Experimental assessment of rotational speed stability in a micro turbojet engine for unmanned aerial vehicles based on fuel physicochemical properties, engine performance, and vibration analysis

This study investigated the rotational speed stability of a micro turbojet engine operating with three different fuels through physicochemical fuel characterization, engine performance testing, and vibration-based stability analysis. Experiments were conducted on a micro turbojet engine used in unmanned aerial vehicle applications under five steady-state operating regimes. Key performance parameters—including thrust, turbine inlet temperature, fuel flow rate, and specific fuel consumption—were investigated. Engine stability was evaluated using two triaxial accelerometers installed near the compressor and turbine bearing planes, while rotational speed variations were analyzed from vibration signals and encoder measurements using short-time Fourier transform, root mean square, and kurtosis indicators. The results showed that diesel fuel containing 5 vol% lubricating oil provided engine performance comparable to Jet A containing 5 vol% lubricating oil, with thrust and fuel flow variations generally below 3%. In contrast, biodiesel containing 5 vol% lubricating oil increased the specific fuel consumption by up to 17%. For all tested fuel–lubricating-oil formulations, kurtosis values remained below 3, indicating no pronounced impulsive or shock-like behavior in the measured vibration signals. However, at high operating regimes (approximately 110000 rpm), biodiesel and diesel fuel containing 5 vol% lubricating oil produced slightly higher vibration amplitudes than Jet A containing 5 vol% lubricating oil. The standard deviation and coefficient of variation ranged from 27 rpm to 869 rpm and 0.06% to 2.01% for engine speed, from 0.11 N to 0.78 N and 0.59% to 7.89% for thrust force, and from 0.07 L/h to 0.20 L/h and 0.38% to 2.91% for volumetric fuel flow rate, respectively. Overall, the diesel fuel formulation containing lubricating oil exhibited performance and vibration characteristics closest to those of Jet A formulation containing lubricating oil under the investigated conditions, while the biodiesel formulation containing lubricating oil resulted in higher fuel consumption and slightly increased vibration amplitudes at high operating regimes. These findings are limited to the short-duration operating conditions investigated in the present study.