Abstract
The interest in turbojet engines was emerging in the past years due to their simplicity. The purpose of this article is to investigate sliding mode control (SMC) for a micro turbojet engine based on an unconventional compound thermodynamic parameter called Turbofan Power Ratio (TPR) and prove its advantage over traditional linear methods and thrust parameters. Based on previous research by the authors, TPR can be applied to single stream turbojet engines as it varies proportionally to thrust, thus it is suitable as control law. The turbojet is modeled by a linear, parameter-varying structure, and variable structure sliding mode control has been selected to control the system, as it offers excellent disturbance rejection and provides robustness against discrepancies between mathematical model and real plant as well. Both model and control system have been created in MATLAB® Simulink®, data from real measurement have been taken to evaluate control system performance. The same assessment is conducted with conventional Proportional-Integral-Derivative (PID) controllers and showed the superiority of SMC, furthermore TPR computation using turbine discharge temperature was proven. Based on the results of the simulation, a controller layout is proposed and its feasibility is investigated. The utilization of TPR results in more accurate thrust output, meanwhile it allows better insight into the thermodynamic process of the engine, hence it carries an additional diagnostic possibility.
Highlights
Aviation industry always have to face the challenge of sustainability [1] which permanently inspires researchers to find novel approaches [2,3].Turbojet engines, even if their utilization has reduced in commercial aviation during the past decades due to their comparably less propulsive efficiency [4], still have a role in different propulsion systems
As a summary of this assessment, one can state that rotor speed and Turbofan Power Ratio (TPR) have enough range to provide smaller magnitude of the signal-to-noise ratio, Engine Pressure Ratio (EPR) and TPR have almost linear correlation to thrust resulting in simpler computation in the control system
The sliding mode control (SMC) control has been adapted to other control laws as rotor speed and EPR
Summary
Even if their utilization has reduced in commercial aviation during the past decades due to their comparably less propulsive efficiency [4], still have a role in different propulsion systems. There is an increasing interest in Unmanned Aerial Vehicles (UAV’s) in the past decade [5], which has led to a considerable interest in developing micro turbojet engines, as stated in [6,7] or [8], because these engines can be applied on military UAV’s, which can include high-speed aircraft (as shown in [9]), where the turbojet already provides fair propulsive efficiency. The non-optimal speed range of sailplanes turbojets can still be an alternative to piston engines, because they feature relatively simple structure along with reduced weight which is a benefit for small units, as it was stated by [14]. As the Energies 2020, 13, 4841; doi:10.3390/en13184841 www.mdpi.com/journal/energies
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