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Article The Use of as an Alternative for Small Engines

Rudolf Andoga 1,* , Ladislav F˝oz˝o 2 , Martin Schrötter 1 and Stanislav Szabo 3

1 Department of Avionics, Faculty of Aeronautics, Technical University of Košice, 040 01 Košice, Slovakia; [email protected] 2 Department of Aviation , Faculty of Aeronautics, Technical University of Košice, 040 01 Košice, Slovakia; [email protected] 3 Department of Air Traffic Management, Faculty of Aeronautics, Technical University of Košice, 040 01 Košice, Slovakia; [email protected] * Correspondence: [email protected]

Abstract: The use of alternative to traditional -based ones in turbo-jet engines is currently being widely explored and researched. However, the application of alternative fuels in the area of small turbojet engines with ratings up to 2 kilo-newtons, which are used as auxiliary power units or to propel small or drones, is not as well researched. This paper explores the use of ethanol as a sustainable fuel and its effects on the operation of a small turbojet engine under laboratory conditions. Several concentrations of ethanol and JET A-1 mixtures are explored to study the effects of this fuel on the basic parameters of a small turbojet engine. The influence of the different concentrations of the mixture on the start-up process, speed of the engine, temperature, and compressor pressure are evaluated. The measurements shown in the article represent a pilot

 study, the results of which show that ethanol can be reliably used as an only when its  concentration in a mixture with traditional fuel is lower than 40%, yielding positive effects on the

Citation: Andoga, R.; F˝oz˝o,L.; operating temperatures and small negative effects on the speed or thrust of the engine. Schrötter, M.; Szabo, S. The Use of Ethanol as an Alternative Fuel for Keywords: ethanol; alternative fuel; small turbojet engine; turbojet engines Small Turbojet Engines. Sustainability 2021, 13, 2541. https://doi.org/ 10.3390/su13052541 1. Introduction Academic Editor: Talal Yusaf In 2017, (GHG) emissions from the transportation sector (including international aviation) in the EU-28 represented 25% of the total, whereas in 1990, they Received: 29 January 2021 represented only 15% [1]. Aircraft produce around 2% to 3% of the dioxide (CO2) Accepted: 20 February 2021 emissions worldwide [2]. This value will probably grow or stagnate after the corona crisis Published: 26 February 2021 is over, as the number of passenger and transport aircraft was expected to increase to 48,000 by 2038 [3]. Publisher’s Note: MDPI stays neutral Traditional aviation fuels are produced by the of crude . There are with regard to jurisdictional claims in also some alternative fuels potentially suitable for aviation, including , ethanol, published maps and institutional affil- , liquid , liquid , and synthetic fuels made from , natural iations. gas, or other (HC) feedstock [4]. Presently, much research is currently focused on aviation . Alternative -based fuels were first proposed in 1907 for possible use in the automotive [5]. Today, the most common alcohol-based fuel for piston combustion Copyright: © 2021 by the authors. engines in automobiles is ethanol [6–8]. This fuel is used in advanced countries as an Licensee MDPI, Basel, Switzerland. additive to normal petrol fuels, usually at a concentration of 10% (E10) but also at 20% (E20), This article is an open access article 85% (), and even 100% (E100) [9–11]. Using such a concentration demands thorough distributed under the terms and testing of these engines. The small content of ethanol in fuel does not significantly affect conditions of the Creative Commons the mechanical and energy characteristics of an engine [12]. However, it increases thermal Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ efficiency and improves the anti-detonation properties of the fuel [5]. CO2 and unburned 4.0/). HC emissions increase with ethanol content in the fuel [13], which produces lower tailpipe

Sustainability 2021, 13, 2541. https://doi.org/10.3390/su13052541 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 2541 2 of 21

emissions in terms of total HCs and carbon monoxide (CO); however, ethanol also causes a significant increase in the emissions of [13]. E10 was observed to cause higher emissions of nitrogen oxide (NOx) than E0 [14]. The use of E20 fuel had an impact on decreasing damage to human health by 1.3% and decreasing damage to the ecosystem by 1.4% and natural resources by 12.9% [13]. The influence of ethanol was also tested in diesel engines, where a significant decrease in performance was observed using a mixture of diesel and ethanol. On the other hand, the smoke values and NOx values were reduced [15]. Mixing ethanol and aviation JET A fuel was done in a four-stroke direct injection , thereby obtaining higher efficiency and ecological benefits [16]. The influence of biofuels and alcohol-based mixtures on gas turbines has also been tested in power engineering. More ethanol content caused greater CO emissions. NOx was reduced for biofuels by up to 70%, and particulate matter (PM) was also reduced [17]. The first tests and certifications of ethanol and mixtures were performed on piston engines with carburettors. It was found that an aviation piston engine can operate with any mixture of aviation gas () and ethanol [18]. However, a recent review emphasised the advantages of the fuel properties of butanol over methanol and ethanol for use in internal combustion engines [19,20]. Testing and comparison of clean JET A fuel with ethanol was also performed on aviation engines, where it was found that the engine had a slower dynamic response when using ethanol in the fuel mixture. This was remedied by a redesign of the controller gains [21]. Other tests were aimed at mixtures of butanol and JET A fuel. The results showed that butanol has several advantages over ethanol, including higher energy content, lower vapour pressure, lower water absorptivity, and fewer corrosive effects. The results show that the performance parameters such as thrust and exhaust gas temperature (EGT) for Jet A-1/ blend are comparable to or smaller than those for pure JET A-1. The fuel consumption and specific fuel consumption were slightly higher, up to 5% for full thrust and 2% for normal operation, for the blend with butanol than for JET A-1. On the other hand, the values of CO, CO2, and NOx concentrations for the JET A-1/butanol blend were slightly lower—3%, 2%, and 2%, respectively—than those of JET A-1 [22,23]. The use of biofuels has also considerably increased in recent years. Different gen- erations of biofuels, methods of production, and ecological and economic benefits have been thoroughly researched [3,4,24]. Bio-jet fuels have good thermal stability, excellent combustion properties, and good low-temperature fluidity but relatively poor oxidative stability and are not compatible with current fuelling systems due to a swell of the sealing materials and lubricity [25]. Alternative aviation fuels have lower energy content per unit weight and volume than traditional fuels. The specific energy of pure ethanol is 58% and its energy is 60% compared to JET A-1. Small turbojet engines can provide a good testing platform for alternative fuels and various fuel blends. A test of using a small turbojet engine with synthesised paraffinic kerosene (SPK) from hydro-processed esters and fatty acids (HEFA) showed a decrease in fuel consumption, as well as a decrease in CO, HC, PM, and NOx emissions [24]. Test of blends containing Jet A-1 and 10–50% d-limonene, 10–25% butyl butyrate, 25% n-butanol, and 25% ethanol were also used on small turbojet engines. Each tested blend resulted in minimal performance changes at most operating throttle settings [26]. In a test of 2-Ethylhexanol (2-EH) and Jet A-1 fuel, it was found that the thrust, fuel consumption, and CO emissions between the tested fuels were negligible, not exceeding 3%. In case of NOx emissions, higher values were obtained for the 2-EH fuel than for clean Jet A-1 [27]. A 20% fatty acid methyl ester (PME) blend with JET A-1 was also tested, where slight performance penalties were observed due to the lower energy content of the biodiesel blend [28]. A small-scale gas turbine engine with a power output of 30 kW fuelled with JET A-1/ethanol blends was also researched using concentrations from 25% to 100% for the ethanol mixture in the fuel/ethanol blend [29]. Researchers found that Sustainability 2021, 13, x FOR PEER REVIEW 3 of 23

