Emission Reduction in Commercial Vehicles Using Selective Catalysts ^†

Abstract

Transportation is a major contributor to air pollution, with vehicles emitting around 65% of manmade hydrocarbons, 64% of carbon monoxide, and 40% of nitrogen oxides. These pollutants harm the environment, human health, and materials. With vehicle populations expected to reach 1.3 billion by 2030, emissions will only worsen. This project focuses on enhancing the efficiency of catalytic converters, which help convert harmful tailpipe emissions like unburned hydrocarbons and CO into less harmful substances (CO₂ and H₂O). Using a selective catalyst alongside a catalytic converter, the study aims to significantly reduce toxic emissions from traditional IC engine vehicles.

1. Introduction

For a sustainable environment, a multipronged emission control strategy is desired in place of purely electric, hydrogen, and fuel cell technologies. Thus, combustion will continue to be a major source of energy, and any improvement is desirable and welcome. Diesel engines, being the most efficient prime movers for heavy-capacity transport, are used in the fields of rail, marine, power generation, and on-road and off-road vehicles. To improve the salient characteristics of diesel engines, several researchers have varied the ignition delay, duration, as well as pressure and angle of injection, in addition to using additives in diesel fuel or other alternative fuels that have been widely reported in the open literature. The most serious problems facing diesel engines are exhaust emissions, knock control, and idle stability. Reducing emissions is a major and critical focus for researchers because of the increasing number of vehicles and stringent regulations all over the world.

The environmental impact can be mitigated by the proper use of after-treatment technologies. However, technologies such as catalytic converters, EGR, thermal converters, etc., may not improve efficiency while reducing emissions. For simultaneous reduction of emissions and increased performance, the injection methods need to be modified in diesel engines. Achieving higher power with lower fuel requirements greatly depends on fuel injection methods [1,2,3,4,5]. For better combustion, fuel particle size is an important factor that depends on the nozzle orifice, angle, pressure, and timing of injection. In the fuel injector, a good nozzle helps achieve proper atomization, distribution of fuel, prevention of impingement on walls, and mixing with air. During injection, several factors need to be considered, such as the injection angle into the cylinder, which is approximately 100 degrees to 160 degrees [6,7,8,9]; fuel injection pressure, commonly varying from 200 bar to 1500 bar [10,11,12]; injection timing, which either advances or retards the time of injection into the cylinder [13,14,15]; and either the orifice diameter or the number of nozzle holes.

In the present study, harmful airborne pollutants were converted to less harmful emissions to the extent of 90% with the help of a catalytic converter. Owing to the requirement for a higher operating temperature (400 °C), it is placed near the engine or, for smaller versions, at the exit of the exhaust manifold.

1.1. Procedure for Preparing the Final Catalyst

Figure 1 describes the sequence in which the final catalyst is achieved. The catalyst was stirred constantly at 700 rpm for 4 h using a magnetic stirrer until it was completely homogenous and formed into a slurry or sol–gel. This slurry was then used to coat a stainless steel wire mesh of 8.5 cm diameter using the dip-coating process. The coated mesh was placed in a hot-air oven and was constantly heated at a temperature of 120 °C for 5 h to completely remove all moisture content from the wire mesh. The stainless steel wire mesh was cut to a diameter of 8.5 cm and then kept at the end of the catalytic converter for the catalysis reaction to take place. After the mixing and heating process, the mesh was removed. Figure 2 shows the stainless steel mesh after it bonded with the final catalyst. Sodium silicate was added to ensure good binding and prevent the loss of catalyst along with exhaust.

Figure 1. Sequence in which the final catalyst was achieved.

Figure 1. Sequence in which the final catalyst was achieved.

Table 1. Molar ratio calculation.

Catalyst	Molar Mass (g/mol)	Weight (g or mL)	No. of Moles
Sodium silicate (Na2SiO3)	122	254 g or 185 mL	2.08
Magnesium oxide (MgO)	40.38	69.3 g	1.71
Chromium oxide (Cr2O3)	152	26.08 g	0.17
The molar ratio (MgO and Cr2O3)—1.71:0.17			10:1

Total weight taken: 350 g.

