Particle Counter Design Upgrade for Euro 7

Abstract

This research article presents an optimized approach to enhance the performance of the APC exhaust gas particle analyzer, a significant instrument used for exhaust emission evaluation in diesel-powered vehicles considering EU regulations on pollutant emissions. The study aimed to address the challenge of particle counter contamination that often occurs during frequent exhaust gas measurements and leads to measurement interruptions until maintenance is conducted. To achieve this, a preparatory unit that extends the operational duration of the measurement system between maintenance intervals while preserving measurement accuracy was developed based on actual exhaust gas experiments. The preparatory unit comprises a condensate drainage system, cooling fan, HEPA filter, membrane pump, and interconnecting pipelines to prevent moisture and larger particle deposition, ensuring uninterrupted and accurate exhaust gas measurements. The research findings underscore the significance of reliable and precise exhaust gas emission measurements, contributing to advancements in particle counting technology and facilitating compliance with emissions regulations in various scientific and industrial applications. This study provides an objective representation of the proposed preparatory unit’s effectiveness in mitigating particle contamination with only 1.9% measurement variance, offering promising implications for the improvement of exhaust gas analysis methods.

1. Introduction

In today’s context, zero-local-emission vehicles like hydrogen and electric options are gaining increasing significance. However, during this transition phase, as initial challenges are being addressed [1,2], internal combustion vehicles and their continued development still hold significant importance. The first emissions directive dates back to 1970 [3], with Euro 1 standards introduced in 1992 [4]. At first, PM mass limits were exclusively imposed on diesel vehicles under Euro 1, later extending to gasoline direct injection vehicles with Euro 5 (2009). In subsequent developments, solid particle number (SPN) limits were introduced for diesel vehicles in Euro 5 in 2011, followed by similar regulations for gasoline direct injection vehicles in Euro 6 in 2014. These SPN limits were subsequently extended to heavy-duty vehicles in 2013 and non-road mobile machinery in 2019. Euro 1 initiated the regulatory oversight of gaseous pollutants (CO, total hydrocarbons, and NO_x) in 1992, indirectly influencing volatile organic compounds and precursors to inorganic aerosols. Total hydrocarbons (THCs), often assessed through a heated line at 191 °C (for diesel vehicles), encompass a broad spectrum of volatile organic compounds, except for components that do not respond to the flame ionization detection utilized in total hydrocarbon analysis.

Over time, Euro emission limits underwent substantial tightening, leading to notable declines in ambient CO and non-methane volatile organic compound levels. However, trends in particle matter (PM), especially considering non-exhaust emissions, and NO_x emissions from road transport exhibited less favorable trajectories [5]. The issue of NO_x abuse [6], particularly in diesel vehicles [7] (e.g., dieselgate [8]), contributed to this scenario [9,10]. The situation improved significantly with the introduction of the real-driving emission (RDE) regulation in 2017 (Euro 6d-temp and subsequent) [11,12]. Concerning PM, diesel particulate filters (DPFs) (around 2009–2011, corresponding to Euro 5) significantly curtailed soot emissions, while fuel advancements alleviated sulfates. Yet, sources such as non-exhaust emissions (brakes and tires) and secondary particles continue to significantly contribute to transport PM emissions. Importantly, the pollutants regulated by Euro standards and air quality standards do not align perfectly, with certain pollutants being regulated by air quality directives but not by Euro standards. It is worth noting that while there is no particle number limit in the ambient air, particle number levels have gained attention in recent atmospheric monitoring initiatives. Future regulations such as Euro 7 based on the latest proposal [13] will include sub-23 nm particles, further increasing the relevance of particle counting technology.

European lab regulations prioritize full-flow condensation particle counters (CPCs) due to their accurate and efficient detection. Particle material significantly impacts detectability, especially for 23 nm and 10 nm cut-point CPCs. Higher saturator–condenser temperature differences reduce material dependency. Commercially available full-flow CPCs reach cut points as low as 4–5 nm, which are crucial for capturing the nucleation mode of modern vehicles. Diffusion-charging-based systems are robust for on-road use and permit real-time size distribution measurements. Instruments like the engine exhaust particle sizer (EEPS), differential mobility spectrometer (DMS), and electrical low-pressure impactor (ELPI) are common in research, though their uncertainty is higher than that of CPCs. Variations exist in volatile particle measurements, warranting technology-specific regulatory considerations. Instruments measuring sub-10 nm [14] particles have been reviewed elsewhere. Additional instruments like the aerosol gas exchange system (AGES) offer research flexibility.

