Theoretical and experimental investigation of the flow field and visualization of the flame in internal combustion engines using renewable fuels.

Abstract

The internal combustion engine will remain the major propulsion system for land transportation, for at least the next decade, as the transition to full electrification will not be imminent. The automotive industry always looks for cleaner and more efficient technologies and biofuels are a promising alternative to fossil fuels that reduce the net production of greenhouse gases (GHG) and they can be used in existing vehicle engines. The change of fuel requires re – inspection of the flow and combustion phenomena to improve the combustion efficiency and emissions. To achieve this, it is important to understand and control the in – cylinder flow evolution, and primarily the Tumble development as it is strongly connected with the mixing preparation, combustion process and emissions production. Flow field measurements were obtained by using low repetition rate PIV and time resolved PIV techniques in a 475cc optical single – cylinder Gasoline Direct Injection (GDI) spark ignition engine. Moreover, flame visualization and emission measurements were performed for gasoline and two biofuel blends. The results include flow fields illustration using these two PIV techniques. Alongside, Large Eddy Simulation and Reynolds Averaged Navier Stokes (RANS) simulations were carried out using real engine geometry and initial conditions similar to the experiment in order to compare the flow field and validate quantities such as Turbulent Kinetic Energy (TKE) and Tumble Ratio (TR). Moreover, quantitative comparison was conducted by using relevance (RI) and magnitude (MI) indices, and the Γ1 criterion was used to identify the center of vortex structures. Subsequently, flame visualization measurements were conducted at different intake pressures (850mBar & 700mBar), engine speeds (1000, 1500, 2000 RPMs), injection strategies (single, double) and two air / fuel ratios (λ=1 & λ=1,2). This work showed that a tumble-like motion with Counterclockwise rotation (CCW) was present when the intake valves were open and continued to evolve at the intake and compression stroke. Comparing the experimental and simulation results the same trend was observed for the TKE and TR at the intake stroke. The RI and MI gave acceptable quantitative comparison of the flow field for values above 60% and 80% respectively, and the Γ1 algorithm estimated the vortex center in both approaches, giving a good agreement in some CADs. Regarding the combustion process the stoichiometric combustion presented higher flame speed and higher rate of flame diameter growth. The ETBE20 presented the fastest flame development compared with other fuels under the same conditions. The emissions showed a decreasing trend by using biofuels blends.