Sustainability 2021, 13, 2541 3 of 21 output of 30 kW fuelled with JET A-1/ethanol blends was also researched using concen- trations from 25% to 100% for the ethanol mixture in the fuel/ethanol blend [29]. Research- ers found that the CO and NOx emissions were comparable between the blended and clean the CO and NOx emissions were comparable between the blended and clean JET A-1 fuel, JET A-1 fuel, and the NOx emissions were lower for the mixture of JET A-1/ethanol be- and the NOx emissions were lower for the mixture of JET A-1/ethanol because of the lower temperaturecause of the atlower the turbinetemperature inlet [at29 ,the30]. turbine inlet [29,30]. BasedBased on on these these different different and and sometimes sometimes controversial controversial effects effects of of burning burning alcohol alcohol and and JETJET A-1 A-1 mixtures mixtures with with similar similar combustion combustion characteristics, characteristics, but but with with much much lower lower soothing soothing propensitypropensity thanthan purepure kerosene,kerosene, ethanol was selected selected for for the the experiments experiments to to apply apply it itas as an analternative alternative fuel fuel in insmall-scale small-scale turbojet turbojet engines, engines, which which is an is anarea area that that remains remains poorly poorly ex- explored.plored. The The aim aim of of this this study study is is to to summarise summarise and and explore explore the the effects effects of of using using ethanol ethanol in in a asmall-scale small-scale turbojet turbojet engine engine and and to explore whetherwhether a mixturemixture ofof JETJET A-1A-1 andand ethanol ethanol is is a a viableviable fuel fuel for for use use in in small small turbojet turbojet engines, engines, as as indicated indicated by by previous previous studies. studies. The The other other aimaim isis toto exploreexplore thethe concentrations that are are vi viableable to to be be used used for for small small turbojet turbojet engine, engine, as aswell well as asthe the effects effects of ofalcohol-based alcohol-based fuel fuel on onthe the operational operational parameters parameters of jet of engines. jet engines. The Theresults results could could helphelp broaden broaden the knowledgebase the knowledgebase for using for usingalternative alternative fuels in fuels small in turbojet small turbojetengines, engines, and some and knowledge some knowledge from the from describ the describeded experiment experiment can also can be alsoused be to used supple- to supplementment knowledge knowledge about aboutthe use the of usealternative of alternative fuels in fuels large in turbojet large turbojet engines engines or gas turbine or gas turbineengines engines in general. in general.

2.2. Materials Materials and and Methods Methods ToTo evaluate evaluate the the hypothesis hypothesis of of the the application application and and effects effects of of different different JET JET A-1/ethanol A-1/ethanol fuelfuel blends blends on on a a small small turbojet turbojet engine, engine, we we used used an an engine engine test test stand stand with with a a digital digital data data acquisitionacquisition system system in in a a laboratory. laboratory. The The engine engine used used in in the the experiments experiments is is a a small small turbojet turbojet engine,engine, MPM-20, MPM-20, with with a a radial radial compressor compressor and and a a hydromechanical hydromechanical control control unit. unit. This This is is a a smallsmall turbojet turbojet engine engine with with a a thrust thrust of of up up toto 500500 Newtons,Newtons, anan annularannular combustioncombustion chamber,chamber, andand a a radial radial single single sided sided compressor. compressor. The The engine engine is is a a one-shaft one-shaft engine engine with with direct direct air air flow flow andand a a single single disc disc uncooled uncooled turbine turbine featuring featuring a a fixed fixed exhaust exhaust nozzle nozzle [31 [31,32].,32]. This This is is a a simple simple andand traditional traditional turbojet turbojet engine engine construction; construction; thus, thus it, it is is expected expected that that the the results results of of the the tests tests willwill be be valid valid for for similar similar class class of of engines, engines, as as well well as as for for engines engines with with small small power power outputs outputs andand airflows. airflows. The The engine engine and and its its installation installation are are shown shown in in Figure Figure1. 1.

Figure 1. MPM-20 Engine,Figure test bench 1. MPM-20 installation. Engine, test bench installation.

TheThe alternative alternative fuel fuel in in our our experiment experiment is is a a blend blend of of JET JET A-1 A-1 fuel fuel and and ethanol. ethanol. Ethanol Ethanol isis a a colourless colourless flammable flammable liquidliquid thatthat can can be be easily easily produced produced from from different different renewable renewable sources.sources. Ethanol Ethanol is is also also less less toxic toxic than than other other alcohol-based alcohol-based fuels. fuels. Its Its main main deficiency deficiency is is its its waterwater binding binding properties, properties, which which can can cause cause corrosion in in engines. engines. This This shortcoming shortcoming can can be be remedied via additives [33,34]. The basic chemical properties of pure ethanol (less than 0.1% of water) are listed in Table1. Sustainability 2021, 13, 2541 4 of 21

Table 1. Basic chemical properties of ethanol [35,36].

Systematic Name Ethanol—Hydroxyethane Rational name Ethanol (99.9%) CAS Registry No. 64-17-5 Summary formula C2H6O Molecular weight 46.07 g/mol −114.4 ◦C Freezing temperature −130.5 ◦C Boiling point 78.3 ◦C (1013 hPa) Density (at 20 ◦C) 0.789 g/cm3 12.7 ◦C (<0.1% of water) Flash point 13.57 ◦C (10% of water) 19.33 ◦C (50% water) Burning temperature 30 ◦C Ignition temperature 366 ◦C Explosion limits 3.4–15% of volume Kinematic 1.5 mm2/s (at 20 ◦C, 1 atm) Dynamic viscosity 1.184 mPa·s (at 20 ◦C, 1 atm) Specific heat 2 460 J/(kg·K) Higher heating value 29.7 MJ/kg

2.1. The Experimental Engine and Measurement System A small turbojet engine, MPM-20, was used as a test engine to evaluate the effects of using ethanol and JET A-1 fuel blends. In the tests, this engine was only partly digitally controlled with a real-time data acquisition system [37]. To test the effects of alcohol- based fuels, a hydro-mechanical fuel pump with a partial digital control was used so the parameters of fuel flow (FF) would not be optimised, and the fuel flow could be constant and start-up fuel flows could have the same curve in each experiment without specific optimisations of a fully digital control system. The parameters and sensors that were used to measure them are shown in Table2. They were dynamically measured with a constant sampling frequency of 10 Hertz. The scheme of the data acquisition system and the arrangement of the sensors from Table2 are shown in Figure2. This figure shows precisely the placement of each sensor on the engine using the engine’s 3D model. The figure also shows connection of the sensors through a bus into NI CDAQ 9717 data acquisition system.

Table 2. Sensors used in experiments.

Parameter—Abbreviation Sensor Unit Basic Calibrated Error Badger-meter MN-2 Fuel Flow—FF (l.min−1) ±1% flow meter Rotational speed of the Optical speed meter (min−1) ±0.2% engine—n Total exhaust gas Calibrated K (◦C) ±0.75% temperature—EGT thermocouple Total compressor QBE2002-P10 (atm) ±0.4% pressure—p2

To sample and digitise data, we used a calibrated National instruments data acqui- sition system NI CDAQ 9172 with the following installed modules for real-time data acquisition: • NI 9263—100 kHz Voltage Output Module, used for the control of analogue engine systems; • NI 9472—8-Channel, Digital I/O Module, used for the control of digital engine sys- tems; • NI 9205—±10 V, 250 kHz, 16-Bit, 32-Channel C Series Voltage Input Module, used for the measurement of analogue sensors, in this case, QBE2002-P10; Sustainability 2021, 13, 2541 5 of 21

• NI 9423—8 Channel Sinking Input, C Series Digital Module, used for the measurement of frequency signals, in this case, optical speed sensor; Sustainability 2021, 13, x FOR PEER •REVIEWNI 9213—NI-9213, 16-channel, Thermocouple Input Module, used for the measure-5 of 23

ment of thermocouples, in this case, K-thermocouple EGT sensors.

Figure 2. SchemeFigure of the 2. measurementScheme of the system measurement and placement system and of sensors. placement of sensors.