Particle counting technology holds paramount importance in after-treatment performance research and industrial applications, such as in approval tests of vehicles, that are subject to solid particle number regulations [15,16]. Solid particle regulations depend on dry measurement methods, where volatile particles are removed before entering the measurement equipment [17]. One significant instrument in the automotive industry is the APC 489 (AVL Particle Counter), which is a product of AVL, a world-renowned automotive supplier and services company based in Austria, Graz. This exhaust gas analyzer is capable of delivering precise data regarding the compliance of diesel-powered vehicle exhaust systems with EU regulations on harmful pollutant emissions [18]. Nevertheless, raw (experiment mode) exhaust gas measurements with the APC may lead to the buildup of larger contaminants on its particle counter, resulting in downtime for the instrument until maintenance is conducted. To address this challenge and optimize the APC’s performance, this study focuses on developing a preparatory unit based on actual exhaust gas experiments. The aim of the development of this preparatory unit is to extend the operational duration of the measurement system between maintenance intervals while maintaining measurement accuracy.

The current state of research in particle counting technology emphasizes the significance of accurate and efficient exhaust gas analysis for addressing air quality concerns and evaluating vehicle emissions [19]. However, the issue of particle counter contamination remains a challenge [20], necessitating innovative approaches to mitigate its impact on measurement results.

In light of these challenges and opportunities, the primary aim of this work is to model and implement an improved aerosol preparation unit for the APC exhaust gas analyzer. The proposed preparatory unit comprises a condensate drainage system, cooling fan, HEPA filter for preliminary particle filtration, membrane pump for pressure control, and interconnecting pipelines. This unit is designed to prevent moisture and larger particle deposition in the APC system, ensuring uninterrupted and accurate exhaust gas measurements. The significance of this research lies in its potential to enhance exhaust gas emission measurements, making them more reliable and suitable for various scientific and industrial applications.

In conclusion, this study aims to optimize the performance of the APC exhaust gas analyzer by introducing a preparatory unit to minimize particle contamination. By providing accurate and reliable measurements, the proposed unit holds promise for broader scientific applications and facilitates compliance with emissions regulations in the automotive industry.

2. Materials and Methods

2.1. Original Layout

The APC (Figure 1), which is based on a condensation particle counting (CPC), is a well-known and widely used device in the automotive research sector.

Condensation particle counting stands as the most widely employed method for determining particle number concentrations of aerosols [21]. In this well-established approach, the aerosol’s solid particles undergo enlargement through the introduction of liquid butanol as a condensing agent. Subsequently, these enlarged particles traverse through a highly sensitive detection chamber, which houses laser light and sophisticated light-sensing optics, meticulously designed for precise particle analysis and quantification [22]. During this transit through the detection chamber, the enlarged particles induce discontinuities in the laser detection mechanism, generating measurable signals that enable the accurate and robust enumeration of particles within the aerosol sample.

In the context of solid particle measurement, the preparation of the sample gas plays a pivotal role in ensuring accurate and reliable data. Notably, for solid particles with diameters over 23 nm, the successful implementation of an evaporation tube (ET) is imperative to facilitate the desired measurements, as shown in Figure 2. Conversely, measuring solid particles with diameters exceeding 10 nm necessitate the utilization of a catalytic stripper (CS) for optimal characterization [23]. It is noteworthy that both the ET and CS elevate the temperature of the sampled gas to levels exceeding 350 °C, thus inducing the evaporation of downstream liquids and ensuring that only solid particles are retained and available for subsequent measurements.

The AVL Particle Counter is employed for the precise determination of vehicle particle number emissions, adhering to the guidelines outlined in “UNECE-R83, Revision 5” and “UNECE-R49, Revision 6, Annex 4 (Emissions of compression ignition and gas-fueled positive ignition engines for use in vehicles)” [24]. These established regulatory frameworks govern the rigorous assessment of particle number emissions across various vehicle categories [25].

