Decarbonisation of transport: options and challenges

This EASAC report reviews options for reducing greenhouse gas (GHG) emissions from European transport. It argues for stronger policies to bridge the gap between the GHG emission reductions that will be delivered by current policies and the levels needed to limit global warming to less than 2°C or even 1.5°C (Paris Agreement). The report focusses on road transport because, in the EU, this contributes 72% of transport GHG emissions. EASAC recommends a combination of transitional measures for the next 10-15 years and sustainable measures for the long term, based on a three level policy framework: avoid and contain demand for transport services; shift passengers and freight to transport modes with lower emissions (trains, buses and ships); and improve performance through vehicle design, more efficient powertrains and replacing fossil fuels with sustainable energy carriers including low-carbon electricity, hydrogen and synthetic fuels. Opportunities for the EU to strengthen its industrial competitiveness and create high quality jobs are also discussed

Decarbonisation of transport: options and challenges

This EASAC report reviews options for reducing greenhouse gas (GHG) emissions from European transport. It argues for stronger policies to bridge the gap between the GHG emission reductions that will be delivered by current policies and the levels needed to limit global warming to less than 2\xc2\xb0C or even 1.5\xc2\xb0C (Paris Agreement). The report focusses on road transport because, in the EU, this contributes 72% of transport GHG emissions. EASAC recommends a combination of transitional measures for the next 10-15 years and sustainable measures for the long term, based on a three level policy framework: avoid and contain demand for transport services; shift passengers and freight to transport modes with lower emissions (trains, buses and ships); and improve performance through vehicle design, more efficient powertrains and replacing fossil fuels with sustainable energy carriers including low-carbon electricity, hydrogen and synthetic fuels. Opportunities for the EU to strengthen its industrial competitiveness and create high quality jobs are also discussed

A 2-D DNS study of the effects of nozzle geometry, ignition kernel placement and initial turbulence on prechamber ignition

A parametric direct numerical simulation study was conducted to investigate the effects of the initial flow field (quiescent or turbulent), nozzle inlet sharpness and width, main chamber composition (lean and stoichiometric), and ignition kernel placement in a two-dimensional prechamber (PC) ignition system. The strongly coupled operating and geometric parameters determine the time at which the flame exits the prechamber, the transient structure and penetration of the initially cold and subsequently hot reactive jet and their impingement on the lower main chamber (MC) wall, affecting the combustion mode and the fuel consumption rate. The temperature of the flame reaching and crossing the nozzle is affected by the flame exit time and is significantly lower than the adiabatic flame temperature of the planar flame, although no quenching is observed. Interaction with the flow field (strong small scale vortices for narrow and sharp entry nozzles, large vortices for wide nozzles) generated close to the exit increases the surface area of the flame and its interaction with the MC mixture. Jet penetration and impingement on the lower MC wall is determined by combustion in the PC and the flow field it generates in the main chamber. Impingement results in large scale vortical structures, which further contribute to the flame area increase and accelerate the consumption of the MC charge at later times. For the conditions studied, budget analysis shows that the main combustion mode is premixed deflagration with locally enhanced or reduced reactivity. Local flame–flame interactions which are more pronounced close to the nozzle exit and the lower MC wall can increase the propagation speed up to six times compared to the planar flame. The evolution of the probability density functions of different quantities is used to characterize the strongly transient process. © 2020 The Combustion Institute

Characterizing the Evolution of Boundary Layers in IC Engines by Combined Direct Numerical and Large-Eddy Simulations

The structure of boundary layers (BLs) and wall heat flux is investigated as they evolve during the compression stroke in an optically accessible, single-cylinder research engine of passenger-car dimensions with a typical four-valve pent-roof design operated at motored and throttled conditions. Three-dimensional direct numerical simulations (DNS) of the compression stroke were carried out, which enable full resolution in space and time of all flow and temperature field structures in the entire domain, including the BLs. Since the high computational cost precludes calculation of the scavenging cycle, scale-resolving simulations were employed to provide initial fields for the DNS at intake valve closure. The analysis revealed that BLs deviate from ideal scaling laws commonly adopted in algebraic wall models, and that the non-zero streamwise pressure gradient correlates with changes in the near-wall profiles. Phenomenologically, such deviations are similar to those for developing BLs, and in particular for impinging flows. The momentum BL structure was found to be affected by the large-scale bulk flow motion, in contrast to the thermal BLs which exhibit a more structured behavior following the density increase due to compression. Inspection of the heat flux distribution confirmed the similarity between the flow and heat flux patterns and identified regions of intense heat flux, mainly in locations of strong directed flow towards the wall. The improved characterization of the boundary layer structure and its evolution during the compression stroke not only constitutes an important step towards improved understanding of near-wall phenomena in internal combustion engines, but the vast dataset also serves as a database for development of improved wall models.ISSN:1386-6184ISSN:1573-198