \\ Laminar flame speed in air \({}^{\mathrm{a,c,d}}\) (m/s) & **1.85** & 0.38 & 0.37–0.43 & 0.37–0.43 s \\ Diffusion coefficient in air \({}^{\mathrm{a,b}}\) (m\({}^{2}\)/s) & \(8.5\times 10^{-6}\) & \(1.9\times 10^{-6}\) & - & - \\ Fammability limits in air (vol\%) & 4–76 & 5.3–15 & 1–7.6 & 0.6–5.5 \\ Adiabatic flame temperature \({}^{\mathrm{a,c,d}}\) (K) & 2480 & 2214 & 2580 & \(\sim\)2300 \\ \hline \end{tabular}  
* at 1 bar, \({}^{\mathrm{b}}\) at 273 K, \({}^{\mathrm{c}}\) at 298 K, \({}^{\mathrm{d}}\) at stoichiometry, \({}^{\mathrm{e}}\) methane, \({}^{\mathrm{f}}\) vapor and \(\mathrm{\SIUnitSymbolMicro n}\)-heptane.  
\end{table} 
Table 1: Hydrogen properties compared with compressed natural gas (CNG), gasoline and diesel. fuels, which indicates that hydrogen can be easily ignited by hot spots or residues in combustion chamber. This may lead to pre-ignition of fuel, which is characterized by combustion during the compression stroke prior to the intended ignition.