Hydrogen internal combustion engines are considered as alternatives to conventional diesel and natural gas engines in the heavy-duty transportation as they emit near-zero carbon dioxides. The hydrogen engines equipped with port-fuel injection (PFI) system are advantageous for market entry because of the high availability of existing mature engine hardware. However, abnormal combustion such as backfire, preignition, and knocking acts as a major obstacle to increasing the power output in hydrogen PFI engines. Therefore, this study investigated intake mixture strategies for suppressing the abnormal combustion in a hydrogen PFI engine. The investigation was conducted experimentally in a single-cylinder spark-ignition engine that was modified from a six-cylinder heavy-duty diesel engine. The compression ratio of the engine was 12:1. Hydrogen was supplied to the intake port during the intake stroke. A piezo-resistive pressure transducer was installed in the intake port to detect backfire. The in-cylinder pressure was measured through a piezo-electric pressure transducer and used for knocking detection. The engine was operated at 1,200 rpm and 1,800 rpm. The engine load was selected as 0.8 MPa gross indicated mean effective pressure for both engine speeds. An excess air ratio (EAR) of 2.8 was reduced by 0.2 during the engine operation until abnormal combustion occurred. High-pressure exhaust gas recirculation (EGR) was implemented to suppress the abnormal combustion. The EGR rate was applied up to 30%. Abnormal combustion did not occur in the hydrogen engine with high EARs at both engine speeds. However, when the EAR was reduced to 2.2 at 1,200 rpm, the optimal operation of the engine was limited because of knocking. Although the knocking was suppressed by retarding the ignition timing, the gross indicated thermal efficiency (ITE) significantly decreased. At 1,800 rpm, the engine operation was limited because of backfire when an EAR of 2.4 was reached. The knocking and backfire were avoided through excess air strategies. However, high intake pressure with intake boosting was required to implement the excess air strategies. The high-pressure EGR strategies suppressed the knocking and backfire without additional intake boosting, enabling stable engine operation. Despite the effective control of the abnormal combustion, the ITEs were maintained at similar levels compared to those of the engine operation without EGR. This study has a limitation in that it is difficult to detect abnormal combustion caused by cylinder interference occurring in multi-cylinder hydrogen engines as it was conducted in a single-cylinder engine. The analysis of the abnormal combustion in multi-cylinder engines will be conducted after finishing the investigation in the single-cylinder engine. Previous studies also suggested methods of abnormal combustion control using EGR in hydrogen combustion engines. In this study, effects of abnormal combustion control using excess air and EGR were compared. Furthermore, intake mixture strategies that can improve the power output of hydrogen PFI engines under conditions of meeting the recent stringent emission regulations were presented. The strategies of excess air and EGR were effective in suppressing the abnormal combustion in hydrogen PFI engines. The excess air strategies required additional intake boosting, which increased the burden on the turbocharger. The high-pressure EGR strategies did not require intake boosting, but the exhaust energy supplied to the turbocharger was reduced. Therefore, it is important to match a turbocharger in a hydrogen PFI engine considering the above two points in order to utilize the intake mixture strategies for controlling the abnormal combustion.
Dr. Hyunwook Park, Senior researcher, Korea Institute of Machinery and Materials