Turbocharger Integrated Assist

Hannu Jääskeläinen

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Abstract: One way to assist a turbocharger is to integrate a motor into the turbocharger itself. In turbocharger-integrated assist, additional power is supplied directly to the turbocharger shaft from an electric or hydraulic motor or even from the engine itself via a gear train or transmission device.


One way to assist a turbocharger is to integrate a motor directly into the turbocharger. Figure 3 shows an example where an electric motor is integrated into the turbocharger. Hydraulic turbines (Figure 11) or pumps as well as a direct mechanical connection to the engine crankshaft via a transmission (Figure 9) are other possibilities for providing turbocharger integrated assist.

Integrating the assist feature into the turbocharger has certain benefits including compact and flexible packaging and in some cases, the potential to incorporate waste heat recovery in the form of turbocompounding using hardware common to the assist function. In addition to the energy recovery aspects of turbocompounding, a turbocharger with both assist and turbocompound functions can be controlled to have less speed variation over the entire engine operating range. This makes it possible to incorporate vanes into the compressor and turbine to improve efficiency. Vanes limit the speed range of the turbocharger and are not normally used in conventional designs [2362].

Incorporating the assist function into the turbocharger is not without its challenges. One very important challenge is the fact that the flow range of the compressor must be expanded, especially at the low flow rate end of the range to provide the air required to boost low engine speed torque. If maximum air flow at high speed is unchanged to maintain engine power, in most cases the compressor surge line must move further to the left to provide the required increase in low engine speed air flow and boost pressure, Figure 1 [2357]. To achieve this expanded compressor map, standard Map Width Expansion (MWE) techniques may not be sufficient and Variable Geometry Compressors (VGC) may be required [2360][2361]. Alternatively, a separate compressor such as a supercharger can be used to supplement the turbocharger [3299].

Figure 1. Effect of increasing compressor pressure ratio and flow range

1: Improvement of surge limit. 2: Rising the charging power. 3: Rising pressure ratio & rotational speed.

Turbocharger integrated assist can also have a detrimental impact on the efficiency of the turbine when the assist feature is active. Turbines are designed to be efficient over the widest possible operating range. Under conditions where supplementary power is provided to the turbocharger, turbocharger shaft speed increases and can lead to a mismatch between exhaust flow and turbine geometry that can decrease turbine efficiency as shown in Figure 2. Aerodynamic optimization of the turbine wheel or a variable geometry turbine may be required if maximization of turbine power (and thus minimization of supplementary power) during periods of turbocharger assist is required [2361].

Figure 2. Effect of supplementary power supply to turbocharger on turbine efficiency

Another challenge with turbocharger integrated assist is that the motor for providing the assist function must withstand the maximum rotational speed of the turbocharger even if it is only needed at low to mid-range turbocharger rotational speeds.