Model based design of a turbo-compound bottomed to internal combustion engine exhaust gas

Abstract

The transportation sector is living a new era, where the conventional powertrains based on thermal engines are flanked by innovative ones, based on electric and hybrid systems. This revolutionizes the behaviour and the driving habits, as well as the figure of the whole propulsive system, which should integrate different energy sources on board and the energy demand for propulsion, auxiliaries, ancillary components, vehicle needs, etc. But, for heavy-duty vehicles, it is very difficult to abandon in the short and mean term the reciprocating combustion engine technology. Also, for passenger cars and light duty vehicles, the pure electric propulsion seems to put in more evidence limits not only technological. In this panorama, the development of very high efficiency engines is mandatory to fit the emissions targets, both referred to pollutant emission and CO2. In this regard, waste heat recovery into mechanical or electrical energy is one of the most promising options to reduce fuel consumption. It is of particular interest for heavy duty engines, where the operation does not suffer so much the transient phases, and hybrid powertrains, where the energy recovered can be stored in electrical form and used for all the necessities of the vehicles. In this paper, a waste heat recovery system based on an additional turbine placed in the exhaust line of a turbocharged internal combustion engine has been studied. The auxiliary turbine is designed thanks to a model-based approach. The performance map of the turbine has been calculated referring to the thermodynamic conditions of the engine exhaust gases as input parameters. The so-designed component is then integrated with an engine model, and the benefits of a turbo-compound technology bottomed to the engine were assessed. In this way, the potential power recoverable from the turbine is evaluated under design and off-design conditions. The integration with engine model allowed to estimate the side effects related to backpressure increase on the engine exhaust manifold (which leads to an overconsumption or an underrating of the engine torque), as well as the equilibrium change on the turbocharger shaft. Definitively, the final overall engine performances are assessed including the need for a bypass which, in certain engine working conditions, must exclude the recovery.