A review on performance of biogas and hydrogen on diesel engine in dual fuel mode

Fuel 260 (2020) 116337

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Review article

A review on performance of biogas and hydrogen on diesel engine in dual fuel mode

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Chinmay Deheri, Saroj Kumar Acharya , Dhirendra Nath Thatoi, Ambica Prasad Mohanty Dept. of Mechanical Engineering, ITER, SOA Deemed to be University, Odisha, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Review Biogas Hydrogen Performance Diesel engine

The growing concern of energy demand and environmental pollution using fossil fuel influences the requirement of alternative fuel for a clean and healthy environment. Numerous research works have been carried out to use alternative fuels in order to optimize the energy requirement. This review presents an in-depth analysis on the impact of alternative fuel in compression ignition (CI) engine in order to develop performance, emission and combustion characteristics. Deterioration in the performance parameters such as brake thermal efficiency (BTE) and exhaust gas temperature (EGT) has been found between 2 and 22% whereas a significant increase in brake specific fuel consumption (BSFC) was reported up to 36% by using gaseous fuel as an alternative source of energy in dual fuel engine. Further the analysis on combustion indicates increase in peak cylinder pressure and heat release rate up to 23% and 30% respectively. Emission analysis shows reduction in nitrogen oxides and smoke emission between 20 and 60%. However the dual fuel engine shows a significant increase in hydrocarbon (HC) and carbon monoxide (CO) emission up to 30% when compared with normal diesel engine.

1. Introduction With the advancement in technologies and modern practices of increasing economy, though the economy and technology have increased but it has created a serious threat to the environment & have raised a big question mark towards developing a healthy & eco-friendly environment. The renewable energy sources such as biomass, geothermal, solar, wind & hydro-wave offers a wide range of attractive prospects. They are unlimited and are of economically viable as compared to other sources. Energy is the basic need for sustainability and development of society. Fuel contains a large amount of energy that is being utilized for our daily life. Conservation of conventional fuel is increasing day by day which is a serious problem especially for a developing country like India and it is expected that a day will come when the requirement of fuel energy will be more than supply. With the increasing demand and growth of power sectors all over the world the conventional sources are decaying at an increasing rate [1]. This is one of the reasons that the researchers are focusing on alternative fuels. Another reason that the world is being tilting towards alternative fuels is environmental pollution. Air pollution and global warming are the major concern to go for alternative fuels. Main sources of air pollution and global warming are oxides of nitrogen, particulate matters, carbon dioxide etc [2]. These

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polluting agents mainly resulted from the combustion of conventional fossil fuels. A large percentage of engines in transportation as well as agricultural sectors are driven by the conventional fossil fuels. The outcomes of engines operated with fossil fuels are serious threat to the environment. This is why engines driven by renewable sources like biogas and hydrogen are getting more attention now a day. Conventional engines can be switched to be operated with biogas or hydrogen along with pilot diesel fuel with very less modification. Schematic diagram of the engines driven by biogas and hydrogen in dual fuel mode are shown in Figs. 1 and 2. Alternative fuels are produced from renewable sources widely available in the environment itself. These are the fuels which can be used as replacement of conventional fossil fuels. These include biofuels, alcohol, natural gas, liquefied gas, biogas, hydrogen, etc [3]. These fuels exhibit a lot of positive impact towards the control of environmental pollution as well as global warming [4]. Sources of these alternative fuels include biological, agricultural, organic, inorganic, waste etc. These renewable sources being waste are used to produce alternative fuels which play a significant role in the environmental and economic growth of society [5]. Biomass is made up of about a large variety of sources which is available in enormous quantity and it is the leading contributor to the

Corresponding author at: Dept. of Mechanical Engineering, ITER, SOA Deemed to be University, Bhubaneswar, Odisha 751030, India. E-mail address: [email protected] (S.K. Acharya).

https://doi.org/10.1016/j.fuel.2019.116337 Received 14 May 2019; Received in revised form 13 September 2019; Accepted 2 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature CI EGT BSFC BP BSEC NOx VE BGES RCCI VCR CR PCCI IT UHC CPP EGR HRR IMEP CNG MO

HCCI BTE HC CO SFC CO2 NA KME DEE DI ID NHRR PCP THC CH4 CD H2 BTDC PM DME ND WI

Compression Ignition Exhaust Gas temperature Brake Specific Fuel Consumption Brake power Brake Specific Energy Consumption Nitrogen Oxide Volumetric Efficiency Biogas Energy Share Reactivity Control Compression Ignition Variable Compression Ratio Compression Ratio Premixed Charge Compression Ignition Injection Timing Unburned Hydrocarbon Cylinder Peak Pressure Exhaust Gas Recirculation Heat Release Rate Indicated Mean Effective Pressure Compressed Natural Gas Maduca longifolia oil

Homogeneous Charge Compression Ignition Brake Thermal Efficiency Hydrocarbon Carbon Monoxide Specific Fuel Consumption Carbon Dioxide Naturally Aspirated Karanja Methyl Ester Diethyl Ether Direct Injection Ignition Delay Net Heat Release Rate Peak Cylinder Pressure Total Hydrocarbon Methane Combustion Duration Hydrogen Before Top Dead Center Particulate Matter Dimethyl Ether Neat diesel Water injection

Fig. 1. Schematic diagram of biogas-diesel dual fuel engine.

2. Performance analysis

renewable energy. The biomass is considered as a worldwide valuable and most effective alternative to fossil fuels. Large quantities of cellulosic biomass, such as food wastes, rice husks, agricultural residues like straws, fruit shells, fodders & nut shells, leaves of green plants, and molasses are being disposed to the environment utilized. The various food wastes produced from different households, hotels and other sources also contribute a potential source for production of biogas. The present review focuses on the utilization of biogas and hydrogen as alternative fuel in CI engine. Utilization includes the optimized performance and low emission characteristics of CI engines by the use of biogas and hydrogen as alternative fuels.

Performance analysis of an engine includes the utilization of fuel energy to produce the required power. This chapter includes the analysis of various operating parameters such as brake thermal efficiency (BTE), brake power (BP), Specific fuel consumption (SFC), brake specific energy consumption (BSEC), Exhaust gas temperature (EGT). Characteristics of compression ignition engine observed by different researchers are presented in Table 1. 2.1. BTE It is explained as the power produced from the engine by utilizing 2

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Fig. 2. Schematic diagram of hydrogen-diesel dual fuel engine.

injection timing because of improved combustion and reduced combustion duration caused by the homogeneous mixture of air-biogas [27–29]. Bora et al. [30] observed that increasing the compression ratio results higher BTE due to increase in temperature and pressure. Preheating the inlet charge also results higher BTE due to improved calorific value that results in improved combustion [31,32]. Feroskhan et al. [33] observed higher BTE by adding cerium oxide in biogas-diesel dual fuel mode due to the increase in energy release by the presence of oxygen. Cacua et al. [34] observed higher BTE by enhancing oxygen in biogas diesel dual fuel engine due to increased flame propagation caused by higher oxidation of fuel in the cylinder. Brake thermal efficiency was found to be increased by using hydrogen as an alternative fuel due to better mixing of hydrogen and air that results in faster burning and higher flame propagation but excess supply of hydrogen leads to drop in BTE due to knocking combustion caused by the high pressure rise during combustion [35–38].

