Comparative Analysis of Aeroshell 500 Oil Effects on Jet A and Diesel-Powered Aviation Microturbines

Abstract

This study aims to analyze the influence of adding Aeroshell 500 oil on physicochemical properties. It was found that the oil’s kinematic viscosity is much higher than that of diesel and Jet A, with a higher density and flash point as well. Elemental analysis revealed a higher carbon content and lower hydrogen content in Aeroshell oil compared to Jet A and diesel, with lower calorific power. Adding 5% oil increases the mixture viscosity, flash point, and density; decreases the calorific power; and increases the carbon content for both diesel and Jet A. In the second part, mathematical models determined the combustion temperatures for Jet A, diesel, Jet A plus 5% Aeroshell 500 oil, and diesel plus 5% Aeroshell 500 oil, based on an air excess from one to five. Elemental analysis determined the oxygen and air quantities for these mixtures and stoichiometric combustion reaction for CO₂ and H₂O. Regarding the CO₂ quantity, adding 5% Aeroshell 500 to Jet A increases it from 3.143 kg to 3.159 kg for each kilogram of mixture burned in the stoichiometric reaction. Similarly, adding the oil to diesel in a 5% proportion increases the CO₂ quantity from 3.175 to 3.190 in the stoichiometric reaction. Through experimentation with the Jet Cat P80 microturbine engine across four operating regimes, it was observed that the combustion chamber temperature and fuel flow rate are lower when using diesel with a 5% addition of Aeroshell 500 oil compared to Jet A with the same additive. However, the thrust is slightly higher with diesel + 5% Aeroshell 500 oil. Moreover, the specific fuel consumption is higher in regimes one and two for diesel + 5% Aeroshell 500 oil compared to Jet A + 5% Aeroshell 500 oil, while the differences are negligible in regimes three and four. At maximum operating conditions, the excess air was determined from the measured values, comparing the combustion chamber temperature with the calculated value, with a 7% error, extrapolating the results for the scenario when oil is not used. Also, during the testing campaign, the concentrations of CO and SO₂ in the exhaust gas jet were measured, with higher concentrations of CO and SO₂ observed for diesel compared to Jet A.

1. Introduction

The lubrication issue of aviation turboengines is crucial for their proper functioning. Thus, for large turboengines, they are equipped with a lubrication system [1]. In the case of microturbojet engines, the lubrication system is absent, and lubrication is performed with Jet A to which aviation oil is added. Typically, this quantity is a few percent, but by adding it, the engine’s performance will decrease because the calorific power of aviation oils is lower than that of Jet A, and at the same time, pollution will increase due to the addition of the oil. The bearings of microturbojet engines are lubricated with a fuel–oil mixture. It is an open lubricating system, but in cases where engine weight reduction is necessary, it is the only feasible solution. Most commonly, a two-stroke motorcycle or outboard oil is used, but some manufacturers prefer turbine oil. The oil–fuel ratio depends on the specific design of the engine and the type of fuel used (Jet A or diesel) and may range from 1.5% to 5.0%. Generally, a higher amount of lubricating oil is added to Jet A-based lighter fuel [2].

There are a few companies actively involved in the development of micromotors, serving various purposes such as research, equipping UAVs, electricity generation, and others. These companies include AMT Netherlands [3], JetCat [4], JetCentral [5], Frank Turbine [6], Toyota Turbine and Systems [7], MTT Microturbine [8], UAV Turbines [9], Blandon Jets [10], Brayton Energy [11], ICR Turbine Engine Corporation [12], TurboTech Energy [13], PBS Aerospace [14], KingTech Turbines [15], and many others. All these manufacturers of aviation microturbojet engines recommend the use of Jet A or diesel as fuel with an addition of 4–5% oil. Each manufacturer specifies a certain type of oil for lubrication. All these types of microturbojet engines do not have a lubrication system, similar to that of large-sized turboengines.

The lubrication issue is also very important for internal combustion engines that use various fuels, and efforts are being made to improve the lubrication properties to achieve better performance and exhaust emissions by adding additives [16] or through biolubrication [17]. The lubrication problem is also very important in the turbocharger test rig setup for low speeds of a centrifugal compressor for an automotive engine and aviation [18]. Another study analyzed and monitored the oil parameters of a turbocharger for internal combustion engines [19].

