Design of an electric supercharger for downsized engines

For many applications, the electric supercharger is used as a secondary charging device in conjunction with a turbocharger. In such cases, the position of the electric supercharger in the engine air-path is an important factor influencing air-charging performance. Different configurations were quickly evaluated using system simulation, ❷. On the one hand, it was observed that an upstream configuration where the electric supercharger was installed at the turbocharger inlet has a lower air-charging performance because the electric supercharger compressed the air first. This means that the turbocharger works with a higher inlet pressure and higher temperature. This pushes the turbocharger’s operating point closer to the surge line with a lower isentropic efficiency. On the other hand, a downstream configuration where the electric supercharger is installed at the turbocharger outlet provides better system performance because it has less influence on the turbocharger. In this case, it also leads to poorer thermal conditions for the electric supercharger, which may even require an additional intercooler between the turbocharger and the electric supercharger. Finally, the choice between configurations is always a multi-criteria decision between thermal combustion tolerance for diesel or gasoline, performance/cost ratio, and other integration problems. Open image in new window ❶ Example of models with upstream and downstream configuration used for system analysis Since the objective was to provide high-level performance results, the engine was equipped with a larger turbocharger in combination with the new electric supercharger. The longer time-to-torque response inherent to the larger turbocharger is compensated by the electric supercharger’s operation. With this kind of configuration, the small 1.2-l PFI gasoline engine provides the driver with a very impressive torque characteristic, ❸. In practice, the larger turbocharger alone is able to match the target of above 2250 rpm, but the benefits of applying the electric supercharger set-up are clearly demonstrated in ➂. There is a tremendous torque increase at low rpm. In addition, the resulting engine still maintains very good torque performance: the time-to-torque is less than 1 s for all engine speeds and significantly better than the baseline engine below 2000 rpm. Open image in new window ❸ Steady-state and transient performance of the 1.2-l PFI engine with the electric supercharger