Dual-Fuel Reactivity Controlled Compression Ignition (RCCI) with Alternative Fuels

Dempsey, A. B. Dual-Fuel Reactivity Controlled Compression Ignition (RCCI) With Alternative Fuels. University of Wisconsin-Madison, 2013.

This research will demonstrate that Reactivity Controlled Compression Ignition (RCCI) has several advantages over other LTC concepts in regards to fuel flexibility and combustion controllability. RCCI is a dual-fuel partially premixed combustion concept. In this strategy, a low reactivity fuel, such as gasoline or an alcohol fuel, is premixed via port fuel injection (PFI) and a high reactivity fuel, such as diesel fuel, is direct injected (DI) during the compression stroke.

Once it was clear that dual-fuel RCCI operation had great potential in terms of combustion controllability, which is a great challenge for LTC strategies, the study focused on alternative fuels with RCCI combustion. The light-duty engine was used to study two different fuel combinations: gasoline/diesel and methanol/diesel. In addition to the fuels comparison, a modified piston bowl geometry was studied and compared to the stock re-entrant bowl. The modified piston featured a wide/shallow bowl with a matched geometric compression ratio to the stock piston of ?17.3. Using the modified piston, the gross indicated efficiency of RCCI combustion was significantly improved at light loads due to increases in combustion efficiency and decreases in heat transfer losses. At higher loads the modified piston also performed better than the stock piston, but the improvements were not as significant.

The final portion of this research looks at the effects of cetane improvers on gasoline, ethanol, and methanol’s fuel reactivity and the implications for RCCI combustion. In all three base fuels it was found that 2-ethylhexyl nitrate is more effective at increasing fuel reactivity (i.e., suppressing the octane number) compared to di-tert-butyl peroxide. However, 2-ethylhexyl nitrate has a potential disadvantage due its nitrate group, which can manifest itself as NOx emissions in the exhaust. The relationship between the fuel-bound nitrate group and the engine-out NOx emissions was extensively characterized. It was also observed that methanol’s response to cetane improvers was better than that of ethanol, in spite of the fact that they have similar reactivities in their neat form.