Recent research has evaluated the physical–chemical properties of aviation fuel blends with biodiesel and alcohols. For example, authors in [20] examined the density, viscosity, and thermal stability of these mixtures. These properties are essential for ensuring compatibility with existing aviation infrastructure and for safe engine operations. The chemical composition influences properties such as density, viscosity, and combustion characteristics, thereby influencing engine performance.

Many studies regarding the assessment of the combustion characteristics of alternative fuels and blends with Jet A have been published. Parameters such as ignition delay, flame stability, and emission profiles during combustion were investigated in [21]. Understanding combustion behavior is crucial for optimizing engine performance and reducing emissions. Additionally, the environmental impact of aviation fuel mixtures involving biodiesel and alcohols has been thoroughly studied. The emissions of particulate matter, nitrogen oxides (NOx), and carbon dioxide (CO₂) have been quantified in [22]. These assessments promote our understanding of the prospective reductions in greenhouse gas emissions and particulate matter, which are critical for air quality and climate change mitigation.

There are several studies and publications that focus on aviation microengines that do not have an oil system, like the engines on civilian or military aircraft. Many of these studies investigate the influence of various biofuels or alternative fuels with the addition of various percentages of oil for bearing lubrication. Thus, the following articles study the influence of various types of blends of alternative fuels and Jet A on microengine performance and emissions [23,24]. In other studies, various types of biodiesel have been studied, from pork fat to recycled sunflower oil, microalgae, etc., [25,26]. All these studies involved the addition of 5% Aeroshell 500 oil.

Some studies have attempted blends of diesel and Jet A or pure diesel in microengines [27], emphasizing a comparative performance.

Another study analyzes the transient processes in microturbojet engines when they operate with diesel [28].

It is known that, compared to Jet A, diesel has some disadvantages, among which one of the most significant is its much higher freezing point than that of Jet A [29,30].

Since no studies have been identified regarding the influence of added oil in fuels for microturbine engines, namely Jet A or diesel, the novelty of this work lies in a theoretical and experimental analysis of the influence of Aeroshell 500 oil on Jet A and diesel as presented below.

This paper analyzes the recommended fuels for engines in the Jet Cat range that have Aeroshell 500 oil added to Jet A or diesel. This paper aims to analyze, in the first part, the influence of adding Aeroshell 500 oil on properties such as flash point, kinematic viscosity, density, calorific power, elemental analysis, and FTIR analysis for Jet A, diesel, and Aeroshell 500 oil.

In the second part, based on mathematical models, the combustion temperatures are determined for Jet A, diesel, Jet A plus 5% Aeroshell 500 oil, and diesel plus 5% Aeroshell 500 oil, depending on the excess air from one to five. Based on the elemental analysis, the amount of oxygen and air for the above mixtures will be determined, and for the stoichiometric combustion reaction, the amount of CO₂ and H₂O for the four mentioned fuels will be determined.

In the third part of this paper, some results obtained with the Jet Cat P80 microengine operating with Jet A + 5% Aeroshell 500 and then with diesel + 5% Aeroshell 500 are presented because, for safety reasons, the engine cannot be started without adding oil. Tractive effort, fuel flow, and combustion chamber temperatures for the two fuels at the top operating condition are presented comparatively, where the excess air values are determined from the measured values, comparing the combustion chamber temperature value with the calculated value by extrapolating the results for the case when oil is not used. Also, during the testing campaign, the concentrations of CO and SO₂ in the exhaust gas jet for four engine operating conditions were measured, with a higher concentration of SO₂ for diesel compared to Jet A being observed.

2. Materials and Methods

To assess the influence of Aeroshell 500 oil on the properties of the fuels used in aviation microturbines, namely Jet A and diesel, a series of analyses were conducted. The properties of Jet A, diesel, Aeroshell 500 oil, Jet A + 5% Aeroshell 500, and diesel + 5% Aeroshell 500 were analyzed. Additionally, functional tests were performed by feeding a microturboengine with the aforementioned fuels, Jet A + 5% Aeroshell 500, and diesel + 5% Aeroshell 500.