Table 1. Molar ratio calculation.

Catalyst	Molar Mass (g/mol)	Weight (g or mL)	No. of Moles
Sodium silicate (Na2SiO3)	122	254 g or 185 mL	2.08
Magnesium oxide (MgO)	40.38	69.3 g	1.71
Chromium oxide (Cr2O3)	152	26.08 g	0.17
The molar ratio (MgO and Cr2O3)—1.71:0.17			10:1

Total weight taken: 350 g.

Figure 2. Existing catalytic converter [16].

Figure 2. Existing catalytic converter [16].

1.2. Catalytic Converter

The catalytic converter of a BS-IV Chevrolet Cruze diesel engine vehicle, shown in Figure 2, was used. After ensuring thorough cleaning and inspection, the flanged joints with a 1.25-inch diameter gasket between them for sealing were welded. The prepared catalyst (Figure 3) was placed, with a 1 cm gap between the existing catalytic converter and the flanged connection.

1.3. Selective Catalysts

After the 1950s, catalytic converters began to be used, and in the late 1970s, they began to gain popularity globally. About 90% of the emissions of HC, CO, and NO_x and a small quantity of SOx, soot particles, and other tailpipe pollutants are converted with their assistance. Although catalytic converters have undergone significant improvements over the years, they nevertheless have some drawbacks and lower efficiency in some situations. We studied the reactions of different metals in the periodic table to find a solution to this problem and came to the conclusion that transition and alkaline earth metals had a greater ability to reduce HC and CO emissions. For this reduction process, transition metals like copper (Cu) can be employed. Existing research procedures like selective catalyst reduction (SCR) and reformed exhaust gas recirculation (REGR) can be employed to achieve improved efficiency for turning these harmful emissions into innocuous tailpipe emissions. The conversion efficiencies of these dangerous tailpipe emissions are further influenced by the problems catalytic converters have with cold starting. We also discovered that employing a hexagonal design with reduced thickness can lower the overall cost of catalytic converters by lowering the amount of metal required.

1.4. Calculation of Molar Ratio

Table 1 represents the molar ratio calculation to prepare a new catalyst (Cr₂O₃, MgO) for a 350 g composite, 254 g of Na₂SiO₃ (2.08 mol), 69.3 g of MgO (1.71 mol), and 26.08 g of Cr₂O₃ (0.17 mol) were used. Based on molar masses, the molar ratio of MgO to Cr₂O₃ is 1.71:0.17, simplified to 10:1. This ratio helps control chemical reactivity and stability in the final product, ensuring optimal performance in material applications.

2. Engine Test Rig

Kirloskar Engine TV 1 was used for the purpose of testing the catalytic converter. The engine specifications are provided in [17]. First, the test was performed for a conventional catalytic converter, which consists of platinum, palladium, and rhodium (PPR) as its catalyst, and the test results were interpreted. Figure 4 shows the test rig setup. The test was performed after the modification of the catalytic converter and the addition of a new catalyst (Cr₂O₃, MgO). The results obtained were compared to those for the converter without the new catalyst (PPR), and various graphs such as conversion efficiency, exhaust gas temperature, HC, and CO emissions were plotted.

3. Results and Discussions

3.1. Smoke Opacity

Figure 5 depicts smoke opacity vs. load. By comparing the characteristics of load and opacity, it is seen that opacity increases with increasing load. In this study, a similar result was observed, a 30% smoke reduction using a chromium–magnesium-based catalyst. Similar result found in the S.P. Venkatesan et al. [18] using copper-based catalytic converters led to a smoke reduction of 32%.

3.2. Conversion Efficiency with PPR Catalyst

Figure 6 depicts the results obtained using the test rig setup with the existing catalytic converter consisting of the PPR catalyst. The values of the air/fuel ratio and the emission levels are noted. With the help of emission levels, conversion efficiencies for each emission are obtained. It is observed that CO and HC conversion efficiency curves increase with higher air/fuel ratios and NO_x conversion efficiency drops with increasing air/fuel ratios, as NO_x occurs as a result of higher engine temperatures. The Bharat standard emission norms for stage VI have HC and CO emissions of 90 ppm and 200 ppm. The emissions at the highest load were 55 ppm, which is within the range of BS4 and BS6 norms, indicating that there is a nominal change in conversion efficiency when the PPR catalyst is used.

