Study of using diesel/high-ON biofuel blends as the pilot fuel with large proportion and split injection in the dual-fuel combustion

Dual-fuel combustion using diesel and high-octane number (ON) biofuel of n-butanol blends to ignite compressed natural gas (CNG) with pilot fuel split injection was investigated. 30% n-butanol/70% diesel (B30D70) and pure diesel (B0D100) as the pilot fuels were compared with the start of pre-injection (SOIp) timings swept from -30 (SOI(p)30) to -80 degrees CA ATDC (SOI(p)80) under the CNG substitution rates of 50% at low load and 65% at medium load. A fixed start of main-injection (SOIm) timing of -9 degrees CA ATDC in B0D100 and various SOIm timings in B30D70 to match CA50 of B0D100 were employed. The experimental results indicated that adding n-butanol in the pilot fuel can significantly reduce the PM emission at advanced SOIp timings even with large proportion of pilot fuel. At low load, B30D70 reveals higher indicated thermal efficiency (ITE) due to its shorter combustion duration compared to B0D100, and it also leads to lower NOx emissions resulted from the more homogeneous mixture. The CO and THC emissions tend to be slightly increased in B30D70 for the advanced SOIp timings. At medium load, B30D70 also obtains higher ITE but yields higher NOx emissions. Relatively low level of NOx emissions can be obtained at early SOIp timings, such as SOI(p)60, SOI(p)70 and SOI(p)80. The CO and THC emissions are similar for the two pilot fuels at these timings. For the comparisons of the two pilot fuels and injection strategies, it can be demonstrated that B30D70 is more favorable to the thermal efficiency and emissions with split injection at SOI(p)40 under low load condition and early SOIp timings under medium load condition.