Investigation of Cyclohexanol as an Oxygenated Additive for Gasoline–Bioethanol Mixtures and Its Effect on the Combustion and Emission Characteristics of Spark Ignition Engines

 The number of vehicles in Indonesia increases every year. In 2019, the number of vehicles in Indonesia reached 133 million units (Gaikindo, 2021). The rapidly increasing number of vehicles has led directly to increased fuel consumption. Most of the energy consumed comes from fossil fuels, such as oil, natural gas, and coal. The consumption of fossil fuels in the transportation sector is said to have reached 64 million kiloliters in 2018. Meanwhile, national oil production has decreased almost every year since 2009 (SKK Migas, 2020). Because the use of fossil fuels is directly related to the increase in the number of vehicles, air quality is also destined to get worse due to the increased number of vehicles because the products of combustion in vehicle engines increase the concentration of toxic gases such as carbon monoxide (CO), hydrocarbons (HC), and sulphur oxide (SOx) in the air. Of the most polluted countries in 2020 (IQAir, 2020), Indonesia was ranked 9^th with an index of PM2.5, air particles that are smaller than 2.5 ?m, reaching 40.7 ?g/m³. This was four times higher than the limit recommended by the WHO, 10 ?g/m³. Hence, according to the US air quality index (AQI), Indonesia’s air was defined as unhealthy for the community.

To maintain the level of energy supply stability in Indonesia and also to reduce fuel emissions, alternative energy sources were required. In recent years, ethanol-based fuels have been favored because their physical properties and characteristics share many similarities with gasoline. Bioethanol is produced from biomass (Hossain et al., 2017) and provides a range of benefits, such as its ability to be mixed with gasoline to increase a fuel’s RON rating. The characteristics of gasoline-ethanol blends include density that increases linearly and a research octane number (RON) that increases with the blend’s percentage volume of ethanol (Wibowo et al., 2020). Bioethanol (C₂H₅OH) is derived from plants that contain starch. The drawbacks of bioethanol are a lower energy density than gasoline (bioethanol has 66% of the energy that gasoline has), its corrosive properties, low flame luminosity, lower vapor pressure (making cold starts difficult), miscibility with water, and toxicity and adverse effects in biological systems (Majid et al., 2016). On top of that, the mixing of gasoline and bioethanol is far from perfect due to the difference in the polar and non-polar properties of the mixture, which causes the mixture to tend to be separated into phases and become inhomogeneous (Srinivasan and Saravanan, 2010). Therefore, an additive is required so that the two fuels can mix together to become homogeneous.

There are several classifications of additives according to their purposes. Antioxidant-cosolvent additives are suitable for increasing phase stability (Srivastava and Hancsok, 2014). There are three types of cosolvents that are common in use: branched higher alcohols, higher aliphatic alcohols, and ethers (Honig et al., 2015). Cyclohexanol is a higher aliphatic alcohol, which are also categorized as secondary alcohols with ring-shaped chains. As a secondary alcohol, cyclohexanol is more stable and reactive than primary alcohols (methanol, ethanol).

An investigation done by Amine and Barakat (2021) on a 95% gasoline, 5% bioethanol fuel mixture (E5) and a 90% gasoline, 10% bioethanol fuel mixture (E10) determined that phase separation occurred at 30ºC. Meanwhile after the addition of cyclohexanol to the fuel mixture, the phase became more stable and no phase separation occurred. Also, with a higher percentage of bioethanol, the addition of cyclohexanol would further improve water tolerance to 1.4% v/v for E20.

Waluyo et al. (2020) studied the role of cosolvent in an isooctane–methanol blend. Assessed visually, the isooctane–methanol fuel blend separated into several parts at room temperature. Then, after the addition of cosolvent, no separation occurred. According to molecular modeling using semi-empirical quantum mechanics provided by HyperChem software, the addition of cosolvent into the isooctane–methanol blend improved molecular interactions among its constituents, making the fuel mixture more homogeneous and stable.

Srinivasan and Saravanan (2010) conducted an experiment using a gasoline–bioethanol mixture with an oxygenated additive on a three-cylinder, 796 cc, four-stroke, SI engine. The combustion process occurred while the maximum load was maintained at 40 N – 3000 rpm. Exhaust gas emission, engine performance, brake thermal efficiency, and heat release parameters were obtained from the experiment. It was found that there was a slight reduction in the exhaust emission content of CO, CO₂, and NO_x, but that HC and O₂ moderately increased. Also, engine power and torque were enhanced compared to fuel alone.

Abikusna et al. (2020) explained the variations in combustion in each engine cycle. The maximum cylinder pressure and the crank angle position when maximum pressure occurs is different for every cycle. A statistical method was used to represent the ratio of standard deviation to the mean of a set of data, also called the coefficient of variation (COV). It provided evidence that the addition of an oxygenated additive into the fuel mixture led to a decrease in combustion variation, lowering the COV.

This experiment aimed to understand the character of gasoline–bioethanol mixtures with cyclohexanol as an oxygenated additive. Previous research had established that the addition of cyclohexanol as an oxygenated additive to a gasoline–bioethanol mixture would reduce the exhaust emission content of CO, CO₂, and NO_x. In this research, the gasoline–bioethanol mixture was set to a ratio that was more applicable to unmodified engines. The bioethanol percentage was set to no more than 20% of the mixture’s total volume. The percentage of cyclohexanol in the mixture was also increased from previous studies to find out if the increase would improve combustion. The experiment was conducted locally with an unmodified SI engine as the experimental object. The COV, engine power, torque, and exhaust emission produced by the engine were also investigated.