During these measurements, the sampling procedure consistently involves the utilization of either a constant volume sampling (CVS) tunnel or a proportional partial flow dilution system. Both of these sampling methodologies are widely adopted among automotive companies for their robustness and accuracy in capturing exhaust emissions from vehicles during testing and Conformity of Production (CoP) evaluations [26].

2.2. N-Layout

In research and development pertaining to engine test bench measurements, elevated exhaust gas particle concentrations are often encountered. In such scenarios, sampling may be conducted at the engine out position, necessitating the use of a specific measurement mode known as the “raw” measurement. This mode is particularly designed to address coarse mode particles and high concentration levels [27]. However, due to the characteristics of these particles, there is a higher likelihood of clogging occurring along the sample aerosol path. Additionally, in environments with high humidity, condensation may occur in the cooler section of the aerosol path.

The presence of condensate and solid particles, following the drying process, can lead to the formation of obstructions, causing passages to narrow and resulting in pressure drops. These pressure drops are detected by the onboard pressure sensors, triggering self-protection mechanisms that inhibit further measurements.

The frequent occurrences of clogging incidents along the aerosol path can have significant implications, necessitating more frequent maintenance procedures and leading to a reduction in the operational lifespan of critical components, such as the Venturi pump, HEPA paper filter, and aerosol pipes. This, in turn, results in increased downtime for testing activities, which can translate to substantial costs. It is important to add that this aerosol path and associated parts are only in use in the case of the raw measurement mode, which is for research purposes.

To effectively address and mitigate the aforementioned challenges, a novel N-layout configuration was devised, as shown in Figure 3.

The N-layout incorporates essential components, such as a condensate drain unit, a dedicated reservoir designed to collect condensed liquids, a high-efficiency particulate air (HEPA) filter, an adjustable power supply, and a diaphragm pump, as visually depicted in Figure 4. By incorporating these elements into the design, the N-layout aims to offer enhanced functionality and performance, facilitating the prevention of clogging incidents, alleviating the negative impacts on component longevity, and reducing the downtime associated with testing activities.

2.2.1. Condensate Drain Unit

The moisture separator or condensate drain unit is designed with specific requirements in mind to ensure optimal performance. Firstly, it should exhibit a low maintenance demand, minimizing the frequency of interventions necessary for its upkeep. Additionally, the unit’s design should facilitate ease of replacement when required, enabling swift and efficient maintenance procedures. Secondly, its core objective lies in the efficient separation of moisture from the aerosol sample, preventing unwanted interference with the measurements performed by the particle counter machine [28].

In addition, a cooling fan has also been incorporated, playing a crucial role in the sub cooling of the condensate unit. This enhancement leads to improved efficiency during the moisture separation process. The cooling fan is connected to a 24 V DC power supply, ensuring automatic activation upon the device’s startup. This automatic operating mode contributes to both efficient and convenient usage, promoting seamless operations throughout the process. The utilization of the cooling fan elevates the performance of the preparatory unit, ensuring reliable and accurate measurements during the aerosol sample separation. The cooling process aids in reducing the extent of condensation, thereby minimizing potential deleterious effects on the aerosol composition [29], further reinforcing the reliability and validity of the measurement results. The cooling fan and the condensation unit can be seen in Figure 5.

2.2.2. High-Efficiency Particulate Air (HEPA) Filter

The HEPA filter plays an important role in the preparatory unit, serving several important functions. Its main purpose is to effectively remove larger particles (with diameters exceeding 0.3 µm) from the system right from the start of the process [30]. These larger particles are not relevant to the measurements but can negatively affect the system’s stability if they enter. When the HEPA filter (Parker, Lancaster NY, USA) is used (as shown in Figure 6), only the particles that are important for the measurements reach the device, ensuring accurate and reliable results. The HEPA filter utilized here is the same one used by AVL GmbH in the sampling/preparation step of the equipment.

2.2.3. Diaphragm Pump

The pump (as shown in Figure 6) for the aerosol system should possess certain desirable characteristics. Firstly, it should not require frequent maintenance to minimize the need for constant interventions. Additionally, it should be easy to replace, allowing for convenient and straightforward upkeep. The pump should also offer the flexibility to adjust the aerosol flow rate, as per the experimental requirements. It should be customizable and controllable, enabling adaptable operation based on specific needs. Furthermore, the pump must demonstrate resistance against the abrasive substances present in the aerosol flowing through the system. This ensures its durability and reliability in challenging environmental conditions. Some specific performance specifications for the pump include an airflow rate of 16 L per min, a maximum pressure value of 2.2 bar, and a maximum achievable vacuum of 220 mbar.

