What is the ideal efficiency of an engine if both its hot reservoir and exhaust are at 400K?

The ideal efficiency of an engine operating between two thermal reservoirs is determined by the Carnot efficiency formula: ( \eta = 1 - \frac{T_{cold}}{T_{hot}} ). If both the hot reservoir and the exhaust are at 400 K, then the temperature of the cold reservoir is also 400 K. This results in ( \eta = 1 - \frac{400}{400} = 0 ). Therefore, the ideal efficiency of the engine in this scenario is 0%, meaning no work can be extracted.