## 1 Introduction
Thermal barrier coatings are used to prevent thermal fatigue of piston surfaces, improve combustion and performance in the engine,, and reduce engine emissions. In a diesel engine, approximately 30% of the fuel energy is lost energy due to the heat losses in the combustion chamber. For this reason, many studies have focused on thermal barrier coating applications in diesel engines. Ceramic coating of the combustion chamber affects combustion, engine performance and exhaust emissions. It has been determined that the efficiency is increased thermally and mechanically by plasma spray thermal barrier coating of the piston [1]. [PERSON] and [PERSON] [10] used zirconium stabilized with Y\({}_{2}\)O\({}_{3}\) as a coating material in their study. They found that the temperature on the coated piston surface in a diesel engine is approximately 44% and 153 \({}^{\circ}\)C higher than the uncoated piston surface temperature. They found that the substrate surface temperature was 43 \({}^{\circ}\)C lower than the uncoated piston and the life of the material was longer than that of the other coating materials. The addition of 6-8% by weight of Y\({}_{2}\)O\({}_{3}\) stabilizes the tetragonal phase (PYSZ). [PERSON] et al. [11] investigated the effect of TBC on engine performance and emission values; a mathematical model was proposed by considering the parameters of the applied TBC material. La\({}_{2}\)Zr\({}_{2}\)O\({}_{7}\), which is one of the rare zirconates that has recently been used in thermal barrier coatings, has been used as a coating material. Combustion and emission values were analyzed numerically by creating the coated and uncoated engine model in the Kiva3V CFD program. The results showed that the heat losses from the piston walls decreased, and the thermal efficiency increased accordingly. Ceramic materials used for thermal barrier coatings are more resistant to oxidation, corrosion, and wear, unlike metals, and also have better insulating properties. Yttriastabilized zirconium is the most widely used ceramic material. Lanthanates and rare-earth oxides are also promising materials for thermal barrier coatings [12]. [PERSON] et al. [13] performed thermomechanical analyses on a diesel engine with a potential yttrium barium zirconate coating. They found a reduction in thermal stresses at the top of the piston bore compared to the uncoated piston. Additionally, improved engine performance, longer life cycles, erosion protection, and thermal stability were all observed. For a successful thermal coating, the melting point, oxidation resistance, thermal expansion coefficient and corrosion resistance of the ceramic material should be high, and the thermal conductivity coefficient should be low [14]. For eliminating the high temperature ablation of the piston substrate made with cast aluminum alloy, it must be ensured that TBCs have an adequate fatigue life throughout their service life [15]. Coating analysis of TBCs can effectively ensure the reliable operation of pistons throughout their lifetime. It has been shown that failure of TBCs is due to crack growth along the bonding coating and ceramic coating interface under thermal cycling and that TBCs will eventually break down under the combined effects of several factors [16; 17]. Because the piston substrate and TBC have different thermal conductivity and different thermal expansion rates, thermal stress may cause separation of materials and catastrophic failure originated by cracks [18]. The present study aims to investigate the effect of partial coating on the thermal and stress distribution of the piston. For this purpose, experimental and finite element studies were carried out. Real-time engine tests were carried out for different piston models, and exhaust emissions, fuel consumption, and in-cylinder temperature and heat transfer coefficient values were determined. Two different finite element analyses were carried out. The first one was a transient thermo-mechanical simulation used for calculating the temperature distribution and stress distribution of the piston.