History of Power Recovery Turbine Engines
Here are two of the famous World War II engines that used PRTs.
One of the most influential radial aircraft engines invented in the United States was the Wright R-3350 duplex (multi-row) Cyclone. This power recovery turbine was developed to deliver reduced fuel consumption, and thereby better economy to manage the civilian market during the war. It saw its most successful use on the B-29 bomber.
Three power recovery turbines were attached to the exhaust piping. This exhaust piping was responsible for delivering exhaust gases from a group of six cylinders, all of which were attached to the crankshaft. These PRTs were attached to the engine crankshaft by fluid coupling to develop more power. Although the invention of PRTs could save about 20 percent of the wasted heat power, it was also a cause of troubles for the engines. The turbines had a negative impact on the engine and its reliability. Due to this, most of the field mechanics nicknamed them “Parts Recovery Turbines."
Actually, use of this turbine caused an increase in back pressure and thus the operating temperature of the cylinder, which are the responsible factors for high wear inside the engine and its failure.
The basic design of the VD-4K, which was similar to the M-253K, was developed from the VD-3TK engine. it contained six monobloc banks, each with four fluid-cooled cylinders. The power recovery turbines were attached between the cylinder banks, and the engine was jointed to a geared centrifugal supercharger and a turbocharger. These parts combined made the VD-4K a power recovery turbine, though not exactly like the PRTs used on the big Cyclone.
This project was started keeping in mind the high power delivery of the engine. However, the engine was not able to provide required power for the aircraft, particularly at altitude. This was the main cause of failure of the project.