The linerless engine technology originated in Europe. It uses thermal spray coating to apply a wear-resistant layer to the inner wall of aluminum engine cylinders, replacing traditional cast-iron liners. This not only reduces engine weight and fuel consumption but also significantly lowers friction and enhances engine performance.
　　After the cylinder block has been sprayed, the final machining of the coating is critical; honing must be performed to ensure the cylinder achieves the specified correct diameter, roundness, and surface roughness. Based on measured reductions in oil and fuel consumption from engine testing, a well-known European thermal-spraying company has made refined adjustments to the recommended technical specifications for the coated surface after honing, as shown in the table below. To ensure that no abrasive debris remains trapped in the pores, it is essential to properly clean the coated surface after honing; burrs at hole edges must be removed, and the honed surface should be free of deposits.
　　Table 1: Recommended Roughness Specifications After Coated Honing
　　Recommended Roughness Specifications After Coated Honing
　　After rough honing and finish honing, the inner surface of the cylinder bore can achieve a mirror-like finish, as shown in Figure 2. Compared with honing of conventional cast-iron cylinder liners, the surface after coating honing exhibits a porous structure rather than a veined texture. Figure 3a shows the microstructure of the surface after honing of a conventional cast-iron cylinder liner, while Figure 3b shows the microstructure after coating honing.
　　Mirror-like finish after coated honing
　　Figure 2: Mirror-like surface after honing of the coated liner Figure 3: Microscopic morphology after conventional cylinder liner honing and after coated liner honing
　　It is precisely because the coating surface has a porous structure that its smoothness can be reduced to 25%, which is one of the reasons this technology enables substantial savings in engine oil. The gently rounded micro‑pores also decrease the area of the combustion chamber and piston rings exposed to oil, reducing the tangential force on the oil scraper ring and allowing the piston ring to reach a hydrodynamic state more quickly, thereby significantly lowering friction, as illustrated in Figure 4. Lower engine friction translates into reduced energy loss from frictional heat, decreased blowby and fuel consumption, and ensures maximum operational reliability. (Blowby refers to the leakage of combustion gases past the piston rings into the crankcase due to cylinder or piston wear or deformation, which adversely affects engine power and fuel economy.) Moreover, owing to the inherent porosity of the final coating, the oil‑storage architecture does not get worn away like a conventional textured surface; instead, as wear progresses, new pores open up within the coating itself, leading to a marked improvement in service life and sustained efficiency.