Analysis of combustion and performance characteristics of multilayer HVOF and plasma ceramic coatings on piston in a diesel engine.
The rapid increase in fuel expenses, the decreasing supply of high-grade fuels on the market and environmental concerns stimulated research on more efficient engines with acceptable emission characteristics. The state-of-art thermal barrier coatings provide the potential for higher thermal efficiencies of the engine, improved combustion and reduced emissions. In addition, ceramics show better wear characteristics than conventional materials. Lower heat rejection from the combustion chamber through thermally insulated components causes an increase in available energy that would increase the in-cylinder work and the amount of energy transported by the exhaust gases, which could be also utilized.
Ceramic coatings are becoming increasingly important in providing thermal insulation for heat engine components. Thermal spraying technology has been widely used for thermal barrier coatings and wear resistant coatings [1-3]. Particularly, zirconia (ZrO) coating has superior property as thermal barrier coatings for high temperature protection of metallic structures. Plasma-sprayed ceramic coatings in combustion chambers of alternative car engines have been successfully developed for the thermal protection of components such as pistons, valves, upper liners, etc. These coatings are used to provide resistance to abrasion and wear at high temperature and reduce friction. Wear is closely related to the adhesion of the coating to the substrate because if the adhesion is poor, the coatings will wear off quite rapidly and the extent of deterioration of the substrates by environmental factors will be greatly accelerated . Adhesion is a very important factor determining the life of coated parts. The coatings must be properly bonded to the substrates. Adhesion depends on substrate preparation. Very few papers have dealt with the effect of preparation on adhesion. The thermal barrier coating is usually a multi-layer system consisting of a ceramic topcoat, a metallic bond coat and a superalloy substrate.
Application of thick plasma-sprayed coatings has a number of disadvantages namely, adhesion of coatings thicker than 0.5 mm is poor, and temperatures of combustion chamber walls raised to excessive values reduce the volumetric efficiency and create additional problems with adequate lubrication at the higher end of piston stroke. Typically, carbon deposits on the combustion chamber surface possess thermal properties comparable with ceramics. Due to higher temperatures in the combustion chamber, lower emissions of unburned hydrocarbons can be expected. On the other hand, however, for the same reasons, nitrous and nitric oxide (NO) emissions would tend to rise. There are contradictory reports on the effect of ceramic coatings on engine performance . In this work, therefore, multilayer ceramic coatings Cr3C2-PS. ZrO2 with bonding coating (Ni-Al) were applied by HVOF and plasma-spraying technique onto the piston top and the combustion and performance characteristics of a diesel engine were analyzed.
HVOF and Plasma spray coating:
In this work, two variants of ceramic powders were selected based on their characterization of particle sizes, dispersive nature and compositions, to be suitable for HVOF and plasma spray coatings. The powder for plasma spraying and HVOF process were, prepared with layer coating format as Cr3C2 powder for wear resistance (top layer) and PS.ZrO2 powder for thermal insulation (base layer). The phenomenon of plasma powder getting through plasma gas and sprayed on substrate is known to be plasma spraying technique. In plasma spray technique, the coating powder can be sent through a plasma gas. Between two electrodes plasma is formed and powder is deposited in plasma arc. The primary gas is usually argon or nitrogen. In gun, with high frequency electrical current, ionization takes place frequently in the system. The formed plasma can withstand up to 2000 A. A direct current with potential of 30-80 V. Standard plasma guns can reach up to 40 kW. The melted and mixed powder is quickly sprayed on the substrate.
In plasma spraying technique, when argon, hydrogen or nitrogen gases are used, oxidation problem is minimized. For this reason, plasma spraying techniques have found useful applications. Even though this system is expensive, in some purpose there is no way to use other method but plasma spray technique. One of the advantages of plasma spraying is that it makes possible to coat with high melting point materials. To coat the coating material, surface should be rough and does not contain oxide, oil and powder on the surface. Surface roughness can be provided by spraying alumina and sand with pressure on to the surface. With the plasma spraying technique the coating thickness can be in the range of 2.5-2500[micro]m.
Experimental multilayer ceramic coating:
Multilayer ceramic coating was applied by using HVOF and plasma spraying techniques. The piston top was coated with first layer Cr3C2of thickness 125 microns and PSZrO2(second layer of thickness 125 microns with bonding material. The bonding material used in this study was Ni-Al. After coating the total thickness was measured and found to be about 250 microns. The photographic view of the uncoated piston and coated piston are shown in Fig. 1 and Fig. 2 respectively.
