Prediction of Pollutant Emissions in Various Cases in Road Transport

Abstract

The road transport sector is a key source of carbon dioxide and air pollutants. Mathematical modeling is frequently used to assess the sector’s contribution to the total national emissions budget (inventory). The present article focuses on studying the impacts of the fuel used (LPG, CNG, gasoline, diesel, and biofuel), the Euro standard, and the structure of vehicles on CO₂, NO_X, and PM2.5 emissions. This paper presents the results of mathematical simulations of the influence of the fuel type and Euro standards on pollutant emissions. Two scenarios were considered in terms of the effect on CO₂, NO_X, and PM2.5 emissions: one focused on changing the current fleet and introducing Euro 6/VI vehicles, and the second scenario focused on cities adding low-pollution zones (only Euro 6 vehicles and PHEVs, HEVs, and BEVs entering the city). The results of the simulations showed that Euro 6/IV vehicles emit significantly less PM2.5 and NO_X, and biofuels can substantially contribute to reducing emissions.

1. Introduction

During the combustion of fuels in internal combustion car engines, complete and in-complete combustion processes occur, resulting in the formation of greenhouse gases (GHGs) and other harmful substances. More detailed information about the environmental issues associated with the combustion of fuels in vehicle engines can be found in the work of Benvenutti [1] and Nickischer [2].

The design of a vehicle’s engine is usually determined by the type of fuel combination applied. Specific methods and types of fuels are used in both spark-ignition engines and compression-ignition engines. The engine’s performance, and thus the engine’s power, is also determined by the fuel used.

Despite the use of various technological solutions for reducing the emission of pollutants, such as EGR (exhaust gas recirculation) systems [3], TWC (three-way catalytic) converters [4], and SCR (selective catalytic reduction) systems [5], and the introduction of the newest (higher) Euro standards [6], emissions from motor vehicles are not being reduced enough. However, work is underway on the use of other types of fuel in internal combustion engines, such as biofuel or biogas [7], and these works provide information on the impact of fuel mixtures on emissions and engine performance.

Several studies have considered the changes in the parameters of internal combustion engines depending on the fuel type. Legue et al. [8] presented experimental and thermodynamic research on the modeling of the combustion in a compression-ignition engine powered by an alternative fuel based on biodiesel (B100) and by a conventional diesel oil (D100). The impact on heat losses in the engine was also analyzed. On the one hand, Koszalka et al. [9] noticed that engines powered by biodiesel had an engine torque and power that was lower by about 3%. On the other hand, Mortadha et al. [10] showed that adding ethanol caused a power increase, and specific fuel consumption and thermal efficiency improved with increasing ethanol concentration. Moreover, it was found that ethanol had a negative effect on volumetric performance.

The combustion of fossil fuels contributes to air pollutant and greenhouse gas emissions. Therefore, the EU has decided to fight to reduce carbon dioxide (CO₂) emissions by developing the European Green Deal recommendations for climate neutrality, with the goal of net zero GHG emissions in 2050. In the road transport sector, climate neutrality can be achieved by developing electromobility or increasing the contribution of renewable energy sources (such as alternative fuels) into the fuel mix [11].

This problem was also noticed by Meyer [12] and Blas et al. [13], who presented modeling results for a transport decarbonization strategy.

Unfortunately, the development of electromobility may not be enough to reduce GHG emissions. Most EU countries generate electricity using coal-based energy mixes [14].

The EU’s final energy consumption in 2020 was 37,086 PJ, 5.6% less than in 2019. This consumption slowly increased from 1994, peaking at 41,445 Mtoe in 2006. By 2020, final energy use had fallen from its peak by 10.5%. There was a significant decrease in the amount and share of solid fossil fuels from 9.6% in 1990 to 3.6% in 2000, and from 2.8% in 2010 to 2.1% in 2020. Moreover, the stake of renewables in the total increased from 4.3% in 1990 to 5.3% in 2000 and 8.8% in 2010, eventually reaching 11.8% in 2020. Natural gas remained relatively stable during this period. Its contribution fluctuated from 18.8% (in 1990) to 22.6% (in 2005), with a total share of 21.9% in 2020 [15].