FiguresTo sample3 and and4 show digitise how data, the engine we used dynamically a calibrated operates National during instruments a single data run usingacquisi- cleantion system JET A-1 NI fuel CDAQ with 9172 a depiction with the of followin its basicg parameters,installed modules which for are real-time evaluated data for acqui- the effectssition: of alternative fuels. As shown in Figure3, the engine was run for approximately 25 s. Figure NI3 shows 9263—100 the course kHz Voltage of the engineOutput speed Module, and us theed Exhaust for the control Gas Temperature of analogue (EGT), engine whilesystems; Figure4 shows the total compressor pressure and the fuel flow during standard operations of the engine at 46,000 min−1. The EGT temperature reaches a peak of 800 ◦C.  NI 9472—8-Channel, Digital I/O Module, used for the control of digital engine sys- Sustainability 2021, 13, x FOR PEER REVIEWThis is an important parameter, present in many turbojet engines, that needs to be analysed6 of 23 tems; when using blends or alternative fuels in general.  NI 9205—±10 V, 250 kHz, 16-Bit, 32-Channel C Series Voltage Input Module, used for the measurement of analogue sensors, in this case, QBE2002-P10;  NI 9423—8 Channel Sinking Input, C Series Digital Module, used for the measure- ment of frequency signals, in this case, optical speed sensor;  NI 9213—NI-9213, 16-channel, Thermocouple Input Module, used for the measure- ment of thermocouples, in this case, K-thermocouple EGT sensors. Figures 3 and 4 show how the engine dynamically operates during a single run using clean JET A-1 fuel with a depiction of its basic parameters, which are evaluated for the effects of alternative fuels. As shown in Figure 3, the engine was run for approximately 25 s. Figure 3 shows the course of the engine speed and the Exhaust Gas Temperature (EGT), while Figure 4 shows the total compressor pressure and the fuel flow during standard operations of the engine at 46,000 min−1. The EGT temperature reaches a peak of 800 °C. This is an important parameter, present in many turbojet engines, that needs to be ana- lysed when using ethanol fuel blends or alternative fuels in general. Figures 3 and 4 also illustrate the three phases of operation of the engine—the start- up phase, the stable operation phase, and the shutdown phase—for which the engine’s parameters were evaluated. The engine’s hydromechanical fuel/oil pump was run at a constant speed, thus producing a constant fuel flow to secure the stable operation of the

engine. Start-up of the engine was controlled hydromechanically based on compressor Figure 3. Exhaust gas temperature and speed of the engine during a single engine run. Figurepressure 3. Exhaust in the gasfeedback temperature to control and speed the fuel of the flow engine during during start-up a single producing engine run. the same fuel flow curve in each run. Fuel flow (l/min) flow Fuel Total compressor pressure - p2 (atm) p2 - pressure compressor Total

Figure 4. Fuel flow and compressor pressure during a single engine run.

2.2. The Experimental Fuel To evaluate the effects of ethanol and clean JET A-1 (E0) fuel blends on the operation of a small turbojet engine, concentrations of 10% (E10), 20% (E20), 30% (E30), and 40% (E40) ethanol in the JET fuel were analysed by the certified research facility, The Centre Of Metrology and Testing in Žilina. The results for the individual blends are presented in Table 3. Tests for density were done according to STN EN ISO 12185 [38], tests for viscosity were done according to STN EN ISO 3104+AC [39], and distillation tests were done ac- cording to STN EN ISO 3405 [40].

Sustainability 2021, 13, x FOR PEER REVIEW 6 of 23

Sustainability 2021, 13, 2541 6 of 21

Figure 3. Exhaust gas temperature and speed of the engine during a single engine run.

Figure 4. FuelFigure flow and4. Fuel compressor flow and pressurecompressor during pressure a single during engine a single run. engine run.

2.2. TheFigures Experimental3 and4 also Fuel illustrate the three phases of operation of the engine—the start- up phase,To evaluate the stable the effects operation of ethanol phase, and and clean the shutdownJET A-1 (E0) phase—for fuel blends which on the the operation engine’s ofparameters a small turbojet were evaluated. engine, concentrations The engine’s of hydromechanical 10% (E10), 20% (E20), fuel/oil 30% pump (E30), was and run 40% at a (E40)constant ethanol speed, in the thus JET producing fuel were a analysed constant fuelby the flow certified to secure research the stable facility, operation The Centre of the Ofengine. Metrology Start-up and ofTesting the engine in Žilina. was The controlled results for hydromechanically the individual blends based are on pr compressoresented in Tablepressure 3. Tests in the for feedback density were to control done according the fuel flow to STN during EN ISO start-up 12185 producing [38], tests for the viscosity same fuel wereflow curve done in according each run. to STN EN ISO 3104+AC [39], and distillation tests were done according to STN EN ISO 3405 [40]. 2.2. The Experimental Fuel

To evaluate the effects of ethanol and clean JET A-1 (E0) fuel blends on the operation of a small turbojet engine, concentrations of 10% (E10), 20% (E20), 30% (E30), and 40% (E40) ethanol in the JET fuel were analysed by the certified research facility, The Centre Of Metrology and Testing in Žilina. The results for the individual blends are presented in Table3. Tests for density were done according to STN EN ISO 12185 [ 38], tests for viscosity were done according to STN EN ISO 3104+AC [39], and distillation tests were done according to STN EN ISO 3405 [40].

Table 3. Basic chemical properties of the studied JET A-1/Ethanol fuel blends.

Concentration Test Units E0 E10 E20 E30 E40 Density (at 15 ◦C) (kg/m3) 814.1 810.6 808.2 805.8 803.6 Viscosity (at 20 ◦C) (mm2/s) 2.052 1.885 1.863 1.820 1.774 Flashpoint in a closed container (◦C) 61 <16 <16 <16 <16 PR * 1 1 1 1 1 Water Degree reaction PL ** 2 3 3 3 3 Crystallisation (◦C) −56.3 <−105 <−105 <−105 <−105 Acidity (mg KOH/100 mL) 0.17 0.15 0.14 0.12 0.08 * Phase range (PR); ** Phase level (PL). Sustainability 2021, 13, 2541 7 of 21

The results presented in Table3 show that the inclusion of even 10% ethanol in the fuel blend yields a distinct decrease in the flashpoint because ethanol is highly flammable. Ethanol is also highly hygroscopic, resulting in a very low point of crystallisation below −105 ◦C, as ethanol will bind all available water in the JET A-1 fuel. Viscosity also decreased; this means that some additives will be needed to improve this factor.

2.3. Calculation of the Gross Heat of the Combustion of Alternative Fuel Blends Combustion is defined as an exothermic reaction in which a compound reacts com- pletely with an oxidant. In the ideal complete combustion of organic compounds reacting with , every carbon atom in the original compound ends up in the CO2, every hy- drogen atom ends up in the H2O, and every nitrogen compound ends up in N2. However, in practice, combustion is usually not complete. The water produced in a combustion process ends up as a vapour or liquid, and the enthalpy of combustion is larger when the produced water is in a liquid state, rather than in a gas state. The gross (or the higher heating value (HHV)) for a fuel is defined as the amount of heat released by a specified quantity (initially at 25 ◦C) once the fuel is combusted and the products have returned to a temperature of 25 ◦C[41]. The HHV includes the latent heat of condensation of the water, which is in a liquid state under these conditions. The HHV can be measured by a calorimetric bomb. The exact procedure is specified by the standard EN ISO 1716:2018 [42]. As the combustion of any substance is an exothermic reaction, the HHV will be a positive value. The gross heat of combustion can be determined by Equation (1):

E(T − T + c) − b Q = m i (1) PCS m where

QPCS is the gross heat of combustion (MJ/kg); E is the water equivalent of the calorimeter, the bomb, accessories, and the water introduced into the bomb (MJ/kg); Ti is the initial temperature (K); Tm is the maximum temperature (K); b is the correction required for the combustion heat of the “fuels” used during the test, i.e., the firing wire, thread, cigarette-making paper, and benzoic acid or combustion aid (MJ/kg); c is the temperature correction factor required for the exchange of heat with the outside, which is nil if an adiabatic jacket is used (K); m is the mass of the test specimen in kilograms (kg).

The QPCS of the tested fuel blends of JET A-1 and ethanol, under various concentra- tions, were determined according to the standard EN ISO 1716:2018 [42] and are presented in Table4 and depicted in Figure5.

Table 4. The gross heat of combustion for blends of JET A-1 with ethanol.

