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 (NOx). With lean combustion the NOx emissions could be significantly reduced, enabling the use of lean NOx 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 (COVIMEP ≤ 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 NOx emissions dropped below 1.4 g/kWh at 4.0 bar IMEP and lambda 1.4.
Several efforts are required to improve engine efficiency and decrease global carbon dioxide (CO2) emissions and local pollutant products. In countries like Brazil, with a four-decade long experience with ethanol, the use of fossil fuels, although widespread, is being put under intense scrutiny. Governmental programs like ROTA 2030 stimulate engine research and development focused on environment-friendly fuels such as bioethanol-gasoline blends. Brazilian gasoline has about a quarter of ethanol, reducing the carbon footprint while maintaining suitable energy density. Regardless of the fuel, the load control method in part load operation of spark ignition engine causes a considerable penalty in fuel conversion efficiency. For these reasons, ozone addition was tested as an ignition enhancer that would allow higher de-throttling to reduce part-load pumping losses. The residual gas was used to dilute the mixture due to the possibility of keeping the three-way catalyst working properly with the stoichiometric mixture. An experimental investigation on efficiency, emissions and combustion related parameters was carried out in a downsized 1.0 l turbocharged direct-injected engine. Experimental tests were performed at 3 bar of indicated mean effective pressure (IMEP), 1500 rpm and stoichiometric air-fuel ratio. Spark timing (ST) was adjusted to achieve maximum indicated efficiency and the fuel used was Brazilian gasoline. Different ozone concentrations were used as a mixture with the intake air to overcome unstable engine operation by enhancing ignition. The results showed that it is possible to increase the gas exchange efficiency with ozone addition by promoting de-throttling operation with residual gases. Furthermore, the ozone addition exhibits the potential to promote autoignition of the end gas with spark assistance, even with a low compression ratio and residual gas fraction higher than 30%. However, the combustion efficiency is impaired by the higher residual gas fraction in some operation points that leave room for improvements.
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