Effect of modifying bowl geometry for IC engine fueled with diesel and Biofuels - Review

Abstract

Keywords: Biodiesel; Combustion Chamber Geometry; IC Engine; Diesel Engine

Introduction

The demand for energy and fuel due to rapid growth in the human population worldwide leads to a hike in the prices of petroleum. This increase in need also leads to a reduction in the stock of fossil fuels as reserves of fossil fuel are limited and maybe it lasts for a few decades [1]. In the majority of the commercial sector, transportation is a key factor. The transportation sector consumes the second largest amount of energy which is considered a key factor for projections of energy demand [2]. A diesel engine (CI)

finds wide usage because of its higher effectiveness and reliability than gasoline engines in the transportation sector. High demand and dependability on diesel as a fuel have put challenges in front of researchers worldwide for finding out alternative fuels with or without making a change in available CI engines. It is desired that novel fuels fulfill the performance parameters and match emission standards. Biofuel is one of that due to local availability, renewability, biodegradability, high flashpoint, high cetane number, low cost and low sulfur value of extracted oil makes it a

*Corresponding author. *E-mail address: dzbhavesh@gmail.com This paper was recommended for publication in revised form by Ahmet Selim Dalkılıç Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

suitable option as a fuel[3–7]. Biofuel is widely accepted as an alternative to crude due to its salient features like ease in availability, higher index for lubricity, and lower values for emissions [8]. Siva Lakshmi et al. [9] observed a reduction in the value of carbon monoxide (CO), hydrocarbon (HC), and smoke emissions. Oxidation of excess HC creates high local temperature which generates thermal NOx [10–12]. They further worked on blended mahua oil for performance and emission analysis and derived that the rise in mixing ratio results in lower HC and CO emissions measurements, brake specific fuel consumption (BSFC), and NOx emission found higher side. They also tested jatropha oil methyl ester and reported improvement in fuel consumption and emissions besides that soot emission was found reduced. Aiming for improvement in fuel consumption and to achieve emission standards there have been some techniques applied like improvisation in fuel quality by little modifications in properties, a small alteration in engine design, and treatments on the exhaust. The objective of modifying fuel properties is aimed at quality mixing formation and enhancing combustion phenomena without any alteration in engine design. Improvement by the usage of different additives like antioxidants (L-Ascorbic) and oxygenated additives was used with biodiesel and resulted in decreased value for emissions. G Balaji et al. [13] used metal-based additives like magnesium-based fuel additives on CI engine outcomes. Monoxide emission and smoke were found to decrease by around 57% and by 35%, respectively. The objective of modifying or redesigning work on the engine is meant to enhance turbulence kinetic energy and which in turn leads to the engine performance along with matching emission standards. Modification in the engine includes various parameters like bowl shape, injection pressure, injection timing, and ignition delay [14–16]. Biofuel Production, its Properties, and Comparison of Biofuel with Conventional Fossil Fuel Based on the Economy Biodiesel production Through chemical reactions and processes, oil is extracted from animal fat and vegetable oils to serve as biodiesel. The feedstocks for extracting oil can be classified in four ways[17]. Transesterification, pyrolysis, direct usage and blending of oils, and micro emulsification are some of the widely adopted techniques for producing yield for biodiesel from various feedstocks [18]. Transesterification This process happens in presence of alcohol, also known as fatty acid alkyl esters (FAAE), which is an accurate method for the extraction of biodiesel. A reaction occurs in the presence of a catalyst like sodium or potassium hydroxide, between the oil containing triglyceride and methanol or ethanol. Glycerol is the end product of this process [19].

