This study investigates the exhaust emissions of the Allison 250-C20B helicopter engine using conventional kerosene (Jet A-1) and alternative fuels. The alternative fuels examined include a sustainable aviation fuel (SAF) based on hydroprocessed esters and fatty acids (HEFA) derived from vegetable oils and animal fats, as well as a synthetic paraffinic kerosene produced via the Fischer–Tropsch process (Fischer–Tropsch Synthetic Paraffinic Kerosene, FT-SPK). The focus is on the chemical and physical characterization of gaseous and particulate emissions. Non-regulated engines, such as the one studied, contribute significantly to air pollution, particularly due to high emissions during ground operation. Alternative fuels offer a promosing option for reducing pollutant emissions. The thermal efficiency of modern gas turbine engines is primarily achieved through higher pressure ratios combined with elevated combustion temperatures. This also enables more complete combustion, resulting in fewer pollutant components such as carbon monoxide (CO), unburned hydrocarbons (UHC), and volatile organic compounds (VOC). However, the higher combustion temperatures also lead to increased nitrogen oxide (NOx) emissions. The conventional combustion chamber technology used in the investigated engine - operating under design conditions with a fuel-rich primary zone - shows a clear influence of fuel preparation (including injection and vaporization) and combustion conditions on the emissions profile. In contrast, more recent combustion chamber concepts such as Rich-Burn Quick-Quench Lean-Burn (RQL) and Lean-Direct-Injection (LDI) demonstrate the potential of modern technologies to reduce pollutant emissions. Gaseous emissions influence particle formation, as aromatic hydrocarbons - primarily polycyclic aromatic hydrocarbons (PAH) - act as soot precursors. While aromatics are chemically bound in petroleum-based kerosene, they are absent in alternative fuels, resulting in different emission characteristics. The measurement results show that emissions per kilogram of burned fuel - referred to as emission indices (EI) - depend on the power setting of the turboshaft engine and the fuel used. At full load, the emission indices for CO, UHC, and VOC decrease due to higher temperatures and pressures, while they increase under low power. In contrast, NOx emissions remain largely constant across all operating conditions. While combustion of HEFA-SAF produces pollutant levels similar to those of Jet A-1, EI-CO is reduced by over 20% and EI-UHC by nearly 37% when using FT-SPK. This behavior can primarily be attributed to the chemical composition of the alternative fuel, which consists mainly of n- and iso-alkanes. Non-volatile particulate matter (nvPM) shows behavior similar to gaseous emissions. While the soot particle number EI is higher at ground-idle than at take-off, the opposite is observed for soot particle mass. The soot mass per kilogram of burned fuel is higher at full load than at partial load. This is partly due to the soot particle diameters, which vary depending on the power setting of the Allison 250-C20B. The increase in particle diameter results from finer fuel atomization and a higher collision probability of particles formed in the fuel-rich primary zone of the combustion chamber. While Jet A-1 produces a comparatively high number of particles at low loading points, the alternative fuels investigated significantly reduce both particle number and mass - by up to 80%. This is also attributed to their chemical composition and PAH formation, as clearly demonstrated through measurements. Scanning electron microscope analyses show no morphological differences in soot particles when using alternative fuels. A comparison with regulated turbofan and turbojet engines with a net thrust above 26.7 kN shows that engines with a similar rich primary zone exhibit emission indices comparable to those of the investigated helicopter engine. In summary, the alternative fuels investigated in this work exhibit an overall positive effect on pollutant formation, particularly with regard to soot emissions. This effect was demonstrated across a wide operating range for the combustion chamber technology used in the helicopter engine. Even blends of 30% and 50% result in a significant reduction of non-volatile soot particles, with the greatest reduction achieved when using 100% SAF or SPK.     «

This study investigates the exhaust emissions of the Allison 250-C20B helicopter engine using conventional kerosene (Jet A-1) and alternative fuels. The alternative fuels examined include a sustainable aviation fuel (SAF) based on hydroprocessed esters and fatty acids (HEFA) derived from vegetable oils and animal fats, as well as a synthetic paraffinic kerosene produced via the Fischer–Tropsch process (Fischer–Tropsch Synthetic Paraffinic Kerosene, FT-SPK). The focus is on the chemical and physi...     »