Ethanol Percentage in JET A-1 (%) HHV (MJ/dm3) HHV (MJ/kg) 0 37.61 46.2 10 36.26 44.55 20 34.92 42.9 30 33.58 41.25 35 32.9 40.42 40 32.23 39.6 Sustainability 2021, 13, x FOR PEER REVIEW 8 of 23

The QPCS of the tested fuel blends of JET A-1 and ethanol, under various concentra- tions, were determined according to the standard EN ISO 1716:2018 [42] and are presented in Table 4 and depicted in Figure 5.

Table 4. The gross heat of combustion for blends of JET A-1 with ethanol.

Ethanol Percentage in JET HHV HHV A-1 (MJ/dm3) (MJ/kg) (%) 0 37.61 46.2 10 36.26 44.55 20 34.92 42.9 30 33.58 41.25 Sustainability 2021, 13, 2541 35 32.9 40.42 8 of 21 40 32.23 39.6

Figure 5. Gross heat of combustionFigure 5. Gross for different heat of combustion ethanol concentrations for different in ethanol JET A-1. concentrations in JET A-1.

2.4. Experimental2.4. Design Experimental Design According to theAccording computed to grossthe computed heat of combustion gross heat of and combustion the defined and hypothesis the defined in hypothesis in the introductionthe of introduction this paper, the of experimentalthis paper, the design experimental was created design as was follows. created We as initially follows. We initially proposed to testproposed concentrations to test concentrations with fuel blends with designatedfuel blends designated as E0, E10, as E20, E0, E30,E10, E20, and E30, and E40. E40. Since preliminarySince preliminary tests showed tests that showed an E40 that blend an E40 will blend be problematic will be problematic for the engine for the engine dur- during start-up,ing the start-up, E35 blend the was E35 added blend to was the added experiment. to the Allexperiment. blends were All mixedblends using were amixed using a certified facilitycertified for creating facility fuel for blends. creating The fuel clean blends. fuel was The compliant with was thecompliant standards with of the standards SN_0501_01/ASTMof SN_0501_01/ASTM D 1665/AFQRJOS D 23 1665/AFQRJOS [43]. Moreover, 23 the [43] ethanol. Moreover, used in the experiments ethanol used in experi- was compliant withments the was conditions compliant set with in the the testconditions for waterless set in the ethanol test for of 99.9%.waterless ethanol of 99.9%. The experimentalThe designexperimental was developed design was as developed a pilot experiment, as a pilot experiment, and the following and the following set- set-up was proposedup was with proposed the following with the number followin ofg experimentalnumber of experimental runs: runs: • E0: Five experimental E0: Five runs experimental and a single runs run and between a single each run blendbetween each blend • E10, E20, E30, E35,E10, E40:E20, ThreeE30, E35, experimental E40: Three runs experimental for each concentrationruns for each concentration Each fuel blendEach E{i} fuel test wasblend operated E{i} test withwas operated the fuel flow with fixed the fuel at 1.1 flow L/min, fixed whichat 1.1 L/min, which −1 resulted in an engineresulted speed in an of engine around speed 45,000 of min around. This 45,000 means min− that1. This the means whole that experiment the whole experiment consisted of fifteenconsisted engine of runs fifteen that engine could runs be analysed that could as be a pilotanalysed study. as a pilot study. The start-up ofThe the enginestart-up was of the controlled engine was hydromechanically controlled hydromechanically using the compressor using the compressor pressure p2 outputpressure in a proportional p2 output in fuel a propor flow feedbacktional fuel loop flow during feedback start-up. loop Afterduring start-up, start-up. After start- the fuel flow was kept at a constant level using the fuel pump. The effects of the fuels upon start-up of the engine and during stable operation were investigated separately, as shown in the results. After each run, the engine’s fuel system was purged with clean JET A-1 fuel, and the engine was operated once using only clean fuel to purge any remnants of the fuel blend from the engine. The engine in the laboratory was maintained at 18 ◦C and 60% humidity during all experimental runs. The basic results and courses of all executed experiments are shown in Table5. The rows outlined in red show the experiments that were not successful, indicating that the general operation of the engine using the JET A-1/Ethanol fuel blends was problematic. The general rule is to first explore the results when starting the engine after a pause, which seemed to be more problematic using the ethanol blends than when starting with pure kerosene fuel. This was observed in the first start-ups of the engine with the E20 and E30 fuels. Table5 also shows that the E40 blend was problematic and that the fuel did not have enough power to accelerate the engine above 22,000 min−1. Sustainability 2021, 13, 2541 9 of 21

Table 5. Experimental logbook *.

◦ −1 Test Day EGT max ( C) nmax (min ) Tested Blend Note Runs Time (s) Operational

1 3 30 812 45,500 E0 3 consecutive runs with E0 1 1 30 690 45,000 E10 — 1 1 10 753 45,000 E10 Run for only 10 s—discarded 1 1 30 735 45,000 E10 — Total 6 The end of the testing day 2 1 30 731 44,700 E10 Engine achieved only 20,000 2 1 30 602 21,000 E10/E20 (min−1)—discarded 2 1 30 710 44,000 E20 — 2 1 30 690 44,000 E20 — Total 10 The end of the testing day Engine achieved only 20,000 3 1 15 570 19,000 E20 (min−1)—discarded 3 1 30 716 44,000 E20 — Remnants of the fuel blend 3 1 30 703 45,000 E0/E20 caused the speed to rise—discarded Total 13 The end of the testing day Double spike in EGT, difficult 4 1 30 626 43,500 E30 startup—discarded 4 1 30 687 43,000 E30 — 4 1 30 687 42,700 E30 — 4 1 30 687 43,000 E30 — Remnants of the fuel blend 4 1 50 680 45,000 E0/E30 caused the speed to rise from 43k to 45k—discarded Total 18 The end of the testing day 5 1 20 610 17,000 E40 Stabilised only at 17,000 (min−1) 5 1 20 617 22,000 E40 Stabilised only at 22,000 (min−1) 5 1 30 617 22,000 E40 Stabilised only at 22,000 (min−1) Remnants of the fuel blend 5 1 60 680 45,200 E0/E40 caused the speed to rise from 21k to 45k—discarded 5 1 30 674 42,500 E35 — 5 1 30 660 42,500 E35 — 5 1 30 660 42,500 E35 — Remnants of the fuel blend 5 1 60 655 45,200 E0/E35 caused the speed to rise from 21k to 45k—discarded Total 26 The end of the testing day * Red rows indicate measurements which were problematic and not included in the analysis.

3. Results To evaluate the effects of different fuel blends, it was necessary to perform an analysis of the engine’s operation separately during its start-up and during a stable operational run. Thus, situational frames for the three engine operational phases were proposed (Figure6 ). The start-up phase is defined as Condition (2). The stable operational phase is given Sustainability 2021, 13, x FOR PEER REVIEW 10 of 23

Remnants of the fuel blend caused the speed to 5 1 60 655 45,200 E0/E35 rise from 21k to 45k—discarded Total 26 The end of the testing day *Red rows indicate measurements which were problematic and not included in the analysis

