What is one purpose of the ring end gap on piston rings?

The ring end gap on piston rings serves a critical purpose in the operation of an engine. The primary function of the gap is to allow for thermal expansion of the piston rings. As the engine operates, the piston rings heat up and expand. If there were no gap, the rings could bind in their grooves or even break due to the increased pressure from expansion.

Additionally, the end gap helps ensure proper sealing between the rings and the cylinder wall, which is vital for maintaining compression within the engine. Proper sealing helps to prevent blow-by (the escape of combustion gases past the piston rings), which can reduce engine efficiency and power.

While the other options may seem plausible in some contexts, they don't align with the specific function of the ring end gap. Holding fuel pertains to the fuel system rather than piston operation, supporting the engine structure relates more to the overall assembly of the engine rather than to the function of the piston rings, and increasing friction would be counterproductive to the smooth functioning of the engine. Thus, the correct understanding emphasizes the role of the ring end gap in accommodating expansion and maintaining effective compression, affirming the correct answer.