rus
Issue #5/2024
V.M.Sitdikov, N.Y.Dudareva
NANOSTRUCTURED COATING FOR REDUCING INTERNAL COMBUSTION ENGINES TOXICITY
NANOSTRUCTURED COATING FOR REDUCING INTERNAL COMBUSTION ENGINES TOXICITY
DOI: https://doi.org/10.22184/1993-8578.2024.17.5.311.319
The effect of a nanostructured coating formed by microarc oxidation on pistons of internal combustion engines on the exhaust gases toxicity has been studied. The role of nanoscale porosity in the mechanism of toxicity reduction has been revealed.
The effect of a nanostructured coating formed by microarc oxidation on pistons of internal combustion engines on the exhaust gases toxicity has been studied. The role of nanoscale porosity in the mechanism of toxicity reduction has been revealed.
Теги: exhaust gases internal combustion engines microarc oxidation nanostructured coating toxicity двигатели внутреннего сгорания микродуговое оксидирование наноструктурное покрытие отработавшие газы токсичность
INTRODUCTION
The most important modern problem, relevant for all countries of the world, is reduction of ecological load on the environment. Exhaust gases (EG) of internal combustion engines (ICE) emitted into the atmosphere are one of the main causes of its pollution [1]. According to statistics, about 60% of air pollution is caused by motor transport. Toxic substances contained in exhaust gases have a detrimental effect on vegetation, as well as human and animal health [2]. Internal combustion engine exhaust gases contain such harmful substances as carbon monoxide (CO), unburned hydrocarbons (CnHm), aldehydes, nitrogen oxides (NOx), particulate matter, etc. [3]. Nowadays, various ways to reduce combustion engine toxicity exhaust gas have been developed: the impact on the engine operating process, the use of alternative fuels, and the use of catalytic converters in the exhaust system [4, 5]. However, the possibilities of all the above methods are practically exhausted, which does not allow modern engines to achieve Euro-6 environmental standards [6].
One of the promising ways to improve environmental friendliness of piston engines is neutralisation of toxic exhaust gas substances directly in the engine combustion chamber. This approach allows increasing efficiency of the already available neutralisation system. A number of studies are known, in which the use of coatings formed by micro-arc oxidation (MAO) technology for thermal protection of aluminium pistons during engines operation resulted in a reduction of exhaust gas toxicity [7–9]. At the same time, the use of coatings of similar composition, but formed by other methods, such as plasma spraying, did not have such an effect on EG [10]. However, as the analysis of scientific literature has shown, no systematic studies in this direction have been conducted.
It is known that MAO coatings formed on aluminium alloys consist of oxides ((a-Al2O3, g-Al2O3) and aluminium silicates [11, 12], which are traditionally used in catalysts. These coatings also have developed porosity [13, 14] and are nanostructured according to GOST R 9.318-2013. In this work, a hypothesis was formulated that reduction of combustion engine exhaust gas toxicity is provided by the structure of MAO coatings represented by nanoscale pores, when diffusion of molecules and the process of catalysis take place. Based on that facts, the aim of this work was formulated: to study the role of the structure of the MAO coating formed on combustion chamber parts in influencing toxicity of internal combustion engines.
RESEARCH METHODS
Two petrol engines were chosen as experimental ones: a two-stroke two-cylinder engine of RMZ-551i brand and a four-stroke one-cylinder engine of UMZ-341 brand [15, 16]. MAO coating was formed on the crowns of pistons of these engines. It is known that MAO coating quality and its structure are significantly influenced by chemical composition of the aluminium alloy of the substrate [17]. The piston of the RMZ-551i engine is made of high-silicon foreign-made alloy of M244 grade (AlSi26CuNiMg: Si 23–26%; Cu 0.8–1.5%; Zn – up to 0.2%; Fe 0.7%; Mn 0.2%; Mg 0.8–1.3%; Ti 0.2%; Ni 0.8–1.3%) [18], and the piston of the UMZ-341 engine is made of eutectic AK12 grade alloy (Al 84.3–90%; Si 10–13%; Fe – up to 1.5%; Cu – up to 0.6%; Mn – up to 0.5%; Zn – up to 0.3%; Mg and Ti up to 0.1%) [19].
