e&e No 25
Greenhouse Effect Gases Emission Parameters of Heavy Vehicles in Brazil
1 – Preliminary Considerations
Our aim in this section is to present emission coefficients that permit the calculation of total emission using the fleet composition and the age of vehicles. In the next section we will calculate the greenhouse effect gases emission in the nineties.
In the 1990-1997 period – to be examined in the present work – the participation of Otto cycle heavy vehicles in the emissions is limited to the small remnant fleet. The emission due to these vehicles (gasoline– and alcohol- fueled) will be evaluated using global parameters.
In the previous sections the parameters necessary to characterize the fleet were studied using data concerning
sales of vehicles by category (light commercial, cars, trucks and buses) and a function that describes the life span of vehicles from their fabrication year on.
In the calculation of emission indexes for the diesel fleet characterized in this way, the diesel oil consumption in road transport used was that of the National Energy Balance (BEN), 1998 issue, and the emission indexes for new vehicles used were those established by the National Council for the Environment (CONAMA). Considering that the Brazilian legislation about the matter is recent, it was necessary to use information from countries where the control of vehicle emissions has a longer tradition , in order to establish hypothesis concerning the situation existing previously in Brazil.It should be observed that in Brazil the control of vehicle emissions is carried out by the Program for Controlling Air Pollution by Automotive Vehicles – PROCONVE – established by CONAMA through the Administrative Rule N° 18 /1985.
The PRONCOVE objectives are:
The PROCONVE established as well the obligatory Conformity Certificate for engines in what concerns the prescribed emission levels.
The implementation of the Program has been carried in a gradual way since the establishment of the Administrative Rule n0 18 /1985. Until two ago there were no vehicle emission laboratories available. Presently, the Institute of Technological Research – IPT, in São Paulo, operates a laboratory equipped with the resources necessary for this purpose. It did not happen to the diesel vehicles the same thing that happened to the Otto cycle vehicles (gasoline and alcohol) which had national laboratories working on emissions of this type of vehicles, among them the Aeronautics Technological Center -–CTA, in São José dos Campos, and CETESB, in São Paulo.
Studies on vehicle emissions are relatively recent. In 1982, the World Health Organization (WHO) published results of the study "Rapid Assessment of Sources of Air, Water and Land Pollution". We have no information regarding the situation in Europe in this respect before the 1991 agreement among the European Union countries, when prescribed levels were established for new engines. The Brazilian legislation is copied from the European one, due to the fact that most of the diesel vehicles assemblers established in Brazil are European (SCANIA, VOLVO and MERCEDEZ BENZ)
The emission limits for New Heavy Vehicles are presented in Annex 3. It should be noticed that the more relevant greenhouse effect gases (CO2 and CH4) are not included in the prescription.
2 – Relevant Characteristics of Diesel Engines.
In what follows, we will recall the relevant characteristics of diesel engines that are of interest to the emission of pollutants
The diesel engine is called “ignition by compression” engine, which means that the fuel mixture is ignited when the fuel cloud is injected by the high-pressure pump into the hot air contained in the cylinder. The air heating is due to the practically adiabatic compression ( without heat exchange with the exterior) carried out by the engine’s piston. In an Otto cycle engine, on the contrary, the ignition is started by the spark that flies off between the spark plug electrodes. The difference between these ignition forms imposes distinct physical-chemical characteristics on the fuel used in one and other type of engines. The Otto cycle fuel uses light petroleum products ( light naphtha, propane, butane, etc.), natural gas, water gas (produced in gas generators), alcohol and other gaseous substances that can be easily vaporized before getting into the engine’s cylinder. On the other hand, these fuels must stand moderate compression, typical of the Otto cycle (from 1 to 12 atmospheres), without ignition which in this case would be explosive due to the high flame propagation velocity in these fuels and due to the molecular decomposition and re-composition. The parameter that characterizes the resistance to ignition by compression is the Octane Number (ON); in Otto cycle fuels a high octane number is desirable. On the other hand, the easiness of combustion by compression is expressed by the Cetane Number (CN). There is a complementary relationship between these two numbers expressed by the equation of the following type
ON = a – b CN
where a and b are constants that are characteristics of “family” fuels, such as light hydrocarbons that constitute gasoline.
