State-of-the-Art of Strategies to Reduce Exhaust Emissions from Diesel Engine Vehicles

Compression ignition engines play a significant role in the development of a country. They are widely used due to their innate properties such as high efficiency, high power output, and durability. However, they are considered one of the key contributors to transport-related emission and have recently been identified as carcinogenic. Thus, it is important to modify the designs and processes before, during, and after combustion to reduce the emissions to meet the strict emission regulations. The paper discusses the pros and cons of different strategies to reduce emissions of a diesel engine. An overview of various techniques to modify the pre-combustion engine design aspects has been discussed first. After that, fuel modifications techniques during combustion to improve the fuel properties to reduce the engine-out emission is discussed. Finally, post-combustion after-treatment devices are briefly discussed, which help improve the air quality of our environment.


Introduction
The development of a country greatly depends upon its transportation, mining, and power generation sectors. Diesel engines, also known as compression ignition (CI) engines, have become a source of power for these sectors due to their having high innate efficiency, durability, and better power output. Hence, it shares a large portion of the passenger car and heavy machinery market [1]. However, with rapid growth in demand, the diesel engine has also become one of the key sources of pollutants. Both the environment and human health are adversely affected by the pollutants resulting from petroleum-derived fuel combustion. United Nation Intergovernmental Panel on Climate Change has reported a rapid surge of global warming due to increased greenhouse gas emission, including methane, nitrogen oxides, and carbon dioxide. It is predicted that more than a hundred million lives will be in danger if the average global temperature increases by more than 2 • C [2].
The main constituents of emission resulted from the combustion of diesel fuel are carbon monoxide (CO), nitrogen oxides (NOX), hydrocarbons (HC), and particulate matter (PM)-these are considered as regulated emission and Polycyclic aromatic hydrocarbon, benzene, toluene, xylene, soot-these are considered as unregulated emission. Diesel engines emit hydrocarbon as the by-product of incomplete or partial combustion [3]. Sneezing, coughing, drowsiness, eye irritation, symptoms akin to drunkenness, several lung diseases-these are the problems that can be caused by HC emission [4]. The incomplete oxidation product of hydrocarbon fuel results in CO emission [5]. Excessive inhaling of CO disrupts the proper functionality of several vital organs such as the brain, nervous tissue and heart by reducing the oxygen-carrying capacity of blood [6,7]. CO emission

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Pre-combustion engine configuration modifications • In-combustion fuel modification • Post-combustion treatment techniques The article's main focus is the in-combustion fuel modification section, where a thorough analysis was provided-pre-and post-combustion modifications discussed with less emphasis. The literature was selected through specific search parameters such as "biodiesel", "performance", and "emission" in "sciencedirect.com". Relevant studies from the past ten years were chosen for this review with very few necessary exceptions. This article intends to present the current scenario of research activities on strategies to reduce exhaust emissions from diesel engine vehicles.

Diesel Engine Emission Standards
In recent years, due to the increased impact of global warming and the adverse effect on human health, governments have imposed stringent anti-pollution laws. These have fostered the development of efficient engines with minimum emissions [19]. In Table 1, Euro emission standards for both passenger cars and heavy-duty vehicles [20] are shown and Table 2 exhibits emission standards followed in Australia from 2002 onwards [21].

