Calibration and Experimental Study of a Self-Developed Particle-Number Measurement Instrument

Abstract

To accurately evaluate the size and distribution characteristics of the emission particles exhausted from in-use motor vehicle engines, we independently developed a condensation particle counter (CPC) known as BHCPC. It was calibrated by conducting the calibration procedures stated in the International Standard ISO 27981. After calibration, we conducted on-site measurements and offline sampling analysis of soot particles exhausted from different engines at a motor vehicle inspection center. The calibration results show that the detection efficiency is 90% when the particle diameter is 20.6 nm and the startup response time of the instrument is 3.53 s. The experiment results show that the self-developed BHCPC demonstrates good consistency in measuring particle-number concentration (PNC) in motor vehicle exhaust, with significant count fluctuations only occurring when PNC is higher than 25,000 P/cc. Under idle conditions, motor vehicles compliant with China VI emission regulations exhibit markedly lower exhaust emission PNC compared to those adhering to China IV emission regulations. Moreover, the results obtained from the offline electron microscope analysis show that fuel content in particle samples significantly decreases as engine speed increases, and a similar variation was also found in particle size. The micro-characteristics of the particle can give potential support to the combustion diagnostics.

1. Introduction

PM_2.5, which is exhausted from combustion sources, including automobiles, airplanes, and power plants [1,2,3], plays a significant role in the formation of the atmospheric haze. For the PM_2.5 generated from motor vehicle engines, the number of nanoparticles whose diameter is less than 100 nm accounts for over 90% [4,5]. These nanoparticles can easily adsorb polluting gases to form secondary aerosols in the atmosphere, carrying viruses and bacteria and posing great harm to the human body. So far, the assessment of the harm of exhaust particulate matter based solely on mass concentration indicators is not comprehensive. Although the mass proportion of numerous ultrafine particles is small, the harm is even higher. Compared to new cars, in-use motor vehicles have a higher level of particulate matter emissions. It is no longer sufficient to evaluate the degree of exhaust emissions by relying solely on the mass concentration of particulate matter. Therefore, some countries, including the Netherlands and Finland [4], have taken the lead in promoting exhaust gas detection using particle-number measurement technology for in-use motor vehicles [6].

The pollutants exhausted from motor vehicles encompass harmful gases such as carbon dioxide (CO₂), nitrogen oxides (NO_x), carbon monoxide (CO), and volatile organic compounds, as well as particulate matter [7,8]. Currently, the detection technologies for gaseous pollutants are relatively mature and include laser gas analyzers for measuring CO₂ and NO_x concentrations [9,10], as well as opacity meters for assessing smoke opacity [11]. However, as the awareness of the hazards associated with particulate emissions has grown, governments and environmental agencies have introduced stricter regulations and standards for motor vehicle emissions, along with heightened testing requirements. Consequently, there has been a significant shift in recent years from traditional particle mass (PM) measurements toward particle-number (PN) measurements.

As a reliable method for particle-number counting, CPC has been widely applied in the accurate measurements of fine particles with diameters less than 100 nm [12]. In a CPC instrument, the saturated working fluid vapor heterogeneously condenses on the surface of the nanoparticle, and then the nanoparticle can grow, and its size can reach the micron scale. Therefore, micro-scale particles can be precisely counted with optical sensors. In comparison to the electrometer [13], CPC offers higher sensitivity and accuracy for particles with smaller sizes. CPC has been commercially applied by several companies, such as TSI Inc. (Shoreview, MN, USA) [14], Grimm Inc. (Berlin, Germany) [15], AVL Inc. (Graz, Austria) [16], and Horiba Inc. (Tokyo, Japan) [17]. The CPC instrument parameters from these companies are listed in

Table 1. Performance Comparison of Leading Commercial CPCs.

Parameters	TSI 3775	TSI 3750	Grimm	AVL	Horiba
sampling temperature (°C)	10~35	10~35	40	5~45	−10~40
detection range (nm)	4~3000	7~3000	>3	23~2500	23~1000
PNC 2 (P/mL)	0~50,000	0~100,000	150,000	0~10,000	0~10,000
D50 1 (nm)	4	7	9	_	_
working fluid	n-Butanol	n-Butanol	n-Butanol	n-Butanol	Isopropanol

¹ PNC: particle-number concentration. ² D50: 50% particle detection efficiency.

