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Increasing fuel economy is highly demanded because of the GHG reduction today. Turbocharged downsized engines have much attention as one of the effective technology for this demand. Turbocharged boost technology enables to increase thermal efficiency, but this also has a response delay known as turbo lag, which may cause lower engine performance and poor drivability. This issue impedes the broader application of this technology. The research discussed in this paper focused on turbo lag, and adopted a numerical approach to analyzing the detailed mechanism of this phenomenon. The study concluded that turbo lag is a delay in the boost pressure response that originates from a combination of factors. The primary factor in turbo lag is a delay that is due to physical properties of the turbocharger system; the secondary factor is a decreased effective turbine energy caused by a shift in the operating point, resulting from the primary factor.

Downsizing or higher compression ratio of SI engines is an appropriate way to achieve considerable improvements of part load fuel efficiency. As the compression ratio directly impacts the engine cycle thermal efficiency, it is important to increase the compression ratio in order to reduce the specific fuel consumption. However, when operating a highly boosted / downsized SI engine at full load, the actual combustion process deviates strongly from the ideal Otto cycle due to the increased effective loads requiring ignition timing delay to suppress abnormal combustion phenomena such as engine knocking. This means that for an optimal design of an SI engine between balances must be found between part load and full load operation. If the knocking characteristic can be accurately predicted beforehand when designing the combustion chamber, a reduction of design time and /or an increase in development efficiency would be possible.