2.1. Characterization

The characterization of the fuels was performed experimentally by measuring various parameters such as density, flash point, kinematic viscosity, low calorific value, elemental analysis, and FTIR analysis.

These measurements were conducted for Jet A, diesel, Aeroshell 500 oil, Jet A + 5% Aeroshell 500, and diesel + 5% Aeroshell 500.

Density was measured for all 5 samples at a temperature of 22 degrees Celsius, following SR EN ISO 3675/2002 standards [31], using a graduated cylinder and a thermo densimeter manufactured by Termodensirom SA, Bucharest, Romania. The flash point was determined for all 5 samples, following ASTM D92 standards [32], using an automatic flash point tester Cleveland provided by Scavini, Italy. The kinematic viscosity was measured for all 5 samples at 40 degrees Celsius, following SR EN ISO 3104/2002 standards [33], using a semi-automatic viscometer provided by Scavini, Italy. The low calorific value was determined for all 5 samples, in accordance with ASTM D240-17 standards [34], using an IKA WERKE C 2000 Calorimeter provided by Cole-Parmer, St. Netos, United Kingdom, and the C 5012 calorimeter bomb produced by IKA Analisentechnik GmbH, Staufen, Germany. FTIR analysis (Fourier Transform Infrared Spectroscopy) was performed for all 5 samples using a Spectrum Oil Express Series 100, v 3.0 spectrometer provided by Perkin Elmer—Romanian representative, Tancabesti, Romania, and dedicated software.

Elemental analysis was conducted for all 5 samples to determine the main elements (C, N, H, and O) in accordance with ASTM D 5291-16 standards [35].

For further information on the fuel characterization process, please refer to [25].

2.2. Theoretical Calculation of the Combustion Process

After determining the elemental composition of the fuel blends, the corresponding minimum air quantities required for stoichiometric combustion were calculated for Jet A, diesel, Jet A + 5% Aeroshell 500, and diesel + 5% Aeroshell 500, and the resulting amounts of CO₂ and H₂O from stoichiometric combustion were also calculated.

To understand the stoichiometric combustion characteristics of the various fuel blends, knowledge of their elemental composition is essential. This study considers hydrocarbons with the general formula C_cH_hO_oN_n [26], with specific fractions of $gC, gH, gO$ and $gN$.

The calculation of the required amount of oxygen for stoichiometric combustion is crucial in providing insights into the combustion process and facilitating a comprehensive understanding of the chemical reactions involved and can be calculated using Equation (1):

$$M_o={\displaystyle \frac{32}{12}}gC+{\displaystyle \frac{32}{4}}gH-{\displaystyle \frac{32}{32}}gO=2.667gC+8gH-gO$$

(1)

$$M_{air}=4.35M_o$$

(2)

$C{O}_{2 }$and $H_{2}O$from the combustion process can be calculated by using the following:

$$C{O}_2=44{\displaystyle \frac{gC}{12}}$$

(3)

$$H_{2}O=9gH$$

(4)

In order to determine the combustion temperature of air with Jet A, diesel, and for Jet A + 5% Aeroshell 500 oil and diesel + 5% Aeroshell 500 oil, a mathematical model was used to determine the adiabatic combustion temperature [36].

The adiabatic flame temperature resulting from the application of this algorithm represents the maximum temperature that the reaction products can reach. Hence, it is desired to build a system that prevents the transfer of heat to the external environment. Considering this aspect, the following thermodynamic scheme has been constructed.

The adiabatic combustion temperature can be determined according to the following scheme in Figure 1:

In Figure 1, two fictive heat exchangers, SC1 and SC2, and the combustion chamber CC are illustrated, where the actual combustion reaction takes place. Heat exchanger SC1 is responsible for extracting preheating heat from the air used for combustion, heat obtained during the compression of the air. Thus, the reactants enter the combustion chamber at the same temperature and pressure. The preheating heat is defined by the following expression, Equation (5):

$$Q_{PI}=m_{aer}\cdot {{\displaystyle \int }}_{T_0}^{T_{PI}}\left(x_{O2}\cdot c_{p,O2}+x_{N2}\cdot c_{p,N2}\right)dT\left[{\displaystyle \frac{kJ}{kg cb}}\right],$$

