Effects of six-carbon alcohols, ethers and ketones with chain or ring molecular structures on diesel low temperature combustion

•Effects of molecular structures (chain/ring) on combustion and emission are tested.•Effects of cetane number and oxygen on combustion and emission are isolated.•Soot reduction effects of different oxygen fundamental groups are compared.•The sequence of soot reduction is alcohols < ketones < ethers.•2,5-Dimethylfuran is more effective to realize high efficiency and clean combustion.

The effect of oxygenated additives with different oxygen functional groups and different carbon chain structures on the combustion and emissions was investigated on a single-cylinder diesel engine. Three different oxygenated hydrocarbons with ring molecular structure including cyclohexanone, cyclohexanol and 2,5-dimethylfuran and other three oxygenated hydrocarbons with chain molecular structure including 2-hexanone, n-hexanol, and isopropyl ether were blended with diesel fuel separately to formulate 6 blended fuels with the same oxygen content (4 wt%). A wide range of exhaust gas recirculation rates (from 0 to misfire) were employed to cover both conventional diesel combustion and low temperature combustion. The results indicate that the effect of oxygenated fuels on combustion and emission get larger at high rates of exhaust gas recirculation. For chain structure oxygenated additives, although n-hexanol presents much longer ignition delay at high rates of exhaust gas recirculation, it exhibits the highest soot peak due to its high boiling point. And isopropyl ether has the lowest soot emission attributing to its better volatility. The ring structure oxygenated additives present more significant difference in combustion than chain structure ones, and the sequence of ignition delay is 2,5-dimethylfuran > cyclohexanol > cyclohexanone > diesel. The soot emissions show a close relation with the ignition delay of the fuel, i.e., the fuel with longer ignition delay always exhibits lower soot emissions. And the lowest soot emission can be obtained with 2,5-dimethylfuran companied with apparent improvement in combustion efficiency and thermal efficiency. For the soot reduction ability of different oxygen functional groups, the sequence is alcohols < ketones < ethers. The oxygenated fuels with ring molecular structure tend to produce more soot compared with the ones with chain structure if only the oxygenated effect is considered, while the soot emissions is also influenced by the position of oxygen functional group. Taking all the effects together, 2,5-dimethylfuran is more effective in realizing high efficiency and clean low temperature combustion.