HCCI of wet ethanol on dedicated cylinder of a diesel engine using exhaust heat recovery 2018-36-0191
Low cost ethanol with high levels of hydrations is a fuel that can be easily produced and that offers the potential to replace fossil fuels and contribute to reduce greenhouse gas emissions. However, it shows several ignition challenges depending on the hydration level, ambient temperature compression ratio and other engine-specific aspects. Advanced combustion concepts such as homogeneous charge compression ignition (HCCI) have shown to be very tolerant to the water content in the fuel due to their non-flame propagating nature. Moreover, HCCI tends to increase engine efficiency while reducing oxides of nitrogen (NOx) emissions. In this sense, the present research demonstrates the operation of a 3-cylinder power generator engine in which two cylinders operate on conventional diesel combustion (CDC) and provide recycled exhaust gas (EGR) for the last cylinder running on wet ethanol HCCI combustion. At low engine loads the cylinders operating on CDC provide high oxygen content EGR for the dedicated HCCI cylinder. At high engine loads insufficient air could be provided by the EGR from CDC and ambient air was bypassed to the HCCI cylinder. Stable HCCI operation was achieved from idle to 5 bar IMEP under different combinations of air/fuel ratio, EGR rate and ethanol-water blends. HCCI combustion could be obtained at low loads due to the heat rejection form CDC and the high stock compression ratio of 16.6:1. This compression ratio, however, provided a narrow range of operation under HCCI due to excessive rate of pressure rise. This was improved with the reduction of compression ratio to 14:1. Heat release and emission analysis were performed, and all acquired data was compared to previous results obtained with spark ignition combustion in the same dedicated cylinder of the engine. Efficiency values exceeding those of the original diesel engine were achieved while reducing NOx emissions.