Non-volatile Particle Research Articles

Methane slip and other emissions from newbuild LNG engine under real-world operation of a state-of-the art cruise ship

Liquefied natural gas (LNG) use as shipping fuel has increased in recent years. While LNG results in lower carbon dioxide (CO2) emissions as well as benefits in terms of air pollutants, the slip of unburned methane, the main component of LNG, has remained a concern. In this study, methane together with other climate warming agents, CO2 and black carbon (BC), as well as other emission compounds were characterized from 4-stroke low-pressure dual fuel engine on-board a newly build cruise ship utilizing LNG as well as marine gas oil (MGO). The brake specific methane slip was found to vary according to engine load, being 2.3–3.0 g/kWh at 54–80% loads, but increasing to 10 g/kWh at 25% load and 21 g/kWh at 12% load. The LNG combustion also resulted in higher formaldehyde emissions compared to MGO, but reduction in formaldehyde levels was observed over the SCR catalyst present in the exhaust line of the dual-fuel engine, without urea injection, suggesting it may provide a pathway for formaldehyde mitigation. In terms of particle emissions, LNG use reduced particle mass (PM) by 87–93% and BC by 94–99% compared to MGO combustion. Non-volatile particle number above 23 nm (PNnv,>23nm) and 10 nm (PNnv,>10nm) were reduced by 88–97% and 97–99%, except at lowest engine load where PNnv,>10nm increased by 26% compared to MGO utilization. When total greenhouse gas (GHG) emissions including CO2 and BC were considered, LNG use resulted in 13–15% lower GHG at high loads, but the benefit was undermined by the escaping methane at low load conditions. Following the engine activity profile during 8-months of vessel operation on the Mediterranean suggested, however, that in a diesel-electric cruise ship, low load conditions are used mainly during arrivals and departures from harbors, as the engine was operated at loads above 40% for 90% of the operation time. Weighted emission factor, representing the actual engine operation, resulted in methane slip of 2.8 g/kWh or 1.7% of the fuel use, which is below the value considered in the FuelEU Maritime. The results suggest that load specific methane slip, together with engine load profile should be considered when evaluating methane slip on vessel or fleet level.

Jet aircraft lubrication oil droplets as contrail ice-forming particles

Abstract. The radiative characteristics and lifetimes of contrails are dependent on the number concentration of ice-forming particles in the engine exhaust plume. Aircraft gas turbine engines produce a variety of particles, yet it is understood that non-volatile black carbon aggregates are the dominant source of ice-forming particles with typical, fossil-derived jet fuel. However, with cleaner combustion technologies and the adoption of alternative fuels (e.g. hydrogen or synthetic aviation fuel), non-volatile black carbon particle emissions are expected to decrease or even be eliminated. Under these conditions, contrail properties will depend upon the concentration and characteristics of particles other than black carbon. Ultrafine (< 100 nm) jet lubrication oil droplets constitute a significant fraction of the total organic particulate matter released by aircraft; however, their ability to form contrail ice crystals has hitherto been unexplored. In this work, we experimentally investigate the activation and freezing behaviour of lubrication oil droplets using an expansion chamber, assessing their potential as ice-forming particles. We generate lubrication oil droplets with a geometric mean mobility diameter of (100.9 ± 0.6) nm and show that these activate to form water droplets, which subsequently freeze when the temperature is below ∼ 235 K. We find that nucleation on lubrication oil droplets should be considered in future computational studies – particularly under soot-poor conditions – and that these studies would benefit from particle size distribution measurements at cruise altitude. Overall, taking steps to reduce lubrication oil number emissions would help reduce the climate impact of contrail cirrus.

