Evaluation of hybrid electric turbocharging for medium speed engines

This paper investigates the effects of hybrid electric turbocharging on the total system efficiency, the transient loading capability and the operational range of maritime engines, including the effects of the air control valves (cylinder bypass & charge air blow-off valve). An existing and validated mean-value first principle engine model has been adapted to simulate the operating principle of a combustion engine with a hybrid electric turbocharger system. The simulation of power-take-off/in together with an excess air ratio control strategy is included by means of torque addition to/removal from the turbocharger shaft and limiting it with seven boundary controllers. The analysis of the simulation results illustrates a trade-off between the increase of the system efficiency on one side and the transient loading capability of the engine on the other side. With turbocompounding, the system efficiency can be increased at the expense of a deteriorated gas exchange process and increased thermal loading of the engine. And assisting the turbocharger for steady state operation leads to a smaller operating envelope due to the limitation of compressor surge. Combining turbocharger assistance, with either the cylinder bypass or the charge air blow-off valve to avoid compressor surge, results in a rise of the engine torque at a lower speed almost up to a constant engine torque. The system efficiency can be further improved for the low-speed region in case the turbocharger is assisted electrically in combination with the air control valves. The load step capability, parametrized by limiting the minimum excess air ratio during the load step event, proved to be dependent on the timing of the electrical support. It was found, that in order to improve the load step capability, it is necessary to start accelerating the turbocharger a few seconds before the load step occurs. The paper shows the effect of hybrid electric turbochargers on the engine characteristics to facilitate enhanced engine dynamics and improved engine torque at reduced engine speed.