Catalysts were added to speed up the reaction. Other factors that influence the transesterification process include catalyst concentration, the intensity of the mixing process, reaction temperature, reaction time, and feedstock type [20]. Pyrolysis Pyrolysis [21] is the process of heating in the absence of oxygen with or without a catalyst to convert one material into another. Comparatively the process is simple, pollution-free, and effective. Biodegradable yield can all be pyrolyzed [22,23]. A blending of oils Biodiesel is not recommended for direct usage because it will result in coked injectors, high deposition of carbon, stuck oil ring, and lubricant thickening after a lengthy period of operation. In a compression ignition (CI) engine, however, blending extracted oils with diesel fuel can overcome the problem of thickening. Furthermore, preheating vegetable oils improves atomization and mixing whereas reduces viscosity, and results in better combustion [24]. Micro-emulsion A microemulsion is a fluid microstructure having molecular dimensions ranging from 1 to 150 nm. It is made of two normally immiscible liquids and one or more than one ionic or non-ionic substances. It is structured as any of an oil phase, an aqueous phase, and a surfactant or mixture of these three phases [25]. Characteristics of Biofuel Viscosity index, cetane number, heating value, flash point, and pour point are usually those properties that are taken into consideration while testing performance. Some researchers have indicated in the literature that the scale of fatty acids and chemical compounds of biodiesel have a measurable impact on the qualities of biofuel [26]. As a result, biodiesel must meet international standards like ASTM D6751 or EN 14214, which are the most often used specifications for biodiesel used as a CI engine fuel [27]. When the fuel has come across the proximity of flame or a spark as biodiesel’s flash point is greater than that of fossil fuel, it is safe to transport, handle, and store. Several factors influence the flash point of a substance, like OH concentration, type of bond, and quantity of carbon atoms [28]. Another key metric that affects combustion quality is the cetane index. The greater the cetane value, the shorter the ignition delays, and hence the longer the duration of combustion. Biodiesel has a greater cetane number than petrol-diesel [29]. The viscosity index of extracted oil is the most critical metric to assess because it has a direct impact on the engine’s injection system. Higher viscosity index forms big droplet sizes, improper vaporization, oil dilution, and narrow-angle of spray pattern in fuel atomization [30,31] thus resulting in poor fuel atomization, imperfect combustion, and increased pollutants [32].

The calorific value of a fuel reveals how much energy it contains. Because of higher oxygen concentration, biofuel has a low mass energy value. According to Jain et al. [25], the net shaft power and torque of a biodiesel-fuelled engine are low compared to those of a petroleum-fuelled engine due to increased density and lower heating value. Bio-oil yielded possesses good lubricity along with negligible sulfur compound. Hence, the wear of components is reduced [33]. Furthermore, the average fatty acid profile has a considerable impact on biodiesel’s physical/chemical properties [34–36]. Economic Analysis of Biofuel in Comparison with Petroleum Having a less negative effect on the environment and having better economic value studied by many researchers› biofuel is recommended as an alternative fuel in some countries. Still, one must check economic feasibility as it is one of the important concerns in long run. Economic analysis should be carried out while using biofuel as it is derived by different manufacturing techniques and from different feedstocks. Rapid advances in biofuel production will have a direct impact on hikes in food-crops prices globally, posing a threat to food storage and security, particularly in countries comes either in poverty or are developing [37]. Feedstock costs climbed considerably after the year 2000, particularly for rapeseed and soybean oil, and peaked in 2008. Due to the drop in the price of crude oil, all feedstock prices have dropped since 2009. Increases in oil prices have an impact on feedstock prices due to transportation, farming, and food distribution expenses; in addition, fertilizer prices have a direct impact on prices. The price of feedstock is seen as having a significant impact on biofuel output and cost. Many countries have established subsidies to cut biofuel prices to prove it is competitive with crude oil [38,39]. Some studies looked from an economic standpoint and evaluate biofuel for promotion in light of the policy’s numerous goals, life-cycle implications, and unexpected effects [39,40]. Many economic issues are related to biofuel production, as indicated in [41]. The cost, which accounts for 80% of the overall cost, is considered the most important economic aspect. Le LT et al. [42] conducted a detailed case study of Vietnam to assess economic and social issues related to biofuel. Comparisons on cost-effectiveness between biomass-based fuel and crude-based fuels still require more research to reach a definite conclusion, as certain studies reveal that biodiesel stability is influenced by a variety of circumstances. Biodiesel is light and temperature sensitive [43], prone to oxidation processes [32,44], naturally hygroscopic [45], and more caustic than diesel [46]. It is supposed to degrade owing to compositional changes. The storage stability of biodiesel is affected when comes to air [47,48]. Non-edible feedstocks are viable resources for producing biodiesel since they do not harm food sources and are diverse. These sources resulted in a 60–90% reduction in spending. It is having several advantages, including low pricing, daily quantities of up to millions of tonnes, and