3. Results To evaluate the effects of different fuel blends, it was necessary to perform an analy- sis of the engine’s operation separately during its start-up and during a stable operational run. Thus, situational frames for the three engine operational phases were proposed (Fig- ure 6.). The start-up phase is defined as Condition (2). The stable operational phase is given as Condition (3). The shutdown phase is initiated both when Condition (4) is met and Condition (2) is false: 𝑑𝑛(𝑡) 𝑑𝐸𝐺𝑇(𝑡) 𝑛(𝑡) 6000 𝑚𝑖𝑛 𝑨𝑵𝑫 50 𝑨𝑵𝑫 𝑑𝑡 𝑑𝑡 (2) 20 °𝐶𝑠 𝑨𝑵𝑫 𝑡2 𝑠 Sustainability 2021, 13, 2541 10 of 21 𝑑𝑛(𝑡) 𝑑𝐸𝐺𝑇(𝑡) ∈ −200; 200 𝑚𝑖𝑛𝑠 𝑨𝑵𝑫 ∈ −5; 5 𝑑𝑡 𝑑𝑡 (3) °𝐶𝑠 𝑨𝑵𝑫 𝑛(𝑡) 35 000 𝑚𝑖𝑛 as Condition (3). The𝑑𝐸𝐺𝑇 shutdown(𝑡) phase is initiated𝑑𝑛(𝑡) both when Condition (4) is met and −40 °𝐶𝑠𝑨𝑵𝑫 −2000 𝑚𝑖𝑛𝑠 (4) Condition (2) is false: 𝑑𝑡 𝑑𝑡 Thedn graph(t) in Figure 6 shows a representativedEGT(t) run of the engine using E0 fuel, with n(t) ≥ 6000 min−1 AND ≥ 50 min−1s−1 AND ≥ 20 ◦Cs−1 AND t ≥ 2 s (2) EGT peakingdt at around 800 °C, and the enginedt running at a constant speed of 45,000 min−1 without a speed controller; only the fuel flow is kept at a constant level. Fuel flow was dn(t) − − dEGT(t) ◦ − − ∈ h−200; 200selectedi min 1 tos be1 ANDconstant to run the∈ engineh−5; 5 ini theCs middle1 AND ofn its(t )power≥ 35 curve 000 min and the1 fuel flow (3) dt dt was fixed at flow of 1.1 L/min in all experiments during its stable operational phase. This means that the hydromechanical fuel pump was set to always maintain this fuel level dEGT(t) ◦ −1 dn(t) −1 −1 during the stable operation≤ of − the40 engineCs AND and environmental≤ −2000 conditions. min s Fuel flow is thus (4) dt dt an independent parameter.

FigureFigure 6. An example 6. An example of the division of the divisionof the engi ofne’s the operation engine’s into operation situational into frames. situational frames.

TheDuring graph stable in Figure operation6 shows of the a engine, representative the following run ofbasic the statistical engine usingindicators E0 fuel,were with EGTcomputed peaking for at aroundeach evaluated 800 ◦C, engine and the parameter: engine running at a constant speed of 45,000 min−1 without a speed controller; only the fuel flow is kept at a constant level. Fuel flow was selected to be constant to run the engine in the middle of its power curve and the fuel flow was fixed at flow of 1.1 L/min in all experiments during its stable operational phase. This means that the hydromechanical fuel pump was set to always maintain this fuel level during the stable operation of the engine and environmental conditions. Fuel flow is thus an independent parameter. During stable operation of the engine, the following basic statistical indicators were computed for each evaluated engine parameter:

1 m Mean x = ∑ xi (5) m i=1

1 m Variance s2 = (x − x)2 (6) − ∑ i m 1 i=1 ! 1 1 m 2 Standarddeviation s = (x − x)2 (7) − ∑ i m 1 i=1

1  1 m 2 2 m−1 ∑i=1(xi − x) Standarderrorofthemean sE = √ (8) m where m is the number of samples. For the start-up situational frame, the following additional indicators were computed: Sustainability 2021, 13, 2541 11 of 21

• Peak time (tP): the time for which the maximum value of variables (peak values) was measured. • Peak value: the measured value at the peak time. • Settling time (tS): the time required for the measured (response) curve to reach and stay within a range of ±2% of the steady state (final) value. The settling time is, in our case, defined by Condition (3). • Integral of value (A): In our case, the areas under the curves were computed using the midpoint rule of the numerical integral:

l A = ∆x ∑ f (xi) (9) i=1 where ∆x is the length of the subinterval (step size), l represents the number of samples, and xi is the midpoint of the ith subinterval. • Overshoot value (percentage overshoot PO):

Tp − Ts PO = . 100% (10) Ts

where Tp is the peak value, and Ts is the settled value. The basic statistical parameters presented in Tables6–9 were calculated as follows. Each parameter—mean (5), variance (6), standard deviation (7), and standard error (8)—was computed across three engine runs for the particular JET A-1/Ethanol fuel blend in a stable situational frame. The following sections analyse and present the results of the observed effects during the individual situational frames for all the main investigated parameters of the engine.

Table 6. Fuel flow during experiments.

Indicator Maximum FF Standard Mean(L/min) Variance Standard Error Fuel Blend (L/min) Deviation E0 1.166 1.130 0.0003278 0.0181 8.5337 × 10−4 E10 1.164 1.124 0.0002135 0.0146 6.8883 × 10−4 E20 1.145 1.116 0.0002354 0.0153 7.2324 × 10−4 E30 1.136 1.117 0.0000852 0.0092 4.3531 × 10−4 E35 1.145 1.126 0.0000477 0.0069 3.2588 × 10−4 E40 0.638 0.618 0.0001162 0.0107 6.2240 × 10−4

Table 7. The effect of ethanol/Jet A-1 fuel on stable speed.

Indicator Maximum Standard Mean (min−1) Variance Standard Error Fuel Blend Speed (min−1) Deviation E0 45556.7 45280.9 38054.0 195.0 9.1 E10 44873.2 44684.6 5963.2 77.2 3.6 E20 43780.4 43514.1 22511.4 150.0 7.0 E30 43299.8 42944.4 36454.7 190.9 9.0 E35 43008.0 42480.9 25084.5 158.3 7.4 E40 22446.6 20251.0 3737437.9 1933.2 111.6 Sustainability 2021, 13, 2541 12 of 21

Table 8. The effect of the alternative fuel on the stable exhaust gas temperature (EGT).

Indicator Maximum ◦ Standard Mean ( C) Variance Standard Error Fuel Blend EGT (◦C) Deviation E0 506.7 501.4 9.4 3.0 0.17 E10 501.5 494.9 15.5 3.9 0.22 E20 489.6 487.8 1.3 1.1 0.06 E30 477.7 474.4 2.2 1.4 0.08 E35 478.8 473.4 14.0 3.7 0.21 E40 510.4 487.9 69.6 8.3 0.54

Table 9. The effect of ethanol/Jet A-1 fuel on stable speed.

Indicator Maximum Standard Mean (atm) Variance Standard Error Fuel Blend Pressure (atm) Deviation E0 3.434 3.366 0.001761 0.041 0.0019 E10 3.280 3.243 0.000198 0.014 0.0006 E20 3.118 3.071 0.000766 0.027 0.0013 E30 3.052 3.007 0.000618 0.024 0.0011 E35 3.033 2.947 0.000676 0.026 0.0012

Sustainability 2021, E4013, x FOR PEER REVIEW 1.418 1.334 0.005004 0.070 0.004013 of 23

3.1. The Independent Variable—Fuel Flow Fuel flow is an independentTable 6.parameter Fuel flow during of the experiments engine. This. parameter was set to a constantIndicator level during stable operation of the engine, fixed at 1.1 L/min, and maintained hydro-mechanically. SomeMaximum smallvariations Meanin the fuel flow canStandard be observed; Standard however, Variance these variations are at theFF level (L/min) of statistical (L/min) errors, which means thatDeviation the fuel flow wasError well- Fuel Blend stabilised and did not have any effect on performance and state parameters of the engine. × -4 Statistical indicatorsE0 of variance,1.166 standard1.130 deviation 0.0003278 and standard error0.0181 have very8.5337 low and10 -4 similar values.E10 Because the1.164 engine failed1.124 to stabilise 0.0002135 at a standard idle0.0146 speed with6.8883 the×10 E40 fuel, fuelE20 flow with this fuel1.145 blend was1.116 considerably 0.0002354 lower and the0.0153 statistical 7.2324 indicator×10-4 show someE30 difference, this state1.136 with E401.117 fuelis analysed0.0000852 in the following0.0092 chapters.4.3531Table×10-64 and FigureE357 prove that the1.145 fuel flow was1.126 kept constant0.0000477 during all experiments0.0069 3.2588 with× very10-4 small variationsE40 during stable0.638 operation of0.618 the engine0.0001162 where the effects0.0107 offuel are6.2240 analysed.×10-4

Figure 7. Fuel flow duringFigure experimental7. Fuel flow during runs of experimental the engine. runs of the engine.

