What type of engine brake employs the exhaust stroke of the piston as its retarding method?

The correct answer is the external compression brake. This type of engine brake utilizes the exhaust stroke of the piston as its means of retarding the vehicle. During the exhaust stroke, the engine's intake valves are closed, and the exhaust is left open, allowing the engine to effectively slow down by creating a high pressure in the cylinder. This pressure works against the motion of the vehicle, providing a braking effect.

External compression brakes are particularly useful in large trucks where they can significantly reduce wear on the service brakes and improve safety, especially during long downhill drives. They operate by modifying the engine's exhaust flow to enhance the retarding characteristics, making them efficient for heavy-duty applications.

Other brake options, like the internal compression brake, operate differently by using compression within the cylinder but do so in a manner that is not primarily focused on the exhaust stroke. Hydraulic retarders function independently of the engine's exhaust processes, using a fluid system to create resistance. The Volvo EPG (Exhaust Brake) is a specific brand or technology that might relate to exhaust braking but does not encompass the broader category of external compression brakes in general terms.