The Allison V-1710 Turbo-Compound
The first V 17 10 produced 750 hp. By the beginning of World War 11 it was producing 1050 hp, then 1125, then 1325, and late model P‑38's got 1475. But engineers recognized that there was a lot of energy going out the exhaust pipes.
An attempt to recover some of this energy resulted in the turbo-compound V1710 shown at the bottom of the page. It was identified as the V1710‑E22 by Allison, and as the V1710‑127 by the government. A turbo-compound engine collects all of the exhaust gasses and runs them through a turbine, with all of the power generated going back into the crankshaft and ultimately to the propeller. It differs from a turbo-supercharged engine, which uses exhaust gas energy to increase the pressure of incoming air. Work on this engine began in about 1944 and continued until 1946, when Allison asked that it be cancelled because turbine engines had greater promise. It was the first successful turbo-compound engine, and probably one of only three to ever be built. This engine was designed to power the XP63H, which, as it turned out, never flew. The V1710‑E22 had a military rating of 2320 hp, and a War Emergency Rating with water/alcohol injection of 3090 hp.
The most successful turbo-compound engine was a version of the air‑cooled, 18‑cylinder, two‑row radial Curtiss Wright R3350. This engine had three turbines, each fed by 6 cylinders, that were geared to the crankshaft. The normal version produced 2700 hp and weighed 2850 lbs. The turbo-compounded version produced 3500 hp and weighted 3440 lb. This engine was used on the Douglas DC‑7 and some versions of the Lockheed Constellation. The other turbo-compound engine is the Napier Nomad, which was built in England. It was a 12‑cylinder, horizontally opposed, liquid cooled, valve-less, 2‑stroke cycle Diesel engine with a 3‑stage turbine. It reached flight test but not production.
The Allison engine collected the exhaust gas from all 12 cylinders and routed it to the turbine at the rear of the engine through two exhaust tubes. The shaft from the turbine ran through the centre of the first stage supercharger impeller, back to the engine and put its power directly into the crankshaft. The turbine could not be connected to the supercharger impeller because the supercharger was driven by a variable speed transmission, which did not run at a fixed speed ratio with respect to either the crankshaft or the turbine. Since this engine was to power a P‑63, it used an extension shaft and a remote gearbox attached to the crankshaft at the front of the engine at lower left in the picture below. This engine represented quite an advancement over the original 750 hp engine.