Reliable experimental procedure for the evaluation of alternative fuel emissions in small-scale turbojet engines: A bio-kerosene blend case study

Most emission studies conducted on micro gas-turbine (MGT) testbeds report inconsistent or contradictory results. The primary cause is the entrainment of ambient air into the exhaust stream during sampling, since the analyzer probe must operate directly in the free jet discharged by the engine. This dilution alters the measured gas composition, increases uncertainty and can mask or even reverse the subtle emission effects associated with bio-kerosene and other Sustainable Aviation Fuel (SAF) blends. Consequently, it becomes difficult to reliably assess, extrapolate or interpret the influence of fuel composition on emission behaviour—despite the importance of accurate SAF evaluation for aviation decarbonization efforts. To address this limitation, an alternative sampling arrangement is proposed and experimentally validated. In the new configuration, the exhaust stream is first guided through a duct & chamber configuration, where the gas decelerates and becomes shielded from ambient dilution before being sampled. This modification significantly improves the stability and repeatability of the measurements. Using Jet-A fuel, the improved setup reduces emission uncertainty by up to 83% for CO 2 and O 2 , and by 25 & 39% for NO x and CO, enabling consistent detection of fuel-dependent emission trends. A series of bio-kerosene blend tests were performed using both the conventional exposed-probe arrangement and the improved shielded-sampling configuration. With both sampling arrangements, the measured thrust–speed profile remains essentially identical across all fuel blends, indicating that neither setup alters engine performance evaluation. However, the corresponding emission profiles differ markedly. In the conventional exposed-probe configuration, the effect of increasing bio-kerosene concentration on emissions appears inconsistent and is often obscured by measurement variability caused by ambient-air dilution. In contrast, the improved shielded-sampling configuration yields a clearer and statistically consistent trend: NO x levels decrease with increasing blending ratio, reaching reductions of up to 16% for the B60 blend. Overall, the proposed sampling method enhances the reliability of emission measurements in small-scale turbojet engines and enables their use as a consistent and meaningful platform for SAF evaluation.