The formation of the coating during MAO takes place in the electrolyte when spark and arc discharges occur on the treated surface under the action of high voltage. As a result, an oxide layer, called modified oxide layer, is formed on the part surface, resulting in the formation of the coating [20]. Coating of piston crowns was performed using a special device that ensures formation of coating exclusively on the piston crown, protecting other surfaces from electrolyte ingress (Fig.1).
The MAO process was carried out in silicate-alkaline electrolyte containing 4 g/l KOH and 4 g/l Na2SiO3 at the capacity of the unit C = 400 µF. The treatment time of the pistons was 90 minutes. Then the pistons were washed in water and after drying the top loose layer of coating was removed with the help of fine-grained sandpaper until the coating thickness ~120 μm was reached. Thickness of coatings on pistons was measured by non-destructive method with eddy current thickness gauge TT-210.
To study the coatings structure, witness samples made of the same alloys as the pistons (M244 and AK12) were processed in the same MAO mode. From the coated witness specimens, micro-slides were made. The porosity and thickness of the coatings were analysed from photographs of the cross-slides obtained with a TESCAN MIRA LMS electron microscope. Porosity of the coatings was studied by processing the images of the cross-slits in the ImageJ software. The share of nanoscale pores with a diameter not exceeding 50 nm was highlighted, since it is in pores of this size that the catalysis process takes place [21].
Studies of the influence of MAO coating on the exhaust gas toxicity were carried out on the engines in the mode of bench engine tests. In the process of tests, measurements of exhaust gas toxicity and other performance indicators of the engines were carried out. The test methodology was in accordance with GOST 14846-2020. To test the RMZ-551i engine, a test rig based on the AVL-DP 80 engine test bench was developed and assembled (Fig.2a). The UMZ-341 engine was tested according to the scheme shown in Fig.2b.
Toxicity was studied in engines with standard pistons (uncoated) and with pistons with MAO coating on the crown (Fig.3). Engine test modes were selected in accordance with their technical characteristics and capabilities of the load bench. On the RMZ-551i engine the tests were carried out on 20 modes, which differed in revolutions and degree of throttle opening. The engine speeds selected are as follows: 2000, 3000, 4000, 5000 and 6000 rpm. At each speed mode the throttle opening degree was: 25, 50, 75 and 100%. On the UMZ-341 engine tests were carried out in 6 modes: crankshaft revolutions were 2400, 2700 and 3200 rpm. Instead of opening a throttle, the generator load was set to 500 W for 2400 rpm, 1000 and 500 W for 2700 rpm, and 1500, 1000 and 500 W for 3200 rpm. In each mode, data on exhaust gas toxicity (СО, NOx, CnHm) and CO2 were recorded at 30 s intervals. Engine performance indicators were measured: fuel consumption, exhaust gas and cylinder head temperatures, quantity of O2 in the exhaust gas, and the excess air ratio. The results were processed taking into account random and instrumental errors of measurements according to the theory of mathematical processing of experimental data.
RESULTS
The study of coatings on microdots of witness samples showed that coatings obtained on different alloys differ in their structure (Fig.4).
The coating on M244 alloy has an irregular structure with many cavities. The coating on AK12 alloy is dense, and cavities are observed only in the surface area. Nanoscale porosity (pores with sizes up to 50 nm) of the MAO coating on M244 alloy was 29.3±4.7%, and on AK12 alloy it was significantly less – 10.8±4.3%. In addition, the coatings differed in the character of pore distribution: on AK12 alloy nanoscale pores are concentrated near the substrate, and on M244 alloy they are distributed throughout the thickness of the coating. At the same time for coatings at a distance from the substrate ~100 μm nanoscale porosity on the M244 alloy was 16.9±4.6%, and on the AK12 alloy – 4.7±2.9%. It is with this area of the piston coating that the engine exhaust gas is in a contact.
