PARTICLE NUMBER (PN) MEASUREMENTS ON FLEX-FUEL VEHICLES WITH DI AND PFI ENGINES

As a heritage from Diesel vehicles, the EURO 6 Light Duty emission standards introduced limits for particle number (PN) of GDI engines. Given the concern of the extremely small particles on health effect, the same limits were set (6.0 x 10 #/km) for both diesel and GDI cars. In Brazil, the current phase for light vehicles of PROCONVE L6 enforced particulate matter (PM) mass emission standard for Diesel vehicles only, applying the limits of 25 mg/km for passenger cars and 30 mg/km for commercial vehicles. Similar to the European concern, even not considering yet the particle number (PN) emissions, future Brazilian phase L7, which is under study, considers a significant reduction on the limit of the particulate matter (PM) mass emission, from current limits down to 6 mg/km for both Diesel and DI engines, for passenger and commercial vehicles. In the goal of generating reference data to foster the important discussion of (PN) emissions in Brazilian passenger vehicles, this article discusses (PN) measurements of Flex-Fuel Vehicles equipped with direct injection (DI) and port fuel injection (PFI) engines, fueled with ethanol (E100) and gasohol (E22). The mechanisms and parameters which influence the particulates formation are discussed according to the bibliography and the advantages that Ethanol usage brings to human health is highlighted. This study shows interesting comparative results of (PN) and can be used as reference for future emissions regulations of Flex-Fuel vehicles in Brazilian market.


INTRODUCTION
World Health Organization (WHO) declared on its 2016 update [1] that more than 80% of people living in urban areas that monitor air pollution are exposed to elevated levels of particulate matter that exceed the air quality limits. This update on air quality database registers that only 2% of cities in low and middle-income countries with more than 100,000 inhabitants meet WHO air quality guidelines. In high-income countries, the scenario increases to 44%. Similarly, according to Irish Environmental protection agency [2], the WHO estimates show that more than 400,000 premature deaths are attributable to poor air quality in Europe annually. Locally in Brazil this matter is not different, the numbers of deaths are also scary. Figures from the Institute for Health Metrics and Evaluation, 2016 related to 2015 showed that the cause of 52,284 deaths were related to exposure to particulate matter (fine particles, PM2.5) [3]. Still according to WHO the Ambient Air Quality Database from 2014 reports annual average PM2.5 concentrations in 40 Brazilian cities [4]. It shows that only one city is below the WHO air quality guideline of 10 µg/m3, the other 39 cities are exceeded, figure 1. Regarding particulate matter produced by flex fuel vehicles, Salvo et al. presented a recent paper with an investigation about the effect of ethanolgasohol fuels usage shifts in ultrafine particles generation (from 6 nm to 100 nm diameter particles) [5]. It was found that the shift from gasohol to ethanol reduced the amount of ultrafine concentration in on third and the opposite occurred when flex fuel vehicles shifted from ethanol to gasohol.
Based on all of these facts, the control of PM pollution is extremely important for human well-being. The effect of ethanol in flex fuel vehicles, as well as the injection technology, PFI or DI, are relevant factors in this topic. This paper aims to contribute in this discussion by generating data with current production vehicles.

Definitions of Particulate Matter & Particle Number
It is important to disambiguate the different meanings that the term particulate matter has been subjected to. Sometimes, the different terms Particulate Matter, Particulates, Particles, PM, PN, Smoke and Soot can be treated interchangeably, on emissions discussions for general purpose, without the rigor of the academic, while discussions of emissions measurements use a distinction between Smoke, PM and PN.
Regarding to this, according to EPA [6] Particulate Matter can be also called particle pollution, which is defined by a mixture of solid particles and liquid droplets found in the air. Some of them are big enough to be seen with the naked eye, such as dust, soot, or smoke, while others are so tiny that they can only be seen on an electronic microscope.
A good explanation is given by EEA [7] when they outline Particulate Matter (PM) as a collective name for fine solid or liquid particles added to the atmosphere by processes at the earth's surface. Particulate matter includes dust, smoke, soot, pollen and soil particles. Related to Smoke, it is defined as an aerosol, consisting of visible particles and gases, produced by the incomplete burning of carbon-based materials, such as wood and fossil fuels. As for Soot, it is an impure black carbon with oily compounds obtained from the incomplete combustion of resinous materials, oils, wood, or coal. At last, the EEA definition for Particle Number (PN) is a variety of measurements characterizing the number of particles in an aerosol sample.

