PH-LIMIT - Use a physical bound before designing a mechanism

• Situation: A proposed cyclic device promises 60 J of work from 100 J of heat taken from a 600 K reservoir, rejecting heat only to a 300 K reservoir.
• Question: Can a different mechanism achieve this with the same resources and restoration conditions?
• First useful result or blocker: The stated second-law comparison bounds net work by 50 J, excluding the 60 J proposal within these conditions.
• Start with: PHY.3:5.1, which constructs the comparison from complete exchanges.
• Stop or return: Use the exclusion without designing every possible engine. Reopen the exchange account if an extra resource or a different end condition is allowed.

The reservoirs have fixed absolute thermodynamic temperatures, the device returns to its initial state, and no other resource is consumed. Under the supplied classical second-law account, W <= (1-300/600)*100 J = 50 J. Energy balance alone would allow the proposed 60 J and therefore would not settle the question. The bound restricts net work; it neither supplies an attaining design nor its power or cost. External work, a nonthermal resource or an unrestored auxiliary changes the account to be compared, not the arithmetic of the original bound.