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It can be observed that the mainstream commercial CPCs have improved the instrument detection limit to below 10 nm, and the highest particle detection concentration can reach 10⁶ particles per milliliter (P/mL) in single-particle counting mode. These commercial CPCs have also been widely used in the detection of motor vehicle engine-emission particles. Wang Jiasong et al. [18] used instruments from TSI Inc., Shoreview, MN, USA, to study the ultrafine particle emissions from diesel, gasoline vehicles, and liquefied petroleum gas taxis under different conditions. The results showed that diesel vehicles contribute more to particles with diameters in the range of 30~150 nm, while liquefied petroleum gas and gasoline vehicles contribute more to particles with diameters of 15~30 nm. Hu Zhiyuan et al. [19] used a TSI scanning mobility particle sizer (SMPS) to study the particle-number and size distribution of cylinder direct injection engines that meet the China VI emission standards under the World Light Motor Vehicle test cycle (WLTC). China VI emission standards were implemented on 1 July 2020 and are still in force. The results showed that under a cold start, instantaneous transition conditions, and high-speed heavy load conditions, the emission of ultrafine particles significantly increased, and the overall emission showed a single peak distribution with a peak value of around 15 nm. Ge Yunshan et al. [20] studied the impact of continuous regeneration diesel particulate filter (CR-DPF) systems on the particle size and mass distribution of diesel engine emissions and found that the number of particles also increases as motor vehicle speed increases. Jiang Jingkun et al. [12] used Dekati’s electrical low-pressure impactor (ELPI) and two types of CPCs from TSI Inc., Shoreview, MN, USA (Model 3790 and Model 3776) to study particulate emissions from gasoline motor vehicles. They found that when using a CPC with a measurement lower limit of 2.5 nm, the particle-number concentration was 35.0% (for cylinder direct injection engine) and 50.4% (for intake manifold injection engine) higher than that measured by a CPC with a cut-off diameter of 23 nm specified in regulations. This implies that current regulatory testing may underestimate the actual number of particles emitted by light-duty gasoline motor vehicles. However, in-depth research on the detection technology for ultrafine particle-number emissions from motor vehicles in China is still insufficient. The relevant measurement technology and equipment are still lacking in China, and domestic institutions mainly rely on imported commercial equipment for related research. Therefore, it is significant to independently develop equipment suitable for detecting particle-number concentrations from in-use motor vehicles that meet China IV emission standards.

In this work, to break the technology limits for particle counter instruments in China, we independently developed a CPC instrument based on the principle of condensation nucleus growth counting. The self-developed BHCPC can be used to detect the PNC in the exhaust emissions of in-use vehicles that meet China VI emission standards. The CPC instrument was strictly calibrated with calibration experiments according to the standard instrument calibration method stated in the International Standard ISO 27981 [21]. Furthermore, to evaluate the reliability and accuracy of the self-developed BHCPC, we conducted an engine-emission measurement experiment with the CPC at the vehicle inspection center. Two different types of engines that meet China VI and China IV emission standards were used in the experiment, respectively. These research results are expected to provide important instruction for the development of exhaust emission detection methods for in-use vehicles.

2. Experimental Apparatus and Methods

2.1. Calibration and Consistency Testing of the Self-Developed BHCPC

A schematic diagram of the self-developed BHCPC is shown in Figure 1. The core components of the BHCPC include a saturator, a condenser, and an optical counting unit. The working temperature of the saturator is 50 °C, which can be used to heat the n-butanol, providing a stable source of saturated steam for particle condensation. Particles undergo condensation growth in the condenser (working temperature 6 °C). When the sample particles enter the condenser, the supersaturated n-butanol vapor will condense on the particle surfaces due to the temperature diffusion, causing the particle diameter to increase from the nanometer level to the submicron or micron level. They then enter the optical counting unit, where they are irradiated by a laser. The scattered signal generated is collected by a photodiode and converted into an electrical pulse, ultimately achieving the goal of particle counting.

The calibration experiment of the self-developed BHCPC instrument was conducted in a clean room certified by the China National Accreditation Service for Conformity Assessment (CNAS), strictly following the method required by the standard document ISO 27891 for CPC instrument calibration [21], as shown in Figure 2. An air compressor generates a pressure of 3 bar and connects to an aerosol generator to produce sodium chloride (NaCl) particles. After removing moisture through a drying tube, the particles are depressurized and then passed into a Differential Mobility Analyzer (DMA) to obtain 50 nm nanoparticles. The self-developed BHCPC and an electrometer (TSI 3068b) are used for number measurement within the particle concentration range of 0~25,000 P/cc. The parameters of the calibrated self-developed BHCPC instrument, such as detection efficiency, startup response, and transient response, can be obtained via the experiment.

