Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Review article Microemulsion fuels for compression ignition engines: A review on engine performance and emission characteristics Iyman Abrar, Ashok N. Bhaskarwar ⁎ Department of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India ARTICLE INFO Keywords: Diesel Microemulsions Vegetable oils Alcohols CI engine Emissions ABSTRACT Fossil fuels are currently the world’s primary source of energy. Their limited reserves and increasing emissions have, however, generated an interest in the development of renewable and cleaner alternative sources of energy. Compression ignition (CI) engine is an extensively used internal combustion engine, with a better fuel economy and efciency as compared to spark ignition (SI) engine. Particulate matter (PM) and NO x emissions generated from diesel fuel are serious environmental and health concerns, which also have led to an interest in the diesel alternatives. Most commonly used diesel alternatives are either uneconomical or require modifcations in current engine and infrastructure. Microemulsifcation is a developing technique, which is economical and does not require any changes in the current systems. In this review, we have focused our attention on diesel, vegetable oil, and alcohol based microemulsion fuels as an alternative to diesel for the CI engines. While most of the micro- emulsions have lower calorifc values and higher fuel consumptions, the thermal efciency is improved due to the presence of water. Moreover, it helps in the simultaneous reduction of PM and NO x emissions. Considering the trends in the costs of fossil fuels in recent past and present, microemulsions seem to be most promising, cleaner, and powerful alternative to fossil fuels, such as gasoline, diesel and aviation fuels. 1. Introduction The compression ignition (CI) engines are more popular than spark ignition (SI) engines due to their better fuel economy and efciency. CI engines run on diesel fuel, which is obtained from the distillation of crude oil, in the temperature range of 180–360 °C. It consists of ap- proximately 75% of saturated hydrocarbons, mainly parafns, naph- thenes, and 25% of aromatic hydrocarbons [1]. Various properties of diesel as per ASTM D975 are given in Table 1, for No. 1 and No. 2 diesel. No. 1 diesel is less viscous and has a better temperature oper- ability, with properties similar to those of kerosene, while No. 2 diesel has lower volatility and better fuel economy, and is therefore used more extensively in CI engines than the No. 1 diesel. CI engines are used in a wide range of operations, such as transportation (road, rail, and marine), agriculture, and construction. The limited reserves and increasing prices of diesel have raised concerns over energy security and economy, while the emissions are leading to the environmental degradation and healthcare concerns as well [3]. Stringent environmental regulations are being enforced to limit the emissions, such as of particulate matter, nitrogen oxides, carbon monoxide, unburnt hydrocarbons, and smoke [4]. The emissions from CI engines are lower than from the SI engines, except for those of PM and NO x . PM poses serious environmental and health concerns, while NO x emissions are responsible for acid rain, smog and ozone formation [5]. Thus, there is a need for renewable and cleaner sources of energy. 1.1. Alternative fuels for CI engines 1.1.1. Vegetable oils Rudolf Diesel ran diesel-car frst on peanut oil in the year 1909. Thus, vegetable oils, which primarily consist of triglycerides of fatty acids, could be potentially used as an alternative fuel. They are en- vironmentally carbon neutral, apart from being renewable and non- toxic [6]. Carbon dioxide is consumed by plants to produce biomass, which can be processed to form vegetable oils, within a life cycle of a https://doi.org/10.1016/j.fuel.2019.115944 Received 30 May 2019; Received in revised form 30 July 2019; Accepted 1 August 2019 Abbreviations: ASTM, American society for testing and materials; BSEC, Brake specifc energy consumption; BSFC, Brake specifc fuel consumption; BTE, Brake thermal efciency; CI, Compression ignition; CPP, Critical packing parameter; CTAB, Cetyl trimethyl ammonium bromide; DTBP, Di-tert-butyl peroxide; EGBE, Ethylene glycol butyl ether; EGT, Exhaust gas temperature; FT, Fischer-Tropsch; HC, Hydrocarbon(s); PIT, Phase inversion temperature; PM, Particulate matter; ULSD, Ultra low sulfur diesel ⁎ Corresponding author. E-mail address: ashoknb@chemical.iitd.ac.in (A.N. Bhaskarwar). Fuel 257 (2019) 115944 Available online 31 August 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved. T