Liquid fuel products had the largest share (35%) of the final energy consumption in 2020. Other sources were electricity (23.2%) and natural gas (21.9%). Solid fossil fuels only contributed 2.1% of the final energy consumption at the end-user level [15]. Moreover, the current Kyoto-based approaches to reducing global GHG emissions might involve looking for successive ways to reduce emissions. The first potential solution to this problem is biofuel, usually treated as a carbon-negative energy carrier [15].

Therefore, it is essential to promote, in parallel with the promotion of electric options, alternative fuels that can reduce GHG emissions. In particular, the contribution of alternative fuels in the EU countries is relatively small.

Apart from the issue of GHG emissions, it is crucial to reduce air pollutant emissions. In this article, nitrogen oxides (NO_X) and particulate matter (PM2.5) were chosen (because of the associated smog and health problems).

Taking into consideration the foundations of climate neutrality, there are a few questions that need to be answered:

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Does a change in fuel type affect CO₂, NO_X, and PM2.5 emissions?

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Does a difference in the vehicle fleet structure impact emissions?

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Which fuel is the most ecologically suitable and will help to achieve goals related to climate neutrality, especially in countries where the energy mix is dominated by hard coal and the development of electromobility may not bring the assumed reduction of CO₂ and pollution?

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Does the climate neutrality trend affect the emission of other air pollutants?

2. Materials and Methods

In this section, the authors used COPERT (Computer Programme to Calculate Emission from Road Traffic), a complex modeling software tool for the calculation of air pollutant emissions from road transport. The applied methodology followed the IPPC (Integrated Pollution Prevention and Control) [16] and the EMEP (European Monitoring and Evaluation Programme) Guidebook 2019 [17] guidelines, as well as those in Ntziachristos [18], which are basic sets of guidance for GHG and air pollutant emissions inventories.

Using the COPERT model made it possible to estimate emissions following international and EU law requirements.

Road transport emissions estimates were based on:

• Fuel consumption;
• Engine size;
• Number of vehicles per category;
• Vehicle weight;
• Emissions control technology;
• Mileage per vehicle class;
• Mileage per road class (urban, rural, and highway);
• Average speed per vehicle type and per road class;
• Monthly temperature (minimum and maximum);
• Fuel characteristics.

The mathematical approach included in the COPERT software is classified as a “Tier 3” approach for quantitative emissions assessment.

The applied Tier 3 (the most complex emissions estimation methodology) approach estimates road transport exhaust emissions as the sum of hot-start and cold-start emissions [18]. Both processes have specific emissions factors, which vary depending on the vehicles’ type and fuel, driving situation (urban, rural, and highway), and the climatic conditions (average monthly temperature) [18].

The general equation for the Tier 3 methodology is presented below [18]:

$$E_{Total}=E_{Hot}+E_{Cold}$$

(1)

• where
• E_Total—total emissions [g] of any pollutant for the spatial and temporal resolution of the application;
• E_Hot—emissions [g] during stabilized (hot) engine operation;
• E_Cold—emissions [g] during transient thermal engine operation (cold start).

It should be emphasized that all of the emissions factors included in the COPERT model were determined based on laboratory testing under the WLTP driving cycle [19].

In this section, the authors investigated the impact of changes in fuel structure on the emission of CO₂, NO_X, and PM2.5 by passenger cars (PCs), light-duty vehicles (LDVs), heavy-duty vehicles (HDVs), buses (urban buses and coaches), and L-category vehicles (mopeds and motorcycles) for particular Euro standards, vehicle segments, and different fuel types (petrol—P, diesel oil—D, liquefied petroleum gas—LPG, compressed natural gas—CNG, and biofuels). It is worth emphasizing that the names of the categories used were consistent with the COPERT model.