Particulate Matter Sources
Particulate matter can come from either natural or anthropogenic source. Natural source is generated by the nature itself, such as salt spray, wildfires, sand, dust, volcanoes, etc. It is considered an anthropogenic source anything that is caused by humans or their activities, highlighting the products of combustion, agriculture, among others. Figure 2 demonstrates that anthropogenic sources, in 2011, were responsible for 44% of the PM2.5 emissions by mass, while natural sources were responsible for 56%. Particulate matter is divided in two distinct categories, primary and secondary. Primary particles are directly emitted and secondary particles form as a result of atmospheric reactions involving gaseous emissions. Both are regulated, though secondary particles are regulated indirectly.

Composition
Once diesel fuel is one of the main responsible for the particulate matter emissions, its composition is highlighted. According to Johnson et al. [8], particle matter diesel emissions is divided in three phases, gas, solid and liquid/vapor phases. The gas phase emission includes NOx, CO, and sulfur dioxide (SO2). Figure 3 demonstrates the components of the solid and liquid/vapor phases. It shows that solid phase emissions are initially constituted of small (10-80 nm) solid carbon cores -SOLand agglomerates (50-1,000 nm). The liquid/vapor phase is composed of the organic, named SOF -Soluble organic fractionand hydrocarbon component and sulfate (SO4), which can be removed by water. Part of the hydrocarbons are absorbed onto the SOL, and part of them remains as a vapor.
Generally speaking, the whole composition involves some hydrocarbons absorbed in an agglomerate of SOL. Those hydrocarbons can be removed by the SOF, which is an organic solvent of the hydrocarbons.

Characterization
It is noticeable that the European Regulatory Agency evolved when went from massbased air pollution regulation for particulate matter to particle number, that is more accurate about the physical properties of the particulate matter regarding to the effects on human health. Surface area can also be measured as a parameter to be regulated.
There is an important correlation between mass, particle number and surface area of the particulate matter that is explained by Kittelson [9], which includes the size range defined for the atmospheric particles as PM10 (diameter < 10 µm), fine particles PM2.5 (diameter < 2.5 µm), ultrafine particles PM0.1 (diameter < 0.10 µm or < 100 nm), and nanoparticles (diameter < 0.05 µm or < 50 nm). A diesel fuel distribution is exposed in figure 4, and according to the author, distribution from a spark ignition engine would be similar but with relatively less material in the accumulation mode region.
The practical meaning of this demonstration is that the total of mass does not reflect properly the number of small particles, which are the ones harmful to human health. Complementing the explanation related to the size of the particles, EPA [6] provides a helpful description about the size of the particles when compared to a human hair. It shows that the average human hair is about 70 µm in diametermaking it 30 times larger than the largest fine particle. EPA calls the particles sizes as: PM10: inhalable particles, with diameters that are generally 10 µm and smaller; PM2.5: fine inhalable particles, with diameters that are generally 2.5 µm and smaller; PM0.1, with diameters that are generally 0.1 µm and smaller, as seen on figure 5.

Impact on health and climate change
Several studies about health of the past decades have pointed out strong evidences that elevated levels of particulate matter air pollution are associated with increased cardiovascular and respiratory diseases [10]. In this matter, small particles from engines are particularly worrisome.
January 2013 issue of Nature magazine [11] stated that soot is a major contributor to climate change. Similarly, defined that soot, also known as black carbon, "is the second most important human emission in terms of its climate forcing in the present-day atmosphere; only carbon dioxide is estimated to have a greater forcing" [12]. Bullis, expound is his article "Cleaning Up Diesel Trucks and Cooking Stoves Could Reduce Climate Change" [13] that selective reductions of particulate pollution might help the climate change issue.

REGULATIONS AND TEST CYCLES
In the U.S. and Europe, PM10 and PM2.5 are currently regulated.

US Emissions Standards
In the United States Emissions standards have two bodies regulating emissionsthe EPA and CARB. Both regulate particulate matter using a mass limit. EPA established Tier III 3mg/mi on FTP driving cycle in 2017, while CARB took LEV III 3mg/mi in 2017, 1mg/mi (begin phase-in 2025), also on FTP driving cycle. Figure 6.