Measurement of the exhaust gas composition during tests of the RMZ-551i engine showed that this engine shows toxicity reduction for all components of the exhaust gas: CO, СnНm and NOx. However, this effect depends on the engine operation mode. The greatest reduction in CO (up to 17.1%) was found at 3000 min–1 and 50% throttle opening (Table 1).
Table 2 presents the results of the exhaust emission measurement in the test of the UMZ-341 engine. This engine showed a reduction of CO2 component, which does not belong to the group of toxic components of the exhaust gas. At the same time, a decrease in fuel consumption, exhaust gas temperature and excess air ratio was observed. In Tables 1 and 2 the differing values of gas emission are marked in bold.
DISCUSSION
The analysis of the obtained results showed that when using pistons made of M244 alloy, significant reduction of exhaust gas toxicity was recorded, especially for CO up to 17%. When using pistons made of AK12 alloy with coating, there is practically no reduction in the exhaust gas toxicity of the internal combustion engine. On the UMZ-341 engine, only a reduction of CO2 was found, which is not related to the catalytic properties of the MAO coating, but is caused by fuel consumption reduction [22]. At present, it is believed that ceramic coating on four-stroke internal combustion engines changes the combustion thermal front, causing reduction in fuel consumption, which entails reduction in the quantity of exhaust gas and, consequently, CO2 [23].
The obtained results can be explained solely by the structure of MAO coatings (Fig.5). It is known that the catalytic effect is always related to porosity [21, 24]. The process of neutralisation of OH occurs due to the diffusion of gas molecules deep into the coating, and diffusion is possible only along pores with a small diameter – up to 50 nm [21]. The nanoscale porosity of the coating on M244 alloy is quite high (16.9±4.6%), more than 2.5 times higher than that of the coating on AK12 alloy – 6.4±3.5%. This can explain the obtained effect.
CONCLUSIONS
It was found that the structure of MAO coatings represented by nanoscale porosity has a significant influence on reduction of emissions of toxic components of exhaust gas and on the efficiency of the coating as a catalyst. Application in engines of pistons with MAO coating with nanosize porosity of 16.9±4.6% leads to reduction of exhaust gas toxicity – emission of CO component is maximally reduced by 17.1%, and on average (taking into account all modes) – by 3.1%. Application in engines of pistons with MAO coated pistons with nanosize porosity of 6.4±3.5% did not cause reduction of exhaust gas toxicity.
ACKNOWLEDGMENTS
The research was supported by the Ministry of Science and Higher Education of the Russian Federation within the framework of the State Assignment No. FEUE-2023-0007 (UUNiT).
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The most important modern problem, relevant for all countries of the world, is reduction of ecological load on the environment. Exhaust gases (EG) of internal combustion engines (ICE) emitted into the atmosphere are one of the main causes of its pollution [1]. According to statistics, about 60% of air pollution is caused by motor transport. Toxic substances contained in exhaust gases have a detrimental effect on vegetation, as well as human and animal health [2]. Internal combustion engine exhaust gases contain such harmful substances as carbon monoxide (CO), unburned hydrocarbons (CnHm), aldehydes, nitrogen oxides (NOx), particulate matter, etc. [3]. Nowadays, various ways to reduce combustion engine toxicity exhaust gas have been developed: the impact on the engine operating process, the use of alternative fuels, and the use of catalytic converters in the exhaust system [4, 5]. However, the possibilities of all the above methods are practically exhausted, which does not allow modern engines to achieve Euro-6 environmental standards [6].
One of the promising ways to improve environmental friendliness of piston engines is neutralisation of toxic exhaust gas substances directly in the engine combustion chamber. This approach allows increasing efficiency of the already available neutralisation system. A number of studies are known, in which the use of coatings formed by micro-arc oxidation (MAO) technology for thermal protection of aluminium pistons during engines operation resulted in a reduction of exhaust gas toxicity [7–9]. At the same time, the use of coatings of similar composition, but formed by other methods, such as plasma spraying, did not have such an effect on EG [10]. However, as the analysis of scientific literature has shown, no systematic studies in this direction have been conducted.