little environmental impact. Algae is a possible biodiesel resource since the oil concentration appears to be excellent and promising. Algae claimed to produce 5 Kilo gallons per acre, compared to 1 Kilo gallons per acre for other vegetable oils [49]. Thus, biodiesel is a possible fuel that can meet both energy and environmental demands. Techniques for Using Biofuels to Improve Combustion and Emission Parameters Although biofuel has emerged as an alternative fuel for diesel, the generation of NOx, low efficiency at the shaft, and high consumption of fuel remain key concerns in unmodified diesel engines. As a result, the focus is on improving the efficiency of diesel engines by changing engine parts for running engines with biodiesel–diesel mixtures. The fluid flow field at the intake manifold entry and end, fuel fluid dynamics at the fuel injection system, combustion process and behavior, heat transfer properties, and process, etc., are the main key factors which are needed to be looked after. As combustion is a complex phenomenon and its control are even very complex and complicated with many variables. So, an effort is made to concentrate on a few key factors for improvement some of which are listed below. Pre-combustion Techniques Mixing additives Chemicals that are blended with fuels for betterment in efficiency value and fuel economy are known as fuel additives. The selection of additives compound is tested based on blending property, feasibility based on an economic scale, solubility based on ease, toxicity value, viscosity index, flash-point, and solubility index of water in the blend. All of these factors play an effective role in the additive selection process. Varieties of additives for a biofuel-run engine are used by the researcher in their research work such as metal-based additives, oxygenated additives [50], antioxidants [51], and cetane number improvers [52]. Recirculating exhaust gas for improvement in emission (EGR) This is a NOx emission reduction method that is now employed with both diesel and biodiesel [53–56]. EGR is one of the promising NOx control methods, according to many researchers [55,57,58]. EGR works by redirecting a portion of the engine’s exhaust smoke back to combustion. The in-cylinder peak temperature is thus minimized by dilution of the oxygen in the intake air stream by providing gases that are inert to combustion to act as heat absorbents. As a result, NOx generation can be dramatically decreased [58]. EGR works on mechanisms of dilution and chemical action which led to raised dissociation during the reaction [57]. As exhaust tailpipe back pressure is often having a high value compared with the intake pressure in normal suction engines, exhaust smoke enters the cylinder. The flow path which is usually in between the exhaust and intake manifolds is controlled by a throttling valve. The action is known as hot EGR when exhaust smoke is redirected to the intake manifold. The gas

leaving from EGR is cooled by a heat exchanger in modern diesel engines to permit the admission of a larger recirculated gas mass; this action is known as cooled EGR. By increasing the EGR rate, NOx emissions are reduced [59,60]. Although employing EGR in a CI engine to reduce NOx emissions is an effective technology, there are certain drawbacks, such as a rise in smoke density, HC proportion, CO proportion, fuel consumption, and reduced efficiency at the shaft unless properly tuned [61]. Injecting Water for emission control (NOx) The presence of water through injecting it in the combustion chamber leads to absorbing the heat of evaporation and thus reduces the local adiabatic flame temperature [62] because of which NOx emission considerably reduces. Injecting water at the entrance upstream of the inlet is known as fumigation. Although this technique has reduced NOx, it has a few demerits, such as considerably increasing other emissions and BSFC. Tauzia et al. [62] tested water injection on an automotive direct injection diesel engine to see how it affected ignition delay, heat release rate, and pollutants. The authors discovered that a larger water flow rate results in a longer ignition delay. It is advisable to combine water injection technology with other technologies. Emulsification (oil, water) The primary goal of the emulsification technique either through oil or through water is meant to improvise combustion efficiency while lowering NOx and other pollutants emissions. A blend of two immiscible fluids is known as an emulsion. Oil-phased emulsion, for example, aids in the uniform distribution of water droplets in the dispersed phase throughout the continuous phase of the fuel oil. The features of an emulsion are similar to those of the continuous phase. As a result, oil-phased emulsions have characteristics that are similar to those of fuel oil rather than water. Surface active agents, often known as «surfactants,» are used in the manufacturing of oil-phased emulsions to keep the emulsion›s composition. These surfactant compounds enclose the water droplets scattered throughout the continuous oil phase in an oil-phased emulsion, preventing them from coming together and coalescing. Water in the biodiesel emulsion affects the heating value of the fuel by increasing its kinematic viscosity. In comparison to B100 and diesel fuels, biodiesel emulsions also reduce PM soot fractions. Additionally, the dissociation of water can produce OH radicals, which can reduce NOx and PM emissions even more [63]. Modification in injection timing and injection pressure Injection at exact timing has a major impact on engine performance and pollutants [64]. Because of the delayed injection time, the combustion process is slowed. The concentration level of thermal NOx is mostly determined by the combustion peak temperature; when the peak temperature remains low, the NOx level will be reduced [65]. However, employing ITR has some drawbacks, including higher HC emissions, higher smoke emissions, higher fuel