3.3. Conversion Efficiency of the (PPR + CrMg) Catalyst

Figure 7 shows the conversion efficiencies of CO, HC, and NO_x emissions plotted against air/fuel ratios, based on the results obtained using a catalytic converter fitted with the (PPR + CrMg) catalyst. The graph shows that there is a significant amount of improvement in the conversion efficiencies of CO and unburned HC compared to the existing PPR catalyst, indicating the oxidation properties of the chromium oxide and magnesium oxide catalyst. It is once again confirmed that excess CO and HC, which are not converted properly by the PPR catalyst, are properly converted to CO₂ and H₂O with the use of the CrMg catalyst. Dillip Kumar Sahoo et al. [19] used a TiO₂ catalyst and obtained a higher conversion efficiency for HC and CO, but there was a slight rise in NO_x emissions, proving that the oxidation reaction was successful on using the CrMg catalyst.

3.4. Unburnt Hydrocarbon and Carbon Monoxide Emissions

Figure 8 indicates that with the help of catalytic converters, there is proper conversion of unburnt HC into H₂O and CO₂ and the HC content decreases with increasing engine speed, almost reaching zero at maximum engine speed. The modified catalytic converter with the CrMg catalyst converted a considerable amount of HC into water and carbon dioxide compared with the PPR catalytic converter without the CrMg catalyst. K. Parthiban et al. [20] achieved a 63% reduction using a copper mesh filter. Through this modified catalyst, it is possible to achieve a 60% reduction in HC using a chromium magnesium catalyst.

Figure 9 represents the CO emission vs. load for three different conditions: without catalyst, with PPR catalyst, and with (PPR + CrMg) catalyst. It is observed that with the help of catalytic converters, the amount of CO emissions reduces drastically, and with our PPR + CrMg catalyst, there is a significant amount of reduction in the occurrence of CO, indicating complete conversion of CO into CO₂. Nitin Rathod et al. proved that there was a reduction in CO of up to 85% when using a ruthenium catalyst. Similarly, CrMg also achieved a reduction in CO levels of up to 80%.

3.5. Nitrous Oxide Emission

Figure 10 represents NO_x emission for three different conditions: without catalyst, with PPR catalyst, and with (PPR + CrMg) catalyst. It is observed that with the help of catalytic converters, there is a significant reduction in NO_x levels, and as CO and HC levels decrease, there is a slight increase in NO_x percentage with the (PPR + CrMg) catalyst. MA Kalam et al. observed a slight rise in the NO_x emissions due to the reduction in HC and CO levels. Similarly, the results for modified catalysts showed a decrease in CO and HC levels with an increase in NO_x levels.

4. Conclusions

The aim of this study was to reduce soot particulates, CO, and HC to control global warming and its adverse effects on the environment. The modified catalyst, consisting of (PPR + CrMg), showed positive results. The modified catalytic converter provided the following results:

• The conversion efficiency of CO was found to be 79%.
• The conversion efficiency of HC was found to be 60%.
• The smoke opacity reduced by 30%.
• CO₂ levels increased by 17%.

When compared to the PPR catalyst, it is evident that the efficiency of CO and HC conversion increased. This was demonstrated by an increase in CO₂ levels, which shows that HC and CO were properly and effectively oxidized to produce H₂O and CO₂. Although CO and HC emissions decreased, it was found that NO_x emissions increased, indicating that the oxidation reaction was successful. Another conclusion that supports this is that there was a 30% reduction in opacity, which shows that the emission of HC reduced. Thus, using (PPR+CrMg), a catalyst based on chromium oxide and magnesium oxide, this catalytic converter achieved our goal of reducing HC and CO emissions with a conversion efficiency of 70% and also achieved better CO conversion efficiencies than those recommended by the BS VI norms.