Turbocharger Assist with External Compressor

Hannu Jääskeläinen

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Abstract: Turbocharger assistance can be supplied by an additional compressor—either a supercharger or a smaller turbocharger—that are used to provide boost when primary turbocharger is unable to do so. Two main types of supercharges can be used: engine driven superchargers and electric superchargers.


While incorporating turbocharger assist directly into the turbocharger would provide a neat packaging solution, numerous challenges must still be overcome before the technology can be adopted for commercial applications in the light-, medium- and heavy-duty engine markets. As an alternative, other approaches that rely on either commercially proven technologies or technologies with fewer commercialization challenges are available. Most of these approaches rely on using another compressor in addition to the turbocharger. This additional compressor can provide higher boost pressures at lower airflows and faster boost pressure transient response than the primary turbocharger can. The additional compressor can be a either a mechanically or electrically driven supercharger. An additional turbocharger, albeit with a smaller flow capacity and lower moment of inertia than the primary turbocharger, can also be used.

While the benefits of integrating a motor (e.g., electric motor) onto the turbocharger shaft are compelling, similar benefits can be had by using a separate compressor, Figure 1. This figure shows the effect of using a wastegated turbocharger and separate electrically driven centrifugal compressor of fixed size at the inlet (a) or outlet (b) of the turbocharger. The turbocharger is sized to boost the high speed output of a 2.0 L SI gasoline engine. The red dashed curve represents the baseline output of a similar engine but that uses a turbocharger plus mechanically driver supercharger instead [3299]. The intent is to match the full load output of the baseline engine with the turbocharger plus electric supercharger combination. The 30 bar (3000 kPa) BMEP (240 Nm/L) of the prototype baseline engine indicates it has been heavily downsized.

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Figure 1. Simulated effect of adding a separate electrically driven centrifugal compressor of fixed size before the inlet (a) and after outlet (b) of a turbocharger compressor

Turbocharger sized to boost the high speed output of an heavily downsized SI gasoline engine. Red dashed curve represents baseline output using a turbocharger plus mechanically driven supercharger.

The power requirements of placing the fixed size compressor after the discharge of the turbocharger compressor are considerably lower than is the case for placing it before the inlet. The primary reason is that the reduced or corrected mass flow (i.e., mass flow adjusted for temperature and pressure) is considerably higher at the turbocharger compressor inlet than outlet; i.e., the density at the inlet is lower. For a given actual mass flow, the compressor placed at the turbocharger inlet needs to move a considerably higher volumetric flow and needs to operate at much higher speed.

Figure 2 shows the load step response of a mechanical supercharger and an electrically driven supercharger, and compares them to more conventional naturally aspirated, turbocharger and supercharger options [2813]. Shown are a 2.0 L turbocharged gasoline direct injection (GDI) engine, the same 2.0 L turbocharged GDI engine also equipped with an electrically driven supercharger (VTES), a turbocharged engine equipped with a clutched mechanical supercharger, a 2.0 L GDI engine with 2-stage turbocharging, a 3.0 L unclutched mechanically supercharged engine and a naturally aspirated engine with variable valve timing. The gray band shows the typical range of series production turbocharged gasoline engines. The figure illustrates the effect of the different boosting systems not only on low speed transient response but on the maximum low speed BMEP as well. As will be discussed later, the response of the turbocharged engine equipped with a clutched mechanical supercharger is affected by the need to manage clutch engagement.

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Figure 2. Gasoline engine transient response for several boosting options

Loadstep from 1 bar BMEP to full load at 1500 rpm

Figure 3, based on data from Burke [3299], shows the impact of an electrically driven supercharger in more detail. The results are simulated for the 2.0 L downsized engine discussed in relation to Figure 1. The baseline is an engine driven supercharger plus turbocharger. For the other cases, the engine driven supercharger is replaced with an electrically driven centrifugal compressor. The electrically driven supercharger is limited to 12 kW. For the standard transient, the actuator inputs for the turbocharger wastegate and electric power to the electrically driven supercharger were stepped almost instantly from their low load setting to their high load setting. For the forced transient, the wastegate and electrical power are initially stepped to the maximum response (i.e., wastegate fully closed and 12 kW to the electrically driven compressor) to promote transient response and subsequently relaxed to avoid overshoot. It is apparent that the electrically driven supercharger provides improved performance compared to the baseline configuration. At engine speeds of 1250 and higher, the post-turbocharger installation of the electrically driven compressor provides superior performance improvements while the performance with the electrically driven compressor in the pre-turbocharger position degrades significantly. Using the flexibility of the electrically driven compressor by driving it briefly at full power combined with flexible wastegate operation even further improves performance and brings the transient performance of the pre and post turbo installations to about the same level.

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Figure 3. Time from start to target BMEP for a 2.0 L gasoline engine

Baseline is an engine driven supercharger plus turbocharger. For the other cases, the engine driven supercharger is replaced with a 12 kW electrically driven centrifugal compressor. All results simulated except Base condition at 1000 rpm.

While Figure 2 shows that the electrically driven supercharger assisted turbocharged engine provides superior transient response to the mechanically driven supercharger assist, it is challenging to operate an electrically driven supercharger for more than a few seconds. Mechanically driven superchargers, however, face no such challenge. Thus the mechanically driven supercharger assisted turbocharged engine could be calibrated for higher levels of low speed-steady state engine torque. Electrically driven superchargers can be combined with an exhaust gas turbine driving an electric generator to form an “electric turbocharger”.