Recommendations for emission testing and control of exhaust particles from a late technology mono-fuel CNG vehicle

The current study characterizes sub-23nm volatile and non-volatile particle emissions from a late technology (Euro 6d-temp) port-fuel injection vehicle, operating with compressed natural gas (CNG) as the primary fuel and E10 gasoline, as a reserve fuel. In part of the testing, gasoline was replaced with alkylate fuel or was doped with an octane booster (MMT), to understand fuel impacts better. Alkylate use resulted to the lowest total particle number (TPN) emission levels, below that of the Euro 6 limit. Increased sensitivity to primary sampling temperature was observed for CNG due to the high H/C ratio that causes water condensation in the exhaust. A minimum sampling temperature of 70oC is recommended to avoid water particles in CNG operation. The study also assessed the potential of a particle filter (PF) in controlling total and solid CNG exhaust particles. Results indicate effective PF filtration of non-volatile and volatile particles, especially in the sub-23 nm size range. Gasoline and CNG tests with PF suggest TPN10 emissions in the order of 5 × 1011 p/km, below the Euro 6 and Euro 7 regulation limit. Adopting PF as part of the aftertreatment layout would be required for control of solid and total particle emissions from CNG vehicles.

Multi-stage oxidation of iron particles in a flame-generated hot laminar flow

The paper presents multi-parameter high-speed optical diagnostics of single iron particle oxidation in a hot laminar flow consisting of O2, H2O, CO2, and N2. Prior to luminous combustion, micron-sized particles are visualized using diffuse-backlight-illumination with high temporal and spatial resolutions, enabling in situ particle sizing. The incandescent burning particles are then monitored by direct high-speed imaging. This approach allows for a successful determination of the multiple oxidation stages of individual iron particles, including the solid phase, melting, and liquid phase. Two characteristic timescales are quantified as a function of the resolved particle diameter at various O2 volume fractions, namely the solid-phase oxidation time (SOT, defined as the time between the onset of rapid particle heating and melting) and the liquid-phase combustion time (LCT, defined as the duration between melting initiation and peak luminosity). SOT shows a linear dependence on the inverse O2 mass fractions at relatively high O2 levels but it tends to plateau when the O2 content is below 20vol.%. LCT is inversely proportional to the O2 mass fraction, suggesting that liquid-phase oxidation of iron particles is limited by external oxygen diffusion. The experimental LCT quantitatively agrees with a theoretical mode developed for nonvolatile particle combustion in the diffusion-limited regime, considering O2 as the only oxidant. This also implies that at the presence of O2, the oxidizing effect of H2O and CO2 on diffusion-limited iron particle combustion is negligible.

Sub-23 nm Particles Dominate Non-Volatile Particle Number Emissions of Road Traffic.

Ultrafine particles (<100 nm) in urban air are a serious health hazard not yet fully understood. Therefore, particle number concentration monitoring was recently included in the WHO air quality guidelines. At present, e.g., the EU regulates particle number only regarding the emissions of solid particles larger than 23 nm emitted by vehicles. The aim of this study was to examine the non-volatile fraction of sub-23 nm particles in a traffic-influenced urban environment. We measured the number concentration of particles larger than 1.4, 3, 10, and 23 nm in May 2018. Volatile compounds were thermally removed in the sampling line and the line losses were carefully determined. According to our results, the sub-23 nm particles dominated the non-volatile number concentrations. Additionally, based on the determined particle number emission factors, the traffic emissions of non-volatile sub-10 nm particles can be even 3 times higher than those of particles larger than 10 nm. Yet, only a fraction of urban sub-10 nm particles consisted of non-volatiles. Thus, while the results highlight the role of ultrafine particles in the traffic-influenced urban air, a careful consideration is needed in terms of future particle number standards to cover the varying factors affecting measured concentrations.

High Particle Number Emissions Determined with Robust Regression Plume Analysis (RRPA) from Hundreds of Vehicle Chases.

Particle number emission factors were determined for hundreds of individual diesel and gasoline vehicles in their real operation on Finnish highways and regional roads in 2020 with one-by-one chase measurements and Robust Regression Plume Analysis (RRPA). RRPA is a rapid way to analyze data from a large number of vehicle chases automatically. The particle number emission factors were determined for four ranges of particle diameters (>1.3, > 2.5, > 10, and >23 nm). The emission factors for most of the measured vehicles were observed to significantly exceed the non-volatile particle number limits used in the most recent European emission regulation levels, for both light-duty and heavy-duty vehicles. Additionally, most of the newest vehicles (covering regulation levels up to Euro 6), for which the particle number emission regulations (non-volatile >23 nm particles) apply, showed emission factors of the >23 nm particles clearly above the regulation limits. Although the experiments included measurements of real-world plume particles (mixture of non-volatile and semi-volatile particles) and not only the non-volatile regulated particles, it is important to note that the emissions of regulated particles were also estimated to exceed the limits, based on non-volatile >23 nm particle fraction from curbside studies. Moreover, the emission factors of the >1.3 nm particles were mostly about an order of magnitude higher compared to the >23 nm particles.

