To accelerate the adoption of renewable fuels in heavy-duty transportation, a conventional diesel engine was retrofitted to operate on gaseous fuels. This approach supports the transition from diesel to renewable energy carriers while maximizing the reuse of existing engine platforms. However, converting a liquid-fuel engine to gaseous operation does not inherently ensure stable or efficient performance. Gaseous fuels require external ignition, and hydrogen, with its low minimum ignition energy and wide flammability range, places particularly high demands on combustion development.
In spark-ignited heavy-duty gas engines, port fuel injection (PFI) is widely used because of its simpler integration and lower fuel-pressure requirements compared with direct injection (DI). However, PFI reduces volumetric efficiency and increases sensitivity to abnormal combustion, including backfire and pre-ignition. DI can mitigate these limitations by enabling fuel delivery after intake valve closure and allowing later injection timings, thereby improving system efficiency and mixture formation control.
Experiments were conducted on a 1991 cc single-cylinder research engine representative of heavy-duty applications. Two fuel supply systems were evaluated: low-pressure PFI up to 15 bar and high-pressure DI up to 200 bar. Two novel injector designs were tested with hydrogen and natural gas to assess the effects of fuel type, pressure level, load, and speed. The cylinder head was instrumented with ten thermocouples to evaluate local thermal distribution. In parallel, exhaust emissions, including NOx, hydrogen slip, and unburned hydrocarbons, were analyzed to link injection strategy, mixture formation, combustion behavior, emissions, and thermal loading.