3.2. The Effect of Alternative Fuel on the Stable Speed of the Engine The effect of all defined JET A-1/Ethanol fuel blends on the engine’s stable speed was evaluated. The stable regime of operation was selected according to the rules defined in Condition (3). The results of various JET A-1/Ethanol fuel blends on stable engine speed are shown in Table 7. The computed means together with the error bars of the standard deviations are visualised in Figure 8. The main effect observable in the data is that the stable engine speed decreased with an increasing concentration of ethanol in the fuel blend. The variance and standard deviation remained stable, which shows that the engine was running in a stabilised mode and that the ethanol concentration did not affect this stability. Some positive effects can be seen with the E10 fuel blend, for which the variance and standard deviation are considerably lower than those for other concentrations. On the other hand, the E40 fuel blend concentration did not allow the engine to run successfully. The engine ran in a regime outside of the envelope regime for this type of engine with a speed much lower than standard idle speed 35,000 min−1. Here, the engine ran in an unstable mode, as can be seen from the high levels of variance and standard deviation. Figure 9 shows the effects of different fuel blends in a single run on speed. Thus, only one run is presented from the three executed runs at each given concentration.

Sustainability 2021, 13, 2541 13 of 21 Sustainability 2021, 13, x FOR PEER REVIEW 14 of 23

3.2. The Effect of AlternativeTable 7. The Fuel effect on of theethanol/Jet Stable SpeedA-1 fuel of on the stable Engine speed.

TheIndicator effect of all definedMaximum JET A-1/Ethanol fuel blends on the engine’s stable speed was Mean Standard Standard Er- evaluated. The stable regimeSpeed of operation wasVariance selected according to the rules defined in (min−1) Deviation ror ConditionFuel Blend (3). (min−1) The resultsE0 of various45556.7 JET A-1/Ethanol45280.9 fuel38054.0 blends on stable 195.0 engine speed 9.1 are shown Sustainability 2021, 13, x FOR PEER REVIEWin Table 7. The computed means together with the error bars of the standard14 of deviations 23 E10 44873.2 44684.6 5963.2 77.2 3.6 are visualisedE20 in Figure43780.48. The main43514.1 effect observable22511.4 in the data 150.0 is that the 7.0 stable engine speed decreasedE30 with an43299.8 increasing concentration42944.4 36454.7 of ethanol in 190.9 the fuel blend. 9.0 The variance and standardE35 deviationTable 7.43008.0 The remained effect of ethanol/Jet stable,42480.9 which A-1 fuel25084.5 shows on stable that speed. the158.3 engine was 7.4 running in a stabilisedE40 mode and that22446.6 the ethanol 20251.0 concentration 3737437.9 did not affect1933.2 this stability. 111.6 Indicator Maximum Mean Standard Standard Er- Speed Variance (min−1) Deviation ror Fuel Blend (min−1) E0 45556.7 45280.9 38054.0 195.0 9.1 E10 44873.2 44684.6 5963.2 77.2 3.6 E20 43780.4 43514.1 22511.4 150.0 7.0 E30 43299.8 42944.4 36454.7 190.9 9.0 E35 43008.0 42480.9 25084.5 158.3 7.4 E40 22446.6 20251.0 3737437.9 1933.2 111.6

FigureFigure 8.8. AverageAveragespeed speedat atdifferent different fuel fuel blend blend concentrations concentrations during during a a stable stable regime regime of of operation. operation. Some positive effects can be seen with the E10 fuel blend, for which the variance and standard deviation are considerably lower than those for other concentrations. On the other hand, the E40 fuel blend concentration did not allow the engine to run successfully. The engine ran in a regime outside of the envelope regime for this type of engine with a speed much lower than standard idle speed 35,000 min−1. Here, the engine ran in an unstable mode, as can be seen from the high levels of variance and standard deviation. Figure9 shows the effects of different fuel blends in a single run on speed. Thus, only one runFigure is presented8. Average fromspeed the at different three executed fuel blend runs concentrations at each given during concentration. a stable regime of operation.

Figure 9. Average speed at different fuel blend concentrations during a stable regime of operation.

FigureFigure 9. 9. AverageAverage speed speed at atdifferent different fuel fuel blend blend concentrat concentrationsions during during a stable a regime stable of regime operation. of operation.

Sustainability 2021, 13, x FOR PEER REVIEW 15 of 23

3.3. The Effect of Alternative Fuel on Stable Exhaust Gas Temperature of the Engine The results of various JET A-1/Ethanol fuel blends on the maximum EGT are shown in Table 8. The computed means together with error bars of the standard deviations are visualised in Figure 10, and a single run for each concentration is shown in Figure 11. Figure 11 shows the effects on speed of different fuel blends in a single run, which means that only one run is presented from three executed runs at each given concentration. The main effect observable in this situational frame is similar to the effect of Jet A-1/Ethanol on speed. It can again be seen that the average exhaust gas temperature decreased with an increase of the ethanol concentration in the fuel. This decrease, however, stopped at concentrations higher than 35%. At a concentration of 40%, an increase in the maximum and average temperature can be observed because, at this concentration, the engine ran outside of its operational envelope in an unstable manner. Moreover, there was an increase in the standard deviation and standard error at a concentration of 35% because a longer time was required for the engine to stabilise, as shown in Figure 11. It can be concluded that increasing the Sustainability 2021, 13, 2541 ethanol concentration in the fuel blend up to a concentration of 35% does not have14 ofan 21 adverse effect on the exhaust gas temperature.

Table 8. The effect of the alternative fuel on the stable exhaust gas temperature (EGT). 3.3. The Effect of Alternative Fuel on Stable Exhaust Gas Temperature of the Engine Indicator TheMaximum results ofvarious Mean JET A-1/Ethanol fuel blendsStandard on theStandard maximum EGT are shown Variance in Table8EGT. The (°C) computed (°C) means together with errorDeviation bars of the standardError deviations are Fuel Blend visualised in Figure 10, and a single run for each concentration is shown in Figure 11. E0 Figure 11 shows506.7 the effects501.4 on speed of9.4 different fuel3.0 blends in a single0.17 run, which means E10 that only one501.5 run is presented494.9 from three15.5 executed runs3.9 at each given0.22 concentration. The E20 main effect489.6 observable in487.8 this situational1.3 frame is similar1.1 to the effect0.06 of Jet A-1/Ethanol E30 on speed. It477.7 can again be474 seen.4 that the2.2 average exhaust1.4 gas temperature0.08 decreased with E35 an increase478.8 of the ethanol473.4 concentration 14.0 in the fuel.3.7 This decrease,0.21 however, stopped at E40 concentrations510.4 higher than487.9 35%. 69.6 8.3 0.54

Sustainability 2021, 13, x FOR PEER REVIEW 16 of 23

FigureFigure 10. 10. AverageAverage EGT EGT at at different fuel blend concentrations duringduring thethe stablestable regimeregimeof ofoperation. operation.

Figure 11. EGT during single engine runs at different ethanol concentrations. Figure 11. EGT during single engine runs at different ethanol concentrations.

3.4. TheAt Effect a concentration of Alternative of 40%, Fuel anon increasethe Stable in Compressor the maximum Pressure and average of the Engine temperature can beThe observed total pressure because, measured at this concentration,behind the compressor the engine shows ran outsidesimilar ofbehaviour its operational to the speedenvelope of the in engine an unstable because manner. both parameters Moreover, there are connected was an increase under inconstant the standard environmental deviation (laboratory)and standard conditions. error at a concentration The total pressure of 35% decreased because a with longer an time increase was required of the ethanol for the concentration in the fuel; this can be observed in Table 9 and is graphically illustrated in Figure 12. The variance, standard deviation, and standard error of the mean are low, which means that the effects are valid. A decrease in the total pressure during operation of the engine can be also seen in the time plot shown in Figure 13 for single concentrations. For E40 concentrations, again, the engine clearly ran in an unstable regime of operation.

Table 9. The effect of ethanol/Jet A-1 fuel on stable speed.

