A former diesel heavy-duty engine was retrofitted to hydrogen operation to simultaneously facilitate the shift from fossil to renewable fuels and maximize the quantity of reusable engine parts. Simply changing the fuel in this case does not make a properly working engine; the burning process needs to be realized in a premixed flame regime, rather than a diffusional flame regime. Therefore, an additional ignition source is necessary. A well-known characteristic of hydrogen is the low need for ignition energy and the wide range of ignitable air/ fuel ratios. Both must be considered to reach a diesel engine equivalent performance.
Port fuel injection (PFI) and direct injection (DI) are commonly used in spark-ignited internal combustion engines. Some disadvantages, such as weak volumetric efficiency and combustion abnormal phenomena like backfire, are connected to PFI. To further improve the volumetric efficiency, high boost pressures are needed. To maximize volumetric efficiency with DI, injection timing after intake valve closure is mandatory.
With a high-pressure level for hydrogen injection, a new field of application possibilities is generated regarding the degree of freedom in the injection timing. Furthermore, combustion anomalies can be prevented or their effects mitigated. The influence of pressure on the mixture formation mechanisms and time scales regarding engine load and speed variation was investigated. Simultaneously, the exhaust gas was analyzed concerning the common emissions, nitrogen oxide (NOx), and hydrogen slip (H₂ slip).
Investigations were carried out on a single-cylinder research engine in two experimental setups. A low-pressure direct injection setup with pressure levels up to 30 bar and a high-pressure direct injection setup with up to 200 bar pressure. While varying the hydrogen injection pressure, two novel injectors were in use. The single-cylinder research engine’s displacement is representative of heavy-duty applications such as trucks, buses, and excavators. In a series application in a four-cylinder diesel engine setup, the following data is representative of the employed engine type: