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Article

Experimental Investigation of Particulate Number Measurement Methodology for Micro-Turbojet Engine Emissions

by
Zheng Xu
1,
Minghua Wang
2,
Guangze Li
1,
Xuehuan Hu
2,
Pengfei Yang
3,
Meiyin Zhu
1,
Bin Zhang
1,
Liuyong Chang
1,*,† and
Longfei Chen
2,*,†
1
International Innovation Institute, Beihang University, Hangzhou 311115, China
2
School of Energy and Power Engineering, Beihang University, Beijing 100083, China
3
Faculty of Science and Engineering, University of Nottingham Ningbo China, Ningbo 315100, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Aerospace 2024, 11(7), 548; https://doi.org/10.3390/aerospace11070548
Submission received: 12 May 2024 / Revised: 24 June 2024 / Accepted: 27 June 2024 / Published: 3 July 2024
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Abstract

:
Increasing aviation activities have heightened concerns about particulate emissions from aircraft engines. Current ICAO standards mainly focus on large civil turbofan engines, leaving a gap in the research on PN emissions from small aircraft engines. This study examined the PN emission characteristics of micro-turbojet engines, including the morphology, PN, and size distribution under different load conditions, compared with a micro-piston engine. The results showed that the nvPM from micro-turbojet engines was larger and more complex than typical aviation soot, likely due to reduced combustion efficiency and incomplete fuel combustion. Micro-turbojet engines exhibited fewer fluctuations in their emission index number (EIn) and emission index mass (EIm) at lower speeds. The geometric mean diameter (GMD) of particles was inversely correlated with thrust, while the geometric standard deviation (GSD) slightly increased with thrust. Quantitative comparisons indicated that PN emissions from micro-turbojet engines were higher, with EIn values ranging from 2.0 to 3.3 × 1016/kg fuel compared with 1.2 to 1.5 × 1016/kg fuel for micro-piston engines. EIm values for micro-turbojet engines ranged from 8 to 40 mg/kg fuel, while micro-piston engines had slightly higher values due to better carbonization. These findings validated the measurement methodology used to accurately assess PN emissions under low-thrust conditions in micro-turbojet engines. These results provide crucial insights and support for the future monitoring and regulation of PN emissions across all thrust conditions in small aircraft engines.

1. Introduction

Global economic expansion has increased transportation demands, driving rapid advancements in aviation [1,2]. Aviation, known for its speed, cargo capacity, and geographical independence, is increasingly popular for passenger and freight transport [3,4,5]. Aviation particulate emissions can be classified into two categories based on their volatility. These are volatile and non-volatile particulate matter (nvPM). However, these particulate emissions within the aviation domain exert discernible effects on the environment and human wellbeing [6,7,8]. These impacts include changes to the atmosphere, degraded air quality, cloud formation effects, increased health risks, and climate implications [9]. Notably, small aircraft engines such as those used in general aviation and unmanned aerial vehicles (UAVs) represent significant sources of particulate emissions [10,11]. Consequently, there is an urgent need to address and mitigate particulate emissions from these engines to safeguard environmental and human health [12].
Aircraft emissions primarily include carbon dioxide (CO2), nitrogen oxide (NOx), hydrocarbons (HCs), particulate matter (PM), and the particle number (PN) [13,14]. These substances originate from the complex combustion processes in aircraft engines: CO2 is generated from carbon combustion, NOx forms under high-temperature combustion conditions, HC consists of incompletely burned hydrocarbon compounds, while PM and the PN primarily stem from combustion-generated fine particles and gaseous pollutants (Figure 1) [15]. PM encompasses inhalable particles such as PM2.5 and PM10, capable of penetrating the respiratory system and exacerbating respiratory diseases such as asthma, chronic obstructive pulmonary disease (COPD), and cardiovascular illnesses, thereby increasing premature mortality risks [16]. The PN directly reflects engine combustion efficiency and fuel completeness; elevated PN levels indicate incomplete combustion and high pollutant emissions, wasting energy and exacerbating air pollution. Therefore, controlling and reducing PM and PN emissions from aviation are crucial for improving air quality and safeguarding public health [17].
To mitigate the environmental impact of particle emissions from small aircraft engines, the International Civil Aviation Organization (ICAO) Committee on Aviation Environmental Protection (CAEP) has developed international standards. Despite these efforts, the traditional smoke number (SN) method has proved to be inadequate for a precise assessment of environmental and health impacts, necessitating regulatory attention to PN emissions alongside PM. PN and PM measurements, known for their repeatability and stability, are crucial for airworthiness certification. The first regulatory standard targeting nvPM emissions from aircraft engines (CAEP/10) was established during the 10th CAEP Conference in 2016 [1]. This standard mandates PN emission reporting for engines with thrust over 26.7 kN. In 2017, the ICAO introduced Annex 16 Volume II, which formalized the nvPM measurement regulations, detailing procedures across the flight cycle using the standard reference system (SRS). This system includes components such as heating, dilution systems, and sampling probes [18]. Officially implemented since January 2023, this standardized methodology highlights the complexity of nvPM handling in sampling and measurement systems, encountering mechanisms like particulate adhesion and loss. Notable loss mechanisms include diffusion, inertia, thermophoresis, and bend loss, often influenced by the particle size [19]. A particulate loss estimation model developed by the United Technologies Research Center (UTRC) incorporates five key loss mechanisms. These are diffusion, thermophoresis, electrostatic, inertial, and bend loss [20]. However, a thorough exploration of these loss mechanisms, particularly in relation to nvPM emissions from small aircraft engines such as turbojet engines, remains a critical area for further investigation.
Significant research has focused on PM and PN emissions from large civil turbofan engines [21,22]. Richard H. Moore et al. demonstrated that blending biofuels could reduce aviation PN and PM emissions by 50% to 70% [23]. Chi Zhang et al. developed methods to formulate alternative jet fuels specifically designed to replicate sooting tendencies in aviation engine combustors, illustrating the effectiveness of non-linear TSI prediction methods in surrogate formulations [24]. Florian Eigentler et al. utilized computational fluid dynamics (CFD) to study the evolution of nvPM in complex fuels, using a computed smoke number (SN) and emission index (EI) at the combustor exit as comparative metrics and finding a good correlation between experimental and simulated results [25]. Niraj Kumar conducted a systematic review and provided a critical analysis of the performance and nvPM-emission characteristics of various oxygenated biofuels using micro-structural characterization techniques. This research highlighted new avenues to reduce both nvPM and NOx emissions from aviation engines [26,27]. Despite these advancements, the ICAO standards and recommendations mainly target carbon and nvPM emissions from large commercial turbofan engines (thrust > 26.7 kN) and do not adequately cover small aircraft engines. Research in this domain is notably scarce. PM from small aircraft engines like micro-turbojets typically has larger sizes and sometimes higher temperatures, complicating testing. Most current studies focus on gaseous pollutant emissions, with limited attention to particulate emissions, generally monitored using smoke number or optical signal methods [28]. Radoslaw Przysowa et al. developed emission factor models employing USA Semtech DS gas analyzers and EEPS spectrometers to predict gaseous emissions [29]. Christiane Voigt et al. reported that the extensive use of low-aromatic fuels and regulations to reduce the maximum aromatic content in fuels could significantly mitigate the climate impacts of aviation [30]. Presently, systematic research specifically addressing PN concentrations in micro-turbojet engines is lacking, indicating a critical need for more comprehensive data to inform policymaking and enhance environmental protection standards.
Therefore, research on PN emissions from micro-turbojets remains limited, with uncertainties in particulate losses during sampling and measurement, and requires comprehensive quantitative studies. Although some researchers have investigated PN emissions using ICAO-endorsed methods, they have mainly focused on large aircraft engines, with minimal research on small aircraft engines. Although some studies have applied ICAO standard measurement methods to small aircraft engine nvPM, discussions on their applicability remain sparse [30,31,32]. Furthermore, considerable uncertainties persist regarding the mechanisms of nvPM emissions and the actual emissions from small turbojet engines, highlighting the need for detailed research [33]. This research should integrate micro- and macro-analyses of the PN and size distributions to quantitatively assess nvPM emissions. Micro-turbine engines, including turbojets, are central to research due to their broad application, involvement in critical technologies, and efficient thrust generation [34]. In comparison, micro-piston engines, similar to automotive engines in their principles and performance, have been extensively studied [31,35]. In contrast, studies on micro-turbojet engines are notably scant. Therefore, an in-depth investigation into nvPM emissions from micro-turbojets is crucial for representative data and future regulatory support.
This study investigated the emission characteristics of non-volatile particulate matter (nvPM) from micro-turbojet engines, employing recommended standard measurement protocols. The efficacies of the nvPM methodology and the UTRC model were validated by measuring particle diffusion and bend loss. The investigation included detailed PN concentration and size distribution measurements from both micro-turbojet and piston engines under part-load conditions. Additionally, the study explored the morphology of nvPM from turbojet engines. These investigations demonstrated the feasibility of using standardized methods to measure nvPM emissions from micro-turbojets, highlighting the robustness of the applied techniques in assessing PM under varying conditions.

2. Engine and Measurement Instruments

2.1. Engine and Operating Conditions

This study measured emissions from a CHN JetCAT P300-Pro turbojet engine. The P300-Pro has a centrifugal compressor, loop-shaped combustion chamber, axial flow turbine, and convergent nozzle. The test setup followed ICAO methods, featuring a full authority digital engine control (FADEC) system, CYB-602S load cells for thrust measurement, exhaust and noise attenuation, and the real-time monitoring of the compressor inlet, turbine inlet/exhaust temperatures, and fuel flow rate. The engine used standard CHN RP3 aviation kerosene with 1% USA Mobil Jet Oil II for lubrication. The operating conditions for the tests are listed in Table 1. For safety and initial methodology testing, maximum relative thrust was limited to 25%.

2.2. nvPM Measurement System

The engine exhaust underwent pre-processing, including dilution and thermal conditioning, following sampling via a USA 316 stainless steel tubing probe (7 mm outer diameter; 5 mm inner diameter). For the micro-turbojet engine, the sampling point was positioned at the tailpipe center; to enhance exhaust gas mixing, the emission-sampling distance was extended to 18 mm. The ICAO-recommended nvPM measurement system [36], as depicted in Figure 2, was utilized in this study, with modifications. This included the dilute front box, constant heat pipe, back-end box, and instrumentation. The exhaust gas within the dilute front box was split into three paths for the gaseous component measurement, pressure control, and the main flow entering diluter 1. The gas flow passed through a 25 m thermally insulated and heated transfer line, including a cyclone separator, and was divided into four paths, each entering an analyzer. Temperature control was employed for the measurement requirements, covering the PN concentration and size distribution.
The experimental procedure for the micro-turbojet engine nvPM measurement comprised four main parts. These were pre-measurement inspection, background nvPM measurement, engine nvPM measurement, and post-measurement processing. The pre-measurement inspection involved instrument calibration, leak, and cleanliness checks. For calibration, the PN concentration analyzers underwent zero calibration using high-efficiency particulate air filter (HEPA)-filtered nitrogen, showing no drift. Leak tightness requirements were met using a measured outlet flow less than 0.3 LPM during sealed-probe and disconnected sampling. These procedures ensured the accurate collection and measurement of the engine exhaust particulate matter for comprehensive analyses and compliance with ICAO standards.
The post-measurement checks involved zero drift verification and the back-blowing of the sampling line, mirroring the pre-test calibration. To mitigate unburnt fuel residuals, back-blowing was performed post-measurement. With valve 1 closed and pressure control valve P1 shut, the gaseous analyzers were removed and nitrogen dilution gas flowed into the sampling probe at ≥20 LPM for ≥3 min.
An ambient nvPM measurement process occurred in the sampling gas treatment and the measured segments of the nvPM-emission standard measurement system. Particulates were collected using two methods, either using a filter to sample the exhaust for a continuous 10 min interval or directly from the inner surface of the sampling probe for black carbon particulates in the turbojet engine experiments. Powder images were obtained using a desktop scanning electron microscope (SEM) (JEOL, NeoScope™ JCM-6000 model, Beijing, China). The nvPM mass was measured using a AT Micro Soot Sensor (MSS)-483, while the nvPM number and particle number (PN) concentration were determined using an AT AVL Particle Counter (APC)-489. A Scanning Mobility Particle Sizer Spectrometer (SMPS) measured the PN concentration and size distribution. With sampling valve 1 and dilution gas valve 2 closed, the overflow outlet of diluter 1 was opened to allow ambient air to enter the system at a flow identical to the engine tests. Once the MSS and APC stabilized, data were recorded for at least 3 min. The results demonstrated a negligible impact of ambient nvPM on engine-emission measurement outcomes.
Data recording started when the measurements stabilized for at least 60 s per test point. The nvPM size distributions from the SMPS were also recorded. The PM and PN emission indices EIm and EIn were calculated using the ICAO Annex 16 methodology [36] as follows:
E I m = 22.4 × n v P M m a s s _ S T P × 1 0 − 6 ( [ C O 2 ] d i l 1 + 1 D F 1 ( [ C O ] − [ C O 2 ] b + [ H C ] ) ( M c + α M H ) k t h e r m o
E I n = 22.4 × n v P M n u m _ S T P × 1 0 6 ( [ C O 2 ] d i l 1 + 1 D F 1 ( [ C O ] − [ C O 2 ] b + [ H C ] ) ( M c + α M H ) k t h e r m o
In the nvPM sampling and measurement process, particle loss is inevitable. To address electrostatic losses, the standard mandates an antistatic transfer line meeting ISO 8031 specifications [36] and utilizing carbon-loaded poly tetra fluoroethylene (PTFE) material with grounding. Active heating is required to maintain the transfer line’s wall temperatures at 60 ± 15 °C to minimize thermophoretic loss. However, diffusional loss is unavoidable, especially given spatial constraints; in the laboratory, this required a coiled 25 m line for the micro-turbojet engines. Additional losses from bends were incurred due to the coiling. This study validated the nvPM diffusion and bend losses by using the 25 m transfer line and a nanoparticle generator. The diffusion loss and bend loss of particles was calculated using the following formula [36]:
η d i f f = e x p − π · D t u b e · L t u b e · V d i f f Q
η b e n d = 1 − s t k · θ
where D t u b e , L t u b e , Q, V d i f f , stk, and θ are the diameter of the pipe, pipe length, gas flow, diffusion deposition speed, Stokes number, and bending angle of the pipe, respectively.
In the validation experiment, a 25 m transfer line coiled with a 180° bend to meet laboratory space constraints was used (1 m maximum allowable bend radius). The PN concentration and size distribution were measured using an AT SMPS-3936. Despite involving up to 270° of bending connections totaling approximately 450° and exceeding the standard’s 0.5 m radius requirement, the bending had a negligible impact. The particulate source maintained a flow rate of 24 LPM.
Using the UTRC model, we computed the nvPM penetration fraction in the transfer line considering diffusion or bend losses, and compared the results with the experimental data (Figure 3). Coiling induced additional losses of 1–4% at the 24 LPM flow rate, diminishing with a larger nvPM size. Micro-turbojet engines, likely emitting larger particulates than high-thrust civil turbofans, enhanced the UTRC model’s accuracy in predicting bend losses.

3. Results and Discussion

3.1. Microscopic Characterization of Exhaust Particles from Micro-Turbojet Engines

Figure 4 presents typical SEM images of micro-turbojet engine and TEM images of RUS PowerJet SaM146 jet engine emission particles under 30% and 70% thrust conditions, respectively. The morphological features of the particles are discussed below.
Figure 4a depicts the emission particle morphology of a micro-turbojet jet engine under 30% thrust conditions. In this scenario, a greater quantity of larger particulates was discernible, featuring intricate shapes such as spheres and chain-like aggregates. However, Figure 4c illustrates the typical particle morphology emitted by a micro-turbojet jet engine under high-thrust conditions. At 70% thrust, the emission particles from the micro-turbojet engine exhibited a state of agglomeration and the size of a single particle was smaller than that under 30% thrust. The main reasons included an increase in combustion temperature, improvement in combustion efficiency, and changes in combustion kinetics. This was primarily attributed to the elevated combustion temperature, facilitating particle fusion and growth.
Figure 4b,d depict the particle emission characteristics of a standard large aero engine (PowerJet SaM146 jet engine) during high- and low-thrust conditions. Both consisted of primary particles forming fractal aggregates of a few hundred nanometers, indicating that the aggregates were rather compact. A comparison revealed that the difference in particle size between the micro-turbojet engine and the large engine was primarily due to the combustion temperature within the micro-turbojet engine being below the threshold required for the onset of non-volatile particulate matter (nvPM), which is higher than 1800 K. The PowerJet SaM146 turbofan engine had a combustion temperature of approximately 2273 K [38,39,40], enhancing combustion efficiency and reducing the size of the particle emissions compared with the micro-turbojet engine, which had a combustion temperature of about 1750 K. This difference in combustion temperature led to more complete combustion in the SaM146, resulting in smaller emission particles. Consequently, these particulates either did not undergo carbonization or experienced only partial carbonization, leading to the observed larger sizes. These findings aligned with previous studies that highlight the complex relationship between combustion temperatures, fuel properties, and emission characteristics in micro-turbojet engines [41,42,43,44]. In our study, the combustion temperature inside the micro-turbojet engine was mostly greater than 1800 K, resulting in most of the PM emissions being carbonized, with relatively few non-carbonized particles.
In a comparison of the particle emission morphology between the PowerJet SaM146 jet engine and micro-turbojet engine, significant differences were evident under similar operating conditions. Additionally, with changes in thrust conditions, there were notable differences in the morphology and particle size of emitted particles. Relatively larger-sized particles are emitted at low thrust by micro-turbojet engines when compared with traditional large turbofan engines, suggesting the need for further investigation into the PN emission characteristics of micro-turbojet engines.

3.2. Comparison of the PN between a Micro-Turbojet Engine and a Micro-Piston Engine

On the basis of particle morphology characterization, the next step involved a comparative measurement of PN emissions from CHN micro-turbojet engines and piston engines (DLE85) under various low-thrust conditions (Figure 5). After a dilution ratio correction, the PN concentrations measured by the APC were observed to be within the engine speed test range. The experimental results indicated that for relative thrust levels not exceeding 25%, the PN concentration from micro-turbojet engines decreased with an increase in thrust. The air–fuel ratio (AFR) significantly impacts combustion efficiency and PM in engines. In piston gasoline engines, the AFR decreases with an increase in thrust, enhancing combustion but raising particulate emissions beyond 25% thrust due to incomplete combustion [45]. Turbojet engines exhibit a more complex AFR–thrust relationship. The AFR rises with initial thrust but decreases at higher thrust levels, promoting combustion efficiency but potentially increasing particulates if not optimized [46,47]. Understanding these AFR variations is crucial for balancing performance and emissions in both engine types. This is attributed to an increase in the fuel flow rate with engine thrust within the measurement range, coupled with an increase in the air–fuel ratio, resulting in enhanced combustion efficiency. Meanwhile, a reduction in the combustion temperature contributes to a decrease in the PN concentration. However, when the relative thrust exceeds 25%, the PN concentration exhibits an increasing trend with thrust, correlating with a decrease in the air–fuel ratio.
It is noteworthy that, for micro-piston engines, the PN concentration decreases across the entire thrust range with an increase in horizontal speed. This phenomenon is analogous to that observed in micro-turbojet engines, where an increase in engine speed leads to higher fuel consumption and a higher air–fuel ratio, promoting more efficient fuel combustion and a concurrent reduction in the PN concentration. The higher PN concentration in turbofan engines at low operating conditions compared with piston engines is primarily attributed to the use of diffusion combustion in turbofan engines, whereas piston engines typically employ spark ignition (SI) combustion. Compression diffusion combustion is more prone to generating high-temperature and high-pressure conditions, potentially resulting in the formation of more PM [45,46,47]. Additionally, incomplete combustion at low loads and low speeds may contribute to increased particle emissions [48,49]. Some studies collectively support the notion that diffusion combustion in turbojet engines leads to higher PN concentrations compared with the spark ignition systems used in piston engines [50,51,52,53,54]. These research findings offer valuable insights for a comprehensive understanding of PM emissions from micro-turbojet engines under different operating conditions.
To further prove the above point, Figure 6a,b present the EIn measurements for turbofan jet engines and micro-piston engines under partial load thrust conditions, with ranges of 2.0 to 3.3 × 1016/(kg fuel) and 1.2~1.5 × 1016/(kg fuel), respectively. As the thrust increased, the EIn initially fluctuated and then steadily decreased, possibly due to improved combustion and reduced nvPM emissions resulting from increased fuel consumption and an increased air–fuel ratio. Compared with turbojet engines, the EIn for piston engines was approximately halved, primarily due to the use of different fuels.
Overall, consistent emission trends demonstrated the applicability of the standard measurement methods for the characterization of PN emissions from micro-piston engines. Although different fuels and engine types led to varying absolute values, the trends within the measurement range were similar. The results indicated that, with an increase in thrust, the EIn for turbojet engines decreased below 25% thrust, attributed to improved combustion efficiency and more complete fuel combustion at high-altitude lean conditions, surpassing the increase in the fuel flow rate. Above 25% thrust, the EIn increased with thrust, accompanied by a decrease in the air–fuel ratio. Despite different engine types, the consistent trends suggested the suitability of the standard nvPM measurement methods for micro-turbojet-engine aircraft.
To further investigate PM emissions from the micro-turbojet engine, we calculated the EIm values (plotted in Figure 7a), ranging from 8 to 40 mg/kg fuel over the test range. Similar to the micro-piston engine results depicted in Figure 7b, the EIm decreased with increasing thrust. Additionally, higher combustion pressures in the piston engines resulted in greater particle carbonization, yielding slightly higher EIm values compared with the turbojet engines. This further proved the enhancement of combustion efficiency and more complete fuel combustion under high-altitude thin-air conditions. The standard nvPM measurement method was applicable to micro-turbojet-engine aircraft. Numerous research studies have explored variations in emission indices for mass (EIm) and number (EIn) under typical operating conditions of two types of engines, similar to our experimental data [45,46,47,48,49]. These results provide strong support for our conclusions by highlighting the distinct behaviors of each engine type under comparable conditions.

3.3. Comparison of the PSD, GMD and GSD between a Micro-Turbojet Engine and a Micro-Piston Engine

The particulate size distribution (PSD) characteristics of the micro-turbojet engine depended on engine thrust (Figure 8a). The GMD was biggest at idle (~105 nm), followed by an initial steep increase up to ~25% thrust (Figure 8c). The decline in the GMD became apparent at thrust levels below 25% due to improvements in combustion efficiency. This decline was followed by an elevation in the GMD beyond 25% thrust, attributed to decreases in the air–fuel ratios (Figure 8e). The role of the AFR in influencing the particulate size distributions should be more clearly articulated, particularly when explaining the changes in the GMD and GSD at different thrust levels. Specifically, variations in the AFR significantly impact the GMD and GSD, with higher AFRs generally leading to smaller, more uniform particles, while lower AFRs result in larger, more variable particle sizes [45,46,54]. The GMD values for aviation turbofans such as CFM56 exhibit a range of 30–100 nm [55], whereas the measurements for the micro-turbojet engine in this study fell between 80 and 130 nm. This discrepancy could be attributed to a lower combustion efficiency and the presence of semi-volatile compounds stemming from incomplete fuel combustion. The GSD of the measured PSD ranged from 1.37 to 1.44, independent of thrust. The GMD decrease with engine thrust was consistent with previous emission measurements directly obtained from behind turbofan engines with conventional (single annular) combustors. This showed that, despite the presence of a larger average particle size, the distribution slightly converged at higher velocities. However, Figure 8b,d show that, compared with the micro-turbojet engine, the PSD and GMD of piston engines were larger at all thrust levels. As the GMD increased with thrust, the particle concentration also increased from idle up to ~30% thrust, but decreased with a further increase in thrust, with the minimum at an initial steep. The trend of the GMD in the micro-piston engine was observable, spanning the range of 100 to 160 nm. This decline was followed by an elevation in the GMD beyond 20% thrust, attributed to decreases in air–fuel ratios. The GSD results for piston engines ranged from 1.36 to 1.46 (Figure 8f). Interestingly, as the engine speed increased, the GSD slightly decreased. This showed that, despite the presence of a larger average particle size, the distribution slightly converged at higher velocities. To sum up, the measurement methodology could effectively capture the representative characterization of PN emissions for micro-turbojet aircraft engines. Several researchers have addressed the issue of PSD in micro-turbojet and piston engines, highlighting the differences in the GMD and GSD under varying thrust conditions [56,57,58]. These studies collectively illustrate the complex relationship between engine operating conditions, the PSD, and the geometric parameters that influence emission characteristics in micro-turbojet and piston engines.

4. Conclusions

This study experimentally investigated PN characteristics in micro-turbojet engines, validating the PN measurement methodology and UTRC model for particulate diffusion and bend losses. A comparative analysis of the particle morphology between a micro-turbojet and a PowerJet SaM146 jet engine revealed distinctions in emissions. The PN concentration and size distributions from the micro-turbojet were measured and compared with piston aircraft engines, focusing on the relationship between PN emissions and part-load engine thrusts. The study demonstrated the feasibility of the methodology for the PN emission measurement of micro-turbojet engines, with the principal findings as follows:
(1)
A comparative analysis of the emission particle morphology between micro-turbojet engines and PowerJet SaM146 jet engines revealed significant distinctions. Under 30% thrust conditions, micro-turbojet engines exhibited higher abundances of larger, intricately shaped particulates, while at 70% thrust, the particles displayed agglomeration with smaller individual sizes. These differences were attributed to elevated combustion temperatures, improved combustion efficiency, and changes in combustion kinetics in the micro-turbojet engines. In contrast, the standard large aero engine displayed compact fractal aggregates. The particle size variation was primarily linked to the combustion temperature, emphasizing the need for further investigations into the PN emission characteristics of micro-turbojet engines.
(2)
The investigation characterized the PN concentrations from a micro-turbojet engine across various operational conditions using APC and MSS instruments. Applying emission index calculations from ICAO regulations resulted in an EIn range of 2.0–3.3 × 1016/(kg fuel) and an EIm range of 8–40 mg/kg fuel. Notably, both the EIn and EIm displayed a trend of an initial decrease, followed by an increase during part-load thrust periods, resembling the patterns observed in the micro-piston engines. Despite the lower EIn in the piston engines compared with the turbojet engines, the difference in combustion pressures led to a greater degree of carbonization within the piston engine.
(3)
The size distributions for the turbojet engine indicated a GMD in the range of 80–130 nm, exceeding the typical 30–100 nm range for turbofan engines. The GSD spanned from 1.37 to 1.44, with both the GMD and GSD exhibiting a trend of initially decreasing and then increasing with increasing engine thrust. Compared with the turbofan jet engines, the piston engines showed higher GMD and GSD values, attributed to factors like a reduced air–fuel ratio, a lower combustion efficiency, and the presence of semi-volatile compounds resulting from incomplete fuel combustion.
In future work, extending the measurement range to higher thrust conditions will lay the foundation to study the number and size distribution characteristics of nvPM emissions under all thrust conditions of these small aircraft engines.

Author Contributions

Conceptualization, Z.X. and L.C. (Liuyong Chang); data curation, Z.X., M.W., X.H., P.Y. and L.C. (Liuyong Chang); formal analysis, X.H. and P.Y.; funding acquisition, Z.X. and L.C. (Longfei Chen); investigation, Z.X. and B.Z.; methodology, G.L. and L.C. (Longfei Chen); project administration, L.C. (Longfei Chen); resources, M.Z.; supervision, L.C. (Longfei Chen); validation, G.L. and B.Z.; visualization, M.W., M.Z. and L.C. (Liuyong Chang); writing—original draft, M.W.; writing—review and editing, Z.X. and L.C. (Longfei Chen). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key R&D Program of China, grant number [2022YFB2602000]; the National Natural Science Foundation of China, grant number [52206131] and [U2333217]; the Zhejiang Provincial Natural Science Foundation of China, grant number [LQ22E060004], and T01 Project.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic diagram of aviation emission hazards [15].
Figure 1. Schematic diagram of aviation emission hazards [15].
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Figure 2. Layout of nvPM sampling and measurement system; the illustrations are actual pictures of a micro-turbojet engine, SMPS, VPR, and APC.
Figure 2. Layout of nvPM sampling and measurement system; the illustrations are actual pictures of a micro-turbojet engine, SMPS, VPR, and APC.
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Figure 3. nvPM penetration efficiency (a) only considering diffusion loss or (b) only considering bending loss.
Figure 3. nvPM penetration efficiency (a) only considering diffusion loss or (b) only considering bending loss.
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Figure 4. Comparison of micro-morphology of micro-turbojet (a,c) and PowerJet SaM146 jet engine (b,d) emission particles [37] under 30% and 70% working conditions. The comparison of the micro-morphology of emission particles from the micro-turbojet engine (a,c) and the PowerJet SaM146 jet engine (b,d) [37] under 30% and 70% operating conditions.
Figure 4. Comparison of micro-morphology of micro-turbojet (a,c) and PowerJet SaM146 jet engine (b,d) emission particles [37] under 30% and 70% working conditions. The comparison of the micro-morphology of emission particles from the micro-turbojet engine (a,c) and the PowerJet SaM146 jet engine (b,d) [37] under 30% and 70% operating conditions.
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Figure 5. PN concentration measurements for (a) micro-turbojet and (b) micro-piston engines using an AVL–nvPM system under different operating conditions.
Figure 5. PN concentration measurements for (a) micro-turbojet and (b) micro-piston engines using an AVL–nvPM system under different operating conditions.
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Figure 6. EIn of micro-turbojet (a) and micro-piston (b) engines with thrust.
Figure 6. EIn of micro-turbojet (a) and micro-piston (b) engines with thrust.
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Figure 7. EIm of micro-turbojet (a) and micro-piston (b) engines with thrust.
Figure 7. EIm of micro-turbojet (a) and micro-piston (b) engines with thrust.
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Figure 8. Thrust-dependent variability in particulate size distribution: GMD and GSD for micro-turbojet engine (a,c,e) and micro-piston engine (b,d,f) emissions.
Figure 8. Thrust-dependent variability in particulate size distribution: GMD and GSD for micro-turbojet engine (a,c,e) and micro-piston engine (b,d,f) emissions.
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Table 1. Working parameters of micro-turbojet engine.
Table 1. Working parameters of micro-turbojet engine.
ParameterUnitsValue
Idling speedrpm35,000
Idling thrustN14
Supercharge ratio-3.55
Idling fuel consumptionmL/min179
Exhaust gas temperature°C480–750
Maximum speedrpm106,000
Maximum thrustN300
Equivalent horsepowerkW90
Maximum speed fuel consumptionmL/min980
Exhaust velocitykm/h2160
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Xu, Z.; Wang, M.; Li, G.; Hu, X.; Yang, P.; Zhu, M.; Zhang, B.; Chang, L.; Chen, L. Experimental Investigation of Particulate Number Measurement Methodology for Micro-Turbojet Engine Emissions. Aerospace 2024, 11, 548. https://doi.org/10.3390/aerospace11070548

AMA Style

Xu Z, Wang M, Li G, Hu X, Yang P, Zhu M, Zhang B, Chang L, Chen L. Experimental Investigation of Particulate Number Measurement Methodology for Micro-Turbojet Engine Emissions. Aerospace. 2024; 11(7):548. https://doi.org/10.3390/aerospace11070548

Chicago/Turabian Style

Xu, Zheng, Minghua Wang, Guangze Li, Xuehuan Hu, Pengfei Yang, Meiyin Zhu, Bin Zhang, Liuyong Chang, and Longfei Chen. 2024. "Experimental Investigation of Particulate Number Measurement Methodology for Micro-Turbojet Engine Emissions" Aerospace 11, no. 7: 548. https://doi.org/10.3390/aerospace11070548

APA Style

Xu, Z., Wang, M., Li, G., Hu, X., Yang, P., Zhu, M., Zhang, B., Chang, L., & Chen, L. (2024). Experimental Investigation of Particulate Number Measurement Methodology for Micro-Turbojet Engine Emissions. Aerospace, 11(7), 548. https://doi.org/10.3390/aerospace11070548

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