Fossil fuels depletion is a globally main issue because it is the primary transportation fuel. Furthermore, environmental issues including global warming and climate changes are problems that need to be dealt with immediately. Therefore, biofuels (bio-based fuels), i.e. bio-alcohols (bioethanol and biobutanol) which are produced from natural materials have emerged as promising transportation fuels because of their sustainability and environmental benefits which can reduce the dependency on crude oil reserves. Today, bioethanol is widely used as an option for transportation fuel or additives to gasoline in spark ignition (SI) engine due to its attractive properties of high octane number and reduced exhaust emissions. The next promising and competitive biofuel is biobutanol which has superior properties to be used in SI engine without engine modification. The aim of the present review is to highlight on the feasibility of the bioethanol and biobutanol as alternative transportation fuels in SI engine. The first section of this paper is an overview on bioethanol and biobutanol as gasoline alternatives. In the next section, comparative physicochemical properties of gasoline, bioethanol and biobutanol and their potential sources of production are presented. The effect of bioethanol and biobutanol with gasoline blends on engine performances, combustion analysis, exhaust emissions, engine durability and their effect on lubricating oil are discussed in the next section. The review study acknowledges that the bioethanol and biobutanol are capable of improving engine performances, combustion and also reducing exhaust emissions. However, the addition of alcohols in fuel blends leads to negative impacts on the engine durability and lubricating oil properties.

This research was aimed to examine the diesel engine’s performance and emission of secondary fuels (SFs), comprising waste plastic oil (WPO) and palm oil biodiesel (POB), and to analyze their tribological properties. Their compositions were analyzed by gas chromatography–mass spectrometry (GC–MS). Five SFs (10–50% POB in WPO) were prepared by mechanical stirring. The results were compared to blank WPO (WPO100) and Malaysian commercial diesel (B10). WPO90 showed the maximum brake power (BP) and brake torque (BT) among the SFs, and their values were 0.52 and 0.59% higher compared to B10, respectively. The increase in POB ratio (20–50%) showed a negligible difference in BP and BT. WPO70 showed the lowest brake-specific fuel consumption among the SFs. The brake thermal efficiency (BTE) increased with POB composition. The maximum reductions in emission of hydrocarbon (HC, 37.21%) and carbon monoxide (CO, 27.10%) were achieved by WPO50 among the SFs. WPO90 showed the maximum reduction in CO₂ emission (6.78%). Increasing the POB composition reduced the CO emissions and increased the CO₂ emissions. All SFs showed a higher coefficient of friction (COF) than WPO100. WPO50 showed the minimal increase in COF of 2.45%. WPO90 showed the maximum reduction in wear scar diameter (WSD), by 10.34%, compared to B10. Among the secondary contaminated samples, SAE40-WPO90 showed the lowest COF, with 5.98% reduction compared to SAE40-WPO100. However, with increasing POB content in the secondary contaminated samples, the COF increased. The same trend was also observed in their WSD. Overall, WPO90 is the optimal SF with excellent potential for diesel engines.