(5)

where $m_{aer}$ represents the mass of air participating in the combustion reaction, which directly depends on the air–fuel ratio coefficient, λ; $x_{O2}$ and $x_{N2}$are the volumetric fractions of $O_2$ and $N_2$ in air, respectively; and $c_{p,O2}$ and $c_{p,N2}$ represent the specific heats at constant pressure of the air components. The heat $Q_0$ released by the combustion of the fuel is extracted, and the reaction products leave the combustion chamber at the reference temperature and pressure using Equation (6):

$$Q_0=x_{C_{12}{H}_{23}}\cdot Q_{C_{12}{H}_{23}}+x_{oil}\cdot Q_{oil}\left[{\displaystyle \frac{kJ}{kg cb}}\right]$$

(6)

where $x_{C12H23}$ and $x_{oil}$ are the volumetric fractions of the fuel and oil used, respectively, and $Q_{C12H23}$ and $Q_{oil}$represent the lower heating values of the reactants.

Finally, through heat exchanger SC2, both the preheating heat $Q_{PI}$ and the reaction heat $Q_0$are returned to the combustion gases. Thus, the temperature of the combustion reaction products is determined from the heat balance of heat exchanger SC2, Equation (7):

$$Q_0{+\mathrm{Q}}_{PI}={\displaystyle \sum _i\left(m_i^{ga}{\displaystyle \underset{T_0}{\overset{T_{ga}}{\int }}{c}_{p,i}^{ga} dT}\right)}\left[{\displaystyle \frac{kJ}{kg cb}}\right]$$

(7)

where $m_i^{ga}$ represents the mass of compound $i$resulting from the combustion process, and $c_{p,i}^{ga}$ represents the specific heat at constant pressure of species $i$. The coefficients of the polynomials describing the variation in specific heats with temperature are provided in [37].

The adiabatic flame temperature is determined by solving the transcendental Equation (7) using numerical methods, such as the Newton–Raphson method. This solving method was chosen because the algorithm requires a small number of iterations and is relatively easy to implement.

The algorithm for determining the adiabatic flame temperature presented in this article addresses the combustion phenomenon in the absence of dissociation. Therefore, due to the fact that energy is not consumed for the formation of multiple product species, the temperature obtained by applying this algorithm will be higher compared to the real one.

2.3. Engine Experimental Procedure

In order to perform a comparative analysis of the experimentally obtained parameters, an experimental campaign was conducted on a Jet Cat class engine, specifically the Jet Cat P80, as shown in Figure 2a. This type of microturbine engine operates on both Jet A and diesel, but the addition of 5% Aeroshell 500 oil is mandatory for lubrication. During the testing campaign, experiments were conducted using Jet A and Diesel + 5% Aeroshell 500. No experiments were conducted with the mentioned fuels without adding Aeroshell 500 oil.

The experiments were conducted for 4 operating regimes of the microengine, namely, idle regime when the throttle gas is at 18.7%, 30%, 60%, and 92%. The value of 92% was chosen for the maximum regime for safety reasons and in accordance with the manufacturer’s recommendations. For each operating regime, the engine was run for approximately 2 min, and experimental values were recorded for both Jet A + 5% Aeroshell 500 and diesel + 5% Aeroshell 500. The temperature, humidity, and ambient pressure were recorded using an Extech instrument, as depicted in Figure 2b. The throttle was set to the aforementioned percentages, and the parameters were recorded for two minutes, averaging over the entire two-minute period to obtain the most accurate values. This two-minute period was specifically chosen to accommodate the gas analyzer, which requires this duration to accurately record the data.

The engine instrumentation allows for the recording of multiple parameters such as traction (F), combustion chamber temperature (Tcomb), fuel flow rate (Qf), air flow rate (Qa), temperature after the compressor (Tcomp), pressure after the compressor, and speed. To obtain more accurate values for the two-minute period, the engine was kept stable for the same percentage of throttle gas, and the recorded values were averaged. The engine speed, controlled by automation and its governing law, was kept constant for the same throttle gas percentage.