consumption, lower BTE, and lower power[62]. Many studies [55,64] have found that delaying injection timing has an impact on both exhaust emissions and engine performance. Because of the lesser calorific value, this strategy hindered engine performance. The shorter ignition delays lowered NOx emissions because of the low burning rate and gradually boosted combustion temperature. In diesel engines, the injection pressure has a measurable impact on pollution control and performance. Because of the enhanced atomization, raising the value of injection pressure produces an early beginning of combustion, resulting in desirable air-fuel mixing. Modification in compression ratio on engine characteristics Change or modification in compression ratio is a critical parameter that needs to be studied. Compression ratio plays a vital role in designing any internal combustion engine. Nowadays there is variable compression ratio design available in the field given the importance of this factor only on the performance of an engine. Several researchers have conducted their studies to determine the impact of compression ratio ranges from 16-20 on the performance and emissions of CI engines. Muralidharan and Vasudevan [66] worked to know the effect of CR on VCR engines fuelled by waste cooking oil and its blends with diesel. At CR 21, the experiment showed that the B40 blend performed better than regular diesel. EL Kassaby and Nemit Allah [67] observed an increase in BTE as CR increased for waste cooking oil blended with diesel. The change in CR between 14-18 resulted in an Approximately 30% increase in BTE for the B20 blend. Sayin and Gumus [68] investigated with CRs 17-19 on diesel engines and concluded the better performance with biofuel and its blend with diesel. According to the study, brake-specific energy consumption improved with an increase in CR. Comparing it to the original CR of 18, CO and HC emissions were shown to decrease as CR increased. For diesel engines driven with a used fried oil blended with methyl ester, Hirkude and Padalkar [69] discovered that as the CR increases, the specific fuel consumption decreases, and the rise in exhaust gas temperature (EGT) is noted. Sharma and Murugan [70] used a mixture of 4/5 proportion of biodiesel and 1/5 proportion of oil extracted from waste tires to investigate the impact of altering the CR. The highest BTE resulted be roughly 7-8% greater at CR of 18.5, compared to the initial CR of 17.5 for palm oil-based biodiesel and emulsified palm oil-based biodiesel in a study carried out by Debnath et al. [71]. The BTE for palm oil-based biodiesel-driven diesel engines improved by 7-8 % for CR between 16-18. Bora et al. [72] found an effect of CR on the output of a raw biogas-fuelled hybrid diesel engine. They found that increasing CR from 16 to 18 at 100 percent load reduced BSEC by 19.38 percent. Modification by Combining two or More Techniques at a Time Applying a combination of two or more technologies may give better combustion efficiency with improved outcomes

and noticeably reduced output in terms of emission for biodiesel-fuelled CI diesel engines. These kinds of experiments for combining two techniques have been applied like emulsion process & EGR, additives & EGR [73–75], and EGR & ITR [55,76]. Saravanan et al. [77] experimented to see how a combination of 10-12 % EGR and 210–240 bar injection pressure influences diesel engines fuelled by rice branbased biofuel and found improved outcomes. Modification in Piston-bowl Geometry or Combustion Chamber Geometry Not only injection parameters and/or other variables but piston bowl shape could also have an impact on engine outcomes. When compared to an open-piston bowl form, computational fluid dynamics (CFD) simulations show improved flow field characteristics during a compression stroke [78]. As a result, the piston bowl shape has a significant impact on swirl, squish, and turbulence in diesel engines [79]. When compared to lip-shaped combustion chambers, bottom-cornered combustion chambers have been shown to have an improvement in thermal efficiency and acceptable outcomes for NOx, CO, and soot emissions [80].

Objectives

Many works of literature show that the geometry and shape of clearance volume has a significant correlation with CI engine outcomes. Due to local flow field changes, the CC geometry ensures adequate air-fuel mixing [81]. The aim of the present study is meant to examine the different geometries utilized for biofuel compatibility and their impact on CI engine performance characteristics. Parameters Influencing Combustion Chamber Design The combustion chamber design, more than any other component of the engine, influences emissions, performance, and knocking features. The type of material used for the cylinder wall, the placement of valves, and the volume of air in various components of the chamber such as the bowl, clearance, valve seat gap, and so on, all play a role in the design of the chamber.

Air turbulence Like CC geometry, which improves whirling and squish action within the CC, air turbulence also improves the mixing of air-fuel and plays a critical role in the combustion phenomenon in internal combustion [81]. The movement of air allows for greater atomization, dispersion, and mixing of fuel, resulting in less afterburning and a shorter delay period, as well as better air utilization in the cylinder [84]. Heat transfer loss through a wall Losses in the form of heat imply a reduction in the engine›s available energy, which has a direct impact on volumetric efficiency and starting issues, particularly in cold conditions in diesel engines. The pressure and temperature in the chamber reduce as an outcome of the higher rate of heat transfer from the combustion chamber wall [82,83]. Geometry/ Shape of Chamber Bowl geometry has a measurable effect on performance and emission metrics. Many academics have been working on combustion chamber geometry designs using diverse fuels as an intake for a long duration [85]. As a result, the combustion chamber shape is classified as a single curve or double curve [86]. It is also determined that a geometry that performs well may be proven to be ineffective for another geometry [84]. DIFFERENT SHAPES/TYPES OF COMBUSTION CHAMBER/PISTON BOWL GEOMETRY Direct Injection Combustion Chamber The high swirl design and the low swirl design are two different design ideas for direct injection combustion chambers. In the first scenario, the injector has fewer holes and a deep bowl, but in the second situation, the injector has more holes and a skin-deep bowl. The following are a few of the most frequent geometries [84]: Omega shape chamber

