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A turbo-compound engine is a reciprocating engine that employs a turbine to recover energy from the exhaust gases. The turbine is usually mechanically connected to the crankshaft, as on the Wright R-3350 Duplex-Cyclone engines fitted to Douglas DC-7B and the Lockheed L-1049 Super Constellation airliners, but electric and hydraulic power recovery systems have been investigated as well. Recovery turbines increase the output of the engine without increasing its fuel consumption, thus reducing the specific fuel consumption. Turbo-compounding was used for commercial airliners and similar long-range, long-endurance roles before the introduction of high-bypass turbofan engines replaced them in this role.
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Most piston engines have a hot exhaust that still contains considerable undeveloped energy that could be used for propulsion if extracted. A turbine is often used to extract energy from such a stream of gasses. A conventional gas turbine is fed high-pressure, high-velocity air, extracts energy from it, and leaves as a lower-pressure, slower-moving stream. This action has the side-effect of increasing the upstream pressure, which makes it undesirable for use with a piston engine as it has the side-effect of increasing the back-pressure in the engine, which decreases scavenging of the exhaust gas from the cylinders and thereby lowers the efficiency of the piston portion of a compound engine.
History
Through the late 1930s and early 1940s one solution to this problem was the introduction of "jet stack" exhaust manifolds. These were simply short sections of metal pipe attached to the exhaust ports, shaped so that they would interact with the airstream to produce a jet of air that produced forward thrust. Another World War II introduction was the use of the Meredith effect to recover heat from the radiator system to provide additional thrust.
By the late-war era, turbine development had improved dramatically and led to a new turbine design known as the "blowdown turbine" or "power-recovery turbine". This design extracts energy from the momentum of the moving air, but does not appreciably increase back-pressure. This means it does not have the undesirable effects of conventional designs when connected to the exhaust of a piston engine, and a number of manufacturers began studying the design. The first aircraft engine to be tested with a power-recovery turbine was the Rolls-Royce Crecy. This was used primarily to drive a geared centrifugal supercharger, although it was also coupled to the crankshaft and gave an extra 15 to 35 percent fuel economy.
Blowdown turbines became relatively common features in the late- and post-war era, especially for engines designed for long overwater flights. Turbo-compounding was used on several airplane engines after World War II, including the Napier Nomad and the Wright R-3350 being examples. The exhaust restriction imparted by the three blowdown turbines used on the Wright R-3350 is equal to a well-designed jet stack system used on a conventional radial engine, while recovering about 550 hp (410 kW) at METO (maximum continuous except for take-off) power. In the case of the R-3350, maintenance crews sometimes nicknamed the turbine the parts recovery turbine due to its negative effect on engine reliability. Turbo-compound versions of the Napier Deltic, Rolls-Royce Crecy, Rolls-Royce Griffon, and Allison V-1710 were constructed but none was developed beyond the prototype stage. It was realized in many cases the power produced by the simple turbine was approaching that of the enormously complex and maintenance-intensive piston engine to which it was attached. As a result, turbo-compound aero engines were soon supplanted by turboprop and turbojet engines.
Some modern heavy truck diesel manufacturers have incorporated turbo-compounding into their designs. Examples include the Detroit Diesel DD15 and Scania in production from 1991.
Starting with the 2014 season, Formula 1 switched to a new 1.6 liter turbocharged V6 formula that uses turbo-compounding. The engines use a single turbocharger that is connected to an electric motor/generator called the MGU-H. On deceleration, the MGU-H acts as a generator, converting wasted mechanical energy from the turbine into electrical energy that is stored in a battery. When the car accelerates, the MGU-H acts as a motor, using the stored electrical energy to spool up the turbine, eliminating lag.