For the detection efficiency experiment, the DMA was used for particle diameter screening, ensuring that the NaCl particle concentration can be stabilized at 3000 P/cc in the range of 8 nm to 50 nm. The self-developed BHCPC and the electrometer are both used to measure the detection efficiency under a particle diameter of 850 nm.

For the instrument response experiment, the diameter of the particle source was set to 50 nm, and the number concentration was fixed at 1000 P/cc. The data were recorded every 0.1 s for a total duration of 5 s. The timing started from the moment that particles entered the intake of the self-developed BHCPC and ended when 90% of the particle source concentration was detected. This time interval is considered to be the startup response time of the CPC instrument.

2.2. In-Use Vehicle Exhaust Emission Testing Apparatus and Procedures

In-use vehicle exhaust emission testing at the Beijing Xueyuan Road Motor Vehicle Inspection Station was conducted in accordance with “China VI Emission Standards—Limits and Measurement Methods for Emissions from Light-duty Vehicles (GB18352.6—2016)” [22]. The sampling and measurement equipment were arranged as shown in Figure 3. A sampling probe was installed at the exhaust pipe outlet of the vehicle to collect the sample gas. The sample gas was then transported through a 5 m conductive transmission pipeline into diluter 1. The supply pressure for diluter 1 and diluter 2 ranged between 2 and 3 bar, and all dilution gases had to pass through high-efficiency air filters (Filters 1 and 2) before entering the diluter. During the experiment, the dilution ratio was adjusted based on the particle-number concentration level of the sampled gas. The diluted sample gas sequentially entered the cyclone separator and the volatile particle remover (VPR). The flow rate at the inlet of the cyclone separator was controlled at 9.5~10.5 lpm to remove particles larger than 2.5 μm. The temperature of the VPR was maintained at 350℃. At the outlet of the VPR, a DKL-2 single-particle sampler was used to collect exhaust particles, which were subsequently analyzed offline using scanning electron microscopy (SEM). The remaining sample gas particles were mixed evenly through a mixing tube and then introduced into the three self-developed BHCPCs and TSI CPC 3775 for particle-number concentration measurements. This process was designed to verify the accuracy and consistency of the self-developed BHCPC instruments. Filter 3 in the diagram was used to balance the pipeline pressure and prevent particles from entering the experimental environment and causing contamination.

Before conducting the experiments, the pipeline leakage check, pipeline cleanliness check, and instrument preheating also needed to be carried out. Moreover, a background measurement of the internal environment of the exhaust pipe under the non-working state of the car engine is also needed to ensure that the particle concentration of the exhaust pipe and experimental pipeline does not affect the counting. The vehicle engine was operated with the double idle speed method. The engine-related parameters of the Volvo S60L T5 and VGV U75PLUS used in the experiment are shown in

Table 2. Parameters of the engine tested in the present study.

Type	HY4C20B	B5204T9
Displacement (mL)	1967	1984
Maximum Power (kW)	165	157
Maximum Torque (N·m)	5500	6000
Speed at Maximum Torque (rpm)	385	300
Maximum Horsepower (Ps)	224	214

, which are engines that meet the China IV and China VI emission standards, respectively.

2.3. Data Processing Method

The detection efficiency of the instrument can be determined by the following equation.

$$E_{ce}=\frac{N_{CPC}}{N_{EPC}}\times 100\%$$

(1)

where E_ce represents the detection efficiency, N_CPC is the particle-number concentration detected by the self-developed BHCPC. N_EPC is the particle-number concentration detected by the electrometer.

Furthermore, to evaluate the measurement consistency of the self-developed BHCPC, we proposed a consistency-related evaluation indicator in this work.

$$A_{ie}=\frac{\left[A_i-F_i\right]}{A_i}$$

(2)

$$\overline{A_{ie}}=\frac{1}{n}\sum _{i=1}^n{A}_{ie}\times 100\%$$

(3)

where A_i is the particle-number concentration measured by the self-developed BHCPC, and F_i is the particle-number concentration obtained by the electrometer or TSI CPC. A_ie represents the proportion of absolute error for each measurement data point, n is the total sample points, and $\overline{A_{ie}}$ is the average percentage error of all measurement points.

$$S_{ie}=\sqrt{\frac{\sum _{i=1}^n{(A_{ie}-\overline{A_{ie}})}^2}{(n-1)}}$$

(4)

where $S_{ie}$ is the standard deviation of the error. Ultimately, $\overline{A_{ie}}$ and $S_{ie}$ are obtained as evaluation indicators for the measurement consistency of the self-developed BHCPC instrument.

