Investigation of dual injection of ethanol fuel in downsized spark ignition engine

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Ethanol fuel, as a bioproduct has become a common option to address the issue of energy sustainability. However, the current method of blending ethanol with gasoline does not take the full advantages of ethanol fuel such as its high octane number and great latent heat which potentially allow the increase of the compression ratio and improvement of engine efficiency. Dual injection of ethanol fuel is currently in development and has aimed to make more effective and efficient use of ethanol fuel in SI engines. Experiments were performed on a small single-cylinder four-stroke SI engine equipped with two dual fuel injection systems to investigate both dual injection of ethanol fuel (DualEI) and ethanol port injection plus gasoline direct injection (EPI+GDI). The effect of EPI+GDI on knock mitigation was also investigated. In the investigation of DualEI, the effects of the ratio of the directly injected (DI) ethanol fuel, spark timing, and DI timing on engine performance, combustion and emissions were analysed. The results demonstrated that the indicated mean effective pressure (IMEP) was improved over all the DI ratios in DualEI engine compared to the original engine with gasoline port injection (GPI) only. This improvement was mainly due to the enhanced combustion quality. However, at higher DI percentages, the over-cooling effect and poor mixture quality adversely affected the combustion performance. The indicated specific nitric oxide emission (ISNO) was reduced by the cooling effect enhanced by ethanol fuel and the DI strategy, but the indicated specific hydrocarbon emission (ISHC) and the indicated specific carbon monoxide emission (ISCO) were raised with the increased DI percentage. As shown by the results for the effect of spark timing, the greatest IMEP and thermal efficiency occurred at spark timing around 30 CAD bTDC at the light load and 23 CAD bTDC at the medium load, which was identified to be the MBT spark timing. The IMEP was increased and the combustion duration was shortened when the spark timing was advanced from 15 CAD bTDC to the MBT timings. The effect of DI timing associated with spark timing was also investigated. Results showed that the early DI timing enhanced the DualEI engine performance. The variation of IMEP with DI timing was not significant either with early DI timing or in most of the tested conditions with late DI timing. However, the results showed different effects of early and late DI timings associated with the spark timing on engine emissions. With late DI timing, the engine emissions of ISCO and ISNO increased with the advance of late DI timing and spark timing. With early DI timing, the engine emissions increased with the advance of spark timing. However, the variation of engine emissions with early DI timing was greater than that with late DI timing, showing more unstable combustion. In the investigation of EPI+GDI, the IMEP did not increase obviously with the increased ratio of EPI. However, the indicated thermal efficiency increased with the increased ratio of EPI because the total heating value of the fuels reduced with the increase of EPI. This was mainly attributed to the enhanced combustion process as the initial and major combustion durations were shortened with the increased ratio of EPI. This also explained why the coefficient of variation of the IMEP reduced with the increased ratio of EPI. As a consequence of improved combustion, the ISCO and ISHC emissions decreased with the increased ratio of EPI. However, the ISNO was increased possibly due to the average combustion temperature increased with and the oxygen added by the increased of EPI. The EPI+GDI effectively mitigated the engine knock and permitted more advanced spark timing. Results showed that every 10% increment (by volume) of EPI permitted about 2.0 CAD advance of knock limit spark timing. When the EPI ratio was 30% and over, the engine knock was entirely suppressed. The knock intensity was decreased with the increased ratio of EPI until the engine knock was completely suppressed when EPI was increased to 30%.
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