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The properties of butanol offer more promising results compared to those of lower chain alcohol such as methanol or ethanol. However, butanol as a biofuel has not yet been commercially produced due to its costly process. Butanol is generally produced via the process of Acetone-Butanol-Ethanol (ABE) fermentation and can only be acquired after it was recovered from the ABE solvent. Despite the efforts and recent developments, obtaining higher butanol concentration from ABE fermentation is still relatively expensive and challenging. The idea of using ABE directly in internal combustion engines is then proposed to eliminate the recovery process. Several preliminary studies have reported several promising results of using ABE blends in both gasoline and diesel engines. However, researches in this area are still in the early stages, and thorough investigations are required. This review paper aims to provide essential findings from the latest development in the addition of ABE both with gasoline and diesel fuel in Spark Ignition (SI) and Compression Ignition (CI) engines. A brief discussion on ABE properties will be firstly given before the effects of its addition on SI and CI engine is comprehensively reviewed. The end of this article highlights some possible contributions and research gaps.

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Several studies on ethanol as a biofuel have been comprehensively carried out. Today, a growing interest in longer chain alcohols has emerged due to their favourable physical and thermodynamic properties. Butanol or butyl alcohol with 4-carbon structure (C4H9OH) is a more promising biofuel as it outweighs methanol and ethanol in several ways. The production cost of butanol has been gradually reduced, and rapid progress in the development of its production technology has allowed butanol to be produced more effectively. However, studies on the effect of butanol on gasoline or spark ignition (SI) engines are not as comprehensive as the abundant research of ethanol. This paper highlights recent novelty and contribution of butanol addition in SI engine. Results of new approaches on the engine’s performance, combustion and emission characteristics are summarised. Reviews from this paper suggest that there are some gaps in the addition of butanol found in the literature. Thus, several encounters and forthcoming research directions are outlined in the final section of this review article, highlighting several possible contributions that have not yet been carried out using butanol as a biofuel in a gasoline engine.

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Due to the tremendous amount of palm biomass residues produced during the palm oil extraction from fresh fruit bunch (FFB), it is inevitable to harness these biomass energy sources to cope with the depletion of fossil fuels and increase in global energy demand scenarios. Densification is one of the favourable techniques to improve the storage and transportation of biomass fuels in order to prevent dumped areas adjacent to palm mills and to prevent from becoming another waste product. This article reviews comprehensively on how type of palm biomass, compaction pressure and temperature, binder, pre- and post-treatments affect the physical and combustion properties of the palm biomass briquettes produced. Based on the previous researches, generally it can be said that the type of palm biomass, the compaction pressure and temperature, and type of binder affect both the physical and combustion performance of densified palm biomass. However, the effect of particle size could be observed only on the physical characteristics of densified products, whereas the effect on the combustion properties remains unclear. In addition, treatments such as pyrolysis, dry and wet torrefaction (hydrothermal treatment), and steam explosion have potential to be applied during briquette production in order to improve the combustion properties. In this review article, it is also suggested that the combination of densification and followed by wet torrefaction will enhance the combustion properties of palm biomass briquettes.