The COPERT software calculations were based on the input data used to compile the Polish national emissions inventory for 2020 [20]. The estimations were conducted based on the following basic assumptions:

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Number of actual driving vehicles per vehicle category (PCs—18,587,297, LDVs—2,224,407, HDVs and buses 819,809, L-category vehicles—2,248,651), engine size or vehicle weight, emissions control technology, and Polish fuel mix consumed by vehicles in 2020 [21];

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Share by road class (urban, rural, and highway) (Figure 1);

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Vehicles’ average velocities (Figure 2) [22];

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Maximum and minimum monthly temperature for Poland in 2020.

The data are given below.

The subject of analysis was related to the annual assessment of air emissions generated by the road transport sector on the regional (country) level. This kind of analysis is impossible to carry out as a laboratory experiment. However, the modeling software used [18] is based on laboratory testing of particular types of vehicles.

In the Polish national inventory of pollutant and greenhouse gas emissions, the share of PC and LDV vehicles in urban traffic was over 35%, and in rural traffic it was about 50%. In cities, the average speed was about 30 km/h, and in extra-urban areas it was 70 and 65 km/h. Most HDVs and coaches traveled at an average velocity of 55 km/h in rural areas. Motorcycles and mopeds operated mainly in urban and rural areas. The share of all categories of vehicles traveling on motorways was relatively small and amounted to about 15%. On highways, these vehicles traveled at the highest average speed. The methodology of the COPERT model for determining emissions from motor vehicles predicts that these emissions will also depend on ambient temperature.

Based on the various traffic situations in Polish cities, the authors wanted to check how significantly the fuel type and Euro standards impact emissions. This was achieved in the following ways:

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Changing the fuel type and investigating the impact on air pollutants, where the emissions were calculated assuming the same amount of fuel and fuel structure (mix) as was used by the number of PCs in 2020; additionally, the authors investigated changes in emissions when using only one fuel type, i.e., diesel oil, gasoline, LPG, CNG, or biofuel (bioethanol and biodiesel);

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Checking the influence of the Euro standard and the type of fuel on emissions—this part of the analysis was carried out for only one car for each Euro category.

Additionally, two scenarios were developed to determine the impact of the Euro 6 standards on CO₂, NO_X, and PM2.5 emissions:

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The first scenario focused on changing the current fleet to vehicles all complying with the Euro 6/VI standards and then evaluating the benefit in terms of CO₂, NO_X, and PM2.5 emissions.

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The second scenario focused on introducing low-pollution zones to cities and their impact on CO₂, NO_X, and PM2.5 emissions. It was assumed that only cars that meet Euro 6 standards and PHEVs (plug-in hybrid electric vehicles), HEVs (hybrid electric vehicles), and BEVs (battery electric vehicles) could enter the cities.

3. Results

The authors investigated the impact of changing the fuel type on air pollutants, where the emissions were calculated assuming the same amount and structure of fuel (mix) as was used by the number of passenger cars in 2020. Additionally, the authors investigated changes in emissions when using only one fuel type, i.e., diesel oil, gasoline, LPG, CNG, or biofuel (bioethanol and biodiesel). The comparison of annual CO₂, NO_X, and PM2.5 emissions from passenger cars depending on fuel type is shown in Figure 3.

The figure shows the dependence between CO₂, NO_X, and PM2.5 emissions and fuel type. It is worth noting that for petrol (gasoline), LPG, and CNG, the generated CO₂ emissions were almost two times higher than those for the currently applied fuel mix and biofuel. Moreover, diesel oil produced only slightly higher emissions than the fuel mix and biofuel. According to the information above, biofuels might qualify as carbon-negative energy carriers.

The emissions of NO_X and PM2.5 caused by the combustion of gasoline, diesel oil, LPG, and CNG were higher than those of the currently applied fuel mix. It is worth noting that the combustion of biofuels will cause a slight increase in NO_X and PM2.5 emissions compared to the presently applied fuel mix. In the cases of NO_X and PM2.5, the relationship between the emissions from biofuel to the emissions from fuel mix was very similar to the case of CO₂. The simulation results clearly show that biofuel caused the lowest emissions of CO₂, NO_X, and PM2.5. In the case of conventional fuels, the lowest emissions were caused by diesel-powered engines.