heat energy available in the fuel. It is the measure of the engine’s performance in terms of conversion of heat energy into mechanical energy. When the engine is supplied with 90% methane and 10% hydrogen along with diesel, BTE was observed to be lower compared to neat diesel fuel. The reduction of BTE was found to be more than 23% at low engine speed, which was improved to 15.45% at higher speed [6]. It was further observed that as the gaseous fuel replacement increased reduction in BTE also increases as shown in Fig. 3 which can be improved by increasing the percentage of hydrogen in the supplied fuel. Rahman and Ramesh [7] observed lower BTE with increase in BGES of biogas. Variation of methane proportion has negligible effect on heat release rate which results in less variation in BTE at low BGES. Brake thermal efficiency was found to be less in low BGES due to decelerated combustion which is observed to be increase above BGES of 60% as shown in Fig. 4. BTE was observed to be decreased in biogas diesel dual fuel engine with the increase in BGES as reported by the authors. Kalsi and Subramanian [8], Nathan et al. [9] found that BTE dropped significantly at low methane proportion and high BGES due to decrease in heat release rate resulted from poor burning and high ignition delay. Prabhu et al. [10], Karagoz et al. [11] observed lower BTE with increase in methane content at full load conditions in dual fuel engine due to incomplete combustion caused by the reduced air supply during suction. Similar results were obtained by many other researchers [12–16] due to low or poor fuel utilization and slower flame propagation resulted from low combustion chamber temperature at all load conditions. Decrease in BTE in biogas-biodiesel dual fuel mode was also found by Barik et al. [17], Yoon and Lee [18] due to incomplete combustion of mixture caused by reduction in volumetric efficiency and low flame propagation. However it was found to be increased at full load because of better combustion at elevated temperature as reported by some authors [19–21]. BTE was also found to be affected by the ratio of fuel and air as Sarkar and Saha [22] observed a decrease in BTE with increase in global fuel air equivalence ratio in a biogas- diesel dual fuel engine due to reduced flame propagation caused by the high flow of biogas which have low heating value that reduce the in-cylinder temperature. Higher BTE in dual fuel mode compared to diesel mode was reported by some authors due to homogeneous mixture of biogas and air at inlet [23–26]. It was also found to be increased with advanced

2.1.1. Summary BTE decreases with high flow rate of biogas i.e. with increase in BGES. It was found to be low at partial load but increases at full load. Also the increase in global fuel air equivalence ratio has a negative impact on BTE of the engine which can be improved by charge preheating. BTE was found to be higher by using diethyl ether in HCCI mode in biogas diesel dual fuel mode. Further it has been increased by advancing the injection timing. It is found by the authors that increasing the compression ratio to a certain limit enhances BTE in dual fuel engine. It is also found from the literature that BTE can also be enhance by adopting some techniques like cerium oxide addition, turbo charging and oxygen enrichment in dual fuel engine that uses biogas and diesel. Nevertheless preheating of the inlet charge is also found to be an effective method to improve BTE of biogas and diesel dual fueled engine. Another method to increase BTE adopted by some authors is to use hydrogen as the supplement agent for biogas or biodiesel dual fuel engine. BTE can be increased significantly by supplying hydrogen up to a certain limit with pilot fuel in dual fuel engines. 2.2. Brake power It is defined as the power available at the output shaft of the engine after some power being lost by friction, gears, lubrication etc. Brake 3

4

Diesel

Diesel

Modified diesel

Feroshkan and Ismail [20]

Feroshkan et al. [32]

Feroshkan et al. [33]

Diesel

Bora and Saha [43]

Diesel-Biodiesel Blend

Diesel KME

Barik et al. [17]

Chintala and Subramanian [58]

Diesel KME DEE

Barik and Murugan [27]

Diesel

Diesel KME

Barik and Murugan [40]

Bora and Saha [13]

Diesel

Barik and Murugan [16]

Diesel Biodiesel

Diesel

Barik and Murugan [41]

Bora and Saha [15]

Diesel

Ambarita [26]

Diesel Biodiesel

Diesel

Aklouche et al. [25]

Bora and Saha [14]

Pilot fuel

Name of the researcher

Biogas

Biogas

Biogas

Hydrogen

Biogas

Biogas

Biogas

Biogas

Single cylinder, 4-stroke, NA, water cooled, CI engine

Single cylinder, 4-stroke, NA, water cooled, CI engine

Single cylinder, 4-stroke, NA, water cooled, CI engine

Kirloskar Single cylinder,4-stroke, NA, Diesel engine

Single cylinder, 4-stroke, DI, NA, Water cooled, VCR Diesel engine

Single cylinder, 4-stroke, DI, NA, Water cooled, VCR Diesel engine

Single cylinder, 4-stroke, DI, NA, Water cooled, VCR Diesel engine

Single cylinder, 4-stroke, DI, NA, Water cooled, VCR Diesel engine

Kirloskar TAF 1, Single cylinder, 4-stroke, NA, Air cooled, CI engine

Kirloskar TAF 1, Single cylinder, 4-stroke, NA, Air cooled, CI engine

Biogas

Biogas

Kirloskar TAF 1, Single cylinder, 4-stroke, NA, Air cooled, CI engine

Kirloskar TAF 1, Single cylinder,4-stroke, NA, Air cooled, CI engine Kirloskar TAF 1, Single cylinder,4-stroke, NA, Air cooled, CI engine

Lister-Petter, Single cylinder,4-stroke, NA, Air cooled, Diesel engine Single cylinder,4-stroke, Water cooled, Diesel engine

Engine used

Biogas

Biogas

Biogas

Biogas

Biogas

Gaseous fuel

Table 1 Characteristics of CI engine operated on dual fuel mode.

–

–

Decreased ID with higher CR Increased NHRR with higher CR Increased PCP with higher CR Increased ID with advanced IT Increased NHRR with advanced IT Increased PCP with advanced IT Reduced ignition delay Reduced combustion duration Higher peak cylinder pressure Higher heat release rate –

Longer ignition delay Lower heat release rate Lower peak cylinder pressure

Higer in-cylinder pressure Higher heat release rate Lower ignition delay Reduced combustion duration Higer peak cylinder pressure Higher heat release rate Longer ignition delay Increased combustion duration Higher peak cylinder pressure Higher heat release rate

Higer peak cylinder pressure Higher heat release rate Longer ignition delay Increased combustion duration –

–

Lower peak cylinder pressure Longer ignition delay –

Combustion

Engine characteristics

Lower BTE Higher BSEC Lower VE Higher EGT Improved BTE Higher BSFC Lower VE Lower EGT

–

Lower BSFC Higher BTE Lower EGT Lower BTE Higher BSEC Lower VE Higher EGT Increased BTE with higher CR Reduced BSEC with higher CR Increased VE with higher CR Reduced EGT with higher CR Increased BTE with advanced IT Reduced EGT with advanced IT

Higher power output Higher BTE at low load Higher SFC Higher BSEC Lower EGT Higher BSFC Lower BTE Lower VE Lower EGT Higher BSEC Lower EGT Lower BGES Lower BSFC Higher BTE Lower VE Higher EGT Higher BSFC Lower BTE Lower EGT

Higher BTE

Performance

–

–

–

(continued on next page)

Lower NOx emission Higher HC, CO and smoke emission

Higher HC, CO2 and NOx emission with advanced IT Lower CO emission with advanced IT

Higher NOx and CO2 emission with higher CR Lower HC, CO emission with higher CR

Higher HC, CO and CO2 emission Lower NOx emission

–

Lower HC, CO, NOx and smoke emission

Lower HC, CO and smoke emission Higher NOx emission

Higher HC,CO emission Lower NOx, smoke emission

Higher HC,CO emission Lower NO, smoke emission Higher HC, CO emission Lower NOx, CO2, smoke emission

Higher HC and CO emission

Reduced HC,CO,CO2,NOx and smoke emission

Emission

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5

Diesel

Diesel

Diesel

Diesel

Lounici et al. [21]

Mahla et al. [12]

Makareviciene et al. [56]

Mustafi et al. [39]

Biogas

Biogas

Biogas

Biogas Natural gas

Biogas

Biogas

Biogas

Methane Hydrogen Hydrogen

Biogas

Biogas

Biomethane

Gaseous fuel

Single cylinder, 4-stroke, NA, water cooled, DI VCR CI engine

Single cylinder, 4-stroke, NA, water cooled, Diesel engine Single cylinder, 4-stroke, NA, water cooled, Diesel engine

Lister Peter single cylinder, 4-stroke, DI, water cooled, Diesel engine

Lister-Petter Singlr cylinder, 4-stroke, NA, DI, Air cooled, CI engine Kirloskar single cylinder, 4-stroke, NA, DI, Diesel engine 4-cylinder, 4-stroke, DI, Diesel engine

Kirloskar single cylinder, 4-stroke, air cooled, Diesel engine Twin cylinder, 4-stroke, NA, water cooled, PCCI engine Single cylinder, 4-stroke, NA, water cooled, Diesel engine Kirloskar single cylinder, 4-stroke, NA, water cooled, Diesel engine