3. Discussion and Analysis

3.1. Comparison of Self-Developed BHCPC and Electrometer Counting

Figure 4 compares the particle-number concentration measured by the self-developed BHCPC and the TSI 3068b electrometer. As can be seen from Figure 4, the counting results of the self-developed BHCPC and the electrometer show a relatively consistent trend over time, with the maximum error not exceeding 20%, showing a positive correlation. When the particle-number concentration is in the range of 12,500 to 15,750 P/cc, there is almost no difference between the measurement results of the self-developed BHCPC and the electrometer. However, when the particle-number concentration is in other ranges, there is a deviation between the measurements of the self-developed BHCPC and the electrometer. In the work, we calibrate the self-developed BHCPC using a quadratic polynomial fitting method according to the standard document ISO 27891 [21]. The final fitting result is shown in Figure 5. The fitting results show that the obtained quadratic fitting curve is:

$$\mathrm{y}=0.72537\times \mathrm{x}+1.7938\times {10}^3$$

(5)

where y represents the count result of the electrometer, and x represents the count result of the self-developed BHCPC. The goodness of fit R² equals 0.999, indicating that the fitting effect is excellent.

3.2. Detection Efficiency

The efficiency test of the self-developed BHCPC was conducted after calibration. Measurements were made using a particle source with a particle diameter range of 8 nm to 50 nm and a number concentration of 3000 P/cc. The average measurement results for each particle diameter were fitted with a polynomial, and the fitting result is shown in Figure 6. The R²-value of the fitting curve is 0.995, indicating that the curve can effectively reflect the trend of experimental measurements [23]. Therefore, according to the fitting curve, when the detection efficiency of the instrument reaches 90%, the corresponding particle diameter is 20.6 nm. When the particle diameter reaches 27 nm, the instrument can achieve a detection efficiency of 99%. In addition, when the particle diameter is larger than 27 nm, the detection efficiency of the instrument fluctuates slightly, but it is all greater than 98%, indicating that the instrument has higher detection accuracy for measuring larger particles. According to the requirements of the China VI motor vehicle emission regulation [22], the detection efficiency of the particle counter should reach 50 ± 12% when the particle diameter is 23 ± 1 nm and should be greater than 90% when the particle diameter is 41 ± 1 nm. The measurement results show that the self-developed BHCPC fully complies with regulatory requirements and can be used for measuring particle-number concentration in vehicle exhaust emissions.

3.3. Instrument Time Response

Figure 7 shows the time response results of the instrument startup and uses the Boltzmann method of the software originLAB 9.0 to fit the instrument measurement results. As can be seen from Figure 7, the sample gas to be measured enters the sample gas inlet of the self-developed BHCPC at 1 s, and the BHCPC instrument reaches 90% detection efficiency at 3.53 s, and after 4 s, the instrument detection efficiency reaches 98%, and then remains stable. From Figure 7, it can be seen that the startup response time of the instrument is 2.53 s. This response time is mainly affected by factors such as the sample gas mass flow rate of the self-developed BHCPC and the structure of the gas pipeline. The China VI motor vehicle emission regulation also stipulates the startup response time of the instrument [22]. The regulations require that the T90 response time of the instrument within the measurement concentration range should not exceed 5 s. The self-developed BHCPC instrument can also meet this requirement.

3.4. Consistency Evaluation

The intake ports of three CPCs were connected in parallel and tested with an unstable particle source. The particle-number concentration range was from 600 to 56,000 P/cc, and the measurement results are shown in Figure 8. As can be seen from Figure 8, when the concentration is less than 10,000 P/cc, the test results fluctuate less. As the particle-number concentration increases, the counting results of the CPC fluctuate more. In the experiment, the measurement result of BHCPC-1 was selected as the reference value, and the other two CPCs were evaluated for consistency. When the particle-number concentration is less than 10,000 P/cc, the value of BHCPC-2 is 5.3%, and that of BHCPC-3 is 2.1%. When the particle-number concentration is higher than 10,000 P/cc, the value of BHCPC-2 is 1.9%, and that of BHCPC-3 is 1.8%. This is because when calculating the percentage error for low concentrations, the error value of the measured number concentration accounts for a higher proportion. In general, the three CPCs are consistent with each other.