Experimental set up:
The experimental setup and the specifications of the test engine are shown in Fig. 3 and Table 1 respectively. The engine was coupled with eddy current dynamometer loaded by electrical resistance bank. Exhaust gas temperature was measured by an iron-constantan thermocouple. A mercury thermometer used to measure the cooling water temperature.
2. Magnetic pickup
3. Exhaust gas analyzer
4. Fuel injector
6. Fuel tank
7. Dynamometer controller
8. Piezo electric transducer
9. Charge amplifiers
10. Storage amplifier
11. Air box
A piezoelectric transducer was installed in the engine cylinder in order to measure the combustion pressure. Signal from the pressure transducer were fed to the charge amplifier. A magnetic shaft encoder was used to give the signal for TDC and the crank angle. The signals from the charge amplifier and shaft encoder were coupled to data acquisition system. Cylinder pressure and other desired data was collected. This paper discusses the variation of combustion, performance and emission parameters with respect to crank angle at full load. The experiment was conducted on test engine at 1500 rpm. The photographic view of the engine set up is shown in Fig. 4.
Later the experiment was carried out in two phases for uncoated and multilayered coated piston engines. The engine was run for approximately 450 h for each engine under the same part load conditions . At the end of the experiments carried out, pistons in both engines were dismantled and same regions of these components were taken for wear analysis. The morphologies were observed by JOEL JSM-6360 scanning electron microscope (SEM).
RESULTS AND DISCUSSION
Experimental investigations were carried out in a single cylinder, four stroke direct injection diesel engine under identical conditions for uncoated piston and multilayered ceramic coated piston. The results were analyzed and presented for the combustion, performance, emission and surface characteristics.
5.1. Variation of Cylinder Pressure:
Figure 5 shows the variation of cylinder pressure with respect to crank angle for the full load conditions for uncoated piston and multilayered ceramic coated piston respectively. The result shows that the cylinder peak pressure for the uncoated piston is 68 bar and for multilayered ceramic coated piston is 72 bar. The increase in peak pressure in the case of multilayered ceramic coated piston is due to the ceramic coating which retains the heat inside the combustion chamber which in turn increases the engine operating temperature resulting in increased cylinder peak pressure. Also in a compression ignition engine, cylinder pressure depends on the burned fuel fraction during the premixed burning phase i.e. initial stage of combustion, cylinder pressure characteristics and the ability of the fuel to mix well with air and burn. High peak pressure and maximum rate of pressure rise corresponds to large amount of fuel burned in premixed stage.
Fig. 6 shows the variation of the cylinder peak pressure with respect to load for uncoated piston and multilayer ceramic coated piston for various load conditions. It is seen that irrespective of the load condition the cylinder peak pressure is higher in case of multilayer ceramic coated piston. The result shows that the cylinder peak pressure for the ceramic coated engine is higher than the uncoated piston due to the higher heat retainment inside the combustion chamber with higher operating temperature which enhances the preparation and reaction rate.
5.2. Variation of Rate of heat release:
Figure 7 shows the variation of heat release with crank angle at full load condition. Under identical conditions, fuel in uncoated piston and multilayer ceramic coated piston experiences the rapid premixed fuel burning followed by diffusion combustion. After the ignition delay period the premixed fuel air mixture burn rapidly releasing the heat at very high rapid rate after which the diffusion combustion takes place. Where the burning rate is controlled by the availability of the combustible fuel air mixture. The peak heat release for the multilayer ceramic coated piston is higher than the uncoated piston diesel engine by 18%. The significance of higher operating temperature associated with ceramic coating shows better performance
5.3 Brake thermal efficiency:
Figure 8 shows the variation of brake thermal efficiency with load. The brake thermal efficiency increases by 4.6% for multilayered ceramic coated piston engine when compared to uncoated piston engine. The increase in brake thermal efficiency in the case of multilayered ceramic coated piston may be due to higher cylinder peak pressure and lower frictional losses. The increase in cylinder peak pressure helps in increase in cumulative work done.
5.4 Exhaust Gas Temperature:
Figure 9 shows the variation of Exhaust gas temperature with load. The Exhaust gas temperature increases by 12.6% for multilayered ceramic coated piston engine when compared to uncoated piston engine at full load conditions. The increase in exhaust gas temperature in the case of multilayered ceramic coated piston may be due to lesser heat transfer to the coolant due to the ceramic coating on the piston top which retains the heat inside the combustion chamber which is partly converted into cumulative work done and the remaining heat is carried away by the exhaust gas.
