## 3 Results and Discussions  
Figure 4 shows the effects of the HMD on the brake thermal efficiency. With the NHMD, the lean burn limit of the engine was the excess air ratio of 1.5. Since the lean burn limit of hydrogen was much greater with hydrogen addition, the lean-burn performance was greatly improved and the excess air ratio could reach 2.8. Since the oxygen content increased with the increase in the excess air ratio, combustion was more complete and the power capacity of the engine was also enhanced. Moreover, the combustion speed of the engine was reduced and the ignition performance deteriorated. Therefore, with the increase in the excess air ratio, the brake thermal efficiency of the engine first increased and then decreased. With the NHMD, the maximum brake thermal efficiency appeared at the excess air ratio of 1.2. The lean-burn performance was significantly improved by adding hydrogen. With the PPHMD, when the excess air ratio was 1.5, the maximum brake thermal efficiency appeared at 26.86%. The efficiencies of the SHMD and the PPHMD were relatively higher. This may have been due to the fact that the stratified hydrogen made the engine more efficient. The stratified hydrogen formed a hydrogen concentration zone around the engine spark plug, making the engine ignition  
Figure 4: Effects of the HMD on the brake thermal efficiency. Figure 3: The best first hydrogen injection proportions under lean-burn conditions. process faster and more stable. Therefore, the efficiency of the stratified hydrogen was higher. With the increase in the excess air ratio, the best HMD changed from the PPHMD to the SHMD, and the engine ignition performance became the key for the engine efficiency. The hydrogen in the SHMD was injected into the engine cylinder at one time and the hydrogen concentration was the highest. Figure 5 shows the effects of the HMD on the cylinder pressure. With the increase in the excess air ratio, since the quantity of fuel in the engine decreased, the maximal cylinder pressure (Pmax) and the combustion speed decreased continuously. As the best ignition timing was adopted, the level of the maximal cylinder pressure was kept within a certain range. With the increase in the excess air ratio, the combustion of the engine became slower, the ignition timing advanced and the combustion duration increased. The engine cylinder pressure with the NHMD was higher, as the best ignition timing with the NHMD was earlier than that with hydrogen addition. Figures 6 and 7 indicate the effects of the HMD on the Pmax and its corresponding position. With the increase in the excess air ratio, the total amount of fuel in the engine decreased, resulting in a continuous decrease in the Pmax of the engine. However, by adjusting the ignition time, the position of the Pmax could be kept within a certain range. The maximal cylinder pressure under the excess air ratio of 2.8 decreased by 46.19% on average compared to that with the excess air ratio of 1.0. The increased excess air ratio resulted in the best HMD (in terms of the Pmax) changing from the PPHMD to the SHMD. This result further verified that the engine ignition performance was the key to lean combustion efficiency. Figure 5: Effects of the HMD on the cylinder pressure: (**A**) NHMD, (**B**) SHMD, (**C**) PHMD, (**D**) PPHMD. Figure 8 shows the effects of the HMD on the coefficient of variation. The stability of the ignition decreased with the increase in the excess air ratio, which reduced the combustion speed and increased the COV of the engine. It was noted that the COV with the NHMD was higher than that with hydrogen addition. In this study, hydrogen addition increased the ignition and combustion stability of the engine significantly, so that the lean burn limit extended from an excess air ratio of 1.5 to 2.8. Figure 8: Effects of the HMD on the COV. Figure 6: Effects of the HMD on the Pmax. Figure 7: Effects of the HMD on the position of the Pmax. Figure 9 displays the effects of the HMD on the CO emissions.