European Emission Standards
The European Union implemented a Particle Number limit in addition to a mass standard. Started with Euro5b, Diesel vehicles were limited to 6x10 11 particles/km. For Direct Injection Gasoline engines, Euro 6, in 2014, initially met a limit of 6x10 12 particles/km, and from 2017, with Euro 6c met full standard of 6x10 11 particles/km. Particle number standard must be met in addition to the 5mg/km particle mass standard. NEDC driving cycle has been replaced in 2017 by WLTP and the RDE onroad cycle was included as a supplemental test to verify real-world compliance with emissions limits.
Particulate measurement protocol -PMPwas established to very carefully define the sampling and measurement conditions in an attempt to obtain repeatable measurements. In order to achieve repeatable measurements, only solid particles are counted, volatile particles are removed by using an evaporation tube or thermodenuder and, to further ensure volatile particles are not counted, only particles between 23 nm and 1,000 nm are counted.

FORMATION IN ENGINES
Basically, the formation of the soot happens during the combustion of hydrocarbons.
In the presence of oxygen (stoich or lean) and sufficient temperature, however, most of it will be oxidized. In rich flames, there is insufficient oxygen to oxidize all of the soot that which remains is emitted.
The mechanism of the formation of the soot particles in diesel engines [14] follows a sequence which starts with pyrolysis, that is the decomposition of the molecules, brought about by high combustion temperatures. As a result, the platelets are formed and then become microscopic crystals, also known as crystallite. All the crystallites together transform in turbostratic particles. After that, there is the coagulation phase and the surface growth. The grown particles, later on, form an aggregation. As mentioned in figure 3, there is the absorption and condensation of hydrocarbons. The increasing on the size of the particles along the mechanism of their formation is also shown on figure 7. They start at 0.35 nm as platelets and end from 0.1 to 10 µm at the aggregation of the grown surface particles. Generally speaking, in internal combustion engines, soot forms as a result of reactions in localized fuel-rich regions. While the mechanisms differ, this is true for both Diesel and Gasoline engines. In Diesel engines soot forms on the fuel-side of the diffusion flame.
In Gasoline engines, the fuel-rich regions may be split into two categories: liquid fuel in-cylinder, in which either aerosol droplets or surface films (or pools) of fuel result from poor fuel spray characteristics; and pockets of fuel-rich mixture, that is a result of poor mixing.
In DI engines the sources of PM formation are directly related to the combustion chamber surfaces. Its influence is impacted by the following characteristics: (1) piston crown (poorly chosen injection timing); (2) bore liner (excessive penetration may be due to poorly chosen injection timing, spray pattern, insufficient or mismatched charge motion); (3) combustion chamber roof (directpoor spray pattern, indirectmay splash off of the piston); (4) intake valves (Incorrect spray pattern); (5) liquid droplets in the chamber volume (poor atomization from poorly designed injector or excessive deposits on the injector); (6) residual fuel on the injector tip (may be leaking, have an excessive sac volume, or fuel may be adsorbed by deposits); (7) fuel (liquid or vapor) may collect in the top land crevice; and (8) vapor-phase fuel-rich pockets (poor mixing may be due to poor injection or charge motion characteristics). Figure 8 [15].