3.5. Analysis of Vehicle Exhaust Measurement Results

The measurements of the exhaust gas of two in-use vehicles were also conducted with the developed CPC. In the experiment, the double idle speed method was used to measure the number concentration of exhaust particles, i.e., the engine condition was quickly raised from low idle speed to high idle speed, maintained at high idle speed for more than 15 s, and then returned to low idle speed. Due to the difference in the emission particle-number concentration, the dilution ratio for the China VI vehicle engine test was set to 15, while for the China IV vehicle, it was 225. A similar approach was also adopted in previous studies [24,25]. The measurements are shown in Figure 9. For both China VI and China IV vehicles, after the engine speed is increased, the particle-number concentration shows an exponential growth trend. With the maintenance of high idle speed, the number concentration decreases, and when it returns to low idle speed, the number concentration quickly returns to a lower level. It is worth noting that the peak number concentration of the China VI vehicle reached 1.17 × 10⁵ P/cc, while that of the China IV vehicle reached 6.41 × 10⁶ P/cc. Under the same idle conditions, the number concentration of particles in the exhaust gas of China VI vehicles is much lower than that of China IV vehicles. Overall, the measurement results of three self-developed BHCPCs are consistent with TSI CPC measurement results. To quantitatively obtain the consistency degree of the self-developed instrument, the TSI CPC 3775 results were adopted as a benchmark. The peak error evaluation results of the self-developed BHCPCs are shown in Figure 10. We extracted 15 s of data around the peak value to calculate the peak error with Formulas (2)–(4). For two different models of vehicle exhaust tests, the maximum discrepancy of BHCPC-1 is 2.52%, BHCPC-2 is 9.26%, and BHCPC-3 is 4.29%, all less than 10%. Therefore, it can be considered that self-developed BHCPC has high accuracy for measuring China VI and China IV vehicles’ exhaust.

The collected exhaust particle samples were analyzed for microscopic morphology and energy spectrum using a scanning electron microscope, as shown in Figure 11. Figure 11a–c correspond to the results obtained when the engine speed is 700 rpm, 2000 rpm, and 4000 rpm, respectively. It can be seen that as the engine speed increases, the fuel content in the collected particle samples significantly decreases. At the same time, the particle diameter also significantly decreases. This is because after the speed increases, the engine has a better combustion performance, so there is less oil and smaller particles in the collected exhaust.

Figure 11d–f are the magnified images of Figure 11a–c, respectively. Figure 11g–i are the corresponding energy spectrum analysis charts of the samples. As can be seen from Figure 11d, when the engine runs at the speed of 700 rpm, most of the particles are wrapped in fuel, and the soot particles are completely submerged, with a small part appearing in a semi-exposed state. This type of soot particle was also found in the studies [26,27]. The energy spectrum analysis results show that the oxygen content of the particles ranges from 40% to 20%, and the carbon-oxygen ratio of the particles corresponding to Figure 11g is 65:35. When the engine speed is 2000 rpm, as shown in Figure 11e, it is still dominated by fully wrapped carbon particles and semi-exposed carbon particles. However, after performing energy spectrum analysis, it is found that the oxygen content of the particles ranges from 35% to 15%. A representative result is shown in Figure 11h, where a carbon-oxygen ratio of 85:15 appears. Figure 11f is a typical situation when the engine speed is at 4000 rpm. It can be seen that there are almost no fuel-wrapped particles. Meanwhile, the size of the collected particles is significantly smaller than that collected at a speed of 2000 rpm, and the soot particles appear exposed, forming a long chain structure composed of typical spherical objects. The typical chain structure was also found in the literature [28,29]. For this operating condition, carbon-oxygen ratio energy spectrum analysis was performed on soot particles. It was found that the proportion of oxygen content in the sample dropped to a range of 0 to 30%. Figure 11i shows a particle with a carbon content as high as 100%. According to the above analysis, it can be concluded that as the engine speed continues to increase, fuel combustion becomes complete, and the oxygen content in exhaust particles gradually decreases.

4. Conclusions

The following conclusions were obtained through the calibration experiment of the self-developed instrument CPC and the exhaust emission detection experiment of in-use vehicles:

• After calibration, the self-developed BHCPC instrument can achieve a detection efficiency of 90% for fine particles with a diameter above 20.6 nm. The 2.53 s startup response of the instrument can be eliminated by time compensation.
• The consistency test results of CPC show that there is a relatively drastic fluctuation in counting when the concentration reaches as high as 25,000 P/cc. Considering that in actual exhaust emission detections, the exhaust particle concentration after dilution will not exceed 20,000 P/cc, it is believed that the self-developed BHCPC can meet the needs of exhaust emission detection experiments.
• Under the same idle conditions, the particle-number concentration of exhaust emissions from in-use vehicles that meet China VI emission standards regulations is much lower than that of in-use vehicles that meet China IV emission standards regulations. The self-developed BHCPC has high accuracy for measuring exhaust emissions from China VI and China IV in-use vehicles.
• The offline electron microscope detection results show that as the engine speed increases, the fuel content in the collected particle samples significantly decreases, and the particle diameter also significantly decreases.