Impact of changing compression ratio on engine characteristics of an SI engine fueled with equi-volume blend of methanol and gasoline

Highlights

• High octane number and oxygen content are two key factors associated with methanol.

• Methanol is a good fuel when operated at higher compression.

• CR change has a significant effect on performance characteristics of M50 fuel.

• M50 fuel with CR10 exhibits excellent combustion efficiency.

• Burning low carbon-hydrogen ratio M50 fuel reduce CO and HC emission.

Abstract

In the present investigation, experiments were conducted in wide open throttle condition (WOT) for different speed ranging from 1200 rpm to 1800 rpm at an interval of 200 on a single-cylinder four-stroke variable compression ratio (VCR) SI engine. The engine fueled with equi-volume blend of methanol/gasoline fuel, while 14° BTDC ignition timing is maintained for all three different compression ratios (8, 9 & 10). Increasing the compression ratio from CR8 to CR10 for the methanol/gasoline blend has improved combustion efficiency by increasing the peak pressure and net heat release value by 27.5% and 30% respectively at a speed of 1600 rpm. The performance results show a good agreement of improvisation of 25% increase in BTE, and BSFC reduction by 19% at compression ratio 10:1. At higher compression ratio 10:1, there was a significant decrease observed in CO and HC by 30–40%, and the same trend is observed at all speeds; however, NO_x emission increased with the increasing CR.

Introduction

Methanol as a fuel in the IC engine is strongly proposed, and its utility as a fuel has been proved by many researchers and documented in the literature. The potential alternative energy sources such as solar energy, nuclear energy, wind energy, have not shown promising capabilities against conventional fossil fuels [1]. Whereas methanol with good combustion properties, renewability, and extraction from a wide range of energy sources, has been considered as a very good alternative fuel. The favorable properties of methanol have allowed it to be directly used in unmodified IC engines. Yang and Jackson (2012) [2], estimation of China’s methanol economy suggest, it is the largest producer and user of methanol according to world statistical data. In 1990, the United States amended bill called Clean Air Act Amendment; methanol came to surface because of its properties such as high octane number and oxygen presence in its chemical structure. M85 fuels became popular in the US market at 1997 [3], as an alternative for leaded gasoline engines. Later critical review was done on the Methanol-fueled engine emissions and the toxicity of Methanol vapors. In the recent past, methanol has a huge demand for development of clean electrochemical energy converters (Alcohol fuel cells) to power electric vehicles; this has led way to develop new active materials for alcohol electro-oxidation process, which is low cost, durable and produce efficient electrocatalytic reaction [4,5]. Further, the favorable chemical and combustion properties of methanol and ethanol have promoted it to be commonly used as an additive with gasoline in SI engines [6,7]. When methanol used as a fuel blend with, certain chemical characteristics makes it best combination for vehicular usage [8] (A. Koenig et al., 1976), such as antiknock property and use as a fuel extender, economical when emissions are considered owing to its positive results. More importantly, it contains zero sulphur, results in reduced tailpipe acids. High load operation in an engine requires high octane number fuel; octane number can be optimized by octane enhancer and thus obtain improved engine efficiency and CO₂ emissions [9]. Methanol blended gasoline provides a good advantage to fuel combustion inside a cylinder, due to the presence of extra oxygen compound, higher flame speed and high latent heat of vaporization, hence attributes to reduced CO, NO_x and HC gases [[10], [11], [12], [13]], while improved combustion increases CO₂ emissions [14]. Sharudin et al. (2016), documented valuable information about the study of fuel properties of methanol and the nature of the combustion process when methanol/gasoline blend is used. The study on Methanol/gasoline blends has extended based on many factors such as, correlation of noise and vibrations on performance and combustion of GDI engine fueled with gasohol [16]. Recent studies are on the development of computational tools for the investigation of advanced mode of combustion, efficiency enhancement techniques, and new technology for emission control [[17], [18], [19]].

References states, the study of pure or blended Methanol were conducted for varying ignition time[[20], [21], [22], [23]] [[20], [21], [22], [23]] [[20], [21], [22], [23]], optimal mixing ratio, and gaseous fuel as additive [[24], [25], [26]] and other parameter changes which affects the characteristics of the engine. The study conducted for different blend ratios of methanol with gasoline has shown a good improvement with performance and reduced emission with lower blend ratios between 5 and 20% [6,27]. Whereas, an increase of methanol blend percentage has a significant effect on power, torque, and specific fuel consumption due to its lower energy content compared to gasoline. However, SI engine operating with M85 has shown a huge drop in HC, CO, and NO_x emission despite the loss of power output[28,29]. Lately, a numerical investigation conducted using computational fluid dynamics coupled to chemical kinetics, has been very handy in studying the effect of change of engine parameters on performance, combustion and emission characteristics[22,30] [[22], [30], [31], [32]] [[22], [30], [31], [32]].

The higher octane number of methanol attributes to improved antiknock characteristics in gasoline, which enables the gasoline-fueled engine to operate at higher compression ratios[33]. Increased compression ratios could yield 5 to 20% more power [34]. When peak pressure and temperature decreases with lower CR, the losses inside the combustion chamber become greater, and this increases BSFC [35]. Increasing CR of SI engine while running with methanol increases brake thermal efficiency and torque substantially, also release significantly less CO, CO_2, and NO_x emissions [36]. The injection and ignition strategies on the SI engine fueled with methanol, and also the effect of injection nozzle parameter on regulated emissions was studied and concluded methanol engine can burn smokeless [37]. The regulated (CO, HC, and NO_x) and unregulated emissions (formaldehyde and acetaldehyde) from M15 and M25 blends were tested, it is found that there was a decrease in CO and HC emissions, while NO_x was higher compared to gasoline and methanol blends yield more unregulated gases [27]. An extensive study conducted with use of alcohol as a neat fuel or as a blend with gasoline has been done; however, effect of compression ratio has a huge influence on engine characteristics, Omar I. Awad et al. [38], in his review included more papers on blends of ethanol, butanol and very few on methanol; whereas, higher volumetric blend ratio of methanol review is not included in the literature.

The scope of this work is to investigate the combined effect of M50 fuel and different compression ratio on performance, combustion, and emission characteristics of single-cylinder four-stroke SI Engine. No major studies were found on the effect of change of engine operating parameters for high fraction methanol blends fueled in standard SI engine model. The study here provides the convenience of use of methanol in IC engines with small operating changes. It also guides the researchers to explore the effect of change of operating parameters with a small modification to the existing engine can operate with neat methanol. Finally, spreading positive intention to use methanol in large scale and appeal for subsidizing and promote methanol as a long-term energy option for the world.