Mechanical Turbocompound

The base turbocharged engine remains unchanged and a power turbine is added to the exhaust stream to extract additional energy from the exhaust. The power turbine is connected to the crankshaft to supply additional power (NESCCAF/ICCT, 2009, p. 81). Typically, the attachment includes a fluid coupling (to allow for speed variation and to protect the power turbine from engine torsional vibration) and a gear set to match power turbine speed to crankshaft speed. Published information on the fuel consumption reduction from mechanical turbocompounding varies, as evidenced by the following: 3 percent, according to the Detroit Diesel Corporation,5 which has a turbocompound engine in production; 2.5 to 3 percent (NESCCAF/ICCT, 2009, p. 54); 3 percent (K.G. Duleep, Energy and Environmental Analysis)6 and 4 to 5 percent (Kruiswyk, 2008, pp. 212-214); TIAX (2009, pp. 4-17) used 2.5 to 3 percent. Some of these differences may depend on the operating condition or duty cycle that was considered by the different researchers. The performance of a turbocompound system tends to be highest at full load and much less or even zero at light load.

Electric Turbocompound

This approach is similar in concept to mechanical turbocompound, except that the power turbine drives an electrical generator (NESCCAF/ICCT, 2009, p. 29). The electricity produced can be used to power an electrical motor supplementing the engine output, to power electrified accessories, or to charge a hybrid system battery. Electric turbocompound is a technology that fits particularly well with a hybrid electric power train for long-haul applications where regenerative braking opportunities are limited. The benefits of electric turbocompound and an electric hybrid power train can be additive. Energy and Environmenal Analyis7 has said that “electric turbo-compound is more efficient and possible as part of hybrid packages.” Fuel consumption reduction benefits as large as 10 percent are claimed. The NESCCAF/ICCT study (p. 54) modeled an electric turbocompound system and estimated benefits at 4.2 percent, including electrification of accessories. Caterpillar, Inc., as part of Department of Energy (DOE) funded work, modeled a system that showed 3 to 5 percent improvement, while John Deere investigated a system (off-highway) that offered 10 percent improvement (Vuk, 2006; TIAX, 2009, p. A-10). None of these systems have been demonstrated commercially. TIAX (2009, pp. 3-5) used a range of 4 to 5 percent for its estimates, which included the benefits of electric accessories. Achieving the full benefit of electric turbocompound requires the electrification of vehicle accessories, the addition of an electric motor to apply turbocompound energy to supplement engine output, and an electric storage system (battery) to store any energy from the power turbine that is not immediately required. Making all of these changes to the vehicle will pose significant development and cost challenges.

Variable Valve Actuation

Variable valve actuation (VVA), also called variable valve timing or discrete variable valve lift, allows the valve actuation to be adjusted independently from the crankshaft angle. There are many implementations of VVA. Some are hydromechanical, such as the system used on some BMW passenger car engines. Other designs use electromagnets or high-pressure hydraulic systems. Some versions offer “full authority,” or unlimited, control of valve timing and lift, while other implementations offer limited control, such as variable duration only, variable lift only, or even more limited control, such as with the system used on some Caterpillar engines to permit a Miller cycle to be used under some operating conditions. VVA technology can also be used for cylinder deactivation. One of the primary drivers for introducing VVA in diesel engines is to facilitate the use of nonconventional combustion modes. According to several sources, variable valve timing can improve fuel consumption by about 1 percent when standard diesel combustion is used (NESCCAF/ICCT, 2009, p. 55).

Low-Temperature Exhaust Gas Recirculation (Also Called Advanced EGR Cooling)

Most medium- and heavy-duty vehicle diesel engines sold in the U.S. market today use cooled EGR, in which part of the exhaust gas is routed through a cooler (rejecting energy to the engine coolant) before being returned to the engine intake manifold. EGR is a technology employed to reduce peak combustion temperatures and thus NOx. Low-temperature EGR uses a larger or secondary EGR cooler to achieve lower intake charge temperatures, which tend to further reduce NOx formation. If the NOx requirement is unchanged, low-temperature EGR can allow changes such as more advanced injection timing that will increase engine efficiency slightly more than 1 percent (NESCCAF/ICCT, 2009, p. 62). Because low-temperature EGR reduces the engine’s exhaust temperature, it may not be compatible with exhaust energy recovery systems such as turbocompound or a bottoming cycle.

Electrification of Engine-Driven Accessories

Accessories that are traditionally gear or belt driven by a vehicle’s engine can be converted to electric power. Ex-


Detroit Diesel Corporation, DD15 Brochure, DDC-EMC-BRO-0003-0408, April 2008.


K.G. Duleep, Energy and Environmental Analysis, “Heavy Duty Trucks Fuel Economy Technology,” presentation to the committee, Washington, D.C., December 5, 2008, slide 17.


K.G. Duleep, Energy and Environmental Analysis, “Heavy Duty Trucks Fuel Economy Technology,” presentation to the committee, Washington, D.C., December 5, 2008, slide 17.

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