FACTOR INFLUENCING SOOT FORMATION
As already described the soot formation is influenced by several parameters such as temperature, air-fuel ratio, chamber pressure, fuel composition, residence time and addition of additives.
The soot formation, as many experimental studies have shown [16] [17], increases with combustion temperature in the region below 1500 K, since the pyrolysis and crystallite rate production depends mainly on temperature, which contributes to aggregation growth and finally to form the structure of the adsorption and condensation of hydrocarbons. Differently, in the high temperature region around 1500 to 1700 K, where the oxidation process is dominant the soot formation is greatly reduced, figure  9. Many experimental investigations confirm that temperature is the dominant influence parameter on soot formation even at elevated pressure levels.
Other parameter that strongly impacts the soot formation is the air-fuel ratio. In general, the soot mass and particle diameter increase with the equivalence ratio (EQR) as shown in figure 9 with the combined effect of temperature and air-fuel ratio (as the C/O ratio). It can be observed a critical C/O ratio below which no soot is formed. The type of fuel and their composition is another important parameter. As the aggregation process results from the fuel thermal composition, its chemical structure strongly impacts the kind and quantity of products. The soot formation increases with the hydrocarbon chain size. Polyaromatics and aromatics chains produce more aggregation than alkenes [16]. In this way, E100 fuel has chemically less tendency to soot formation than E22 fuel since its chemical structure is small (C2H6O) and without aromatic molecules (E22 fuel contains 15 to 35% in volume of aromatic chain).
As result of this efficiency on soot formation, some recent studies have shown the environmental and social benefits of E100 fuel. Salvo et al. [5] reported that the ultrafine particle (7-100 nm diameter) fall by one-third during the morning commute when higher gasoline price induces 2 million drivers in the real-world megacity of São Paulo to refuel their vehicle with E100 fuel instead of E22 fuel. The opposite trend was measured for fueling shifts from E100 to E22, figure 10.
In addition, the soot formation can also be changed by additives. When inert additives like H2O, CO2 or SO2 are added to the fuel (from dilution via internal or external gas recirculation) the profiles of species concentration and temperature are changed, decreasing the shooting tendency and overriding the temperature effect. Studies showed that the addition of water is an efficient way to reduce the soot formation [17].

MITIGATION
One way to mitigate particulate matter is by reducing engine-out emissions. Design and calibration efforts can be aimed at reducing the amount of particulate matter entering the exhaust stream. Related to exhaust aftertreatment, the use of devices in the exhaust stream is necessary to capture or oxidize the particles after they have entered the exhaust stream. Unfortunately, it is not totally effective, because it is tightly dependable on engine out reductions and generally it has larger impacts on vehicle performance and/or fuel consumption, and requires the use of particulate filters. An example of the efforts that come from improvements obtained on engine hardware combined with improvements made on engine calibration, in order to reduce the PN emissions [18] on a Gasoline DI engine at NEDC driving cycle, is shown on figure 11. These improvements together achieved 78% reduction on PN emissions at NEDC driving cycle.

EXPERIMENTAL METHOD
Once exposed the mechanisms and characteristics of the particulate matter and its importance and relation with the human health and climate change, developed countries adopted regulations for a very careful PN control. Considering PN is generated in all engine technologies, and with the use of all fuels, it is certain that discussions about PN emissions for passengers' vehicles in Brazil will eventually consider Flex-Fuel engines of all injection technologies that run with E100 and E22, and any blend that comes from that. As an attempt to provide reference data to help the initial discussions, a group of tests were conducted on four normal production Flex-    A total of 26 tests were run to generate all test batches and allow the comparison between different injection technologies and fuel blends.

RESULTS FROM DI AND PFI FLEX-FUEL VEHICLES
For the measurements with DI engines, two normal production passenger vehicles of the same model were tested, both equipped with the same engine type (vehicle A run 3 test batches and vehicle B run 6 test batches with each fuel), and also one light commercial vehicle with a different DI engine (1 test batch with each fuel), figure 13. Another passenger vehicle assembled with a PFI engine was tested (1 test batch with each fuel), figure 14.
The results of this research showed that PN emissions with E100 fuel is much lower than PN emissions with E22 fuel in all tests. E100 PN emissions with both DI and PFI engines are lower, as seen in figures 13 and 14.  The majority of the PN are generated during cold engine operation of the emissions test. Figure 15 shows the PN picks with both E100 and E22 fuels after cold start and during cold drivability transient accelerations.  Table 3 shows the average results with each Flex-Fuel engine technology, DI and PFI show that E100 fuel produces significantly less PN emissions. On both DI engines, E100 combustion produces about 90% less PN than E22. On PFI engine, PN emissions is much lower than seen on DI engines and E100 performance is even better, generating 30% less than E22.

CONCLUSIONS
Tests confirmed that DI engines produce higher amount of particle numbers than PFI engines. However, E100 particle number in both DI engines is one order of magnitude lower than E22 emissions level and gets closer to the amount of PNs generated by the PFI engine.
Based on the results E100 is a strategic fuel for reducing and controlling PM pollution. Its application is a key factor for obtaining environmental, social and economic sustainability.