Performance of a Wet Electrostatic Precipitator in Marine Applications

Emissions of marine traffic can be lowered by switching to less polluting fuels or by investing in exhaust aftertreatment. Electrostatic precipitation is a widely used method for particle removal but it is not currently used in combination with marine engines. This study presents the particle filtration characteristics of an emission reduction system designed for marine applications and consisting of a scrubber and a Wet Electrostatic Precipitator (WESP) in series. Partial flow of exhaust from a 1.6 MW marine engine, operated with light and heavy fuel oil, was led to the system. Particle concentrations were measured before the system, after the scrubber and after the WESP. Particle removal characteristics were determined for different engine loads. The scrubber alone removed 15–55% of non-volatile particle number, 30–40% of particle mass and 30–40% of black carbon mass depending on engine load, when HFO fuel was used. By studying particle size distributions, scrubber was found also to generate particles seen as an additional mode in 20–40 nm size range. The system combining the scrubber and WESP removed over 98.5% of particles in number, mass and black carbon metrics when HFO fuel was used. With MDO fuel, 96.5% of PN and 99% of black carbon were removed.

Targeted Use of Sustainable Aviation Fuel to Maximize Climate Benefits.

Sustainable aviation fuel (SAF) can reduce aviation's CO2 and non-CO2 impacts. We quantify the change in contrail properties and climate forcing in the North Atlantic resulting from different blending ratios of SAF and demonstrate that intelligently allocating the limited SAF supply could multiply its overall climate benefit by factors of 9-15. A fleetwide adoption of 100% SAF increases contrail occurrence (+5%), but lower nonvolatile particle emissions (-52%) reduce the annual mean contrail net radiative forcing (-44%), adding to climate gains from reduced life cycle CO2 emissions. However, in the short term, SAF supply will be constrained. SAF blended at a 1% ratio and uniformly distributed to all transatlantic flights would reduce both the annual contrail energy forcing (EFcontrail) and the total energy forcing (EFtotal, contrails + change in CO2 life cycle emissions) by ∼0.6%. Instead, targeting the same quantity of SAF at a 50% blend ratio to ∼2% of flights responsible for the most highly warming contrails reduces EFcontrail and EFtotal by ∼10 and ∼6%, respectively. Acknowledging forecasting uncertainties, SAF blended at lower ratios (10%) and distributed to more flights (∼9%) still reduces EFcontrail (∼5%) and EFtotal (∼3%). Both strategies deploy SAF on flights with engine particle emissions exceeding 1012 m-1, at night-time, and in winter.

Nucleation-accumulation mode trade-off in non-volatile particle emissions from a small non-road small diesel engine.