Indicator Maximum Mean Standard Standard Pressure Variance (atm) Deviation Error Fuel Blend (atm) E0 3.434 3.366 0.001761 0.041 0.0019 E10 3.280 3.243 0.000198 0.014 0.0006 E20 3.118 3.071 0.000766 0.027 0.0013 E30 3.052 3.007 0.000618 0.024 0.0011 E35 3.033 2.947 0.000676 0.026 0.0012 E40 1.418 1.334 0.005004 0.070 0.0040 Sustainability 2021, 13, 2541 15 of 21

engine to stabilise, as shown in Figure 11. It can be concluded that increasing the ethanol concentration in the fuel blend up to a concentration of 35% does not have an adverse effect on the exhaust gas temperature.

3.4. The Effect of Alternative Fuel on the Stable Compressor Pressure of the Engine The total pressure measured behind the compressor shows similar behaviour to the speed of the engine because both parameters are connected under constant environmental (laboratory) conditions. The total pressure decreased with an increase of the ethanol concentration in the fuel; this can be observed in Table9 and is graphically illustrated in Figure 12. The variance, standard deviation, and standard error of the mean are low, which means that the effects are valid. A decrease in the total pressure during operation of the Sustainability 2021, 13, x FOR PEER REVIEW 17 of 23 Sustainability 2021, 13, x FOR PEER REVIEWengine can be also seen in the time plot shown in Figure 13 for single concentrations.17 of For 23

E40 concentrations, again, the engine clearly ran in an unstable regime of operation.

Figure 12. 12. AverageAverage compressor compressor pressure pressure at at different different fuel fuel blend blend concentrations concentrations during during the thestable stable Figure 12. Average compressor pressure at different fuel blend concentrations during the stable regime of operation. regimeregime ofof operation.operation.

Figure 13. Compressor pressure during single engine runs at different ethanol concentrations. FigureFigure 13. 13.Compressor Compressor pressure pressure during during single single engine engine runs runs at differentat different ethanol ethanol concentrations. concentrations. 3.5. The Effect of Alternative Fuel on The Start-Up of the Engine 3.5. The Effect of Alternative Fuel on The Start-Up of the Engine The start-up phase of any plays a critical role in its technical life and The start-up phase of any jet engine plays a critical role in its technical life and performance. This phase places a high on the components of the engine. This strain performance. This phase places a high strain on the components of the engine. This strain is defined mainly by the temperature and rotational speed. The effects of alternative fuel is defined mainly by the temperature and rotational speed. The effects of alternative fuel on these parameters were, therefore, investigated. The temperature strain on a jet engine on these parameters were, therefore, investigated. The temperature strain on a jet engine during its start-up is mostly characterised by the engine’s peak temperature, which is during its start-up is mostly characterised by the engine’s peak temperature, which is obtained during start-up (Peak EGT). It is also important to evaluate how much total heat obtained during start-up (Peak EGT). It is also important to evaluate how much total heat the components of the engine absorb during this phase. This can be done by computing the components of the engine absorb during this phase. This can be done by computing the integral value of the EGT during the phase according to Equation (9). The other the integral value of the EGT during the phase according to Equation (9). The other parameters tied to the absorbed heat are the times at which the temperature reaches its parameters tied to the absorbed heat are the times at which the temperature reaches its maximum and the settling time. maximum and the settling time. The settling time defines how long it takes the temperature to stabilise. The last The settling time defines how long it takes the temperature to stabilise. The last parameter tied to the peak value of EGT is the overshoot value, which defines how far the parameter tied to the peak value of EGT is the overshoot value, which defines how far the peak was from the stabilised temperature value. In terms of temperature strain, these peak was from the stabilised temperature value. In terms of temperature strain, these Sustainability 2021, 13, 2541 16 of 21

3.5. The Effect of Alternative Fuel on the Start-Up of the Engine The start-up phase of any jet engine plays a critical role in its technical life and performance. This phase places a high strain on the components of the engine. This strain is defined mainly by the temperature and rotational speed. The effects of alternative fuel on these parameters were, therefore, investigated. The temperature strain on a jet engine during its start-up is mostly characterised by the engine’s peak temperature, which is obtained during start-up (Peak EGT). It is also important to evaluate how much total heat the components of the engine absorb during this phase. This can be done by computing the integral value of the EGT during the phase according to Equation (9). The other parameters tied to the absorbed heat are the times at which the temperature reaches its maximum and the settling time. The settling time defines how long it takes the temperature to stabilise. The last parameter tied to the peak value of EGT is the overshoot value, which defines how far the peak was from the stabilised temperature value. In terms of temperature strain, these parameters can give a good picture of thermal strain during start-up. The results from three runs for each fuel blend concentration are shown in Table 10. The mean value from all three runs was also computed, and the results for the integral and peak values of EGT are visualised in Figures 14 and 15. With an increase of the ethanol concentration in the fuel blend, the peak value of EGT decreased, as seen in Figure 15. This decrease was around 130 ◦C, which is a significant value. This can be evaluated as a positive effect of ethanol on the operation of a small turbojet engine. On the other hand, the integral value of EGT increased during start-up, as shown in Figure 14. This means that the engine experienced high temperatures during start-up for a longer time. The positive effect of lowering the temperature peak was, therefore, somewhat negated by the higher integral value and total heat energy absorbed by the engine during start-up.

Table 10. Exhaust gas temperature during the start-up phase.

Parameter No. of Integral Value Peak EGT Mean EGT Peak Time Settling Overshoot Fuel Blend Measurement (◦C.sec) (◦C) (◦C) (sec) Time (sec) Value (%) 1. 7074.2 790.4 504.2 2.70 10.09 56.7 E0 2. 7455.0 813.0 500.9 2.90 10.26 62.3 3. 6133.6 769.8 498.3 3 9.07 54.4 Mean 6887.6 791.1 501.1 2.86 9.81 57.8 1. 7198.5 677.7 492.6 2.30 10.69 37.5 E10 2. 7529.3 741.5 490.7 2.70 10.87 51.1 3. 7159.0 731.5 499.7 2.40 10.06 46.3 Mean 7295.6 716.9 494.3 2.46 10.54 45.0 1. 7662.4 710.9 487.9 2.50 11.16 45.6 E20 2. 7559.2 691.8 486.0 2.50 11.72 42.3 3. 8145.7 718.0 488.0 2.80 11.71 47.1 Mean 7789.1 706.9 487.3 2.6 11.53 45.0 1. 7900.9 688.0 477.1 2.50 11.91 44.2 E30 2. 8163.8 687.6 473.8 2.50 12.67 45.1 3. 7964.6 681.8 473.7 2.60 12.72 43.9 Mean 8009.7 685.8 474.8 2.53 12.43 44.4 1. 7905.6 674.7 478.3 2.60 12.66 41.0 E35 2. 9503.8 660.9 472.7 2.40 15.91 39.8 3. 9974.9 661.2 469.3 2.40 16.53 40.8 Mean 9128.1 665.6 473.4 2.46 15.03 40.5 1. 3753.5 610.6 496.9 2.20 5.79 22.8 E40 2. 5306.4 614.1 480.3 2.80 8.52 27.8 3. 4645.2 616.8 486.5 2.50 7.52 26.7 Mean 4568.3 613.8 487.9 2.5 7.28 25.8 Sustainability 2021, 13, 2541 17 of 21 Sustainability 2021, 13, x FOR PEER REVIEW 19 of 23 Sustainability 2021, 13, x FOR PEER REVIEW 19 of 23

Figure 14. Integral value of the exhaust gas temperature during start-up. FigureFigure 14. 14. IntegralIntegral value value of ofthe the exhaust exhaust gas gas temperature temperature during during start start-up.-up. Peak EGT (°C) EGT Peak