Single cylinder, 4-stroke, Water cooled, VCR Diesel engine

Engine used

Increased ID Reduced CPP Reduced NHRR Higher CD

Higher in-cylinder pressure Higher heat release rate

Higher Cylinder pressure Higher heat release rate Longer ignition delay Lower heat release rate

–

–

Higher heat release rate Lower BGES Higher in-cylinder pressure Higher heat release rate Higher peak cylinder pressure Increased ignition delay Reduced combustion duration Higher heat release rate –

Lower cylinder pressure Lower rate of pressure rise Lower heat release rate –

Combustion

Engine characteristics

Higher BP Lower BTE Higher BSFC Higher BGES Lower BTE

Lower BTE

Higher BTE Higher BSFC Higher BSEC Lower BTE Higher BTE Lower BSFC Higher SFC

Increased BTE Increased BSFC Increased BTE Reduced EGT

Reduced CO, HC and NOx emission Increased CO2 emission

Lower NOx and smoke emission Higher CH4 emission Higher CO, THC and NOx emission Lower smoke emission

Lower NOx and soot emission Higher HC and CO emission Lower NOx and smoke emission Higher HC and CO emission Lower HC, CO and smoke emission Higher NOx emission Higher UHC, CO emission Lower NOx emission

Higher CO and THC emission Lower NOx and smoke emission Reduced NOx and smoke emission Increased HC and CO emission

Lower HC, CO, NOx and smoke emission

–

–

Higher BTE Reduced BSEC No effect on VE and EGT –

Higher BSFC Higher BTE Higher BTE

Emission

Performance

NA: Naturally aspirated, DI: Direct injection, VCR: Variable compression ratio, PCCI: Premixed charge compression ignition, ID: Ignition delay, CR: Compression ratio, NHRR: Net heat release rate, PCP: Peak cylinder pressure, IT: Injection timing, BGES: Biogas energy share, CPP: Cylinder peak pressure, CD: Combustion duration, BTE: Brake thermal efficiency, SFC: Specific fuel consumption, BSEC: Brake specific energy consumption, EGT: Exhaust gas temperature, BSFC: Brake specific fuel consumption, VE: Volumetric efficiency, BP: Brake power, HC: Hydrocarbon, CO: Carbon monoxide, CO2: Carbon dioxide, NOx: Nitrogen oxide, THC: Total hydrocarbon, UHC: Unburned hydrocarbon, CH4: Methane.

Diesel

Biodiesel

Kumar et al. [37]

Sarkar and Saha [22]

Diesel

Karagoz et al. [11]

Diesel

Diesel

Ibrahim et al. [57]

Sandalci et al. [6]

Diesel

Girish et al. [23]

Diesel

Diesel

Gaikwad et al. [50]

Rahman and Ramesh [7]

Pilot fuel

Name of the researcher

Table 1 (continued)

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methane mixture) diesel dual fuel engine due to higher heating value of methane and hydrogen. 2.2.1. Summary It is noticed from the literature survey that brake power of dual fuel engine is identical to diesel engine and it is further enhanced by increasing engine speed and biogas flow into the cylinder. Introduction of hydrogen along with methane into the cylinder also leads to increase brake power of the engine due to higher heating value of the gas. 2.3. Brake specific fuel consumption Specific fuel consumption is the measure of engine’s ability to produce power by utilizing the energy contain of the fuel. The specific fuel consumption of an engine should be minimized for the desired power output in order to optimize the engine performance. Brake specific fuel consumption is highly dependent on fuel particle diffusivity or formation of homogenous charge inside the engine cylinder. It is also being regulated by the heating value of the fuel. It was observed that supply of gaseous fuel leads to increase in fuel consumption and the possible reason may be due to the formation of homogenous mixture and improved combustion. As shown in Fig. 6, the increase in fuel consumption is maximum compared to neat diesel fuel at high engine speed which is further found to be increased with high gaseous fuel replacement [6]. Brake specific fuel consumption was found to be high at low load of the engine. Further it was found to be higher at high compression ratio during dual fuel operation [30]. This is due to the poor burning efficiency of gaseous fuel. With increase in load BSFC was found to be decreased due to improved combustion. At full load condition it was found to be almost identical at all compression ratios as shown in Fig. 7. Generally specific fuel consumption of dual fuel mode using biogas is found to be increased compared to diesel mode. It was found that biogas operated engine consume more fuel with decrease in load compared to diesel engine due to the low calorific value of biogas and presence of CO2 in biogas which slow down the combustion process [16,23,26,29,39]. Feroskhan et al. [32] observed higher BSFC in a biogas diesel dual fuel engine by preheating the charge due to higher biogas flow rate into the cylinder. Similar observation for biogas-biodiesel dual fuel mode was found by the authors [17,18] due to slower combustion in premixed phase. Karagoz et al. [11] also observed higher BSFC with increase in hydrogen and methane mixture flow in a dual

Fig. 3. Variation of brake thermal efficiency with engine speed [6].

Fig. 4. Variation of BTE with different proportions of CH4 in biogas [7].

power plays an important role in determining engine performance as it the power used for mechanical work. Brake power of the engine was observed to be increased with increase in gaseous fuel (hythane) replacement at all speed of the engine as shown in Fig. 5. At higher speed of the engine the maximum increase of brake power was found to be 14.25% with hythane replacement of 50% compared to neat diesel fuel. It may be due to the high heat value contain of hythane which is supplied into the cylinder along with the pilot diesel fuel [6]. Girish et al. [23] found brake power of the dual fuel engine increases with load and there is no significant difference from diesel engine. Similar observation was found by Ambarita [26] that power output of the engine operating on dual fuel mode increases with engine speed due to increase in energy input to the engine as increase in biogas flow rate reduce the diesel flow into the cylinder. Further it was increased by increasing the fuel flow as Prabhu et al. [10] observed higher brake power with increase in methane content in dual fuel engine due to higher percentage of methane. Also it was observed by Sandalci et al. [6] that higher power output in hythane (hydrogen and

Fig. 5. Variation of brake power with engine speed [6]. 6

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Fig. 8. Variation of brake specific energy consumption with engine load [12]. Fig. 6. Variation of brake specific fuel consumption with engine speed [6].

injection timing, equivalence ratio and use of DEE with fuel it can be reduced to a significant level. Further it can be reduced by preheating the fuel but preheating with EGR leads to increase in biodiesel fueled dual fuel engine. 2.4. Brake specific energy consumption It is the ratio between the energy released by burning the fuel to the power produced by the engine. It is the measure of engine performance on the basis of energy utilization. Brake specific energy consumption was found to be increased with fuel flow rate in biogas-diesel dual fuel engine [12]. It can be observed from Fig. 8, with higher engine load energy consumption decreases at all flow rates. The possible reason for the variation in BSEC may be due to the formation of lean fuel-air mixture with higher flow rate which results in lower combustion chamber temperature inside the engine cylinder. Brake specific fuel consumption was found to be lower in diesel fuel mode at low loads compared to dual fuel mode, but it was reduced significantly with higher load as shown in Fig. 9. It was found to be very close to diesel fuel mode of operation with increment in methane percentage in the supplied biogas [44]. The possible reason may be the higher combustion rate due to the presence of methane and less CO2 in

Fig. 7. Variation of brake specific fuel consumption with engine load [30].

fuel engine. Specific fuel consumption may be reduced by adopting different technique as reported by some authors. Barik and Murugan [27] observed reduced BSFC in dual fuel mode using DEE with advanced injection timing because DEE supply more energy and improve combustion inside the cylinder. Aklouche et al. [25] observed decrease in energy specific fuel consumption with increase in equivalence ratio in dual fuel mode due to complete combustion of charge inside the cylinder. Mekonen and Sahoo [31] observed reduction in BSFC by preheating the fuel in a biodiesel blended dual fuel engine due to improved mixing of air and fuel at higher temperature. However it increases with EGR due to the replacement of oxygen with CO2 in the cylinder during combustion. 2.3.1. Summary It is clear from the literature survey that specific fuel consumption of biogas fueled engine is higher compared to diesel fuel at all load conditions. Though the difference is less at higher loading condition but still it is quite higher at low and intermediate loads. Presence of methane leads to higher fuel consumption due to high energy consumption. However using different technique such as advancement of