Small (< 8kW) non-road engines are a significant source of pollutants such as particle number (PN) emissions. Many small non-road engines do not have diesel particulate filters (DPFs). They are so designed that air-fuel ratio (AFR) can be adjusted to control visible diesel smoke and particulate matter (PM) resulting from larger accumulation mode particles. However, the effect of AFR variation on smaller nucleation mode nanoparticle emissions is not well understood. Several studies on larger engines have reported a trade-off between smaller and larger particles. In this study, AFR was independently varied over the entire engine map of a naturally aspirated (NA) non-road small diesel engine using forced induction (FI) of externally compressed air. AFR's ranged from 57 to 239 compared to the design range of 23-92 for the engine, including unusually high AFR's at full-load operation, not previously reported for conventional combustion. As expected, larger accumulation mode particles were lowered (up to 15 times) for FI operation. However, the smaller nucleation mode nanoparticles increased up to 15 times. Accumulation mode particles stopped decreasing above an AFR threshold while nucleation particles continuously increased. In-cylinder combustion analysis showed a slightly smaller ignition delay and higher burn rate for FI cases relative to NA operation. Much higher peak cylinder pressures were accompanied by much lower combustion and exhaust gas temperatures (EGT), due to higher in-cylinder mass during FI operation. Peak nucleation mode emissions were shown to be negatively correlated to EGT for all the data, collapsing on a single curve. This is consistent with some other studies reporting increased nucleation mode emissions (and higher accumulation mode particles) with decreased load, lower speed, lower EGR, advanced combustion phasing, and higher injection pressure, all of which reduce EGT. The nucleation-accumulation trade-off has been explained by the 'adsorption hypothesis' by some investigators. In the current work, an alternative/supplemental argument has been made for the possibility that lower cylinder temperatures during the late-burning phase (correlated to lower EGT) phase hampers oxidation of nucleation mode particles and increases nucleation mode emissions.

Aircraft engine particulate matter emissions from sustainable aviation fuels: Results from ground-based measurements during the NASA/DLR campaign ECLIF2/ND-MAX

The use of alternative jet fuels by commercial aviation has increased substantially in recent years. Beside the reduction of carbon dioxide emission, the use of sustainable aviation fuels (SAF) may have a positive impact on the reduction of particulate emissions. This study summarizes the results from a ground-based measurement activity conducted in January 2018 as part of the ECLIF2/ND-MAX campaign in Ramstein, Germany. Two fossil reference kerosenes and three different blends with the renewable fuel component HEFA-SPK (Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosene) were burned in an A320 with V2527-A5 engines to investigate the effect of fuel naphthalene/aromatic content and the corresponding fuel hydrogen content on non-volatile particle number and mass emissions. Reductions up to 70% in non-volatile particle mass emission compared to the fossil reference fuel were observed at low power settings. The reduction trends to decrease with increasing power settings. The fuels showed a decrease in particle emission with increasing fuel hydrogen content. Consequently, a second fossil fuel with similar hydrogen content as one of the HEFA blends featured similar reduction factors in particle mass and number. Changes in the fuel naphthalene content had significant impact on the particle number emission. A comparison to in-flight emission data shows similar trends at cruise altitudes. The measurements highlight the importance of individual fuel components in regulating engine emissions, particularly at the low thrust settings typically employed during ground operations (e.g. during idle and taxi). Therefore, when selecting and mixing SAF blends to meet present fuel-certification standards, attention should be paid to minimizing complex aromatic content to achieve the greatest possible air quality and climate benefits.

Calibration Uncertainty of 23nm Engine Exhaust Condensation Particle Counters with Soot Generators: a European Automotive Laboratory Comparison

Calibration of condensation particle counters (CPC) to measure non-volatile particle number (PN) from vehicle emissions is a significant source of uncertainty of the regulated particle number measurements. In this work, the calibration uncertainty of automotive and calibration laboratories was determined in a first-of-its-kind comparison. For this purpose, the counting efficiency of a reference CPC for automotive exhaust emission measurements was determined at seven participants across Europe with ten soot aerosol generators. Calibration uncertainty was found to be very different in the CPC’s cut-off regime (around the D50 of 23nm) with a coefficient of variation (CoV) of 11% and the plateau regime (from the D90 of 41nm upwards) with a CoV of 4.5%. The uncertainty was higher for a group of soot generators with poorly optimized operating points with a CoV of 31% at 23nm and 5.8% at ≥41nm. Specific influence factors on the calibration uncertainty (measured as the inter-lab variability) could be identified. The calibration of the laboratories’ reference counters accounted for most of the variability in the plateau regime, while 20% of the variability was attributed to the sample flow measurement. Differences between soot generators were the main cause of variability in the cut-off regime due to the increased material sensitivity of the CPC at this particle size but had only secondary relevance in the plateau regime. The calibration uncertainty found in this inter-laboratory exercise should be a guideline for users and legislators, as it provides a typical value for the expected measurement uncertainty of a CPC for automotive exhaust PN.

