CFD Simulation to Understand Auto-Ignition Characteristics of Dual Fuel Strategies using High- and Low-Octane Fuels: A Step Towards The Octane-On-Demand Engine

Reduction of CO₂ emissions is becoming one of the great challenges for future gasoline engines. The aim of the current research program (OOD: Octane On Demand) is to use the octane number as a tuning parameter to simultaneously make the engine more efficient and reduce CO₂ emissions. The idea is to prevent knock occurrence by adapting the fuel RON injected in the combustion chamber. Thus, the engine cycle efficiency is increased by keeping combustion phasing at its optimum. This is achieved by a dual fuel injection strategy, involving a low-RON base fuel (Naphtha or Low RON cost effective fuel) and a high-RON octane booster (ethanol). The ratio of fuel quantity on each injector is adapted at each engine cycle to fit the RON requirement as a function of engine operating conditions. A first part of the project, described in [18], was dedicated to the understanding of mixture preparation resulting from different dual-fuel injection strategies. The present part is focused on the evaluation, the understanding, and the characterization of Auto-ignition (AI) propensity (mostly knock occurrence) as a function of the injection strategy.

This work relies on experimental and 3D-simulation investigations. Port Fuel Injection (PFI) and Side Direct Injection (SDI) were studied separately and in dual-injection modes. Detailed investigations on auto-ignition occurrence were carried out with 3D CFD calculations. After validating the simulation results using dedicated experimental measurements, several observations were made. Those observations enable to explain the high auto-ignition tendency with low RON fuel in Port-Fuel Injection (PFI) mode and to give indications on the “octane boosting effect” of RON booster (ethanol) in direct-injection and PFI mode. Finally, some hypothetical but very relevant “perfect heterogeneous” cases were studied in order to help selecting the injection strategy for octane booster and base fuel.