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Variable valve trains offer the opportunity to apply advanced combustion process strategies such as the Miller cycle. As is well known, applying Miller timing for CI engines is an effective way to reduce NOX emissions and can lead to an increase in engine efficiency. Because of the intended future NOX and GHG limits for on-road HD CI engines, the use of variable valve trains become more and more inevitable. Previous studies of the authors have shown that the improvement potential highly depends on the achievable cylinder charge level. Increasing this (through additional increase in boost pressure) results in a significant decrease in ISFC as well as in an improved NOX-PM trade-off. However, in these considerations the pressure difference of the charge air and the exhaust back pressure was kept on the same level. The present paper investigates the improvement potentials for heavy duty CI engines taking a two-stage turbocharging group into account.

A variable air path on diesel engines offers further potentials to manage the challenges of engine development - such as reduction of emissions and fuel consumption, as well as performance increase. The Miller cycle is one of the possibilities, which is well known as an effective way to reduce process temperatures and so NOX emissions. The present paper discusses the potentials of this strategy for heavy duty diesel engines by identifying and analyzing the effects caused. The investigations were carried out in the upper load range. First the isolated effect of the Miller cycle was analyzed. The results show reduced NOX emissions, although increased PM and CO emissions were measured. Further, the Miller cycle caused a reduction in peak cylinder pressure. This pressure reserve can be used to combine the Miller cycle with further measures while maintaining the maximum cylinder pressure of the reference operation point. On the one hand, a performance increase of about 10% was achieved.