Boost system selection for a heavily downsized spark ignition prototype engine

The Ultraboost project outlined in this paper seeks to develop a highly pressure-charged, downsized, spark ignition engine that is capable of a 35% reduction in tailpipe CO2 emissions over a naturally aspirated 5.0L V8 while still maintaining performance, emissions and transient response. This project is especially ambitious since, in order to achieve this level of fuel economy improvement, a 60% reduction in engine displacement is targeted with a BMEP of greater than 30bar. What is more, achieving these targets in a gasoline engine with stoichiometric fuelling requires careful design and component selection in order to address the many challenges surrounding gasoline combustion under such high boost pressures. This paper describes a critical examination of the competing requirements of a heavily downsized, gasoline engine with a specific focus on the role of the boosting system in delivering these requirements. From this analysis, the optimal base boosting system configuration is investigated. In addition, a number of boosting technologies, ranging from the novel to the more traditional, are discussed in view of their ability to fulfil a role on the Ultraboost engine. A set of assessment criteria is presented in order to facilitate the selection process. Finally, a 1-D GT-Power model of the Ultraboost engine equipped with the different boosting systems was used to generate an informed rating of all boosting options and permit a reliable comparison with respect to the targets of the project.