Ignition control for liquid dual-fuel combustion in compression ignition engines
Abstract This paper investigates the characteristics of combustion and emissions for various combinations of liquid fuels under dual-fuel clean combustion modes. The effects of fuel properties on the ignition control are examined along with the optimization of combustion control strategies. With diesel direct injection (DI) as an effective ignition control, the increased use of premixed ethanol in the auto-ignition resistant background can significantly improve the trade-off between the nitrogen oxides (NOx) and the soot emissions. Because of the relatively low reactivity of ethanol fuel, a single-shot diesel micro-pilot injection shows a limited ability to ignite the very lean ethanol mixture at medium loads. When using gasoline to form a relatively higher reactivity premixed background, lower compression ratios and exhaust gas recirculation (EGR) are necessary to avoid the premature auto-ignition that can cause high pressure rise rate and increased NOx and soot emissions. At a compression ratio of 14, ultra-low levels of NOx and soot emissions are achieved simultaneously for diesel-gasoline dual-fuel combustion, with the ignition and combustion phasing being responsively controlled by DI timing. DI butanol is examined as a means of ignition control for dual-fuel combustion. Owing to the relatively lower fuel reactivity, the compression ignition of butanol is difficult and sensitive to the thermal load of the engine and the EGR rate applied. With double-shot butanol DI as the ignition enabler and the primary power producer, butanol-ethanol dual-fuel combustion is realized at an indicated mean effective pressure (IMEP) of 13 bar.