Fig. 9. Variation of brake specific energy consumption with engine load [44]. 7

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[27,29]. Mekonen and Sahoo [31] observed higher EGT by preheating the fuel in a biodiesel blended dual fuel engine due to earlier burning caused by higher cetane number and high oxygen content of biodiesel. Rahman et al. [35] observed higher EGT in H2-biodiesel dual fuel mode compared to only diesel mode due to rise in combustion temperature caused by better combustion of fuel.

biogas. Feroskhan and Ismail [20] observed lower BSEC in dual fuel mode compared to diesel mode at full load due to improved combustion by enriching biogas with methane. It is being further lowered by adding cerium oxide in biogas as Feroskhan et al. [33] observed lower BSEC in biogas-diesel dual fuel mode due to improved combustion by the extra energy released from cerium oxide. Also it is observed by Bora et al. [30] that lower BSEC at higher CR in dual fuel mode due to better combustion of biogas at high CR. Bora and saha [14], Barik and Murugan [40], Barik and Murugan [41] observed higher BSEC in dual fuel operation due to lower volumetric efficiency and presence of CO2 in biogas which reduce the burning velocity. Rahman et al. [35] observed higher BSEC in H2-biodiesel dual fuel mode compared to only diesel mode at low load due to lower heating value and higher viscosity of biodiesel which was reduced by the increase of hydrogen flow because of its higher heating value.

2.5.1. Summary It is found from the literature that EGT of biogas fueled engine is comparatively low due to presence of carbon dioxide which absorbs some heat during combustion because of its higher specific heat. It also reported that enrichment of methane or hydrogen in the supply fuel leads to decrease in EGT due to improved combustion. Further it is found that biodiesel used dual fuel engine produce lower EGT due to delayed combustion in which the fuel get enough time to burn completely. Increasing the compression ratio can also results low EGT due to improved burning. In order to increase the engine performance some of the authors implemented various technique such as advancement of injection timing and use of DEE or cerium oxide supplemented fuel in which they ended up with a higher EGT due to high in-cylinder temperature and flame velocity. Preheating the inlet charge also leads to high EGT which may be compensated by using EGR to a limited proportion into the engine cylinder.

2.4.1. Summary Brake specific energy consumption of biogas fueled dual fuel engine is found to be lower at low load due to poor fuel utilization, but with increase in load it is found to be increased which can be reduced by enriching biogas with methane. It is being effectively decreased by adding cerium oxide with biogas as well as by increasing the compression ratio. It is further reported by some authors that BSEC can be reduced by increasing hydrogen flow rate into the cylinder of a biodiesel fueled dual fuel engine in which it is primarily found to be higher due to low burning velocity.

3. Combustion analysis Engine performance and emission are highly dependent on the combustion of fuel inside the cylinder. Combustion characteristic of the charge in the cylinder changes by introducing gaseous fuel along with pilot diesel fuel due to difference in various parameters and components which takes part in the combustion process. The effect of various combustion characteristic such as in- cylinder pressure, ignition delay, heat release rate, combustion duration was analyzed in the following sections.

2.5. Exhaust gas temperature Exhaust gas temperature of engine signifies the utilization of heat energy available in the fuel. Higher exhaust gas temperature represents poor thermal efficiency of the engine. It is resulted by the incomplete combustion of the fuel so that the heat is not being properly utilized in the combustion process remains in the exhaust gas causing rise in the temperature. In order to enhance the engine performance exhaust gas temperature should be as minimum as possible. Exhaust gas temperature was found to be increased with increase in load for all blends of fuel using hydrogen and biogas along with diesel and biodiesel in dual fuel engine with and without exhaust gas recirculation [35]. Highest EGT was observed to be 390 °C while supplying hydrogen along with biogas in dual fuel mode of engine operation. EGT was increased due to improved combustion caused by the high flame speed of hydrogen. However it was found to be decreased with exhaust gas recirculation as shown in Fig. 10, due to deteriorate combustion rate and less availability of oxygen in the supplied charge. Exhaust gas temperature was observed to be lower with increase in methane content in dual fuel engine due to the reduction of CO2 in biogas as the concentration of methane increases [10,16,17]. Verma et al. [42] observed lower EGT with EGR in biogas diesel dual fuel engine due to low charge temperature after expansion caused by lower combustion temperature. It was also found to be reduced with high compression ratio because of the increase in burning velocity of biogas air mixture that leads to improved combustion [13–15,30,43]. Further it was found to be lower in biogas biodiesel dual fuel mode compared to biodiesel mode due to reduced charge temperature of biogas and delayed combustion phase [18,40,41]. Kumar et al. [37] observed lower EGT by supplying hydrogen in a biodiesel dual fuel engine due to the reduction of late burning caused by the high flame speed of hydrogen. Higher EGT was observed in dual fuel mode with increase in methane content compared to diesel mode at full load due to higher combustion temperature caused by the higher heating value of methane [20,33]. It was further being found to be higher in dual fuel mode with advanced injection timing compared to diesel mode due to high cylinder temperature caused by the enhancement of combustion rate

3.1. In-cylinder pressure In-cylinder pressure is an important parameter to be analyzed as it impact pressure rise, heat release rate, ignition delay and so on. As a high octane fuel is used in dual fuel engine the behavior of in-cylinder pressure needs to be studied more carefully. Peak cylinder pressure inside the engine cylinder using gaseous fuel was found to be increased compared to diesel fuel with advanced injection timing [45]. It was observed that dual fuel operation using hydrogen & diesel shows

Fig. 10. Variation of exhaust gas temperature with engine load [35]. 8

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speed of hydrogen. It also leads to the better mixing of gaseous and liquid fuel particles inside the cylinder which ultimately results in higher heat release rate and peak cylinder pressure [47]. Longer ignition delay period was observed which is further enhanced with BGES in dual fuel mode due to increase in heat capacity of charge and decrease in reaction rate of air fuel mixture [8,13–15,17,18,30,44,51]. It was also found to be increased with increase in fuel air equivalence ratio in a biogas diesel dual fuel engine due to higher amount of carbon dioxide in biogas which have high specific heat and absorb the heat during premixed combustion [22,25]. Further longer ignition delay was observed in dual fuel mode with advanced injection timing compared to diesel mode due to the late ignition of pilot fuel caused by the low in-cylinder temperature and reduced oxygen concentration [16,29,39]. Increased ignition delay period was observed with more amount of hydrogen in the supplied fuel due to more hydroxide ion radicals that leads to more reactive mixture formation in a RCCI engine using hydrogen and landfill gas along with diesel fuel [52]. Same result was obtained by using natural gas and diesel in RCCI engine due to the low reactivity of natural gas [53]. However short ignition delay was observed using DEE with advanced injection timing due to higher cetane number of DEE which increases its auto ignition temperature [27]. Higher oxygen in biogas diesel dual fuel engine also shortens ignition delay due to increased preignition reaction by the presence of more oxygen that leads to improved combustion [34]. It was found that ignition delay is further reduced with increase in CR in biogas diesel dual fuel engine due to higher temperature at the end of compression that leads to low pilot fuel requirement [42]. Verma et al. [47] in hydrogen supplemented biogas diesel dual fuel engine found ignition delay was slightly decreased with increase in hydrogen due to better mixing and faster burning ability.

