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Providing assistance to the turbocharger in turbocharged engines has long been an option that engine designers have utilized to address two undesirable characteristics of many turbocharged engines—namely low boost pressure at low engine speeds and turbocharger lag. This assistance can take one of several forms. Additional power can be supplied directly to the turbocharger shaft from an electric or hydraulic motor or even the engine itself . Alternatively, an additional compressor, either a supercharger or even a smaller turbocharger, can be used to provide boost when primary turbocharger is unable to do so. Helhmoltz resonators and pulse chargers have also been proposed for providing this assistance.
While commercial applications of assisted turbocharging (also known as supported turbocharging) have been available since the early days of the internal combustion engine, increased interest in the technology appeared after about 2010. This renewed interest has been driven primary by the potential of this technology to provide fuel efficiency improvements through engine downsizing and downspeeding by improving engine performance at lower engine speeds (torque curve shaping) and ensuring that transient response does not suffer.
It should be noted that many applications have avoided the need for turbocharger assistance by employing turbochargers with reduced inertia rotors, variable geometry turbines and other means to improve turbocharger performance. Whether they will continue to do so in the future remains to be seen.
One commercial application that has long utilized assisted turbocharging is turbocharged two-stroke engines. Because of their operating principle, two-stroke engines are unable to use the motion of the piston to induce gas flow into and out of the cylinder. Instead, pressurized intake air must be provided at all operating conditions. At low load and start-up conditions, the exhaust energy is too low for the turbocharger of a turbocharged two-stroke to provide the needed scavenge flow and some form of assistance is required. While superchargers are common, electrically assisted turbochargers have seen some limited use for these applications.
Torque Curve Shaping. Figure 1 shows the principle of providing some external power assist to the turbocharger at low speeds to improve steady-state full load torque. In this case, the objective is to achieve a torque curve shape with a downsized engine that is equivalent to a larger displacement naturally aspirated engine. Starting with the naturally aspirated downsized engine, it is relatively straight forward to achieve the required torque characteristics from about rated torque speed to the maximum rated speed of the engine using conventional turbocharging technology. As is apparent, at engine speeds below rated torque speed, the challenge is considerably greater. While a variable geometry turbocharger helps, there is still a consider torque deficit at low speeds. Providing some external assistance to the turbocharger, in this case via a motor connected to the turbocharger’s shaft, is one way to generate the additional boost required to overcome this torque deficit .
Figure 1. Turbocharger Assistance for Improving Low Speed Torque in Downsized Engines
1: Downsized engine. 2: Conventional turbo. 3: Variable geometry turbo. 4: VGT with motor assist. 5: Large displacement engine.
The external power demands for turbocharger assisted torque curve shaping can be significant and depend on the amount that the torque at low engine speeds needs to be increased. For steady-state torque curves, the amount of power that needs to be supplied depends on the difference between the engine torque that can be generated by the turbocharger alone and the desired low speed torque output. Figure 2 shows the effect of providing up to 3 kW of turbocharger assistance for a 2 L diesel engine . A 50% increase in low speed torque can be achieved with 1 kW assist. In another case where the target boost pressure for a 2 L engine was 300 kPa for the entire speed range from 1000 rpm and up, a maximum of 5.6 kW of motor assist was required .
Figure 2. Effect of Turbocharger Assistance on Relative Engine Output and Efficiency in Light-Duty Diesel Engine
Figure 2 also illustrates an important point for engine downsizing applications. The limit on low speed engine output can be strongly dependent on how much assistance power can be supplied to the turbocharger. This can limit the downsizing potential, and thus the thermal efficiency improvements, of various turbocharger assist technologies. It should be noted that the maximum turbocharger assist that can be provided can also be limited by the need to avoid compressor surge. This will be discussed later.