Experimental and numerical study of surface texturing for low power combustion engines

Abstract

The proliferation of small off-grid internal combustion (IC) engines in mobile power equipment has led to an alarming rise in smog-forming emissions and greenhouse gas levels, surpassing those emitted by passenger vehicles with an anticipated doubling by 2031. To address this concerning trajectory and meet stricter regulations, there is a global push to enhance the fuel efficiency of these engines. One promising approach involves integrating surface texturing with low viscosity lubricants to enhance engine liner tribological performance. However, establishing empirical design rules remains unclear due to its dependence on engine operating conditions. Small-size engines have lower operating pressures and higher speeds than large-size engines, necessitating different criteria for surface texturing compared to current solutions established on large engines. This research endeavours to advance the efficiency of small-scale IC engines by introducing surface texturing on engine liners through an electrochemical jet processing (EJP) texturing technique. A methodology has been devised to design and optimise surface texturing patterns using full-scale engine measurements, numerical RINGPAK engine modelling and computational fluid dynamics (CFD) simulations. Desired uniform and variable texture patterns (as a function of crank angle) were fabricated on engine liner materials, and their tribological performances was evaluated using laboratory reciprocating TE77 tribometer. An experimentally validated numerical engine model based on a Honda GX200 generator engine was utilised to underpin the engine load and temperature conditions for designing 2D multiphase hydrodynamic CFD numerical simulations and tribological testing. The CFD simulations suggested features with depths ranging from 5 to 15 µm and widths between 500 and 700 µm can increase lift forces up to 58.7 %, further suggesting a shallow angle design criteria that the inlet angle (IA) should be between 0.82° and 3.34° to improve performances. Desired texture patterns were fabricated using the EJP on three cylinder liner materials: hardened ASP 2023 steel, EN-GJS 400-15 spheroidal graphite cast iron and a hyper-eutectic Al-Si A390 alloy, revealing significant material microstructural influence on surface finishing. Tribological testing was conducted against two counter surface materials (AISI 52100 and cast iron) materials in PAO4 with a stroke length of 25 mm under different loads (11, 50 and 60 N) and two temperatures (30 and 100 ᵒC). The testing results highlighted tribological performance sensitivity to geometric parameters, testing conditions and surface topography. Under recentgular contact configuration at 11 N, the cast iron surface exhibited a maximum decrease of 21.8 %, whilst at 50 and 60 N, this increased to 36.4 and 23.8%, respectively. At 50 N, ASP 2023 surface exhibited a 22.0 % decrease, while the A390 surface decreased up to 10.6 %. Under a liner contact configuration at 50 N and 30 ᵒC, the ASP 2023 surface exhibited up to a 50.9 % reduction in the COF. However, there is no apparent improvement on the textured cast iron surface at 100 ᵒC due to low viscosity of PAO4. The CFD friction predictions exhibited an error up to 36.05 %, 143.21 % and 36.57 % when compared to experimental results for ASP 2023, cast iron and A390 surfaces, suggesting that a lower surface roughness exhibited less impact on predictions. Results also indicated cavitation's adverse effects on tribological performances in specific lubrication regimes, along with surface roughness. Surface analysis revealed chemical etching initiating two-body abrasion, while texture features could capture wear-off debris.

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Identifiers

ORCID for Peng Jiang: orcid.org/0000-0002-7151-029X

ORCID for Ranga Dinesh Kahanda Koralage: orcid.org/0000-0001-9176-6834

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