Piston head shape The primary function of the piston head is to generate momentum for turbulence, which leads to perfect combustion. Its shape varies depending on the engine; for example, a flat head is used in low-cost engines, while a raised dome, such as a wedge, domed, or recessed head, is used to boost the combustion chamber›s compression ratio [82]. Nozzle dimensions To optimize CI engine emissions and performance, the fuel injector nozzle is critical. It has essential design parameters such as hole arrangement, quantity, and size. Higher moments and better mixing conditions were induced by the nozzle [83].

Figure 1 shows the geometry of the omega shape cylinder (OCC) that is found to be more practical for attaining solid squeeze in a short amount of time [87]. At medium to high engine speeds, the OCC’s results were shown to be superior to those of a hemispherical combustion cylinder (HCC) or a shallow-profundity combustion cylinder (SCC). The effect of swirl power and turbulent force on engine behavior has revealed that using a bowl with very violent dynamics increases the capacity for effective fuel contact while reducing whirl movement [84].

creates forces for swirling during the compression into the bowl which creates a turbulent environment [82].

Square combustion chamber (SqCC) Figure 2 shows a square cavity chamber. With this geometry, powerful squish motion can be achieved especially at the corner region of the square chamber which

Shallow depth re-entrant combustion chamber [SCC] The cavity depth in SCC is kept modest. Furthermore, because the cavity diameter is big, the squish and swirl actions are minimal. However, it atomizes biodiesel more

Hemi-spherical combustion chamber [HCC] Figure 3 shows a conventional or basic design for diesel operated engine. The depth-to-diameter ratio in this combustion chamber allows adjustment to produce effective squish for improved outcomes. When operating with biodiesel as a fuel, however, it is ineffective. Because biodiesel has a more viscosity index than diesel, it cannot be atomized or burned properly in such geometry [82,84].

Figure 4. Shallow depth chamber. Figure 2. Square cavity chamber.

effectively than HCC. At low speeds, this is mostly used in huge engines [82]. Cylindrical combustion chamber (CCC) Figure 5 shows a cylindrical shape which is derived and hence a variant of a conical chamber. Here 180° turn of the complete circumference is used to create the swirl. The depth of squish can be altered [82]. Toroidal (re-entrant) combustion chamber [TCC] Figure 6 shows a toroidal shape cavity whereas figure 7 shows a toroidal re-entrant shape cavity. The re-entrant cavity is nothing but a small modification in the basic toroidal shape [88]. Because TCC has a better squish motion than other geometries, the proper current of air in the CC results in proper combustion. The cone angle of TCC varies between 145 and 165 degrees [83,84].

Wave-shaped combustion chamber A wave-shaped piston as shown in figure 8 has recently been demonstrated to increase late-cycle air mixing during diffusion combustion by efficiently diverting the nearwall jet flow back toward the chamber center. Increased turbulence in the reaction layer increases charge mixing and promotes faster and more complete combustion giving higher thermal efficiency and lower soot emissions [83]. Trapezoidal combustion chamber Figure 9 shows a trapezoidal shape cavity that gives good air-fuel mixing comparatively and is thus used in some of the research for comparison with other geometries [84].

Other Modified piston bowl geometries Novel swirling grooves have been provided and experimented with in the piston top face to inhibit the swirling motion. Dished piston, Dome piston, Mexican Hat design, and Trunk piston design are many other designs that have been tried for getting good results in some of the cases. Indirect Injection Combustion Chamber There are two parts to the indirect injection type of combustion chamber. The cylinder head contains one part, while the main cylinder contains the other. The cylinder head is injected with fuel. It produces the pressure difference during the combustion process [89]. Swirl combustion chamber Swirl combustion chambers are typically seen in tiny CI engines with cylinder diameters around 80-100 mm or less. More heat losses are found in this type of geometry [90]. Pre-combustion chamber The main chamber is connected by several holes to other parts in this combustion chamber. Fuel is injected at a pressure below 450 bar through the pintle nozzle. The main chamber holds 35-45 % of the overall capacity, and it is here that combustion takes place. Air enters the main chamber first and then flows to the secondary chamber through a hole. This results in increased turbulence in the flow field and improves fuel combustion [91].