The diesel fuel cetane number characterizes in a certain way the combustion kinetics and therefore influences the specter of substances emitted by the engine. Diesel fuel is a mixture of hydrocarbons whose molecules are heavier than those of gasoline and consequently have a lower hydrogen / carbon mass ratio which determines a high emission of carbon compounds by final energy unit delivered to the engine. However, the diesel cycle characteristics that assure a superior thermal yield than that of the Otto cycle ( such as the fact that it operates with a large air surplus) greatly compensates the disadvantages due to the fuel composition when the parameter of interest is emission/useful energy. The comparison between the two cycles from the point of view of other aspects is vague due to the difference of destination of the respective vehicles. In the present composition of the Brazilian fleet, the Otto cycle vehicles are mainly used for individual passenger transportation and light loads whereas the diesel cycle is used for heavy load services..
In Brazil,where consumption of automotive fuels is practically centered on diesel oil (about 1/3 of the petroleum products uses) due to the structure of the collective transport system of passengers and freight transportation, the cetane number has varied between large limits from the petroleum price rise in the seventies on, reflecting the efforts for accommodating the commercial balance. From the beginning of nineties on, there was a move aiming at improving the diesel quality due to the legislation regarding the air quality. Presently, there are four ranges for cetane number specifications for road, urban, metropolitan, tests and other uses. The range is CN= 40 to 45. In countries where the legislation is more rigorous, the urban CN = 50.
Other relevant characteristics of the diesel fuel (regarding emissions) are density, viscosity, composition (specter of the H atoms/ C atoms ratio, reflected in the distillation curve), the sulfur content, the presence of contaminants, the cyclic hydrocarbons contents (aromatic ones, derived from the benzene fundamental series)
And, obviously, the calorific power.
The mentioned characteristics are not independent, as the acquired experience in Brazil has shown. In the attempts to extend the diesel fuel offer by adding gasoline (or, in other words, the incorporation of lighter hydrocarbons to diesel in the refining process) and alcohol (ethyl and superior ones), the cetane number decreases .whereas the emission profile is modified, with advantages regarding CO emissions and black smoke (soot) but with disadvantages regarding HC emissions. Within certain limits (up to 10% in volume) diesel can receive heavy naphtha without moving away from the specifications of road diesel (CN=40), keeping the specific volumetric consumption and decreasing the CO and smoke emission with a small increase of HC emission.
Of particular interest for Brazil is the addition of alcohol that could act as “lung” for the automotive fuel supply system. The Ministry of Science and Technology coordinated the Alcohol- Diesel Program that considered two addition modalities, hydrated alcohol in emulsion (imported emulsifier) and anhydrous alcohol in solution, with better results for the solution with co-solvent derived from soybean oil, produced in Brazil. The emulsified version was abandoned mainly because of occurrence of carbonization in the injecting pump. Tests carried out at the Institute of Technological Research – IPT, have shown that for the mixture with 10% anhydrous alcohol there was a significant reduction, about 20% in smoke (Bosch index), 12% in CO and 6 –7 % in HCApparently, the direct cost factor is the one that dictates the use of automotive fuel in Brazil. Attempts to modify the status quo , assigning priority to the environmental aspects have collided with corporate interests since they reduce the revenue of predominant groups. It can be shown that the preference for alcohol engines in cars liberates enough gasoline to prepare gasoline-diesel mixture with 10% of the former without using additives, acceptable for road use, reserving the better quality diesel for urban use. This mixture would result in a gain of 3% in thermo-mechanical efficiency of the whole road fleet and reduction of 12% in CO emission. The compensation would be an increase of 6% in HC and 7% in NOx emissions, considering the efficiency gain. The viability of this change would demand a revision of diesel oil prices, subsidized in Brazil and justified by its social importance. However , this subsidy contributes to the concentration of the national wealth in the Southeast Region (mainly São Paulo), which probably annuls the alleged social benefits. One feels the lack of appropriate methodology to debate this question..