Characteristics of sub-10 nm particle emissions from in-use commercial aircraft observed at Narita International Airport

Abstract. The characterization of ultrafine particle emissions from jet aircraft equipped with turbofan engines, which are commonly used in civil aviation, is an important issue in the assessment of the impacts of aviation on climate and human health. We conducted field observations of aerosols and carbon dioxide (CO2) near a runway at Narita International Airport, Japan, in February 2018. We used an ultrafine condensation particle counter (UCPC) and a condensation particle counter (CPC) with unheated and 350 ∘C heated operation modes to investigate the contributions of sub-10 nm size ranges to the total and the non-volatile particle number concentrations. The performance of the 350 ∘C heated mode was tested in the laboratory to verify the consistency with existing methods for non-volatile particle measurements. We also used a scanning mobility particle sizer with unheated and 350 ∘C heated modes and an engine exhaust particle sizer for the measurements of particle number size distributions. Spiked increases in the particle number concentrations and CO2 mixing ratios were observed to be associated with the directions of wind from the runway, which can be attributed to diluted aircraft exhaust plumes. We estimated the particle number emission indices (EIs) for discrete take-off plumes using the UCPC, CPC, and CO2 data. The median values of the total and the non-volatile particle number EIs for diameters larger than 2.5 nm as derived from the UCPC data were found to be 1.1×1017 and 5.7×1015 kg per fuel, respectively. More than half the particle number EIs were in the size range smaller than 10 nm for both the total and the non-volatile particles in most of the cases analyzed in this study. The significance of sub-10 nm size ranges for the total particles in the diluted plumes was qualitatively consistent with previous studies, but that for the non-volatile particles was unexpected. Possible factors affecting the similarities and differences compared with the previous findings are discussed.

Exhaust non-volatile particle filtration characteristics of three-way catalyst and influencing factors in a gasoline direct injection engine compared to gasoline particulate filter

Three-way catalyst (TWC) integrated gasoline particulate filter (GPF) has become a standard configuration for the aftertreatment of a gasoline direct injection (GDI) engine. Although filtration effects on particulate emissions of GPFs have been widely investigated, effects of TWCs on non-volatile particulate emissions are still ambiguous. In the present work, particle size distributions were measured by DMS500 with hot sampling at three locations: before TWC, after TWC and after GPF. The particle filtration characteristics of the TWC and GPF are compared, and the influencing factors and filtration mechanism are analyzed. The results show that the TWC have obvious filtration effects on exhaust particles, and the total filtration efficiency is 20–35% for the TWC compared to 70–85% for the GPF. The TWC exhibits high efficiency in the small and large particle size ends, indicating the particle collection process in the TWC may resemble the collection process in the GPF. Regarding the influencing factors, the filtration efficiency of the TWC is related to both the exhaust temperature and exhaust flow rate, while that of the GPF mainly depends on the exhaust flow rate. With high engine-out PN concentrations, the filtration efficiency of the TWC on AM particles largely decreases, while the filtration efficiency of the GPF on AM particles increases to >95%. It is believed that in TWCs firstly the small particles are collected by diffusion and the large particles by inertia or gravity collection on the washcoat, and then the collected particles get oxidized with the help of the catalyst.

A miniature Portable Emissions Measurement System (PEMS) for real-driving monitoring of motorcycles

