Hannu Jääskeläinen, W. Addy Majewski

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Abstract: Turbocompounding is the use of a power turbine to extract additional energy from the exhaust. Mechanical turbocompounding has been used commercially in diesel engines for various applications for many decades. In heavy-duty engines, series turbocompounding—where a power turbine is connected in series with turbocharger turbine—is the most important configuration. The technology can provide efficiency benefits of a few percent, but these benefits can be negatively affected by EGR, which diverts gas flow from the power turbine. Parallel turbocompounding is suitable when exhaust energy in excess of that needed by the turbocharger is available and would otherwise need to be bypassed around the turbo.


Turbocompounding is the use of a power turbine to extract additional energy from the exhaust. The extracted exhaust energy can be added to the engine crankshaft or converted to electric energy:

Mechanical turbocompounding has been used commercially in diesel engines for various applications for many decades. In North America, 10% of new heavy-duty on-road engines sold in 2011 and 2012 had turbocompounding but the figure decreased to 2% by 2015 after Daimler (Detroit Diesel) phased it out in favor of asymmetric turbocharging for their DD15 engine in 2013 [3788]. The US EPA estimates that penetration will again reach 10% by 2027 [3789]. Mechanical turbocompounding was applied to aircraft engines in the 1950s and ground vehicles starting in the 1960s. More historical details on work prior to the 1990s can be found in the literature [3791].

Electrical turbocompounding has been under development for heavy-duty diesel engines. However, in order to have a significant impact on efficiency, it would require relatively high electrical load in the range of 50 kW. For on-road vehicles, such a load could only be realized with a hybrid drivetrain and thus needs to be accompanied by other major technological changes. In power generation and some marine applications, where a sufficiently high electrical load is readily available, electrical turbocompounding is a commercial technology [1945][1929][2369][3790][3821][3822].

Mechanical Turbocompounding

In turbocharged engines, mechanical turbocompounding can be realized with several different configurations:

In heavy-duty engines, series turbocompounding—depicted schematically in Figure 1—is the most important configuration.

[SVG image]
Figure 1. Schematic representation of mechanical series turbocompounding

Figure 2 shows more detail of two different series turbocompound systems. The Volvo system uses an axial flow power turbine while the older Scania system has a radial flow power turbine.

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Figure 2. Series turbocompounding systems used in some Euro III and Euro IV engines: Volvo D12 and Scania DT12

(Source: Volvo & Scania)

For applications with exhaust flow rates beyond that required to meet the turbocharger demand, the power turbine can be arranged in parallel with the turbocharger turbine. Figure 3 shows such a system that was introduced into Sulzer RTA engines in the early 1980s; the Sulzer Efficiency Booster System (η-Booster) incorporated a different turbocharger in addition to a power turbine connected in parallel [3816][2586][3792]. At the time, newer turbochargers with improved efficiency were coming onto the market; the higher turbocharger efficiency meant that additional exhaust energy was available under some engine operating conditions that could be used for other purposes. A power turbine installed in parallel with the turbocharger turbine has become common in large four stroke medium speed and two-stroke low speed engines. In Figure 3, the upper curve shows the BSFC reduction of the Sulzer RTA engine introduced in 1983 compared to the previous version. The lower curve shows the additional BSFC reduction available in the 1983 RTA engine with the Efficiency Booster System consisting of a rematched turbocharger and power turbine. With the power turbine engaged above about 40-50% power, an additional BSFC reduction of up to 5 g/kWh is shown. With the power turbine disengaged at low load, a BSFC reduction is still possible because of a smaller total turbine nozzle area. Parallel turbocompounding has also been studied for use in light-duty engines [3793][3794][3795][3796][3797].

[SVG image]
Figure 3. Parallel turbocompounding in Sulzer RTA engines

System schematic and the reduction in BSFC relative to the previous engine version. Sulzer’s η-Booster system, introduced in the early 1980s, consisted of a rematched turbocharger and power turbine.

A prototype system where a turbocharger shaft is connected to the crankshaft through a continuously variable transmission (CVT) is shown elsewhere. In principle, this would not only allow excess power from the turbine to be supplied to the crankshaft but also allow crankshaft power to be supplied to the compressor under conditions where exhaust enthalpy is too low to create adequate boost pressure [2259].