## 2 Materials and Methods  
A schematic diagram of the test engine is shown in Figure 1. The experimental prototype was the EA888 engine from Volkswagen Automotive Company Limited, and the engine specifications are listed in Table 1.  
The measurement instruments and the experimental condition are illustrated in Tables 2 and 3, respectively. The dynamometer used was a CW160 hydraulic dynamometer. The experiment was carried out under these conditions since the hydrogen would obviously improve the engine. The excess air ratio was set at 1 to 2.8. When the excess air ratio was 2.8, the COV of the engine was more than 5%. Therefore, the excess air ratio of 2.8 was considered as the lean burn limit. The energy fraction of the hydrogen was set to 20%. Previous studies have shown that addition of a small proportion of hydrogen in an engine can significantly improve the performance, while continuing to increase the hydrogen ratio has little effect on the engine performance [41]. As split injection was used, too small a volume fraction for the hydrogen would have led to too short an injection duration. Therefore, a 20% energy fraction for the hydrogen was selected. The energy fraction of hydrogen is defined as follows:  
\[\varphi=\frac{\mathrm{Q_{H2}}}{\mathrm{Q_{H2}+Q_{gasoline}}} \tag{1}\]  
where \(\varphi\) is the energy fraction of the hydrogen, \(\mathrm{Q_{H2}}\) is the energy of the hydrogen and \(\mathrm{Q_{gasoline}}\) is the energy of the gasoline.  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{
\begin{tabular}{c} **Item** \\ \end{tabular} } & \multicolumn{1}{c}{**Error**} & **Measurement Instrument** \\ \hline Gasoline consumption & \(\leq\)\(\pm\)0.01 g/s & Ono Sokki DF\(-\)2420 \\ Crank angle (CA) position & \(\leq\)\(\pm\)0.01 \({}^{\circ}\)CA & Ono Sokki DS 9028 \\ Cylinder pressure & \(\leq\)\(\pm\)0.3 bar & Ono Sokki DS 9028 \\ Speed & \(\leq\)\(\pm\)1 rpm & CW160 \\ Brake power & \(\leq\)\(\pm\)0.4\% & CW160 \\ Hydrogen consumption & \(\leq\)\(\pm\)0.2\% & DMF\(-\)1\(-\)1 AB \\ Emissions & \(\leq\)\(\pm\)0.1\% & AVL DICOM 4000 \\ Excess air ratio & \(\leq\)\(\pm\)0.15 & LSU4.2 oxygen sensor \\ \hline \hline \end{tabular}
\end{table}
Table 2: Information on measurement instruments.  
Figure 1: Schematic diagram of the test engine.  
To demonstrate the differences between injection strategies clearly, the injection parameters and ignition timing were set to the best values on the basis of efficiency.  
Figures 2 and 3 show the best injection parameters for the SHMD and the PPHMD. With the increase in the excess air ratio, the best hydrogen injection timing for the SHMD and the best second hydrogen injection timing for the PPHMD were delayed. Moreover, the fuel in the engine became thinner and harder to ignite. Delayed hydrogen injection made more hydrogen concentrate around the spark plug, and the engine ignition performance was improved. Therefore, the engine worked more steadily and efficiently. The best second hydrogen injection timing for the PPHMD was later than that for the SHMD. Due to the two injections with the PPHMD, the amount of hydrogen in the second hydrogen injection is less than that of the SHMD, so a greater hydrogen concentration area is needed to promote the ignition stability. The amount of hydrogen around the spark plug was the key to ensure the engine ignition performance. Since ignition became more difficult with the increase in the excess air ratio, the best first hydrogen injection proportion for the PPHMD decreased from 33% to 25%.