For the measurement of the parameters, standard instruments were employed. Type K thermocouples were utilized for temperature measurement, and a root-extracting static pressure sensor was employed for measuring the nozzle pressure at the air inlet, using the Pressure-Converter UNICON-P produced by GHM GROUP—Martens, Germany. A root-extracting pressure sensor from Huba Control, Switzerland, was used for the pressure measurements within the combustion chamber. A force transducer, model KM701 K 200 N 000 Z with 2 mV/N, produced by MEGATRON Elektronik GmbH & Co. KG, Germany, was used to measure the thrust. Speed measurements were conducted using a tachometer, and fuel flow measurements were performed using an actuator from the fuel pump.

Furthermore, to record some of the gas emissions, an MRU Vario Plus analyzer, as shown in Figure 2c,d, was used. The analyzer has the following parts: gas temperature measurement up to 2012 µF (use stainless steel up to 1200 µF and use Inconel tubes up to 2012 µF), integrated gas cooler and automatic condensate draining pump/poly tetra-fluor-ethylene filter, air purging pump for CO-sensor protection, built-in speed printer with easy paper loading, and SD card support for recorded values at 1 s.

It was placed in the gas stream for the four studied regimes and for Jet A + 5% Aeroshell 500 oil and diesel + 5% Aeroshell 500 oil, and the values for the concentrations of CO and SO₂ were recorded. The measuring range for sulfur dioxide (SO₂) was 0–2000 ppm with an accuracy of ±10 ppm or 5% of the reading for concentrations below 2000 ppm and 10% of the reading for concentrations above 2000 ppm. For carbon monoxide (CO), the measuring range was 0–4000 ppm with an accuracy of ±10 ppm or 5% of the reading for concentrations below 4000 ppm and 10% of the reading for concentrations above 4000 ppm.

It is very important to know the environmental parameters (Figure 2b), such as temperature, pressure, etc., as these have a significant impact on the performance of turboengines in general. Lower air temperature results in higher air density and, consequently, a higher airflow rate entering the compressor, contributing to the thrust force. Additionally, the inlet temperature to the turboengine dictates the efficiency of the compressor. More details about the use of microturboengines under different environmental conditions can be found in [28].

3. Results and Discussion

3.1. Experimental Results for the Physicochemical Properties of the Analyzed Fuels

Table 1. The experimentally determined values of the parameters.

Sample	Flash Point [°C]	Viscosity at 40° C, [cSt]	Density at 22 °C, [g/cm3]	Low Calorific Value [MJ/kg]	Elemental Analysis, [%]
Jet A	40.1	1.27	0.808	42.392	C% = 85.71 H% = 14.22 N% = 0.07 O% = 0.00
Diesel	56	2.7	0.850	42.783	C% = 86.6 H% = 13.5 N% = 0.00 O% = 0.00
Aeroshell 500 Oil	262.3	25.11	0.999	32.260	C% = 94.50 H% = 3.50 N% = 0.00 O% =2.00
Jet A + 5% Aeroshell 500 Oil	51.2	2.46	0.818	41.885	C% = 85.20 H% = 14.63 N% = 0.07 O% = 0.1
Diesel + 5% Aeroshell 500 Oil	66.3	3.82	0.857	42.257	C% = 86.99 H% = 12.91 N% = 0.00 O% = 0.10

presents the experimentally determined values of the parameters.

Based on the experimental results, it is observed that the lower calorific value for Jet A and diesel is very close, while for Aeroshell 500 oil, it is much lower than for the other two mentioned above. The properties for diesel and Jet A are highly comparable. An advantage for diesel is its higher density and slightly higher calorific value compared to Jet A. As for the disadvantages, it is known that diesel has a much higher freezing point than Jet A, making it unsuitable for aviation, and diesel has a higher carbon content than Jet A.

Aeroshell 500 oil has a higher carbon content in its composition, which leads, when mixed with Jet A and diesel, to higher amounts of CO₂ and CO. Also, due to its lower calorific value, the mixtures of Jet A + 5% Aeroshell 500 and diesel + 5% Aeroshell 500 will provide lower burning energy.

FTIR spectroscopy is a very useful tool in assessing chemical modifications within a substance. By adding Aeroshell 500 oil to Jet A and diesel, its chemical composition changes. The plots below show the absorbance as a function of wavelength. Figure 3 shows the FTIR spectra for Jet A, Aeroshell 500, and Jet A + 5% Aeroshell 500.

