We have come to number four in our series. Turbo compounding is a method for recovering otherwise lost energy from the exhaust of a normal internal combustion engine (ICE). The design puts a turbine in the exhaust manifold which collects the kinetic energy (energy of motion) of the escaping exhaust gas. This turbine then transfers the power it generates to the crankshaft. The transfer is usually made by a hydrodynamic linkage, like in a transmission.

There are two basic types of turbines that operate by extracting energy from either the velocity (kinetic energy) of the working fluid or the pressure of the working fluid. In the case of pressure turbines there must be a large pressure drop across the rotor blades. This type is not used in turbo compound engines because the pressure drop restricts exhaust outflow, smothering the engine. Instead of pushing exhaust out against atmospheric pressure, the engine has to push it out against atmospheric pressure plus the turbine pressure drop. Using kinetic turbines avoids this problem.

Note that this is different from a turbocharger. In turbocharged engines there is a turbine powered by the flow of exhaust gases, but instead of adding this power to the driveshaft of the engine directly it is used to run a compressor which pressurizes the intake air. This results in a density boost, filling the cylinders with more air (and thus more oxygen) per charge. Since the ultimate limit on the energy you can get out of the combustion is set by the amount of oxygen present, turbochargers also increase power output. The mechanism is different though.

Turbo compounding allows for more power output given the same fuel input because it captures energy that would otherwise escape as exhaust gas velocity. However, the power per weight ratio is lower due to the turbine. The engine is also bulkier. But it is possible to greatly increase either the power available or the fuel economy or a mixture of both.

Although some World War II era aircraft before the development of turboprops used turbo compounding the technology has not been used by automakers. That is now changing. For example, the Daimler Trucks Detroit Diesel DD15 uses turbo compounding. The video above talks about the turbo compounding at about the 3:10 minute mark. Note there is also a turbocharger on this engine. Once again, turbo compounding and turbocharging are two different methods for recovering energy from the exhaust gas.

Perhaps someday soon car engines will also feature turbo compounding.

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