Thermal Effects and Combustion Behaviour of Hydrogen and Natural Gas in a Heavy-Duty Gas Engine

To accelerate the usage of renewable fuels in the heavy-duty transportation sector, a conventional diesel heavy-duty engine was retrofitted for gaseous fuel operation. The aim of this approach was to support the transition from diesel to renewable energy carriers while maximizing the reuse of existing engines in the field. A direct conversion from liquid to gaseous fuel usage does not guarantee stable or efficient engine operation. Gaseous fuels require an external ignition source to initiate combustion. Hydrogen exhibits a low minimum ignition energy and a wide flammability range in air. These characteristics lead to high demands on combustion development. In spark-ignited internal combustion engines, port fuel injection (PFI) is widely used due to its simpler system integration and lower fuel pressure level compared to direct injection (DI). PFI is associated with several inherent disadvantages, including lower volumetric efficiency and an increased sensitivity to abnormal combustion phenomena such as backfire and pre-ignition. To compensate this loss in volumetric efficiency, elevated boost pressures are typically required. In order to maximize volumetric efficiency with gaseous fuels under boosted operation, fuel injection must be timed after intake valve closure. Experimental investigations were conducted on a 1991 cc single-cylinder research engine, representative of heavy-duty applications. Two distinct fuel supply configurations were evaluated: a low-pressure PFI system operating at pressures up to 15 bar, and a high-pressure DI system with injection pressures of up to 200 bar. Two novel injector designs were tested to study the influence of fuel gas pressure and fuel type. The engine was operated with natural gas and hydrogen as gaseous fuels. In addition, the cylinder head was instrumented with ten thermocouples to assess differences in the thermal distribution within the combustion chamber and their effect on measured cylinder head temperatures. The high-pressure level injection system enables new applications by increasing injection timing flexibility and enabling combustion anomalies to be prevented or mitigated. The influence of pressure on mixture-formation mechanisms was investigated at varying engine load and speed. Simultaneously, the exhaust gas was analysed for regulated emissions, including nitrogen oxides (NOx), hydrogen slip (H₂ slip), and unburned hydrocarbons.