Which coolant heater is installed internally in the engine block in place of a freeze plug?

The correct answer is the immersion heater. This type of heater is designed to be installed directly within the engine block, replacing a freeze plug. By being located internally, the immersion heater is able to directly heat the engine coolant, ensuring that the coolant reaches optimal temperatures efficiently. This is particularly important in colder climates, as it helps in preventing the engine from freezing and allows for easier starts and better fuel efficiency once the engine is running.

Circulating heaters, on the other hand, typically work by heating the coolant in a separate reservoir or tank, which is then circulated to the engine. Radiator heaters are situated outside the engine and are meant to warm the radiator, which indirectly heats the coolant. Exterior heaters also operate externally to influence the engine’s temperature but do not replace any components within the engine itself. Therefore, immersion heaters are uniquely positioned to provide direct heating for optimal engine performance.