Outcomes Of Modified Chamber Geometry On Combustion Characteristics

The effect of change in the geometries of a bowl on several CI engines (performance, and emission characteristics) is discussed in this section. Combustion Parameters Variations occurred in-cylinder pressure A highly pressurized injection is a prerequisite for finetuned atomization of biodiesel because of its high capillarity value and viscousness of veg oil [92]. The DI diesel engine’s combustion, performance, and exhaust parameters are influenced by injection pressure [83]. According to Jaichandar and Annamalai [93], an increase in BSFC for injection pressures beyond 220 bar reported due to inefficient combustion happened because of low penetration and lacking oxygen. Further, they evaluated both the Toroidal Re-entrant combustion chamber (TRCC) and hemispherical open combustion chamber (HCC) and compared results for both by varying injection pressures in the range of 185 to 230 bar using a 20% blend of Pongamia oil methyl ester (POME20). They have noted that TRCC gives improved performances when compared to HCC due to the quality of air mixing and high atomization. Puhan et al. [94] tested the effects of IPs 200-240 bar on CI engine parameters utilizing a conventional hemispherical chamber design for linseed oil methyl

ester (LOME) as fuel. The BSFC was found dropped and BTE was found marginally improved at 240 bars. Evaporative heat coefficient, quality of air-fuel mixing, and effective atomization are all factors that affect fuel combustion in diesel engines [95]. Better results were obtained by raising the rate of evaporation, improvising the quality of the A-F mixture, and atomization. A high value of injection pressure improves atomization, combustion, and smoke emissions [96]. Mamilla et al. [84] investigated three different chamber geometries of CI engines fuelled with a Jatropha methyl ester blend. At all loads, all these chambers followed the same pattern for pressure variation as diesel. Cylinder pressure for B20 JTME measured lower than diesel for all geometry. With SCC geometry, the increase in peak at 20° aTDC for B20-B100 was reported at 2.0%-7.7%. Venkateswaran and Nagarajan [97] employed a CFD code (STAR-CD) to know in-cylinder pressure at different conditions. The experimental and computational in-cylinder pressures and crank angles were plotted and found to be identical. Park [98] optimized chamber geometry and operating conditions of a CI engine fuelled by dimethyl ether using a micro-genetic algorithm. Because of the increased pressure, the optimized engine has a higher cylinder pressure. Banapurmath et al. [99] noted the impact of three different chamber geometry on the performance engine fuelled by mahua and neem blends. They found that mahua biodiesel has a greater peak pressure in TCC than neem biodiesel. It also demonstrates that when cylinder pressure rises, combustion acceleration rises dramatically as the piston approaches the BDC and converts fuel energy to work. The influence of changed geometry on engine performance, combustion, and emission characteristics was investigated by Manikalithas et al. [100]. The pressure variation at various crank angles and 100% load was tested and observed that the modified bowl piston reported high in-cylinder pressure of around 57 bar compared to 50 bar for the existing bowl geometry. Prasad et al. [79] studied the effects on engine parameters for modification in the shape of the bowls from hemispherical. They observed high values for peak pressure with modification in a bowl shape. Heat release rate The heat release rate was found better in the modified chamber geometry compared to the traditional combustion chamber fuelled with biodiesel. Biodiesel has a lower heating value, higher viscousness, and capillary due to which the heat release rate of biodiesel is low [101–105]. Mamilla et al. [84] used a customized combustion chamber to evaluate JTME blends and found that the HRR of diesel for B10B100 were 65.5-71.5 J/°CA with 60°-80° CA bTDC, they also found that HRR for B20 with TCC was 40.21 J/°CA, while SCC and RCC were 38 J/°CA and 36 J/°CA, respectively, with changed geometry. According to Lalvani et al. [106], at crank angles between 6°-10° bTDC and injection pressures between 180-230 bar the maximum values for HRR are observed between 68-85 J/°CA. With half load, Li et al. [107] examined the heat escape rate for engine