2.2.4. Adjustable Power Supply

The adjustable power supply played a significant role in the N-layout design by enabling the adjustment of the diaphragm pumps’ performance. This adjustability was particularly helpful, as despite the utilization of a condensation unit and a HEPA filter, particle contamination and clogging could still occur over time. Such clogging would lead to an increase in pressure drop within the aerosol preparatory unit, triggering warning signals and eventually stopping the particle counter.

To address this issue, the standard procedure involves cleaning and replacing parts; however, in urgent situations, the adjustable pump offers a faster temporary solution. By increasing the pump’s performance, the adjustable power supply can help to overcome the elevated pressure drop caused by clogging. It allows operation without significant downtime in urgent cases. It is important to note that this is only a temporary solution, as the increased pump performance may lead to potential damage in the long run. Therefore, it is essential to implement proper maintenance and long-term solutions to prevent adverse effects on the pump. Nevertheless, the adjustable power supply can be invaluable in urgent situations, facilitating uninterrupted research testing despite potential clogging issues.

The adjustable power supply powers both the aerosol preparatory diaphragm pump and the CPC diaphragm pump simultaneously. This ensures seamless coordination between the two pumps, promoting efficient research processes. Monitoring pump performance is vital during concurrent operation to avoid issues like excessive pressure drop. Effective management of the power supply and pumps optimizes the aerosol sampling system’s performance and reliability across different experiments.

2.2.5. External Condensation Particle Counter (CPC)

The dilution and volatile particle removal in the N-layout were accomplished using an APC (APC Plus Advanced 10 nm) from AVL List GmbH, Graz, Austria. The APC in this setup is capable of 10 nm measurements and incorporates a catalytic stripper (CS) to enhance the volatile particle removal effect [31].

The secondary particle counter for 23 nm parallel particle measurements utilized in the N-layout is a standalone AVL condensation particle counter (CPC 488), which lacks a sample gas preparation unit, as shown in Figure 7. For measurements of 23 nm and larger particles, the evaporation tube (ET) was employed for volatile particle removal. However, this could create a potential conflict since, with the N-layout, the CPC receives aerosol that has already been treated by the CS. To address this concern, C. Kandlhofer of AVL conducted a comparison experiment [32]. The results demonstrated that the sample gas for measurements below 23 nm can be treated either in the ET or the CS, with a deviation ranging from −6% to +1%. As depicted in Figure 8, any excess sample gas is directed to the ambient air of the test cell after filtration. It is worth noting that the outgoing gas volume flow of the APC is 9 L/min, while the CPC only requires 1 L/min [33]. Thus, the inclusion of an excess air outlet becomes crucial to prevent overpressurization to safeguard the devices.

2.3. Measurement Layout for Verification

To validate the results obtained from both the N-layout and the original layout, practical measurements were conducted. The engine bench tests took place at the Exhaust System Evaluation Department of Ibiden Hungary’s Technical Center. For this purpose, an AVL HD 500 kW engine dynamometer test cell, equipped with advanced engine fluid conditioners, was utilized.

The subject of the tests was a Euro VI-d-compliant diesel engine, and it is shown in Figure 8. These engine bench tests served as a reliable means to compare and verify the outcomes obtained from the two layouts, ensuring the accuracy and credibility of the experimental data.

The experimental layout of the engine bench test is shown in Figure 9.

3. Results

3.1. Measurement Layout for Verification

The N-layout, depicted in Figure 10, was constructed based on the CAD drawings. It comprises several essential components, as follows:

• Figure 10a shows the APC 489 with the N-layout;
• Figure 10b houses the condensate unit and the power supply;
• Figure 10c shows the condensate unit accompanied by a cooling fan;

The CAD-based construction of the N-layout ensures precise and accurate implementation of the experimental setup. The arrangement of these components facilitates efficient aerosol sampling and particle counting, supporting reliable research outcomes.