maximum peak cylinder pressure of 87.23 bar at an advanced injection timing of 320BTDC as shown in Fig. 11. The possible reason for the increment may be the higher ignition delay in case of gaseous fuel. As the ignition delay period is increased more accumulation of fuel occurs inside the cylinder resulting higher pressure. Peak cylinder pressure in dual fuel mode was found to be higher than diesel mode due to better combustion in premixed phase resulted from longer ignition delay [17,36,44]. It was further found to be increased with advanced injection timing compared to diesel mode due to higher pressure rise in premixed phase caused by less oxygen concentration in biogas and rapid burning caused by higher ignition delay [16,27–29,45]. Yilmaz and Gumus [46] found increase in cylinder pressure in thermal barrier coated engine operating on dual fuel mode due to reduced heat loss from the cylinder as the increase in pressure depends up on the increase in temperature. Cacua et al. [34] observed higher cylinder pressure by enhancing oxygen in biogas diesel dual fuel engine due to increased reactivity of mixture. Higher in-cylinder pressure was observed in hythane (hydrogen and methane mixture) diesel dual fuel engine due to high flame speed of hydrogen. This was further improved by using blend of ethanol and hydrogen due to combined burning of primary and pilot fuel caused by the high octane rating of primary fuel [6,11,37,47]. Increased peak cylinder pressure was observed using hydrogen along with DME/CH4 due to more hydroxide ion radicals that leads to more reactive mixture formation in a RCCI engine [48]. However it was also found that in cylinder pressure in dual fuel mode decreases compared to single fuel mode due to decrease in combustion rate by high specific heat and less reactive gaseous fuel charge [8,13,14,39,43]. Aklouche et al. [25] observed decrease in cylinder peak pressure with increase in fuel-air equivalence ratio in dual fuel mode due to reduction in volumetric efficiency and increased ignition delay. Further decrease in peak in-cylinder pressure was observed with increase in EGR in biomethane-diesel dual fuel engine due to reduction in cylinder temperature which retards the ignition timing caused by the replacement of air by CO2 in the exhaust gas [49,50].

3.2.1. Summary The above literature survey confirms that ignition delay period is generally longer in biogas fueled dual fuel engine due to the presence of CO2 which have high specific heat that absorbs the heat during premixed combustion which is further increased by higher fuel air equivalence ratio. Advancement of injection timing and reduction in oxygen concentration also results in higher ignition delay as explained by the authors. However it is also reported that use of DEE as fuel additive or increasing oxygen content in biogas leads to decrease the delay period. It is further reported that higher compression ratio decrease the ignition delay as the temperature at the end of compression is high. Hydrogen supplemented fuel having high flame velocity may also

3.1.1. Summary It is clear from the above literature study that in-cylinder pressure is a function of fuel accumulation in premixed phase which leads to higher ignition delay and heat release rate. Therefore thermal barrier coating and advanced injection timing using DEE has a positive impact as reported by some authors. Higher oxidation or flame propagation using oxygen, hydrogen or biodiesel enriched fuel in dual fuel engine results in higher in-cylinder pressure. In-cylinder pressure is found to low in some of the cases. The reason for the reduction is lower combustion rate with increase in equivalence ratio, lower volumetric efficiency and higher EGR. 3.2. Ignition delay Ignition delay is defined as the time taken for the start of combustion from the start of inection of fuel. It is represented in the form of crank angle duration. Ignition delay consist of physical delay which represents the atomization, vaporization and mixing phase and chemical delay that represents the reaction prior to main combustion. The literature survey consists of the optimization done by various researchers on dual fuel engine for improved combustion. Ignition delay in case of dual fuel operation was found to be increased compared to normal diesel operation as shown in Fig. 12. The maximum ignition delay of 230 was observed in case of biogas and diesel dual fuel operation at advance injection timing of 320BTDC [45]. The presence of co2 in biogas may be the reason for the enhancement which absorbs some heat leading to lower charge temperature. Ignition delay period was observed to be decreased with increase in hydrogen percentage in the supplied fuel at all load conditions as shown in Fig. 13. Reduced delay period was the result of higher flame

Fig. 11. Variation of peak cylinder pressure with injection timing [45]. 9

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of DEE while Ibrahim et al. found retardation of injection timing is the cause for increase in HRR in biogas diesel fueled HCCI engine due to less available time for mixture preparation which increase the combustion rate [16,25,27,29,45]. Further it was observed to be higher with increase in hydrogen and methane mixture flow due to higher burning rate of hydrogen [6,11,36]. Higher heat release rate was also observed with more amount of hydrogen in the supplied fuel due to high flame speed and auto ignition temperature of hydrogen in RCCI engine using hydrogen and landfill gas (50% CH4 and 50% CO2) along with diesel fuel [52]. Same results were obtained using hydrogen in RCCI engine operated by CNG and diesel [54]. It was found that peak heat release rate in biogas fueled engine is lower compared to diesel engine at all load condition due to the presence of CO2 in biogas and lower flame temperature [9,13–15,43,51]. Shan et al. [49] found that heat release rate decreases with increase in EGR due to induction of excessive CO2 which absorbs the heat during combustion. Further it was found that peak heat release rate decreases with increase in BGES at all loads due to reduced reaction rate of biogas [8,24]. Same result was obtained by using natural gas and diesel in RCCI engine due to the low reactivity of natural gas [53].

Fig. 12. Variation of ignition delay with injection timing [45].

3.3.1. Summary It is identified from the above literature survey that heat release rate is highly dependent on ignition delay. Longer the ignition delay rapid higher the pressure rise which results in high HRR. It is further increased by the thermal barrier coated engine as the heat transfer being reduced due to coating. Altering the injection timing also increases the HRR because of change in mixing concentration of charge inside the cylinder. Hydrogen enrichment in the inducted biogas fuel is reported as the cause for higher HRR by some authors due to higher flame propagation. The reverse trend in HRR is also reported by some authors due to the presence of CO2 in biogas which reduce the flame temperature by absorbing the heat of combustion. The lower calorific value of biogas is also outlined as the reason for lower HRR which can be increased with increase in compression ratio. With increase in BGES and EGR the reaction rate is decreased and heat is consumed by CO2 which ultimately results in lower HRR. 3.4. Combustion duration Fig. 13. Variation of ignition delay with percentage of hydrogen in fuel [47].

Combustion duration is defined as the time taken from 10% to 90% combustion of the fuel. In other words it is the start and end of combustion process inside the cylinder. In order to improve the engine performance the combustion duration is required to be short so that all the charge undergoes complete burning. It depends upon the quality of inlet charge mixture as well as turbulence inside the cylinder produced by the motion of fuel particle. Combustion duration was found to be highest (56°CA) using neat Maduca longifolia oil (MO) in single fuel mode. It was observed to be reduced to 50°CA using ethanol and

be an alternative method to shorten ignition delay period. 3.3. Heat release rate Heat release rate in engine cylinder depends upon the peak pressure rise, rapid premixed combustion, ignition delay etc. Generally longer ignition delay leads to more accumulation of pilot fuel during premixed combustion phase which in turns raise the pressure and temperature during rapid uncontrolled combustion. Combustion process in dual fuel mode is quite different from pure diesel mode. So it needs a conscientious inspection of heat release rate during dual fuel operation for an optimum engine performance. Nathan et al. [9] observed lower heat release rate with increment in biogas percentage at a charge temperature of 135 °C in biogas-diesel dual fuel HCCI mode of engine operation as shown in Fig. 14. The possible reason may be due to the presence of carbon dioxide in biogas which has a higher specific heat. Higher heat release rate in dual fuel mode was found due to more accumulation of biogas charge resulted from longer ignition delay which gives combined combustion of pilot fuel and gaseous fuel which is further increased by thermal barrier coated engine due to high combustion chamber temperature [17,39,44,46]. It was also observed higher HRR in dual fuel mode using DEE with advanced injection timing due to increased premix combustion caused by the early burning