The results of the influence of the Euro standard and the type of fuel on emissions indicated the following in the case of replacing the Polish fuel mix:

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Gasoline would increase CO₂ emissions by 118%, NO_X by 81%, and PM2.5 by 54%;

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Diesel oil would increase CO₂ emissions by 7%, NO_X by 22%, and PM2.5 by 28%;

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LPG would increase CO₂ emissions by 154%, NO_X by 94%, and PM2.5 by 77%;

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CNG would decrease CO₂ emissions by 1% and NO_X by 2%, and increase PM2.5 by 3%;

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Biofuel emissions of CO₂, NOx and PM2.5 would remain at the same level.

The above results show that replacing fuels from the current mix with pure gasoline, diesel oil, or LPG would increase the emission of all pollutants tested. Replacing the current fuel mix with CNG or biofuel would cause emissions to decrease slightly or stay at the same level. It is worth remembering, however, that biofuels are treated as climate-neutral fuels, so CO₂ emissions from their combustion are not treated as CO₂ emissions of anthropological origin.

The comparisons of CO₂ emissions depending on Euro standards and fuel types are shown in Figure 4, Figure 5, Figure 6 and Figure 7.

Figure 4, Figure 5, Figure 6 and Figure 7 show the dependence between CO₂ emissions and technology (Euro standard) for various vehicle types: PCs (Figure 4), LDVs (Figure 5), HDVs and buses (Figure 6), and L-category vehicles (Figure 7). For the Euro standards 1–5, in the case of PCs, LDVs, and L-category vehicles, biofuel (bioethanol) caused higher CO₂ emissions than petrol-fueled vehicles. For Euro standards 1–5 in the case of PCs and LDVs, and for L-category vehicles under Euro standards 1–4, the emissions from biodiesel (bioethanol) were considerably more extensive than the emissions from conventional fuels (petrol, diesel oil, LPG, and CNG). The opposite situation was observed for Euro 6 PCs and LDVs, and Euro 5 L-category vehicles. In the case of diesel oil, the combustion of biofuel resulted in lower emissions for all vehicle categories. The CO₂ emissions caused by the combustion of LPG and CNG in PCs were almost on the same level as those from petrol and diesel oil. It is also worth noting that for HDVs and buses, the emissions were lower for biofuel, and that emissions decreased along with the increase of the Euro standard.

The authors then investigated the influence of the Euro standard and fuel type on emissions. The comparisons of NO_X and PM2.5 emissions depending on Euro standards and fuel types are shown below in Figure 8 and Figure 9.

In the cases of PCs and LDVs, the emissions of NO_X (Figure 8 and Figure 9) and PM2.5 (Figure 10 and Figure 11) from diesel oil were higher than those from biofuel. It is also worth noting that, similarly to CO₂ emissions (Figure 4, Figure 5, Figure 6 and Figure 7), the emissions of NO_X and PM2.5 due to biofuel combustion were higher than those for gasoline, which was the opposite situation to diesel-fueled PCs and LDVs.

The PM2.5 emissions from diesel PCs and LDVs (Figure 12 and Figure 13) were higher than those from biofuel. PM2.5 emission decreases with the increase of the Euro standard.

In the cases of HDVs and buses, NO_X (Figure 10) and PM2.5 (Figure 14) emissions from biofuel were lower than those from diesel oil. The issue of PM2.5 emissions from Euro VI HDVs (Figure 14) demands further analysis.

In the cases of NO_X and PM2.5 emissions from L-category vehicles (Figure 11 and Figure 15), the lower emission from biofuel was observable only for the newest vehicles (Euro 5).

As in the case of CO₂ emissions, NO_X and PM2.5 emissions decreased in line with the development of the Euro standards.