In addition, Figure 11 shows the diaphragm pump, which is an important component of the N-layout. The diaphragm pump plays a significant role in enabling the adjustable performance of the aerosol preparatory unit and addressing potential clogging issues.

3.2. Comparative Test Results

The World Harmonized Stationary Cycle (WHSC) (as shown in Figure 12) test was employed in the experiment for specific reasons. This test constitutes a steady-state engine dynamometer schedule that has been established as part of the global technical regulation (GTR) No. 4 [33] by the United Nations Economic Commission for Europe Group of Rapporteurs on Pollution and Energy (UN ECE GRPE) [34]. The primary purpose of the GTR is to establish a harmonized heavy-duty certification (WHDC) test procedure for engine exhaust emissions on a global scale.

The WHSC test is structured around two distinct cycles: a hot-start steady-state test cycle (WHSC) and a transient test cycle (WHTC), both encompassing cold- and hot-start requirements. In this case, only the cold start has been used, since it historically results in higher emissions and is sufficient for comparison purposes [35]. These test cycles are designed to simulate typical driving conditions found in various regions, including the EU, USA, Japan, and Australia.

This cycle comprises a sequence of steady-state engine test modes, each defined by specific speed and torque criteria, with predetermined ramps connecting these modes [36]. The normalized modes have been denormalized for the specific test engine.

As can be seen in Figure 13, there is no significant difference between the 10 nm original and N-layout in terms of 10 nm particle measurement.

4. Discussion

Throughout the comprehensive evaluation process, the test outcomes were examined for each distinct mode within the WHSC protocol. This enabled a prudent comparison to be established between the computed final results.

It is important to emphasize that each test was subject to an analysis of transient particle concentrations of the aerosol. The specific emphasis on the 10 nm particle diameter stems from its inherent advantages. By focusing exclusively on particles with a diameter of 10 nm and above during the testing phase, a broader range of particle emissions were available for evaluation. This lower cut point approach superseded the previous 23 nm standard, providing a more refined perspective.

In the experimental results that were obtained through the N-layout configuration, a notable alignment was observed concerning the trajectory of the original layout’s results. It is of significance to highlight that while maintaining a consistent distribution pattern, a marginal decrease was noted solely in two distinct peaks occurring at 850 s and 1200 s (Figure 13). This variation, however, can be reasonably interpreted as part of the normal distribution within the context of the tests. This subtle discrepancy, which remains well within acceptable parameters, does show the results are comparable and the utilization of N-layout does not alter the measurement.

The calculated values obtained throughout the WHSC cycle for both the original and N-layouts exhibit a marginal difference of 1.9%, as presented in Figure 14. It is worth noting that these calculations were performed following the UN/ECE guidelines, with Equation (1) serving as the foundational basis for the computations [37]. The closeness of the results reaffirms the methodology introduced with N-layout and its alignment with established standards. This close correspondence underscores the reliability of the derived values and their relevance in the assessment of the WHSC cycle.

$$\mathrm{SPN}=\frac{\mathrm{EVF}\times \mathrm{PN}}{\mathrm{P}} \left[\#/\mathrm{kWh}\right]$$

(1)

EVF = exhaust volume flow over the WHSC cycle (cm³); PN = average PN count over the WHSC cycle (#/cm³); P = work done over the WHSC cycle (kWh); SPN = solid particle number (#/kWh).

Figure 14. Calculated particle count over WHSC cycle (source: Norbert Biro).

Figure 14. Calculated particle count over WHSC cycle (source: Norbert Biro).

During the initial 6-month probation period of the N-layout, monitoring of breakdowns and failures was conducted. Maintenance procedures specific to the N-layout were implemented, resulting in the absence of breakdowns or failures. Any downtimes that did occur were attributed to mistakes by testing operators, as the new procedures required a learning curve. Routine tasks such as emptying the water separator, evaluating and renewing HEPA filters, and examining aerosol delivery pipes are part of the daily routine. Additionally, the aerosol diaphragm pump undergoes evaluation and refurbishment every two months as necessary. This meticulous maintenance routine serves to uphold the system’s integrity and reliability. Furthermore, the operational expenses of the machine have been reduced by 50%. Altogether, these results demonstrate that the N-layout is a feasible and advantageous upgrade for the APC 489 particle counter in research applications.