Fig. 14. Variation of HRR with crank angle at 135 °C charge temperature [9]. 10

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presented hereafter. Engine using biogas in dual fuel mode of operation shows lower NOx emission compared to only diesel mode due to high specific heat of biogas at all loading condition as shown in Fig. 16. It was further observed that when hydrogen is supplied along with biogas NOx emission slightly increased [51]. The possible reason may be the high flame velocity which increases the in-cylinder pressure and temperature. Lower NOx emission using dual fuel mode was observed in contrast to diesel and biodiesel mode due to lower combustion as well as heat release rate of gaseous fuel that leads to reduced combustion temperature caused by the high heat capacity of CO2 in biogas [7,8,12,16–18,21,25,39,40,41,44,56]. Further it was found to be decreased in biogas diesel fueled engine due to reduced combustion rate and lower in-cylinder temperature by the presence of CO2 [9,49,57,58]. Higher NOx emission was observed using hythane (hydrogen and methane mixture) in dual fuel engine as a result of increased cylinder pressure and temperature caused by the high heating value of hydrogen and methane [6,11]. It was also identified that NOx emission increases with enhanced compression ratio that results in higher in-cylinder temperature [13,15,30,42,59]. Same result was found by the advancement of injection due to formation of homogeneous mixture and higher flame propagation as a result of early injection of pilot fuel [29,45]. High pressure and temperature of the cylinder results more NOx formation in dual fuel mode using thermal barrier coating as reported by Yilmaz and Gumus [46]. Further it was observed to be high in dual fuel engine using DEE-HCCI mode because of improved combustion caused by more availability of oxygen as well as rapid combustion in HCCI mode [27,28]. Moreover higher NOx emission was observed in hydrogen supplemented dual fuel engine due to higher rate of heat release and replacement of CO2 in biogas [35–37,47]. Higher NOx emission was observed with more amount of hydrogen in the supplied fuel due to high flame speed and auto ignition temperature of hydrogen which results in higher in-cylinder temperature in RCCI engine using hydrogen and landfill gas (50% CH4 and 50% CO2) along with diesel fuel [52]. Further increase in NOx emission was identified using natural gas and diesel In RCCI engine due to the higher reaction rate [60]. Same results were obtained using hydrogen in RCCI engine operated by CNG and diesel [54]. It is also found to be increased using hydrogen along with DME/CH4 in RCCI engine due to higher combustion temperature caused by the advanced ignition timing by the presence of hydrogen [48].

hydrogen in dual fuel mode as shown if Fig. 15. Reduction in CD was due to the short diffusion phase of combustion which leads to improved premixed combustion. Further it was found to be reduced by injecting water and ethanol with hydrogen but still found to be higher than pure hydrogen in dual fuel mode of operation [37]. Kalsi and Subramanian [8] found combustion duration of dual fuel engine increases with BGES due to the replacement of oxygen with high specific heat gaseous fuel. It was also observed to be longer in dual fuel mode with advanced injection timing compared to diesel mode due to reduced diffusion of charge caused by less oxygen concentration in biogas [16,29]. Further longer combustion duration was observed in biogas-biodiesel dual fuel mode compared to biodiesel mode due to reduced oxygen concentration in biogas leads to slower pre-ignition reaction that results in slow burning during combustion [17,18]. However reduced combustion duration was observed in duel fuel mode due to longer ignition delay caused by less supply of pilot diesel fuel [39,44]. Aklouche et al. [25] observed reduction in combustion duration with increase in equivalence ratio in dual fuel mode due to higher burning velocity near to stoichiometric condition. Barik and Murugan [27] observed shorter combustion duration in dual fuel mode using DEE with advanced injection timing due to faster ignition and high flame speed of DEE. Reduced combustion duration was also observed with more amount of hydrogen in the supplied fuel due to high flame speed and auto ignition temperature of hydrogen in RCCI engine using hydrogen and landfill gas (50% CH4 and 50% CO2) along with diesel fuel [52]. Same results were obtained using hydrogen in RCCI engine operated by CNG and diesel [54]. 3.4.1. Summary It is clear from the literature review that gaseous fuel having high specific heat replace oxygen concentration causes reduced flame propagation and burning velocity that leads to increase in combustion duration. Longer ignition delay results accumulation of more pilot fuel during premixed combustion phase which burn rapidly when the main combustion starts and makes the combustion duration short. Further it is being reduced by increasing the equivalence ratio that caused higher burning velocity. Also advancing the injection timing in DEE supplemented dual fuel mode produces high flame speed and reduce the combustion duration. 4. Emission analysis Emission from engine has significant impact on environmental pollution as well as global warming. A large number of experiments have been conducted by the researchers to improve the engine emission. Alternative fuel having less carbon atom plays an important role to control the hazardous emission to a great extent. Emission also signifies the combustion and performance characteristic of the engine. A detail literature survey on emission behavior of dual fuel engine is presented in the following sections.

4.1.1. Summary It is clear from the literature survey that NOx emission can be

4.1. NOx emission NOx emission generally results from the reaction of nitrogen and oxygen at high temperature. NOx generally refers to one atom of nitrogen and one or more atom of oxygen named as nitric oxide (NO) and nitrogen dioxide (NO2). These gases produced inside the engine cylinder at high temperature are the reason for smog and acid rain. The common sources for NOx emission are thermal, fuel and prompt. Thermal NOx formation occurs by the oxidation of diatomic nitrogen during the combustion of fuel at a temperature above than 1800 K [55]. Fuel NOx is produced by the combustion of fuel which contains excess nitrogen. Prompt NOx contributes as a negligible source that occurs due to the reaction between atmospheric nitrogen and hydrocarbon particle in the fuel. Thermal NOx is considered to be a dominant source in internal combustion engine. A detail literature review on NOx emission is

Fig. 15. Variation of combustion duration with energy share [37]. 11

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Fig. 17, due to replacement of carbon particle in the fuel with hydrogen and also due to the oxidation of soot particle after combustion which is resulted by the higher combustion temperature. Lower smoke emission was observed in dual fuel mode in contrast to diesel mode at all load condition as a consequence of homogeneous mixture of biogas and air resulted from high diffusivity of gaseous fuel that leads to enhanced oxidation [7,8,12,17,44]. Yilmaz and Gumus [46] found decrease in smoke emission in thermal barrier coated engine operating on biogas-diesel mode as a result of improved burning caused by high cylinder temperature. It was also observed to be less with enhancement of injection timing due to increased rate of combustion and oxidation in combustion chamber [16,27,29,40,45]. It was further reduced by using biodiesel due to higher oxygen content leads to improved combustion [18]. Verma et al. [42] observed lower smoke emission by enhancing the compression ratio in biogas diesel engine caused by improved combustion and oxidation as a result of short ignition delay and high in-cylinder temperature. Sandalci et al. [6] observed lower smoke emission in hythane (hydrogen and methane mixture) diesel dual fuel engine due to least carbon contain and high diffusivity of methane and hydrogen. Kumar et al., Rahman et al. Moreover smoke release in H2-biodiesel mode was observed to be lower in contrast to only diesel because hydrogen replaces the carbon atom in the fuel. This was further improved by using blend of ethanol and hydrogen due to higher oxygen and reduced carbon atoms in the fuel [35,37]. Verma et al. [47] observed increase in smoke emission by supplementing hydrogen in biogas diesel engine at high load due to requirement of pilot diesel as well as shorter ignition delay and less oxygen concentration. Chintala and Subramanian [58] observed increase in smoke emission by injecting water into H2 dual fuel engine due to formation of heterogeneous mixture inside the cylinder resulted from poor mixing owing to the presence of water vapor.

Fig. 16. Variation of nitrogen oxide emission with indicated mean effective pressure [51].

reduced in biogas supplemented dual fuel engine owing to the following reasons. Biogas contains carbon dioxide which has high specific heat that results in lower combustion rate and peak cylinder temperature. With increase in biogas flow rate oxygen concentration is also reduced. Exhaust gas recirculation results in low in-cylinder temperature and low NOx emission by absorbing heat of combustion. Water injection in hydrogen supplemented engine leads to replace oxygen concentration and decrease NOx emission. Turbocharging is also suggested as an alternative to reduce NOx emission which increases the heat capacity of the charge. However some authors have also reported higher NOx emission due to the following reasons. High methane concentration leads to increase in in-cylinder temperature. Thermal barrier coating reduce heat transfer from the cylinder and increase combustion temperature. The temperature and pressure of the cylinder were also enhanced by advancing the injection timing of pilot fuel. Supply of hydrogen into the cylinder increases the flame propagation and combustion temperature which can be controlled by EGR in a fixed proportion. Oxygen concentration is increased in the fuel by using DEE that results in improved combustion and high in-cylinder temperature. Increasing the compression ratio also results in short ignition delay and high in-cylinder temperature.