2 – Emission Coefficient Methodology
The calculation is based on the fleet composition, section 2 and 3, obtained from the sales of diesel vehicles in Brazil (national and imported) and on the lasting curve for each category (scraping curves). The information about typical power of each category was supplied by Mercedes Benz do Brasil S. A and they can be summarized as shown in Table 2
The percent sales presented in the table above are close to that of the Diesel fleet composition by category obtained for the last year of the series (1979) cf. Figure 2.6 and Table A1.8, considering only one category of trucks (grouping semi- and extra-heavy ). But along the period, the participation of the cars and light commercial vehicles, aggregated in only one category, has grown while that of trucks has decreased. This trend was taken into account in the calculation of the consumption by category that is presented in what follows.
Due to the similarities of the performance curves supplied by MBB for these categories and the relative constancy of specific consumption in a large rotation range , it is supposed that in Diesel engines (Figure 1) the specific consumption in conditions of use is proportional to the minimum specific consumption.
Figure 1: Specific consumption as a function of the engine rotation
Once this hypothesis is established, the diesel oil consumption in the Road Transport Sector obtained from the National Energy Balance /1997 is distributed by categories according to a division factor defined below. Using the previous results, the substitution of old vehicles manufactured in a specific year is calculated, year by year from 1994 on, by the balance equation applied to each category:
Fleet in year i = fleet in year (i-1) + sales in year i – scraping in year i.
The CO, HC, NOx and particulate emission limits, presented in Table 1 in g/kWh are converted to g/liter by a postulated proportionality between the specific consumption (g/liter) and the minimum specific consumption (G/kWh).
The difference between the possible calculation methodologies, the one presented here and the calculation through the diesel oil consumption lies on the possibility of evaluating with the former the effect of short-term measures aiming at stimulating or restricting new transport vehicles such as vans used in urban transport or extra-heavy trucks used in roads.
The CO2 emission must be calculated through the balance of carbon, including the consumed fuel and the CO and HC emissions. It would be desirable to know the effects of diesel oil composition on the emissions, but the preliminary character of the present study does not permit to obtain this information, since there is no standardization of petroleum products in Brazilian refineries which process petroleum from different origins and characteristics. The specifications refer only to minimum requirements in order to assure the engine performance and this has been met by adjusting the refining profile, whenever possible, by varying the composition of the product. A gain in quality, in what concerns the fuel, must be searched for by implementing the program based on PROCONVE’s proposal. CETESB has been evaluating in a positive way the improvement verified in the certified vehicles (“PROCONVE, results and perspectives” – CETESB/1995).
A simplified way to elaborate the carbon balance consist in using the cetane number as the main index of oil quality. The methodology of cetane test refers to the n-hexadecane (C16H36) as the diesel fuel of excellence ( the most easily ignited by compression) and to alpha-methyl-naphthene (C11H10) as that of most difficult ignition. By preparing mixtures of these hydrocarbons with different contents, a comparison is made between the oil under test and each one of these mixtures in engines with variable compression ratios. The cetane number is indicated by the C16H34 content of the mixture that ignites in the same compression conditions as those of the oil under test. The hydrocarbons emitted by the diesel engine, referred to generically as HC, include the original non-burned fuel and other hydrocarbons resulting from its thermal cracking, as well as other substances resulting from chemical reactions with oxygen in the engine. The legislation does not require a discrimination of these different hydrocarbons. The different nitrogen oxides that constitute NOx are not discriminated as well in the determination of emission level and it is not possible to present specific indexes for them.
3 – Agrupamento de Parâmetros
3 –Grouping of Parameters
In order to simplify calculations and facilitate the presentation of results, some parameters were grouped in average values that are valid for the whole fleet. Each grouping is justified and qualified with the calculated relative uncertainty.
In order to calculate the emission by power category, the diesel oil consumption in each year is divided among the categories according to a division factor given by:
r i = xi Pi c i / S i xi Pi c i
where x is the participation of the category in the fleet, P is the typical power of the category and c is the corresponding specific consumption. The participation of the category in the fleet (in %) is shown in the graphic of Figure 2 and in Table 3 below, extracted from Table 2.11 of the first part of the present work. The participation variation affects only the emission by category through the consumption division
but the emission coefficients are maintained since the legislation does not discriminates (not yet) the maximum emission levels by power range.
Figure 2: Participation of light commercial vehicles, trucks and buses in the diesel fleet.
Table 3: Participation in the Diesel Fleet(%)