5.5 Nitric oxide emission:
Figure 10 shows the variation of nitric oxide emission with load. The nitric oxide emission increases by 15.67% for multilayered ceramic coated piston engine when compared to uncoated piston engine. The increase in nitric oxide emissions is due to higher operating temperature during the combustion cycle because of the increase in preparation and reaction rate
5.6 Microstructure of coatings:
Fig. 11 and 12 shows the SEM images of the uncoated piston and multilayer coated piston after engine test run. It is seen that uncoated piston showed the presence of cracks on its surface and this is due to the thermal stresses generated due to the high operating temperature in the combustion chamber. On the other hand, multilayered coated piston (PSZrO2 and Cr3C2) coating reveals the presence of dense and compact microstructures indicating better stability to the coating.
The multilayer ceramic coatings with Cr3C2-PS.ZrO2over the piston crown ensures the good resistance to wear and thermal insulation on engine combustion and performance characteristics, Following results were obtained from single cylinder diesel engine operation
1. Cylinder peak pressure of multilayered coated piston increased from 68 bar to 72 bar when compared to standard piston.
2. Rate of heat release during premixed combustion was higher and faster than the uncoated piston.
3. Thermal efficiency of multilayered coated piston increased by 4.6% when compared to uncoated piston.
4. Exhaust gas temperature of multilayered coated piston increased by 12.6% when compared to uncoated piston.
5. Nitric oxide emission increased by 15.67% when compared to uncoated piston.
6. Surface characteristics of the piston top reveals the presence of dense and compact microstructures indicating better stability to the coating.
Based on this investigation, we recommend the application of this multilayered ceramic coating PSZ/[Al.sub.2][O.sub.3] in future for improved engine performance and enhanced corrosion resistance.
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(1) V. Dattatreya, (2) B.R. Ramesh Bapu, (3) B. Durga Prasad
(1) Research scholarJNTUniversity, Anantapur, Andhra Pradesh
(2) Prof., Department of Mechanical Engineering, Chennai Institute of Technology, Chennai, India.
(3) Prof., Dept. of Mechanical Engg., JNTUniversity, Anantapur, Andhra Pradesh, India.
Received 28 January 2017; Accepted 22 March 2017; Available online 28 April 2017
Address For Correspondence:
V. Dattatreya, Research scholar JNTUniversity, Anantapur, Andhra Pradesh
E-mail: email@example.com Tel : 9444252328
Caption: Fig. 1: Photographic view of piston before coating.
Caption: Fig. 2: Photographic view of piston after multilayer ceramic coating.
Caption: Fig. 3: Experimental setup.
Caption: Fig. 4: Photographic view of the test engine set up.
Caption: Fig. 5: Cylinder Pressure Vs Crank angle for uncoated piston and multilayer ceramic coated piston.
Caption: Fig. 6: Cylinder peak pressure Vs load for uncoated piston and multilayer ceramic coated piston.
Caption: Fig. 7: Rate of heat release Vs Crank angle for uncoated piston and multilayer ceramic coated piston.
Caption: Fig. 8: Brake thermal efficiency Vs Load.
Caption: Fig. 9: Exhaust gas temperature Vs Load.
Caption: Fig. 10: Nitric oxide emission Vs Load.
Caption: Fig. 11: SEM images of uncoated piston after engine test.
Caption: Fig. 12: SEM images of multilayer ceramic coated piston after engine test.
Table 1: Test engine specifications Item Specification Make Kirloskar engine Type of engine Four stroke, Single cylinder, Naturally aspirated, Water cooled, constant speed engine Bore, mm 80 Stroke, mm 110 Compression ratio 16.5 : 1 Rated power 3.7 kW @ 1500 rpm Type of fuel Diesel Type of injection Direct Injection (DI) Fuel injection pressure, bar 200 No. of nozzle holes 3 Nozzle hole diameter 0.26 mm Inlet valve opens (IVO) 15 bTDC Inlet valve closes (IVC) 45 aBDC Exhaust valve opens (EVO) 45 bBDC Exhaust valve closes (EVC) 10 aTDC
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|Author:||Dattatreya, V.; Bapu, B.R. Ramesh; Prasad, B. Durga|
|Publication:||Advances in Natural and Applied Sciences|
|Date:||Apr 30, 2017|
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