4.2.1. Summary It is clear from the above literature review that smoke emission is reduced significantly under dual fuel operation due to the subsequent reasons. Gaseous fuels have high diffusivity than liquid fuel. Therefore homogeneous mixture is formed in dual fuel mode which results in improved combustion and reduced smoke emission. Thermal barrier coated engine also reduce heat transfer from the engine cylinder which improve the combustion temperature that leads to complete combustion of biogas fuel mixture.

4.2. Smoke or particulate matter emission Smoke emission causes skin problem, eye problem and respiratory problem due to the presence of soot particles of carbon in the air which is harmful to the environment. It is generally high for diesel engine which can noticeable as block smoke. It is due to the improper combustion of rich fuel air mixture due to the deficiency of oxygen. Smoke emission is more in case of high carbon to hydrogen ratio as presence of more carbon leads to less oxidation of fuel. As diesel is a heavier fuel it requires comparatively high oxygen for complete combustion. Therefore smoke emission is high for diesel engine compared to dual fuel engine. As gaseous fuel is generally used in dual fuel mode it produces a homogeneous mixture that results improved combustion which is not the case for diesel engine. Particulate matter emission was found to be increased rapidly at higher load with diesel fuel mode as more fuel is injected into the cylinder. As the engine was supplied with biogas enriched with methane PM emission decreases due to formation of homogenous mixture and less oxygen replacement with CO2 [51]. The induction of hydrogen leads to further decrease in PM as shown in

Fig. 17. Variation of particulate matter with indicated mean effective pressure [51]. 12

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Better oxidation of charge is achieved by supplying higher oxygen contain DEE with pilot diesel fuel, advanced injection timing and high compression ratio. Carbon atom in the fuel can be replaced by increasing hydrogen and methane flow rate that ultimately reduce smoke emission. Biodiesel contain more oxygen and less sulphur as well as carbon atom which results in less smoke emission. This can be further improved by supplying hydrogen and ethanol which has no carbon atom. It is also reported that smoke emission is increased by exhaust gas recirculation because oxygen concentration is reduced which cause incomplete combustion. It is further increased by injecting water into the hydrogen supplemented dual fuel engine due to formation of heterogeneous mixture inside the cylinder.

was found to be less by enhancing the injection timing resulted due to the formation of homogeneous mixture by the early injection of pilot fuel [27–29]. Verma et al. [47] in hydrogen supplemented biogas diesel dual fuel engine found UHC decreases with increase in hydrogen proportion as hydrogen replaces some hydrocarbon as well as it reduce ignition delay resulting better combustion of fuel. Also it was observed to be lower in H2-biodiesel mode in comparison to only diesel caused by the improved combustion by hydrogen supply [35,36]. Reduced unburned hydrocarbon was obtained using hydrogen in RCCI engine operated by CNG and diesel due to higher flame speed of hydrogen that easily spread inside the cylinder resulting improved combustion [54]. Same results were obtained using natural gas and diesel in RCCI engine [60].

4.3. HC emission

4.3.1. Summary It is found from the above literature review that HC emission is higher in biogas fueled engine compared to diesel engine because of the partial combustion resulted from the lower calorific value of biogas that reduce the combustion temperature. Incomplete combustion of the charge may also results by the high specific heat of biogas in which CO2 concentration is more that leads to reduce the flame velocity. HC emission is further increased by EGR due to the presence of CO2 in exhaust gas that has no contribution during combustion rather it absorbs the heat of combustion. However it is being decreased at higher load due to improved combustion caused by the high in-cylinder temperature and pressure. The authors observed increasing the equivalence ratio produces high flame speed and also high compression ratio results in short ignition delay which ultimately leads to improved combustion and reduced HC emission. Thermal barrier coating may be another alternative to reduce HC emission because it decreases the heat transfer from engine cylinder causes better oxidation of the charge. It is also reported that advanced injection timing produces early premixed combustion and form homogeneous mixture inside the cylinder. Hydrogen or DEE supplement in pilot fuel replace the hydrocarbon component with carbon less element that improve the oxidation and reduce HC emission.

Hydrocarbon emission creates ground level ozone by reacting with nitrogen oxide in the presence of sunlight which has a great impact on environmental pollution. It is also responsible for respiratory problem, irritation to the eyes and lungs problem. It is the upshot of the incomplete combustion of unburned hydrocarbon particle. It is usually found at low temperature areas like cylinder wall where the temperature is low compared to the center of the cylinder. Hydrocarbon particles are present in both liquid and gaseous fuels. The analysis of HC emission includes the outcome reported by the researchers by using biogas and hydrogen in dual fuel CI engine. It also highlights various methods applied to control the same in dual fuel engine. Biogas diesel dual fuel operation shows higher HC emission compared to diesel due to the presence of CO2 in biogas which decrease the flame speed that leads to poor combustion of fuel. It was observed that advancing the injection slightly decrease HC emission. Further it is decreased by using hydrogen as fuel in dual fuel operation due to the replacement of carbon atoms in the supplied fuel by hydrogen which has high flame speed that results in improved combustion as shown in Fig. 18. The maximum decrease in HC emission was found to be 46 ppm using hydrogen in dual fuel mode compared to 80 ppm in diesel mode at an advanced injection timing of 320BTDC [45]. Hydrocarbon emission was found to be increased in biogas-diesel dual fuel mode compared to only diesel mode. It was further found to be increased with increase in load and fuel flow rate [41]. HC emission was increased due to the poor combustion of fuel resulted from the higher delay period caused by the low self-ignition temperature of biogas. Highest increment in HC emission was found to be 21% at full load condition with biogas flow rate of 0.6 kg/h as shown in Fig. 19. Higher hydrocarbon emission was observed using dual fuel engine at partial load because of incomplete combustion resulted from slow flame front propagation which decreases by increasing the load [8,12,17,18,21,24,26,39–41,44]. Nathan et al. [9] found higher HC outrush in HCCI mode due to increased biogas and less oxygen concentration. Shan et al. [49] found slight increment in HC emission with rise in EGR due to the reduced temperature inside the cylinder. It was also found to be higher with advancement of injection timing in biogasdiesel mode owing to presence of CO2 and longer ID [45,57]. Chintala and Subramanian [58] observed small increase in HC emission by injecting water into H2 supplemented engine caused by reduced in-cylinder pressure and temperature. Karagoz et al. [11] observed higher HC emission with increase in hydrogen and methane mixture flow due to higher methane composition. Aklouche et al. [25] observed decrease in HC emission with higher equivalence ratio due to improvement in combustion process and high flame propagation. Yilmaz and Gumus [46] found decrease in HC emission in thermal barrier coated engine because of improved oxidation of fuel caused by the higher cylinder temperature. HC emission was observed to be lower with higher compression ratio in biogas diesel engine owing to improved combustion resulted from short ignition delay and high combustion temperature [13,15,30,42,59]. Further it

4.4. CO emission Carbon monoxide emission is hazardous to the environment because results in the formation of ground level ozone and climate change by reacting with pollutant present in the air. It is also the consequences of incomplete combustion like HC emission. CO emission is produced due to the incomplete oxidation of carbon atoms caused by the fuel rich

Fig. 18. Variation of hydrocarbon emission with injection timing [45]. 13

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engine caused by the reduced fuel air equivalence ratio and higher preignition reaction of biogas. It was also found to be decreased with enhanced compression ratio caused by the improved combustion and oxidation resulted from short ignition delay and high in-cylinder temperature [13,15,30,42,59]. Further CO emission was observed to be less in biogas-diesel mode by enhancing the injection timing that results improved oxidation between carbon and oxygen and higher cylinder temperature [27,29,45,57]. It was observed to be lower by supplying hydrogen in biodiesel engine owing to reduced carbon in the fuel caused by hydrogen supplementation. This was further improved by using blend of ethanol and hydrogen due to higher oxidation of the fuel [35–37]. Also lower CO emission was observed in diesel engine by supplying hydrogen as the gaseous fuel resulted from the reduced amount of carbon in the fuel [6,38,47]. Reduced CO emission was also observed with more amount of hydrogen in the supplied fuel due to replacement of carbon with hydrogen in RCCI engine using hydrogen and landfill gas (50% CH4 and 50% CO2) along with diesel fuel [52]. It is also found to be reduced by using natural gas and diesel due to higher reactivity and improved combustion [60]. Same results were obtained using hydrogen in RCCI engine operated by CNG and diesel [54]. It is also found to be reduced using hydrogen along with DME/CH4 in RCCI engine due to better oxidation of CO resulted from the higher combustion temperature [48].

