Exploring the part load lean limit of a direct injection spark ignition engine fueled with ethanol

Abstract

Stringent environmental legislation demands development of internal combustion engines seeking higher thermal efficiency to reduce greenhouse gas emissions. Brazilian governmental programs like ROTA 2030 and RenovaBio drive the research and development of highly efficient engines fueled with biofuels, like sugarcane bioethanol, that has a near to zero carbon footprint. Some technologies such as gasoline direct injection, variable valve train, and turbocharging were implemented in downsized Spark Ignition (SI) engines in order to reach higher thermal efficiencies and decrease CO₂ emissions. Homogeneous lean combustion in SI operation can improve fuel consumption and reduce exhaust gas emissions. However, it is limited by combustion instability and the three-way-catalyst (TWC) inability to work properly converting nitrogen oxides (NO_x). With lean combustion the NO_x emissions could be significantly reduced, enabling the use of lean NO_x trap (LNT) as an effective lower cost after treatment system. To explore the limits of homogeneous lean combustion in SI engines, experimental tests were performed to investigate performance, efficiency, combustion, and emission parameters of a multi-cylinder Ford 1.0l Ti-VCT EcoBoost engine fueled with hydrous ethanol. Hydrous ethanol is a commercial Brazilian fuel which has greater laminar flame speed and higher dilution tolerance than gasoline, and thus can increase combustion stability. Different fuel direct injection strategies were explored at part load (below 8.0 bar IMEP) at engine speed of 1500 rpm. Air-to-fuel ratio was varied from stoichiometric (lambda 1.0) to lambda 1.5. The limit of stable homogeneous lean combustion (COV_IMEP ≤ 3.0%) was achieved in lambda 1.4. The lowest indicated load of 2.1 bar IMEP was limited by combustion stability at lambda 1.3. The maximum indicated efficiency was 36.9% at indicated load of 8.0 bar IMEP and lambda 1.4. The NO_x emissions dropped below 1.4 g/kWh at 4.0 bar IMEP and lambda 1.4.