Additionally, two scenarios were developed to determine the impact of the Euro 6 standards on CO₂, NO_X, and PM2.5 emissions:

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The first scenario focused on changing the current fleet by introducing Euro 6/VI vehicles, and then evaluating the benefit of fuel type;

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The second scenario focused on cities adding low-pollution zones (only Euro 6 vehicles and PHEVs, HEVs, and BEVs entering the city) and the effect of changing the fuel type on the CO₂, NO_X, and PM2.5 emissions.

The authors studied the impact of conversion of the entire fleet on CO₂, NOx and PM2.5 emissions. The first scenario assumed that all vehicles in Poland from the pre-Euro to Euro 5 standard were converted to Euro 6. Assuming that the same amount of fuel would be used, the impact on emissions was checked. The simulation results are presented below in Figure 16, Figure 17 and Figure 18.

The simulation case in which all vehicles in Poland from pre-Euro to Euro 5 standard were converted to Euro 6 did not change the result in terms of CO₂ emissions (Figure 16). This is because the vehicles used the same amount of fuel. However, emissions of NO_X and PM2.5 decreased significantly.

The second scenario focused on introducing low-pollution zones to cities and their impact on CO₂, NO_X, and PM2.5 emissions. It was assumed that only cars that met Euro 6 standards and PHEVs, HEVs, and BEVs could enter the cities. The results of the simulation are presented in two variants (for city traffic and all traffic, treated as urban, rural, and highway) and shown below in Figure 19, Figure 20 and Figure 21.

The case of the second scenario showed that the conversion impacted emissions. Emissions of CO₂, NO_X, and PM2.5 in cities were significantly reduced. The ban on entry to cities of cars other than those meeting the Euro 6/IV standard also impacted the total emissions.

4. Conclusions and Discussion

The results of the present simulations indicated that in the cases of CO₂, NO_X, and PM2.5, replacing fuels from the current mix with pure gasoline, diesel oil, or LPG would increase the emission rates of all pollutants tested. Replacing the current fuel mix with CNG or biofuel would cause pollutant emissions to decrease slightly or stay at the same level. It is worth remembering, however, that biofuels are treated as climate-neutral fuels, so CO₂ emissions from their combustion are not treated as CO₂ emissions of anthropological origin.

In the simulations carried out depending on the Euro standard, the CO₂, NO_X, and PM2.5 emissions were decreased for all fuel types when all vehicles met the Euro 6 standard. It is essential to replace the current vehicle fleet with a newer one. The authors believe that the vehicle fleet should be renewed in addition to promoting electromobility.

The emissions from biofuel combustion were considerably lower than the emissions from the combustion of fossil fuels, which has severe implications for coal-based economies. Moreover, it should be emphasized that biofuels consistently emit CO₂ which is not treated as CO₂ from fossil fuels, and thus is neutral for the environment. The use of biofuels is in line with the recommendations of the REDII Directive, the European Green Deal, and the pursuit of carbon neutrality.

Replacing conventional vehicles with electromobility is justified in economies (countries) where a significant amount of electricity is generated from renewable energy sources (solar, wind, water) or nuclear power plants; otherwise, it is worth promoting the development of alternative fuels such as biofuels.

For this reason, a climate-neutral economy should be pursued not only by promoting electromobility but also through the intensive promotion of biofuels. Moreover, the rise of biofuels should be associated with fleet renewal, which would mean compliance with the newest (highest) Euro standards.

Promoting vehicles that meet the latest Euro standards, as well as PEHVs, HEVs, and BEVs, will have a positive effect on reducing emissions. However, it is worth remembering that BEVs, despite the fact that they do not emit from the exhaust system, are a source of emissions from the tires, brake wear, and road abrasion.

The data presented in the article should be used to develop a new road transport development strategy for Poland and to achieve climate neutrality, especially in the era of the intensifying energy crisis in Europe.

Further work should better reflect the actual structure of road transport in Poland.