Research on the oscillating cooling characteristics of small-bore pistons in opposed piston two-stroke engines for auxiliary power units

The horizontally opposed piston two-stroke (OP2S) engine replaces the cylinder head with a piston, resulting in increased thermal loads and a higher likelihood of fatigue failure. However, current studies on piston oscillatory cooling primarily focus on large-bore or four-stroke engines with vertical piston motion, leaving the unique horizontal oscillatory cooling in small-bore OP2S engines unexplored. This study establishes a dynamic mesh simulation model of a small-bore OP2S piston cooling gallery to investigate oil distribution and heat transfer characteristics under horizontal oscillations. The results reveal that the average heat transfer coefficient (AHTC) in the cooling gallery ranges from 2157 to 2235 W/(m²·K), with a fluctuation amplitude of 3.6 % per cycle. The highest local AHTC reaches 4351 W/(m²·K) at 90°CA. The oil filling ratio stabilizes at approximately 0.715 after 20 cycles. Increasing oil injection flow rate from 1.5 to 4.0 L/min enhances AHTC by up to 32 %, while increasing engine speed from 1600 to 3200 rpm reduces AHTC by about 70 W/(m²·K). A 2 mm nozzle diameter achieves the highest AHTC under the tested conditions. A new heat transfer correlation was proposed, overcoming limitations of classical models, and providing a valuable tool for OP2S piston cooling design.