Anwendungspotenziale des Miller-Verfahrens an Nutzfahrzeugdieselmotoren

Recent legislative trends in the EU and in the US enforce a parallel decrease in NOX emissions as well as a reduction in CO2 emissions, i.e. a drop in fuel consumption. Miller cycle has shown benefits in terms of both NOX and CO2 according to the available literature. However, a systematic investigation based on experimental measurements including a detailed analysis of the effects resulting from applying Miller cycle could not be found for heavy duty diesel engines. Accordingly, the investigations in the present work target the evaluation of the potentials of Miller cycle regarding efficiency increase as well as pollutant emission decrease. A single cylinder research engine fitted with electro-hydraulic variable valve actuation (VVA) system was applied for the experimental tests while model-based investigations were carried out to extend the results for full engines. First, a detailed analysis based on experimental results was carried out targeting the characterization of the changes caused by Miller cycle. For that purpose, the boundary conditions such as cylinder charge were kept unchanged, and measurements were conducted at different engine speeds at full load. The results show a drop in fuel consumption of up to 1 % and a significant decrease in NOX emissions – with an increase in PM emissions. Afterwards, a strategy aiming at load increase was investigated. The enabler for increasing the engine load is the drop in PFP (peak firing pressure) achieved by applying Miller cycle. Load increase potentials of up to 10 % were demonstrated using late intake valve closing (IVC) by 80 degCA. Next, the mentioned margin in PFP was used to combine Miller cycle with further measures targeting more optimal operation. Results showed that by applying an additional charge increase, a drop in fuel consumption by 8 g/kWh (~4 %) was reached and a break-through in PM–NOX trade off was achieved. However, during the previously described investigations, the charge exchange process of the single-cylinder test engine differed from the charge exchange process of in-series full engines. Thus, a coupling approach was developed to extend the experimental results for full engines by means of model-based investigations. As a first step, the effect of turbocharger efficiency on the potentials of Miller cycle was analyzed. Afterwards, Miller timing was applied on the full engine while taking the in-series two-stage turbocharger group into account. As a result, improvement in fuel consumption was evaluated at 1400 rpm but a deterioration in BSFC was found at high engine speeds. As a last step, adjustment of the charging group was performed based on geometric similarity and Miller timing was applied. As a result, it was possible to further reduce fuel consumption in the relevant operating range of commercial vehicles in long hauling applications – without significant disadvantages at rated speed.