Potentials of Miller Cycle on HD Diesel Engines Using a 2-Stage Turbocharging System

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 NO_X emissions and can lead to an increase in engine efficiency. Because of the intended future NO_X 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 NO_X-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. The combustion process and the pollutant emissions are taken over from measurements on a single cylinder test engine equipped with a VVA and are used in a 1D simulation model to expand the results for six cylinder engines. The first part of the investigations shows the application of Miller timing on a HD diesel engine, where charge air pressure increase was achieved through modified waste gate control and presents the change in fuel consumption as well as in pollutant emissions. The second part focuses on an optimized strategy with scaled turbocharger size to maximize the improvements, in which TC-scaling was based on geometrical similarities.