Boosting Systems

Hannu Jääskeläinen

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Abstract: The use of turbocharging in gasoline engines, historically limited to high performance cars, has become a standard practice in downsized engines, where boosting allows for a substantial increase of specific torque. There are significant differences in the boosting system requirements for gasoline and diesel passenger car engines. In diesels, more airflow and higher boost pressure are required for a given fuel flow, and wastegated and 2-stage boosting systems are required at lower torque densities compared to gasoline engines.

Light-Duty Gasoline Engine Boosting Systems

While turbochargers have been applied to production gasoline engines for many decades, they have mainly been used on high performance cars for which customers have been willing to pay the added costs. Production volumes for these vehicles have typically been relatively small. With the introduction of downsized, direct injection gasoline engines to meet various greenhouse gas and fuel economy regulatory limits, this has changed. Turbocharged gasoline engine volumes have increased rapidly while customer’s willingness to pay for performance has perhaps not changed as much. This combination of increased volumes, pressure to keep costs down as well as a combination of relatively new engine technologies has dramatically changed the approach to incorporating a turbocharger into a production gasoline engine in a relatively short time.

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Figure 1. Full load specific torque curves for several turbocharged direct injection gasoline engines

In order to get a better sense of how modern turbocharging technology for downsized gasoline engines has evolved and were it is going, it is useful to examine a few examples of full load torque curves for some boosted gasoline engines in the sub-2.0 L category, Figure 1.

Consider first the two examples of single turbocharger engines from the mid-2000s, Volkswagen’s 2.0 L FSI (280 Nm/147 kW) and 1.4 L FSI (200 Nm/90 kW). These engines had maximum BMEPs of about 1.8 MPa and power densities less than about 75 kW/L. Note also that there is a trade-off in power density and the minimum engine speed at which maximum torque is achieved. These values form a convenient baseline that reflects the technology available to engine manufacturer’s to economically mass produce a gasoline fueled direct injection engine for this period. To achieve a higher BMEP of 2.2 MPa, a wide peak torque engine speed range and a higher power density of 90 kW/L in the mid-2000s required two compressors as reflected by the example of Volkswagen’s 1.4 L TSI (240 Nm/125 kW) engine that used a supercharger and turbocharger combination.

By the beginning of the second decade of the 21st century, this had changed significantly. In 2011, Ford announced their 1.0 L EcoBoost engine (170 Nm/93 kW) whose steady-state specific torque and specific power values were very close to those for Volkswagen’s 1.4 L TSI but that required only one wastegated turbocharger (in transient operation, this 1.0 L EcoBoost delivered 200 Nm of torque). The 1.0 L EcoBoost also showed a considerable decrease in the minimum engine speed at which maximum torque could be achieved—an important achievement considering the higher BMEP compared to single turbocharger engines of only a few years earlier. A low engine speed for maximum torque is a critical requirement to keep fuel consumption low in downsized engines.

To realize this gain in performance, the 1.0 L EcoBoost along with a number of its other contemporaries relied upon a series of available engine technologies, some new developments as well as a design approach that much more closely integrated the engine and turbocharger into a single package than had been done in the past.

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