Fig. 19. Variation of hydrocarbon emission with brake power [41].

mixture in which oxygen availability is less. It is also formed to due lean mixture where the combustion temperature is below 1450 K [55]. Carbon monoxide emission in biogas diesel dual fuel operation was found to be higher compared to diesel operation due to the presence of co2 in biogas which can be reduced by advancing the injection timing [45]. When the injection timing was advanced to 320BTDC from 200BTDC, CO emission was reduced by 30% at full load. Hydrogen in dual fuel operation shows lowest CO emission compared to other fuels due to high flame velocity that result complete combustion of fuel inside the cylinder. Highest decrease of 64.28% CO emission was observed compared to diesel mode at full load condition of the engine as shown in Fig. 20. Carbon monoxide emission in dual fuel engine is generally found to be higher compared to diesel engine. It may be due to the poor combustion resulted by the presence of CO2 in biogas. Barik and Murugan [41] observed that CO emission in dual fuel engine was considerably increased with increase in load and biogas flow rate as shown in Fig. 21. The possible reason may be due to the replacement of fresh air by carbon dioxide present in biogas. A detail literature review on CO emission from dual fuel engine is discussed below. It was found that CO emission of biogas-diesel engine increases at partial load owing to the presence of biogas residual in the cylinder resulted from lower combustion temperature and high heat capacity of biogas which dilutes the charge concentration [8,12,16,17,21,26,39,40,41,44]. Which is further increased with increase in EGR due to presence of more CO2 which again reduce the oxidation of CO [49,56]. It was also found to be increased using HCCI engine due to increased biogas, low temperature combustion and less oxygen concentration [9,28]. Yoon and Lee [18] observed higher CO emission in biogas biodiesel mode of operation in comparison with biodiesel mode because of higher CO2 concentration in the mixture which minimizes the concentration of fresh air. Chintala and Subramanian [58] observed increase in CO emission by injecting water into H2 supplemented engine as a result of slower chemical reaction caused by the low combustion temperature. Karagoz et al. [11] observed higher CO emission with increase in hydrogen and methane mixture flow in a dual fuel engine due to higher methane composition. However lower carbon monoxide emission was observed by enhancing the equivalence ratio because of complete combustion resulted from high flame propagation and rich mixture [10,25]. Cacua et al. [34] observed lower CO emission by enhancing oxygen in biogas diesel

4.4.1. Summary With the review of the above literature it is identified that CO emission in biogas fuelled engine is high at all loading condition. The reasons for the increase are listed below. The specific heat of biogas is quite high compared to conventional fuel. So it requires a high combustion temperature to burn completely and with increase in load higher amount of biogas is supplied into the engine cylinder. Therefore some fuel undergoes incomplete combustion leads to high CO emission. With increase in CO2 contain in biogas the flame speed is reduced and incomplete oxidation of charge takes place which results in higher CO emission. It is further increased by EGR due to high CO2 concentration inside the cylinder. Different methods to minimize CO emission can be attributed from the literature survey. Biodiesel have higher oxygen contain and also is a high cetane number fuel. Due to these properties biodiesel reduce CO emission by improving the combustion inside the cylinder. Hydrogen and ethanol blend also possess the same quality to reduce CO emission. Higher compression ratio and advanced injection timing causes

Fig. 20. Variation of carbon monoxide emission with injection timing [45]. 14

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4.5.1. Summary It is identified from the above literature review that CO2 itself is the main reason for CO2 emission as it is not involved in the combustion process and biogas having less methane concentration produces high CO2 emission if used in dual fuel engine. CO2 emission also resulted from complete combustion of fuel as carbon and oxygen completely oxidized to form CO2 at high in-cylinder temperature caused by higher compression ratio. Advanced injection timing also results in complete combustion by forming homogeneous mixture as the charge gets enough time to mix properly. Further complete combustion is achieved by higher equivalence ratio because the fuel gets sufficient oxygen to burn completely. Engine operated with EGR also produce high CO2 emission as the concentration of CO2 in exhaust gas is quite high. Another reason for higher CO2 emission is incomplete combustion of fuel that leads to higher CO emission rather than CO2 emission. Biogas having higher methane percentage also causes low CO2 emission. This is the reason for which biogas needs to be purified in order to have high methane and low CO2 percentage prior to use in dual fuel engine. Fig. 21. Variation of carbon monoxide emission with brake power [41].

5. Conclusion early ignition and homogeneous mixture of the charge that results in lower CO emission. DEE as a supplement with conventional fuel provides higher oxygen release that leads to better oxidation and improved combustion.

The present literature review serves as a gateway between environmental issue and energy requirement for society. A detail analysis on utilization of alternative fuel reveals the following conclusion.

• Generally use of biogas in CI engine leads to decrease brake thermal

4.5. CO2 emission Carbon dioxide emission is undesirable to the environment because it is primary cause for global warming. The principal reason for this emission is the existence of carbon dioxide itself. In case of dual fuel engine CO2 emission is found to be high if the inducted fuel contains less methane or high CO2 percentage. CO2 emission also results from the complete burning of fuel caused by the improved oxidation of carbon in the presence of adequate oxygen and higher in-cylinder temperature. Carbon dioxide emission in biogas diesel dual fuel engine increases with higher CO2 contain in the biogas. It was observed that CO2 emission is almost identical with only diesel fuel operation when the biogas was enriched with 80% methane and 20% CO2. Further reduction in CO2 emission was observed when the supplied fuel is enriched with hydrogen as shown in Fig. 22, because it replaces some CH4 and CO2 [51]. CO2 emission from dual fuel engine observed by various researchers is discussed below. Kalsi and Subramanian [8] found CO2 emission increases with BGES due to the presence of large mass of CO2 which does not participate in combustion process. It was found to be higher CO2 emission in dual fuel mode due to the presence of CO2 in biogas and it is further enhanced with higher compression ratio caused by the complete combustion and higher in cylinder temperature which was further increased by the advancement of injection due to formation of homogeneous mixture by the early injection of pilot fuel [13,15,30,51,56,59]. Aklouche et al. [25] observed higher CO2 emission with enhanced equivalence ratio owing to improved combustion process. Barik and Murugan [16] observed lower CO2 release using dual fuel engine due to incomplete combustion caused by the less oxygen and lower combustion chamber temperature which rather increase CO emission. Prabhu et al. [10] observed lower CO2 emission with higher methane concentration using dual fuel engine as a result of reduced CO2 in biogas as the concentration of methane increases. Verma et al. [47] found less CO2 emission using hydrogen as supplement in biogas diesel engine due to replacement of hydrocarbon fuel by hydrogen. Rahman et al. [35] observed lower CO2 emission in H2-biodiesel dual fuel engine caused by the replacement of carbon by hydrogen which leads to better oxidation which slightly increased by EGR due to presence of CO2 in the exhaust gas.

•

•

efficiency up to 13% while increasing the fuel consumption up to 36% which can be improved up to 10 to 12% by using certain techniques such as advanced injection timing or higher compression ratio. Combustion parameters such as peak pressure inside cylinder and rate of heat release can be increased up to 30% while the duration of combustion and ignition delay can be minimized up to 4 to 5% by supplying biogas and hydrogen into the cylinder with simultaneous use of advanced injection timing and higher compression ratio. DEE is also identified as an effective supplement along with these alternative fuels to improve combustion characteristics. Dual fuel operation results significant decrease in NOx and smoke emission up to 60% whereas HC and CO emission got increased may be up to 30%. However these emissions can be controlled by using the advanced methods and supplement element with the pilot fuel.

Fig. 22. Variation of carbon dioxide emission with indicated mean effective pressure [51]. 15

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• Hydrogen is considered as a high source of energy due to its higher

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heating value, higher flame speed, low ignition energy. Along with these qualities it does not contain carbon atom which makes it valuable source for emission control as well as performance enhancement of CI engine.

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