Abstract. We present an exploratory study carried out with a new miniature portable emission measurement system (Mini-PEMS) specifically designed at the Technical University of Liberec (CZ) for applications on two-wheeler vehicles owing to its reduced size (45cm×30×20cm) and weight (≈15 kg). It measures the exhaust gas concentrations of hydrocarbons and carbon mono- and dioxide with a nondispersive infrared method and nitrogen mono- and dioxides and oxygen using an electrochemical cell. In addition, the instrument acquires the engine speed, manifold absolute pressure, inlet and exhaust gas temperature, geo-localization, and vehicle speed. The exhaust mass flow rate is calculated from engine and emission data. The Mini-PEMS was validated on three two-wheelers (one moped and two motorcycles) against laboratory-grade instrumentation in the Vehicle Emissions Laboratory of the European Commission in terms of measured concentrations, exhaust flow, fuel consumption, and mass emission of pollutants. The mean absolute deviations of gas concentrations were 8 % for HC, 8 % for CO, 13 % for NOx, and 2 % for CO2, while the mass emissions (which include the exhaust flow determination uncertainty) were 7 % for HC, 7 % for CO, 9 % for NOx, and 5 % for CO2. An agreement of 2 % was achieved between the fuel consumption measured in the laboratory and calculated by the Mini-PEMS. As an application, the instrument was tested on board the vehicles during on-road trips. The emissions measured on the road were consistent among repeated runs, with differences between laboratory and on-road tests much larger than those between the Mini-PEMS and laboratory. We found similar or larger HC and NOx real-driving emissions and larger CO emissions from motorcycles and smaller ones for the moped. Considering its size and weight, the Mini-PEMS proved to be an efficient tool for vehicle monitoring, research and development and could be tested for in-service monitoring applications related to carbon monoxide and nitrogen oxides emissions. A tentative approach to characterize particulate mass and particle number was presented and compared to the existing filter method and nonvolatile particle number protocol.

Particle emissions measurements on CNG vehicles focusing on Sub-23nm

The current study investigates the nonvolatile particle number emissions of three bi-fuel passenger cars (Euro 6b, 6d-temp), which operate on compressed natural gas (CNG), as a primary fuel, and ga...

Non-Volatile Particle Number Emission Measurements with Catalytic Strippers: A Review

Vehicle regulations include limits for non-volatile particle number emissions with sizes larger than 23 nm. The measurements are conducted with systems that remove the volatile particles by means of dilution and heating. Recently, the option of measuring from 10 nm was included in the Global Technical Regulation (GTR 15) as an additional option to the current &gt;23 nm methodology. In order to avoid artefacts, i.e., measuring volatile particles that have nucleated downstream of the evaporation tube, a heated oxidation catalyst (i.e., catalytic stripper) is required. This review summarizes the studies with laboratory aerosols that assessed the volatile removal efficiency of evaporation tube and catalytic stripper-based systems using hydrocarbons, sulfuric acid, mixture of them, and ammonium sulfate. Special emphasis was given to distinguish between artefacts that happened in the 10–23 nm range or below. Furthermore, studies with vehicles’ aerosols that reported artefacts were collected to estimate critical concentration levels of volatiles. Maximum expected levels of volatiles for mopeds, motorcycles, light-duty and heavy-duty vehicles were also summarized. Both laboratory and vehicle studies confirmed the superiority of catalytic strippers in avoiding artefacts. Open issues that need attention are the sulfur storage capacity and the standardization of technical requirements for catalytic strippers.

Comparison of Particle Number Emissions from In-Flight Aircraft Fueled with Jet A1, JP-5 and an Alcohol-to-Jet Fuel Blend

The aviation sector has begun to adopt alternative fuels in an effort to reduce net greenhouse gas (GHG) emissions and reduce their impact on climate change. While many lab and flight-based studies have been completed for hydro-treated esters and fatty acids (HEFA) and Fischer–Tropsch (FT) alternative fuels, only one lab study has been conducted on alcohol-to-jet synthetic paraffinic kerosene (ATJ-SPK) fuels. Here we report results from the Civil Aviation Alternate Fuels Contrails and Emissions with high blend Biojet (CAAFCEB) project which was conducted in order to gather in-flight emissions data and compare an ethanol-based ATJ-SPK fuel blend and conventional JP-5 fuel to conventional Jet A1 fuel. A research aircraft was flown while fueled with the different fuels and in-flight cruise measurements were made by a second research aircraft gathering emissions and contrail data. In this study we report particle number emission index ratios with effective cutoff diameters of 15 and 7.7 nm for total particles and 13 nm for nonvolatile particles for GE CF700-2D2 engines at cruise. The ATJ-SPK blend was found to significantly reduce total and nonvolatile particle number emissions by up to 97% compared to Jet A1 fuel, likely due to the much lower aromatic and sulfur content and higher hydrogen content of the fuel. On the other hand, the total particle emissions for the JP-5 were found to have been 4% smaller than for Jet A1; this small difference is likely due to the similar fuel compositions.

