Use of natural gas/hydrogen blends in heavy-duty spark ignition engines: outcomes from an experimental campaign and methodologies for on-board H2 blending ratio determination

In the present scenario of global warming and energy-transition emergencies, the scientific community is investigating alternative and cleaner energy sources to reduce the environmental impact of the transportation industry. As a matter of fact, various alternative fuels have been explored, such as natural gas, biomethane and, most recently, hydrogen. Among them, hydrogen is one of the most promising fuels for use in internal combustion engines due to its lack of carbon and its combustion properties, which are capable of maintaining high engine performance. Hydrogen can be used in its pure form in engines specifically designed for this purpose, or blended with Compressed Natural Gas (CNG) in conventional engine technologies—with limited mass percentage—to increase combustion efficiency and overall engine performance, as well as to reduce carbon-based emissions. In this study, an experimental campaign was carried out to investigate the impact of natural-gas/hydrogen blends on the performance and emissions of a spark-ignition heavy-duty engine. Experimental tests were conducted on CNG and on two natural-gas/hydrogen blends containing 15% and 25% hydrogen by volume. The hydrogen addition was first analysed while keeping the standard CNG engine calibration, and then by adjusting the spark advance to achieve the same combustion-center timing or the same nitrogen-oxide emissions as the reference CNG case. The testing activity highlighted the need to reduce the ignition timing to counterbalance the rapid combustion evolution when hydrogen is burned, as well as the expected increase in nitrogen-oxide emissions. The optimal parameter setting depends on the hydrogen percentage. Therefore, achieving a flexible fuel supply—with mixtures ranging from a minimum to a maximum hydrogen content—relies on the ability to measure the hydrogen concentration inside the cylinders. In this scenario, two methodologies to estimate the hydrogen percentage in CNG/H₂ blends have been proposed. The description of the calculation methods under development and the discussion of the initial results, which have proven to be very promising for the implementation of future on-board procedures, are the focus of this paper.