The above figure represents the FTIR spectra of all three liquids taken into consideration: Jet A, Aeroshell 500 oil, and Jet A + 5% Aeroshell 500.

As can be observed, the main differences occur between 600 and 800 cm⁻¹ absorbance, where single bond vibrations occur strongly due to C-C, C-O, and C-x. The difference between the molecular lengths of Jet A (shorter) and oil (longer) is clearly observed within the spectra. Another region of high differences is the region between 800 and 1400 cm⁻¹, where in the case of Aeroshell 500, glycol adsorption appears (1000 cm⁻¹), and the additives show their influence (anti-wear, anti-oxidation, etc.). In the case of Jet A, methyl and methylene groups appear at 1350 and 1450 cm⁻¹. Another important modification appears at 1750 cm⁻¹, representing the presence of oxygen bonded by a C atom (C-O). The difference shown at 3200–3600 cm⁻¹ means that hydroxyl (-O-H) has been brought into the structure.

Continuing, in Figure 4, the main differences between Jet A and Jet A + 5% Aeroshell 500 oil are presented.

As can be assessed, differences occur at different wavelengths and are significant. Thus, at 700 cm⁻¹, the mixture gains some C-N and/or C-x bonds from the presence of additives within the oils, but at 800 cm⁻¹, it loses short-chain molecules, being replaced by long-chain molecules from the oil. At 1450 cm⁻¹, the presence of methylene groups (-CH₂) is showing a slight decrease compared with Ke. The radiation absorbed at 1350 cm⁻¹ is showing an increase in methyl groups (–CH₃). Another large difference is shown at 1000 cm⁻¹, representing the C-OH bond. Another important modification appears at 1750 cm⁻¹, representing the presence of oxygen bonded by a C atom (C-O). Another difference can be observed at 2850–2960 cm⁻¹, representing a strong alkene-type bond between C-H.

Figure 5 represents the FTIR spectra of all three liquids taken into consideration: diesel, Aeroshell 500 oil, and diesel + 5% Aeroshell 500.

As can be observed, main differences occur between 600 and 800 cm⁻¹ absorbance, where single bond vibrations occur strongly due to C-C, C-O, and C-x. The difference between the molecular lengths of diesel (shorter) and oil (longer) is clearly observed within the spectra. Another region of high differences is the region between 800 and 1500 cm⁻¹, where in the case of Aeroshell 500, glycol adsorption appears (1000 cm⁻¹), and additives show their influence (anti-wear, anti-oxidation, etc.). In the case of diesel, methyl and methylene groups appear at 1350 and 1450 cm⁻¹. Another important modification appears at 1750 cm⁻¹, representing the presence of oxygen bonded by a C atom (C-O). The difference shown at 3200–3600 cm⁻¹ means that hydroxyl (-O-H) has been brought into the structure.

In Figure 6, the main differences between Jet A and Jet A + 5% Aeroshell 500 oil are presented.

As can be assessed, differences occur at different wavelengths and are significant. Thus, at 700 cm⁻¹, the mixture gains some C-N and/or C-x bonds from the presence of additives within the oils. At 1450 cm⁻¹, the presence of methylene groups (-CH₂) are showing a slight decrease compared with diesel. The radiation absorbed at 1350 cm⁻¹ is showing an increase in methyl groups (–CH₃). Another large difference is shown at 2850–2960 cm⁻¹, representing a strong alkene-type bond between C-H.

3.2. Combustion Process Results

Based on Equations (1)–(4), the centralized values are presented in

Table 2. Obtained values for 1 kg of fuel blend.

Fuel	MO [kg]	Mair [kg]	CO2 [kg]	H2O [kg]
Jet A	3.42	14.89	3.143	1.280
Diesel	3.38	14.71	3.175	1.207
Jet A + 5% Aeroshell 500 Oil	3.39	14.75	3.159	1.232
Diesel + 5% Aeroshell 500 Oil	3.35	14.58	3.190	1.162

, where Mo represents the amount of oxygen required for the stoichiometric reaction, Mair is the amount of air required, and CO₂ and H₂O represent the quantities of CO₂ and H₂O resulting from the combustion of one kg of fuel at stoichiometric combustion.

