Numerical Comparison of an Electric Turbo Compound Applied to a SI and a CI Engine

In a medium term scenario Internal Combustion Engine (ICE) downsizing and hybrid powertrain will represent the actual trend in vehicle technology to reduce fuel consumption and CO2 emission. Concerning downsizing concept, to maintain a reasonable power level in small engines, the application of turbocharging is mandatory both for spark ignition (SI) and compression ignition (CI) engines. Following this aspect, the possibility to couple an electric machine to the turbocharger (electric turbo compound, ETC) to recover the residual energy of the exhaust gases is becoming more and more attractive, as demonstrated by several studies around the world and by the current application in the F1 Championship. The present paper shows the first numerical results of a research program focused on the comparison of the benefits resulting from the application of an ETC to a small twin-cylinder SI engine (900 cm3) and to a four cylinders CI engine (1600 cm3), both of the same maximum power. Starting from the experimental maps of several turbines and compressors, complete model of both turbocharged engines were created using the AVL BOOST one-dimension code. Concerning the SI engine, first numerical results show that ETC can improve the average overall efficiency at the highest engine speeds and loads. Besides, boost range extension in the lowest engine rotational speed region and a possible reduction of turbo lag represent other benefits related to ETC application. On the other hand, the adoption of an ETC to a CI engine shows larger benefits in term energy recovery at the highest engine speeds, with consequent reduction of fuel consumption, mainly due to the absence of throttling effects in the intake manifold and related pumping losses.