speeds between 1200-3600 rpm and found peak HRR at (54 J/°CA) 12°CA aTDC for square geometry at 1200 rpm. With average load, HRR drops to 25 J/°CA from 54 J/°CA at 2400 rpm and 3600 rpm. According to Park et al. [98], the heat release factor in the pre-mixed event of combustion was found high for diesel engines than for dimethyl ether engines because the diesel engine started combustion earlier, resulting in proper mixing of air-fuel in the chamber. Banapurmath et al. [99] found that TCC has a greater HRR than CCC and TrCC geometry. Heat loss and depth of the piston bowl directly vary with each other, according to Rakopoulos et al. [108], with values of 0.7 J/°CA and 0.6 J/°CA for d/D around 64 percent, 0.80 J/°CA and 0.67 J/°CA for d/D around 54 percent, 0.8 J/°CA and 0.68 J/°CA for d/D around 44 percent found in CFD models. Manikalithas et al. [100] studied that due to the good quality of atomization and vaporization of diesel, as well as improved swirl and turbulence conditions. Better mixing gives complete combustion with modified geometry which in turn gives a higher HRR than the existing bowl piston. Ignition delay Ignition delay is a very crucial factor that influences the knocking tendency in diesel engines. The ignition latency is reduced when the cetane number increases. Experiments have demonstrated that as blending increases, atomization quality degrades, and the mean droplet width & break up time of sprayed fuel increases. Moreover, several types of research conducted on the delay period and behavior of the combustion process for biodiesel-fuelled engines, as well as other variable parameters like engine load, compression ratio, pressure& timing of the injection process, and so on. High cetane rating and low calorific value of biofuel produce shorter delays and lower HRR [109]. Mamilla et al. [84] experimented and noted a reduced delay period at full load for a different blend. They found a 1.9 %-9.2% reduction for B20-B100 fuel. Due to the greater cetane number, Lalvani et al. [106] found that the delay period of the conventional piston found more while comparing to the turbulence inducer piston. Furthermore, Banapurmath et al. [99] found shorter delay while increasing brake power for all geometries (TCC, HCC, CCC, and TrCC) because more quantity of fuel is burned, resulting in a rise in the wall temperature of the cylinder. Because of the higher combustion temperature and fuel-air mixing, TCC had a lower ID than other combustion chamber shapes. Because of the greater cetane number of biodiesel blends, Jaichandar and Annamalai [110] evaluated 20 percent POME in engines with different chamber geometries and found delay duration of 20 percent POME was much lower than in an engine fuelled with diesel. They also found that at all loads, the ID times for re-entrant CC were shorter than those for open-type combustion chambers. Peak pressure The peak pressure is rated from the rate of heat released and the heating capacity of the fuel in general. As biofuel

possesses a low heating value to diesel, the peak pressure recorded with any chamber is lower than diesel for biofuel [111,112]. An increase in peak pressure value with increasing brake load was reported for different fuels, during a test conducted by Mamilla et al. [84], since more fuel is burned at a high load. They also noticed a reduction in peak pressure and temperature as the exhaust gas absorbs more heat due to its greater specific heat. Jaichandar and Annamalai [16] reported that the peak pressure of B20 in a conventional engine is lower than that of diesel because of incorrect mixing of B20, high viscosity index, and a low calorific value of B20 POME. Performance Parameters Brake-specific fuel consumption The BSFC is the mass of gasoline utilized per brake power output also it noted that fuel viscosity, rise in density of fuel, and low heating value of biofuel have resulted in a rise in BSFC. Because biofuel owns a low heating value, more quantity of the charge should be pumped for combustion to get the specified power output [113]. Also, it is observed that when 20 percent mixes of palm oil biodiesel and pure biodiesel (B100) were used instead of diesel, BSFC increased by

Results And Discussion

The future of fossil fuel is very short because of almost the end of reserves, uncontrolled increase in demand leads to economic aspects and some serious environmental aspects. Biofuel extracted from nonedible seeds with and without further blending captured the attention of many researchers as an alternative to crude. Further, it has shown some promising results in terms of emission content like UBHC and CO, also found better for performance parameters like thermal efficiency and fuel consumption. The performance of biofuel as a fuel still needs to be improved in many aspects by many other techniques and this challenge has been taken by many researchers in the current scenario. Many works of literature show that the geometry and shape of clearance volume has a significant correlation with engine outcomes. Due to local flow field changes, the geometry ensures adequate air-fuel mixing. The combustion chamber design, more than any other component of the engine, influences emissions, performance, and knocking features. Bowl geometry has a measurable effect on performance and emission metrics. Many academics have been working on combustion chamber geometry designs using diverse fuels as an intake for a long duration. Table 1 and Table 2 shows a summary of the experimental and

numerical work of some researchers in which the influence of change in geometry considered along with engine control parameter and outcomes are listed accordingly.

Table 1. Summary of experimental work Literature Studied Fuel used

Result

20% Jatropha DI engine- 4S, Single cylinder, Spherical, Re-entrant, methyl esters Bore-87.5, Stroke-110 mm, CRand Toroidal 17.5:1, Rated Power-4.4 kW, Tested with different blends (20%-80%)

Toroidal geometry was found better with minimum smoke density, HC, and CO2 emissions and a better value of efficiency (33.92%) compared to the other two. NOX emission is more which was a demerit of using toroidal geometry

At 220 bar IP, the turbulent inducer piston produces the best results. 34.3 percent thermal efficiency was reported. Emission products were found lower, except for NOX value.

