Reactivity Controlled Compression Ignition (RCCI) Using Premixed Hydrated Ethanol and Direct Injection Diesel

Previous research has shown that a Homogeneous Charge Compression Ignition (HCCI) engine with efficient heat recovery can operate on a 35 to 65% volumetric mixture of ethanol-in-water while achieving high brake thermal efficiency (∼39%) and very low NOx emissions [4]. The major advantage of utilizing hydrated ethanol as a fuel is that the net energy gain improves from 21 to 55% of the heating value of ethanol and its co-products, since significant energy must be expended to remove water during production. This is required because wet ethanol is not suitable for conventional combustion engines. For example, spark ignition engines demand the use of pure ethanol because the dilution caused by water reduces the flame speed, resulting in misfire and problems due to condensation. The present study uses numerical simulations to explore the use of wet ethanol for Reactivity Controlled Compression Ignition (RCCI) operation in a heavy duty diesel engine. RCCI uses in-cylinder blending of a low reactivity fuel with a high reactivity fuel and has demonstrated significant fuel efficiency and emissions benefits using a variety of fuels, including gasoline and diesel. Combustion timing is controlled by the local blended fuel reactivity (i.e. octane number), and the combustion duration can be controlled by establishing optimized gradients in fuel reactivity in the combustion chamber. In the present study, the low reactivity fuel was hydrated ethanol while the higher reactivity fuel was diesel. First, the effect of water on ethanol/water/diesel HCCI was investigated using GT-Power and single-zone CHEMKIN simulations. The results showed that the main impact of the water in the ethanol is to reduce the IVC temperature due to vaporization cooling. Next, multidimensional engine modeling was performed using the KIVA code at engine loads from 5 to 17 bar IMEP at 1300 rev/min with various grades of hydrated ethanol and a fixed diesel fraction of the total fuel. The results show that hydrated ethanol can be used in a RCCI engine with gross indicated thermal efficiencies up to 55% and very low emissions. A 70/30 ethanol/water mixture (by mass) was found to yield the best results across the entire load range without the need for EGR.