. "4 Power Train Technologies for Reducing Load-Specific Fuel Consumption." Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles. Washington, DC: The National Academies Press, 2010.
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Technologies and Approaches to Reducing the Fuel Consumption of Medium- and Heavy-Duty Vehicles
FIGURE 4-1 Energy audit for a typical diesel engine. SOURCE: Adapted from Vinod Duggal, Cummins, Inc., “Industrial Perspectives of the 21st Century Truck Partnership,” presentation to the committee, Dearborn, Mich., April 6, 2009, Slide 14 (and TIAX (2009), p. 4-3, Table 4-1).
2009, pp. 3-5 and 4-14). NESCCAF/ICCT (2009, p. 83) estimates the fuel savings of an improved-efficiency single-stage turbocharger at 1 percent. Another source projects that a high-pressure-ratio axial compressor will reduce fuel consumption by 1.1 to 3.6 percent.2
Almost all heavy-duty diesel engines sold in North America today use high-pressure loop EGR for control of engine-out NOx levels. To get EGR to flow from the exhaust manifold to the intake manifold, the pressure in the exhaust manifold must be higher than the pressure in the intake manifold. When the exhaust manifold pressure is higher than the intake manifold pressure, this is called having a negative ∆p, where ∆p refers to the difference in pressure between the intake and exhaust manifolds. High-efficiency turbochargers naturally produce a positive ∆p over much of their operating range, so turbocharger efficiency must be intentionally compromised in order to facilitate EGR flow. If it is possible to produce adequate EGR flow without reducing turbo efficiency, the overall engine efficiency will increase.
Dual-Stage Turbocharging with Intercooling
Modern engines use high-pressure ratios, which limit the efficiency of turbochargers. Using two turbochargers in series with intercooling would allow higher turbocharger efficiency, but this adds cost and packaging complexity and requires an EGR pump or other device such as a turbocompound system to facilitate EGR flow. Air-to-water intercooling is used after the first-stage compressor, in some applications, and air-to-air aftercooling is used after the second-stage compressor.
Conventional two-stage turbocharging employs two turbochargers working in series at all times. True sequential turbocharging switches turbochargers in and out of use as required, but they are normally connected in parallel. A modulated two-stage system brings some of the benefits of each of these two approaches. At low engine speeds it works as a two-stage system, delivering high-boost pressure despite the low engine speed. At high engine speeds it bypasses the small high-pressure turbocharger, allowing the bigger, low pressure turbocharger to work on its own and produce the higher flows at high engine speeds. Modulated two-stage systems offer the benefits of both high-boost pressure and wide-flow range, mainly due to the fact that two different-sized compressors are used. Using two compressors replicates the effect of a variable compressor without the need for a complex housing. The modulated two-stage system can have a high-pressure turbocharger far smaller than that of a conventional two-stage system, improving transient performance by reducing the turbo lag that affects both drivability and emissions.3
Dual-stage turbocharging is used in production by Navistar, Daimler Trucks, and Caterpillar in the United States and by MAN and Mercedes in Europe. Ford has announced that the 2011 diesel engine used in its Class 2b to 7 trucks will use a twin-compressor turbocharger (back-to-back compressors on the same shaft). Another source estimates a 2 to 5 percent reduction in fuel consumption.4 These benefits are only available if a way to provide the required EGR flow is available.
Personal communication between Steve Edmonds and David F. Merrion, committee member, September 2008.