Diffusiophoresis and Photophoresis of Heated Large Nonvolatile Aerosol Spherical Particles

This paper describes a theoretical study of the steady motion of a large solid nonvolatile aerosol spherical particle, which contains thermal sources within itself, in a concentration gradient of binary gas mixture components. It is assumed that an average particle surface temperature significantly differs from the temperature of the binary gas mixture surrounding it. Equations of gas dynamics are solved taking into account the power-law dependence of the molecular transfer coefficients (viscosity, thermal conductivity, and diffusion) and the density of the gaseous medium on temperature. Under boundary conditions, diffusion and thermal slip are taken into account. Numerical estimates show that the diffusion and photophoretic forces and velocity substantially depend on the average particle surface temperature.

Typical and Atypical Morphology of Non-volatile Particles from a Diesel and Natural Gas Marine Engine

Non-volatile particle emissions from a marine engine fueled by either diesel or natural gas (NG) blended with diesel pilot gas were investigated via transmission electron microscopy (TEM). The most common particles (> 95% by number) were soot aggregates. These “typical” aggregates exhibited primary particle diameters of 20.7 ± 1.9 and 26.9 ± 1.7 for 100 nm aggregates when diesel and NG fuel were used, respectively. Highly non-uniform aggregates, with distinct groups of smaller and larger monomers, were visible in all of the samples but occurred most frequently with diesel fueling at high loads. The observed “atypical” particles included super-aggregates, small compact aggregates, spheres, mineral-like polyhedral particles, and fibers. Such particles, although rare (averaging 3% by number, as calculated by counting the number of particles for each type depicted in all of the collected images), were found in most of the samples and could have been produced by a variety of mechanisms. For instance, the spheres (approximately 300 nm in diameter) most likely arose from metals within the lubricating oil.

Particle number (PN) emissions from gasoline, diesel, LPG, CNG and hybrid-electric light-duty vehicles under real-world driving conditions

The current experimental study presents solid (non-volatile) particle number emissions with a size cut-off at 23 nm (SPN23, referred to as PN hereinafter) under real-world driving conditions for 8 passenger cars powered by diesel, gasoline, Liquefied Petroleum Gas (LPG), Compressed Natural Gas (CNG) and hybrid-electric powertrains. The objective was to identify the highest emitters and assess whether PN focus should be extended to engine and fuel types other than diesel and gasoline direct injection (GDI). Results (under the cold-start RDE-compliant route) show that a Euro 6b gasoline vehicle equipped with port fuel injection (PFI) engine, comprised the highest PN emitter (4.5 times higher than the Euro 6 limit of 6 × 1011 p/km), while similar emission levels (up to 3.7 times higher than this limit) were detected in the older-technology (Euro 3 and 4) gasoline and (retrofitted) LPG PFI vehicles. This limit was also exceeded (by 1.7 times) by one gasoline direct injection (GDI) Euro 6b non-GPF vehicle. PN emissions from the hybrid-electric vehicle (equipped with gasoline PFI engine) were lower than the other Euro 6b PFI vehicle, but still high compared to typical PFI engines found in literature. The lowest emissions were detected in diesel and CNG engines (as low as 7.8 × 109 and 2.4 × 1010 p/km respectively). Euro 6b LPG vehicle was found to be a low emitter (3.3 times lower than the 6 × 1011 p/km limit), while similar emission levels were detected in the other Euro 6b non-GPF GDI vehicle. Among the different driving sections (urban, rural, motorway), urban and especially the cold phase had the highest contribution in most cases. Finally, a significant PN reduction potential was observed with LPG and CNG over gasoline (up to 93% and 90% respectively), although the opposite trend was detected in an old-technology retrofitted LPG vehicle. These results indicate that close attention should be focused on other engine and fuel types, apart from diesel and GDI. Regulatory initiatives towards air pollution reduction should include measures for the reduction of high-emission old-technology gasoline and LPG vehicles, while modern gasoline PFI engines should also be further scrutinized.