DI engine- 4S, Single cylinder, Bore-87.5, Stroke-110 mm, CR-17.5:1, Rated Power-5.2 kW, Injection pressure varied between 185 to 230 bar.

DI engine- 4S, Single cylinder, Hemispherical and Bore-87.5, Stroke-110 mm, Cylindrical Injection pressure set to 205 bar for all fuels used in testing

Toroidal geometry results in increases in BTE while decreasing the emission level

DI engine- 4S, Single cylinder, Bore-87.5, Stroke-110 mm, Rated Power-4.4 kW, Tested engine for low, medium, and high loads

At full load, both geometries reported the best results showing an increase in BTE while reducing emissions.

DI engine- 4S, Six Re-entrant cylinder, Bore108mm, Stroke-118 mm, CR-17.5:1, Rated Power-118 kW, N-2400 rpm

By keeping the Compression ratio, Maximum diameter of the bowl, Squish clearance, and Injection rate fixed

Re-entrant type geometry creates more swirls compared to baseline geometry. As a result, it reports high thermal efficiency and lower emission levels of CO, UBHC, and NOX.

DI engine- 4S, Single cylinder, Bore-85, Stroke-82 mm, CR-17.5:1, Rated Power-5.2 kW, Single injection

The diameter-to-depth ratio of 55% and an initial swirl ratio of 2.5 gives optimal results in terms of performance and emissions.

DI engine- 4S, Single cylinder, Bore-135, Stroke-150 mm, CR-17.5:1, Rated Power-14.7 kW

The swirl ratio varies from 0.2 to 3.2. The flow of air is found best with a swirl ratio of 0.8. At 0.8, the mass fraction of NO and Soot decreases.

DI engine- 4S, Single cylinder, Bore-85, Stroke-82 mm, CR-17.5:1, Rated Power-5.2 kW, Single injection

d/D varied from 44% to 64% and engine speed varied from 1500 to 2500 rpm

Maximum heat release value increases with decreases with a ratio of d/D %.

DI engine- 4S, Single cylinder, Bore-102, Stroke-116 mm, CR-16.3:1, Rated Power-7.5 kW

Swirl number varied For better swirl and lower between 1 to 3.5 emissions, Re-entrant geometry without central projection was found best.

Conclusion

The present study reviews valid reasons for accepting bio-fuels as fuel for CI engines by considering outcomes derived from experiments and numerical analysis with changes in the shape of the piston bowl. Some important outcomes are listed below. Modifying or redesigning work on the engine is meant to enhance turbulence kinetic energy and which in turn leads to the engine performance along with matching emission standards. Better performance from an engine could be obtained by working on variations in injection, combustion chamber geometry, and spray characteristics. The presence of effective flow fields in the combustion phenomenon widely depends on flow parameters like swirl, squish, and tumble. Change in bowl shape shown improvement in many aspects some of which are listed: A re-entrant piston bowl is the best for swirling during the compression stroke. TRCC found the best geometry for air-fuel mixing due to improved airflow.

nm NOx OCC OH PM POME PPM rpm rps SCC SO2 SqCC TCC TDC TRCC UBHC VCR W

Nanometer Oxides of Nitrogen Omega Shape Combustion Chamber Hydroxide Particulate Matter Pongamia Oil Methyl Ester Parts Per Million Revolution per Minute Revolution per Second Shallow Depth Combustion Chamber Sulfur Dioxide Square (Cavity) Combustion Chamber Toroidal Combustion Chamber Top Dead Center Toroidal Re-entrant Combustion Chamber Unburnt Hydrocarbon Variable Compression Ratio Watt

Nomenclature

4-S ASTM aTDC B 100 B 20 BSFC bTDC BTE CA CC CCC CFD CI CO CR DI EBP EGR EGT EN FAAE HC HCC HRR IC ID IP ITR JTME kW LOME MBP

4-Stroke American Society for Testing and Materials After Top Dead Center Blend 100 Blend 20 (80% Diesel) Brake-Specific Fuel Consumption Before Top Dead Center Brake Thermal Efficiency Crank Angle Combustion Chamber Cylindrical Combustion Chamber Computational Fluid Dynamics Compressed Ignition Carbon Monoxide Compression Ratio Direct Injection Existing Bowl Piston Exhaust Gas Recovery Exhaust Gas Temperature European Standard Fatty Acid Alkyl Esters Hydro Carbon Hemispherical Depth Combustion Chamber Heat Release Rate Internal Combustion Ignition Delay Injection Pressure Ignition Time Retardation Jatropha Methyl Ester Kilowatt Linseed Oil Methyl Ester Modified Bowl Piston

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

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