It is known that MAO coatings formed on aluminium alloys consist of oxides ((a-Al2O3, g-Al2O3) and aluminium silicates [11, 12], which are traditionally used in catalysts. These coatings also have developed porosity [13, 14] and are nanostructured according to GOST R 9.318-2013. In this work, a hypothesis was formulated that reduction of combustion engine exhaust gas toxicity is provided by the structure of MAO coatings represented by nanoscale pores, when diffusion of molecules and the process of catalysis take place. Based on that facts, the aim of this work was formulated: to study the role of the structure of the MAO coating formed on combustion chamber parts in influencing toxicity of internal combustion engines.
RESEARCH METHODS
Two petrol engines were chosen as experimental ones: a two-stroke two-cylinder engine of RMZ-551i brand and a four-stroke one-cylinder engine of UMZ-341 brand [15, 16]. MAO coating was formed on the crowns of pistons of these engines. It is known that MAO coating quality and its structure are significantly influenced by chemical composition of the aluminium alloy of the substrate [17]. The piston of the RMZ-551i engine is made of high-silicon foreign-made alloy of M244 grade (AlSi26CuNiMg: Si 23–26%; Cu 0.8–1.5%; Zn – up to 0.2%; Fe 0.7%; Mn 0.2%; Mg 0.8–1.3%; Ti 0.2%; Ni 0.8–1.3%) [18], and the piston of the UMZ-341 engine is made of eutectic AK12 grade alloy (Al 84.3–90%; Si 10–13%; Fe – up to 1.5%; Cu – up to 0.6%; Mn – up to 0.5%; Zn – up to 0.3%; Mg and Ti up to 0.1%) [19].
The formation of the coating during MAO takes place in the electrolyte when spark and arc discharges occur on the treated surface under the action of high voltage. As a result, an oxide layer, called modified oxide layer, is formed on the part surface, resulting in the formation of the coating [20]. Coating of piston crowns was performed using a special device that ensures formation of coating exclusively on the piston crown, protecting other surfaces from electrolyte ingress (Fig.1).
The MAO process was carried out in silicate-alkaline electrolyte containing 4 g/l KOH and 4 g/l Na2SiO3 at the capacity of the unit C = 400 µF. The treatment time of the pistons was 90 minutes. Then the pistons were washed in water and after drying the top loose layer of coating was removed with the help of fine-grained sandpaper until the coating thickness ~120 μm was reached. Thickness of coatings on pistons was measured by non-destructive method with eddy current thickness gauge TT-210.
To study the coatings structure, witness samples made of the same alloys as the pistons (M244 and AK12) were processed in the same MAO mode. From the coated witness specimens, micro-slides were made. The porosity and thickness of the coatings were analysed from photographs of the cross-slides obtained with a TESCAN MIRA LMS electron microscope. Porosity of the coatings was studied by processing the images of the cross-slits in the ImageJ software. The share of nanoscale pores with a diameter not exceeding 50 nm was highlighted, since it is in pores of this size that the catalysis process takes place [21].
Studies of the influence of MAO coating on the exhaust gas toxicity were carried out on the engines in the mode of bench engine tests. In the process of tests, measurements of exhaust gas toxicity and other performance indicators of the engines were carried out. The test methodology was in accordance with GOST 14846-2020. To test the RMZ-551i engine, a test rig based on the AVL-DP 80 engine test bench was developed and assembled (Fig.2a). The UMZ-341 engine was tested according to the scheme shown in Fig.2b.