Figure 15. Peak value of the exhaust gas temperature during start-up. Figure 15. PeakFigure value 15. of Peak the exhaustvalue of gasthe exhaust temperature gas temperature during start-up. during start-up. The effects of ethanol in jet fuel on performance were investigated by analysing the TheThe eff effectsects of of ethanol ethanol in injet jet fuel fuel on on performance performance were were investigated investigated by analysing by analysing the the rotational speed of the engine during the start-up phase. One of the most important pa- rotationalrotational speed speed of of the the engine engine during during the the start start-up-up phase. phase. One One of the of themost most important important rameters in jet engines is their starting time. This settling time is shown in Figure 16. When parameters in jet engines is their starting time. This settling time is shown in Figure 16. parametersusing clean inJET jet A-1 engines fuel, the is theirengine starting was able time. to Thisstart settlingin 6 s. By time increasing is shown the in ethanol Figure 16. When using clean JET A-1 fuel, the engine was able to start in 6 s. By increasing the ethanol Whenconcentration using clean in the JET fuel A-1 blend, fuel, the starting enginewas time able increased to start considerably, in 6 s. By increasing being the the worst ethanol concentration in the fuel blend, the starting time increased considerably, being the worst concentrationat a concentration in the of 35%. fuel blend,With the the concentration starting time of increased ethanol at considerably, the level of 40%, being the thestarting worst at at a concentration of 35%. With the concentration of ethanol at the level of 40%, the starting atime concentration was shorter. of However, 35%. With the the engine’s concentration speed reached of ethanol only at the the sub-idle level of level. 40%, It the can starting be time was shorter. However, the engine’s speed reached only the sub-idle level. It can be timeconcluded was shorter. that an However,increase in thethe engine’sethanol co speedncentration reached has only a detrimental the sub-idle impact level. on It en- can be concluded that an increase in the ethanol concentration has a detrimental impact on concludedgine starting that times. an increase in the ethanol concentration has a detrimental impact on engine enginestarting starting times. times. Sustainability 2021, 13, 2541 18 of 21 Sustainability 2021, 13, x FOR PEER REVIEW 20 of 23

FigureFigure 16. 16.Starting Starting time time of of a a small small turbojetturbojet engine engine with with different different ethanol ethanol concentrations. concentrations.

InIn general, general, it can it can be concluded be concluded that the that effects the effects of ethanol of ethanol in JET A in-1 JETfuel A-1are generally fuel are generally adverseadverse during during the the start start-up-up phase phase of the of engine. the engine. Ethanol Ethanol prolongs prolongs the initiali thesation initialisation of the of the engine up to the point that the speed of the engine hangs below its idle speed. The positive engine up to the point that the speed of the engine hangs below its idle speed. The positive effect of ethanol is its ability to decrease the peak temperature during start-up, which couldeffect be of beneficial ethanol isfor its the ability materials to decrease of turbine the blades. peak temperature during start-up, which could be beneficial for the materials of turbine blades. 4. Discussion 4. Discussion The present study explored some effects of the application of JET A-1/ethanol blends in smallThe turbojet present engines. study These explored fuel blends some effectsprovide of less the gross application heat energy of JET than A-1/ethanol clean JET blends Ain-1 smallfuel, so turbojet some performance engines. These drops fuel were blends expected. provide These lessresults gross are heatvalid energy for a class than of clean JET singleA-1 fuel, strea som turbojetsome performance engines, but drops our conclusion were expected. will likely These also results be valid are for valid larger for a class turbojetof single engines. stream turbojet engines, but our conclusion will likely also be valid for larger turbojetWe found engines. that this type of engine with a hydromechanical fuel control system is not able runWe with found concentrations that this type of ethanol of engine higher with than a 40% hydromechanical in the fuel blend. fuel All controlother effects system is not wereable analysed run with in concentrations two basic situational of ethanol frames: higher the start than-up 40%and s intable the regime fuel blend. of operation. All other effects Itwere was found analysed that induring two basicthe stable situational operation frames: of the engine, the start-up the ethanol and stablein the fuel regime caused of operation. a drop in the stable operating speed of the engine, a drop in the exhaust gas temperature, It was found that during the stable operation of the engine, the ethanol in the fuel caused a and, consequently, a decrease in the pressure behind the compressor. With a concentrationdrop in the of stable 35% percent operating of ethanol speed in of the the fuel engine, blend, athe drop speed in dropped the exhaust by 7500 gas min temperature,−1 atand, a constant consequently, fuel flow. a This decrease means in that the ethanol, pressure as ex behindpected, the has compressor. a negative impact With on a concentration the −1 performanceof 35% percent of a turbojet of ethanol engine in the but fuel that blend, the engine the speedis still able dropped to run by in 7500a stable min mannerat a constant atfuel such flow. a concentration. This means that ethanol, as expected, has a negative impact on the performance of aThe turbojet effects engineof ethanol but on that the thestart engine-up of the is stillengine able were to runalso innegative, a stable causing manner the at such a startconcentration.-up to last longer and increasing the total temperature load on the components of the engine.The The effects positive of effect ethanol of ethanol on the was start-up that the of engine the engine peaked were at a alsolower negative, exhaust gas causing the temperaturestart-up to during last longer start- andup, which increasing could thehave total a positive temperature effect on load the lifetime on the durability components of the oengine.f the turbine The blade positives. effect of ethanol was that the engine peaked at a lower exhaust gas temperatureMost of these during effects start-up, could whichlikely be could offset have via the a positive implementation effect on of the digital lifetime fuel durability control systems and speed control algorithms. In this way, the thrust output of the engine of the turbine blades. could be increased and maintained at a desired level, albeit at the cost of increased fuel consumption.Most of The these results effects show could that likely ethanol be is offset a viable via additive the implementation to JET A-1 fuel; of however, digital fuel control thissystems addition and decreasesspeed control the algorithms.performance In of this the way, turbojet the thrust engine output and increases of the engine fuel could be consumption,increased and if the maintained performance at a remains desired a level, constant albeit level. at the cost of increased fuel consumption. The results show that ethanol is a viable additive to JET A-1 fuel; however, this addition decreases the performance of the turbojet engine and increases fuel consumption, if the performance remains a constant level. Given this result, the hypothesis for emissions of the engine is not clear and should be explored in follow-up experiments for the concentrations of ethanol in JET A-1 investigated in this study for this class of engines. Ethanol as fuel additive might also negatively impact seals in fuel and lubrication systems of turbojet engines [44], which is another area that Sustainability 2021, 13, 2541 19 of 21

needs to be further tested specifically for every engine where JET A-1 fuel with ethanol is applied. On the other hand, some additives might solve these problems like investigated in [45]. We can thus conclude that it is operationally possible to use ethanol JET A-1 fuel blend with ethanol concentrations up to 35% for small single stream turbojet engines with radial compressors, but the effects on emissions, seals, and other components need to be researched further in long term testing.

Author Contributions: Conceptualization, R.A., L.F., M.S., and S.S.; data curation, R.A. and L.F.; formal analysis, R.A., L.F., M.S., and S.S.; funding acquisition, R.A., L.F., and S.S.; investigation, R.A., L.F., and M.S.; methodology, R.A., L.F., and M.S.; supervision, R.A. and S.S.; validation, R.A., L.F., and M.S.; visualization, L.F. and M.S.; writing—original draft, R.A., L.F., and M.S.; writing—review and editing, R.A., L.F., M.S., and S.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by KEGA agency, grant number KEGA 044TUKE-4/2019—a small, unmanned airplane—the platform for in the area of intelligent avionics. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: This work was supported by project no. 019/2019/1.1.3/OPVaI/DP with ITMS code 313011T557 under the title Support of the Research and Development Potential in the Area of Transportation Vehicles. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

2-EH 2-Ethylhexanol AVGAS Aviation Gas CO Carbon Monoxide CO2 E Ethanol EGT Exhaust Gas Temperature FF Fuel Flow GHG Greenhouse Gas H2O Water HC Hydrocarbon HEFA Hydro-processed Esters and Fatty Acids HHV Higher Heating Value JET A Jet Fuel MPM-20 Small Turbojet Engine n Rotational Speed of the Engine N2 Nitrogen NI National Instruments NOx Nitrogen Oxide p2 Total Compressor Pressure PL Phase Level PM Particulate Matter PME Palm Methyl Ester PR Phase Range SPK Synthesized Paraffinic Kerosene Sustainability 2021, 13, 2541 20 of 21

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