Development of a concept to achieve Euro6d standards for a high performance SI engine

Environmental sustainability is the macro-trend that is shaping the future of automotive industry. In this scenario, the continuous tightening of emission standards has led car manufacturers to further improve the efficiency of both engine and exhaust gas aftertreatment systems. However, despite an undeniable reliability of the three-way catalyst, its conversion efficiency, which strongly depends on the composition of the exhaust gases, reaches a high level only for stoichiometric mixtures. From the perspective of a further reduction of pollutant emission limits, a potential ban on scavenging and high-load enrichment is put into focus. The first one, result of intake and exhaust cam timing overlapping at low engine speed to improve low-end torque behaviour, is responsible for a lean pre-catalyst exhaust gas composition. On the other hand, the strategy of injecting more fuel than required in order to limit the exhaust temperature and protect components such as the turbine of turbochargers, causes rich pre-catalyst exhaust gas mixtures together with a degradation of fuel consumption. The task of this work was to define a set of different technologies, which enables stoichiometric operation in the whole map of a cross-plane twin-turbocharged high performance V8 engine. For this aim, a commercial 1D-CFD engine simulation software has been used in order to analyse and compare different methods to reduce the exhaust gas temperature, and thus the enrichment requirement, without jeopardizing the performance of the engine. The first assessed solutions concerned the adoption of Miller and Atkinson cycles. The consequent increased boost pressure requirement has been satisfied by the selection of a proper matching of the turbocharger. Moreover, the employment of electrically assisted turbocharging and variable valve lift system has been considered as an option to improve low-end torque behaviour. Further investigations have been performed on two different cooling concepts: the integrated exhaust manifold, which removes heat directly from the exhaust gases, and the charge air subcooling: the latter refers to a particular strategy that exploits the air conditioning circuit of the vehicle in order to strongly reduce the intake air temperature before it enters the cylinders. A steady-state analysis at wide-open throttle operating condition has been carried out and favourable technology combinations have been defined. Both benefits and feasible integration in the reference engine have been considered. The reduction of turbine backpressure and the increase of compression ratio represented the most viable way to reduce the need for enrichment as well as to increase the thermodynamic efficiency of the engine. After the update to future emission legislation compliance (i.e. stoichiometric combustion with no scavenging in the entire operating map), the transient performance of the proposed engine concepts has been evaluated for several tip-in manoeuvres and the fuel consumption at part-load has been assessed for different operating points.