Toxicity was studied in engines with standard pistons (uncoated) and with pistons with MAO coating on the crown (Fig.3). Engine test modes were selected in accordance with their technical characteristics and capabilities of the load bench. On the RMZ-551i engine the tests were carried out on 20 modes, which differed in revolutions and degree of throttle opening. The engine speeds selected are as follows: 2000, 3000, 4000, 5000 and 6000 rpm. At each speed mode the throttle opening degree was: 25, 50, 75 and 100%. On the UMZ-341 engine tests were carried out in 6 modes: crankshaft revolutions were 2400, 2700 and 3200 rpm. Instead of opening a throttle, the generator load was set to 500 W for 2400 rpm, 1000 and 500 W for 2700 rpm, and 1500, 1000 and 500 W for 3200 rpm. In each mode, data on exhaust gas toxicity (СО, NOx, CnHm) and CO2 were recorded at 30 s intervals. Engine performance indicators were measured: fuel consumption, exhaust gas and cylinder head temperatures, quantity of O2 in the exhaust gas, and the excess air ratio. The results were processed taking into account random and instrumental errors of measurements according to the theory of mathematical processing of experimental data.
RESULTS
The study of coatings on microdots of witness samples showed that coatings obtained on different alloys differ in their structure (Fig.4).
The coating on M244 alloy has an irregular structure with many cavities. The coating on AK12 alloy is dense, and cavities are observed only in the surface area. Nanoscale porosity (pores with sizes up to 50 nm) of the MAO coating on M244 alloy was 29.3±4.7%, and on AK12 alloy it was significantly less – 10.8±4.3%. In addition, the coatings differed in the character of pore distribution: on AK12 alloy nanoscale pores are concentrated near the substrate, and on M244 alloy they are distributed throughout the thickness of the coating. At the same time for coatings at a distance from the substrate ~100 μm nanoscale porosity on the M244 alloy was 16.9±4.6%, and on the AK12 alloy – 4.7±2.9%. It is with this area of the piston coating that the engine exhaust gas is in a contact.
Measurement of the exhaust gas composition during tests of the RMZ-551i engine showed that this engine shows toxicity reduction for all components of the exhaust gas: CO, СnНm and NOx. However, this effect depends on the engine operation mode. The greatest reduction in CO (up to 17.1%) was found at 3000 min–1 and 50% throttle opening (Table 1).
Table 2 presents the results of the exhaust emission measurement in the test of the UMZ-341 engine. This engine showed a reduction of CO2 component, which does not belong to the group of toxic components of the exhaust gas. At the same time, a decrease in fuel consumption, exhaust gas temperature and excess air ratio was observed. In Tables 1 and 2 the differing values of gas emission are marked in bold.
DISCUSSION
The analysis of the obtained results showed that when using pistons made of M244 alloy, significant reduction of exhaust gas toxicity was recorded, especially for CO up to 17%. When using pistons made of AK12 alloy with coating, there is practically no reduction in the exhaust gas toxicity of the internal combustion engine. On the UMZ-341 engine, only a reduction of CO2 was found, which is not related to the catalytic properties of the MAO coating, but is caused by fuel consumption reduction [22]. At present, it is believed that ceramic coating on four-stroke internal combustion engines changes the combustion thermal front, causing reduction in fuel consumption, which entails reduction in the quantity of exhaust gas and, consequently, CO2 [23].
The obtained results can be explained solely by the structure of MAO coatings (Fig.5). It is known that the catalytic effect is always related to porosity [21, 24]. The process of neutralisation of OH occurs due to the diffusion of gas molecules deep into the coating, and diffusion is possible only along pores with a small diameter – up to 50 nm [21]. The nanoscale porosity of the coating on M244 alloy is quite high (16.9±4.6%), more than 2.5 times higher than that of the coating on AK12 alloy – 6.4±3.5%. This can explain the obtained effect.
CONCLUSIONS
It was found that the structure of MAO coatings represented by nanoscale porosity has a significant influence on reduction of emissions of toxic components of exhaust gas and on the efficiency of the coating as a catalyst. Application in engines of pistons with MAO coating with nanosize porosity of 16.9±4.6% leads to reduction of exhaust gas toxicity – emission of CO component is maximally reduced by 17.1%, and on average (taking into account all modes) – by 3.1%. Application in engines of pistons with MAO coated pistons with nanosize porosity of 6.4±3.5% did not cause reduction of exhaust gas toxicity.
ACKNOWLEDGMENTS
The research was supported by the Ministry of Science and Higher Education of the Russian Federation within the framework of the State Assignment No. FEUE-2023-0007 (UUNiT).
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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