It can be observed that the amount of air required for Jet A is lower than for diesel as well as for Jet A + 5% Aeroshell 500 oil compared to diesel + 5% Aeroshell 500 oil. Regarding the resulting CO₂ quantity, it can be noted that the amount of CO₂ generated for Jet A is lower than for diesel as well as for Jet A + 5% Aeroshell 500 oil compared to diesel + 5% Aeroshell 500 oil.

Based on the mathematical model for the combustion reaction, the following graph is presented (Figure 7).

The maximum excess air was set to 5 because below this value, the combustion temperature is too low to be significant in the field of turbine engines. The interval between the values of 3 and 3.4 of excess air is highlighted in the above graph, as this range corresponds to microturbine engines operating at maximum capacity.

It can be observed that the combustion temperature of Jet A is slightly higher than the combustion temperature for diesel across the entire range of excess air values. By adding Aeroshell 500 oil, it can be noted that the combustion temperature decreases slightly for both Jet A and diesel.

3.3. Microturboengine Test Bench Experiments

Following the experiments with the Jet Cat P80 microturboengine using Jet A + 5% Aeroshell 500 oil and diesel + 5% Aeroshell 500 oil, a dataset was obtained. The engine operated at four idle regimes: Regime 1 with the throttle gas at 18.7%, Regime 2 with the throttle gas at 30%, Regime 3 with the throttle gas at 60%, and Regime 4 with the throttle gas at 92%; these regimes corresponded to rotations per minute (rpm) of 35,000, 55,000, 88,800, and 114,150 rpm, respectively. From the recorded data collected by the microturboengine instrumentation, the combustion temperature in the combustion chamber (Tcomb), fuel flow rate, and thrust are of interest for our study and will be presented graphically in Figure 8, Figure 9 and Figure 10.

Analyzing the graphs from the figures above, it can be observed that the combustion chamber temperature for the four regimes is lower when using diesel + 5% Aeroshell 500 oil, which is consistent with the theoretical model calculations. Regarding the fuel flow rate, it is also lower when using diesel + 5% Aeroshell 500 oil compared to Jet A + 5% Aeroshell 500 oil. Of particular interest is the variation in thrust for the two fuels across the four regimes, where a slightly higher value is observed when using diesel + 5% Aeroshell 500 oil. This can be explained by the fact that the lower calorific value of diesel is slightly higher than that of Jet A, and the density of diesel is higher than that of Jet A.

Additionally, the variation in CO concentration is presented in Figure 11, and the variation in SO₂ concentration is presented in Figure 12, both recorded by the gas analyzer for these operating regimes. It is widely known that both CO and SO₂ have negative impacts on the environment as follows.

Sulfur dioxide (SO₂) has several significant environmental impacts: Acid Rain—forms sulfuric acid, damaging ecosystems, aquatic life, soil, and structures; Air Quality—causes respiratory problems, aggravates lung diseases, and reduces air quality; Visibility Reduction—contributes to fine particulate matter (PM2.5), leading to haze; Climate Change—has a cooling effect by reflecting sunlight but indirectly affects climate through cloud formation and particle interaction.

Carbon monoxide (CO) has several significant environmental impacts too: Air Quality—CO is a toxic pollutant that reduces blood’s oxygen-carrying capacity, posing health risks, especially to those with heart and respiratory conditions; Ozone Formation—CO contributes to ground-level ozone (smog), harming respiratory health and damaging vegetation; Climate Change—While not a greenhouse gas, CO influences methane and tropospheric ozone levels, indirectly impacting climate change.

Overall, CO and SO₂ emissions harm air quality and human health and indirectly contribute to climate change.

It can be observed that, in the idle regime (Regime 1), the concentrations of both CO and SO₂ are higher compared to the other regimes, likely due to the instability of the idle regime. Additionally, the concentrations of CO and SO₂ are higher when using diesel + 5% Aeroshell 500 for all operating regimes.

Since the microturboengine cannot be started experimentally without adding Aeroshell 500 oil to the fuel, an extrapolation of the theoretical results based on experimental validation will be performed. The maximum tested regime, which is also one of the most stable regimes, was chosen for comparison between the experimentally recorded temperature in the combustion chamber of the microengine and the value obtained from the mathematical model. Utilizing the experimental data of fuel flow and air flow, the excess air can be determined. Thus, for the maximum regime, the excess air for the case of using Jet A + 5% Aeroshell 500 oil was approximately 3.19, while for diesel + 5% Aeroshell 500 oil, it was approximately 3.25. Based on these excess air values, the temperature of combustion was calculated using the mathematical model, resulting in 1065 K for Jet A + 5% Aeroshell 500 oil compared to 990 K experimentally and 1045 K for diesel + 5% Aeroshell 500 oil compared to 982 K experimentally. Therefore, the error between the theoretical calculations and experiments is approximately 6–7 percent, thus, extrapolating the theoretically calculated values to the case where Aeroshell 500 oil is not used in the mixtures, we can say that the results are quite relevant.

Another important parameter that characterizes the performance of turbojet engines is the specific fuel consumption S, which is defined by Formula (8):

$$\mathrm{S}=3600·{\displaystyle \frac{\dot{\mathrm{M}}\mathrm{f}}{\mathrm{F}}}\left[{\displaystyle \frac{\mathrm{kg}}{\mathrm{N}·\mathrm{h}}}\right]$$

(8)

In the formula above, F represents the force measured by the microengine instrumentation, and $\dot{\mathrm{M}}\mathrm{f}$ represents the fuel flow rate in [kg/s].

Figure 13 represents the variations in S.

It can be observed that the specific fuel consumption is higher when using diesel + 5% Aeroshell 500 oil as fuel in Regimes 1 and 2, while in Regimes 3 and 4, the differences become almost negligible between the two fuels.

4. Conclusions

The main conclusions drawn from this study are as follows.

The addition of 5% Aeroshell 500 oil to Jet A and diesel as aviation microturbine fuels alters their properties. The flash point and viscosity increase similarly for both mixtures, as does the density. Adding 5% Aeroshell 500 oil to Jet A and diesel decreases the low calorific power and increases the carbon concentration while decreasing the hydrogen concentration in the mixtures.

Regarding the CO₂ quantity, adding 5% Aeroshell 500 to Jet A increases it from 3.143 kg to 3.159 kg for each kilogram of mixture burned in the stoichiometric reaction. Similarly, adding the oil to diesel in a 5% proportion increases the CO₂ quantity from 3.175 to 3.190 in the stoichiometric reaction.

Through the experiments with the Jet Cat P80 microengine at the four regimes, it was found that the combustion chamber temperature and fuel flow rate are lower for diesel + 5% Aeroshell 500 oil compared to Jet A + 5% Aeroshell 500 oil, while the traction force is slightly higher when using diesel + 5% Aeroshell 500 oil. This can be explained by the higher density of diesel and its higher low calorific power compared to Jet A. Similarly, the specific fuel consumption in Regimes 1 and 2 is higher for diesel + 5% Aeroshell 500 oil compared to Jet A + 5% Aeroshell 500 oil, while in Regimes 3 and 4, the differences are insignificant.

Based on the mathematical model, the combustion temperatures of Jet A, diesel, Jet A + 5% Aeroshell 500 oil, and diesel + 5% Aeroshell 500 oil were calculated as a function of excess air, and a comparison was made between the recorded values from the microengine instrumentation and the calculated ones, resulting in an error between theoretical and experimental values of approximately 6–7%.

To assess the gas emissions of the two tested fuels in the microengine, the concentrations of CO and SO₂ in the exhaust gas jet were measured, showing higher concentrations for diesel + 5% Aeroshell 500 oil compared to Jet A + 5% Aeroshell 500 oil across all four operating regimes.

In conclusion, adding 5% Aeroshell 500 oil to Jet A or diesel does not significantly diminish the physicochemical properties of the mixtures but slightly increases the CO₂ quantity resulting from combustion and carbon-based compounds due to the higher carbon concentration in Aeroshell oil compared to Jet A and diesel.

Future research directions include further development of the mathematical model and comparisons with commercial simulation software to simulate the combustion properties of the tested mixtures in aviation microturbines.