Pre-Combustion Engine Design Considerations
The engine performance and pollutant emissions depend upon diesel engine configuration. By modifying the chamber geometry, compression ratios, inlet swirl location, injection parameters (pressure, timing, and duration), etc., improved engine performance and better fuel efficiency can be achieved [22][23][24]. Reduction of friction between elements, use of sophisticated bearings, and new lubricants will reduce mechanical losses, thus increasing engine efficiency [25,26].
For diesel engine, injection timing is one of the key parameters, which heavily influence engine performance, emissions, and combustion characteristics. Many studies reported the effect of retarding or advancing the injection timing from the standard value. Park et al. [27] reported that NOx emission was reduced when the injection timing is retarded. As more fuel is burnt after top dead centre (TDC), it reduces peak cylinder pressure and temperatures, which as a result reduces NOX emission, which is also supported by Sayin et al. [28]. However, Sayin et al. [28] also reported that retarded injection timings results in increased unburned HC and CO emissions. Muralidharan and Govindarajan [29] reported that when the injection timing is retarded, fuel injection starts later; thus, complete combustion is not possible to attain at this mode. Consequently, brake thermal efficiency (BTE) decreases and brake specific fuel consumption (BSFC) increases.
When the injection timing is advanced from the standard, it reduces the HC and CO emissions due to increased combustion temperature, increased oxidation of carbon and oxygen and decreased flame quenching thickness. However, this also increases CO 2 emission [28]. Moreover, advanced injection timing reduces PM emission, which is reported by [29][30][31]. However, with the advanced injection timing, the volumetric efficiency decreased, and the overall equivalence ratio increased [32]. On the contrary to the claim of increased CO 2 emission due to advanced injection timing, Nwafor [33] reported that advanced injection timing produced the lowest CO 2 emissions due to early combustion resulting in ash formation, a result of high cylinder pressure and temperature, which was supported other researchers [34][35][36][37][38][39][40]. Raheman and Ghadge [41] reported the reduction of mean BSFC and improvement of BTE when injection timing was advanced due to having enough time for proper combustion. The exhaust gas temperature (EGT) dropped continuously as injection timing was advanced. This can be attributed to a favourable pressure-temperature profile. Hariram and Kumar [42] similarly reported that advanced injection timing exhibited a significant reduction in BSFC, unburned HC, CO, and smoke, and increase of combustion pressure, rate of heat release, brake mean effective pressure (BMEP) and NO X emissions. However, some researchers have reported that any change of injection timing from the standard value results in decreased BTE and increased BSFC [30,35].
Varying the injection pressure is another viable option to control engine performance and emission. When injection pressure is increased, it reduces the ignition delay [43]. Several researchers have reported that increased injection pressure results in an increase of peak in-cylinder pressure [44][45][46], and more fuel being burnt in the premixed combustion phase. Canakci et al. [44] reported better combustion when injection pressure is increased as it increases atomisation of fuel at the nozzle outlet and results in a more-distributed vapour phase. Sayin et al. [47] reported better BTE and reduced BSFC at higher injection due to improved atomisation and better mixing, which other researchers support, too [48][49][50]. Purushothaman and Nagarajan [51] stated that NO X emission decreased when the fuel injection pressure increases due to corresponding changes in the in-cylinder gas temperature and the lower intensity heat release rate in the premixed combustion phase and longer combustion duration, which is also supported by other researchers [52,53]. The increasing injection pressure causes a good fuel-air mixing, easy and complete combustion of the smaller droplets, which resulted in CO and HC emission reduction [44,45,[54][55][56][57]. Canakci et al. [44] attributed the reduction to easier combustion of the smaller fuel droplets because of increased injection pressure. In the case of particulate emissions, increased injection pressure reduces number concentration of particulates [45].
Mohan et al. [53] reported retardation of injection timing when injection pressure was increased, which can be attributed to an increase of required time to build up the nozzle opening pressure. Some researchers reported that if injection pressure is increased beyond a certain level, ineffective combustion occurs due to a decreased depth of penetration of fuel particles, and as a result, BTE decreases [50,58]. Furthermore, some researchers have reported engine performance deterioration when injection pressure is increased or decreased [44,45,59]. Canakci et al. [44] reported an increase of BSFC with any alternation of injection pressure. In the case of decreasing injection pressure, fuel particle diameters enlarge, and the ignition delay period increases, which in turn increases the BSFC. On the other hand, increased injection pressure causes a shorter ignition delay period, and as a result, opportunities for homogeneous mixing decrease and BSFC increases. An increase in injection pressure increases NO X emission, which is reported by several researchers [44,46,54,56,60]. Higher injection pressure helps to decrease the fuel droplet diameter, and hence fuel spray vapourises quickly. Therefore, the fuel spray cannot penetrate deeply into the combustion chamber. Consequently, it generates faster combustion rates and higher combustion temperature initially. This phenomenon increases the NOx emission. Jindal et al. [49] reported that CO emissions increased due to poor diffusion flame combustion with the increase in injection pressure. Similar results were reported by others [51,60,61].
The compression ratio is another factor that can affect the performance of a diesel engine. It is the ratio between the combustion chamber volume when the piston is at the bottom and at the top. A higher compression ratio means more power can be extracted from the small amount of fuel, which increases the engine's efficiency. For diesel engine compression ratio is typically 14:1 to 16:1 (Direct injection) and 18:1 to 23:1 (Indirect injection). Jindal et al. [49] reported that increasing the compression ratio reduces the BSFC and increases BTE. Raheman and Ghadge [41] likewise reported comparable findings. The authors reported that an increase in compression ratio increases BTE and reduces BSFC and EGT. Several other studies have found similar results regarding the relation between BTE and increased compression ratio [62,63]. Furthermore, Jindal et al. [49] reported that a higher compression ratio reduces CO emission and smoke opacity. In contrast, several studies reported that increase in compression ratio increases HC and NO X emission [49,62].
Use of advanced materials in the engine chamber's construction, by using advanced coating materials and lubricating oils, engine losses can be reduced, which in turn help improve the combustion within the engine chamber [1]. Based on several studies, the part of fuel energy devoted to mechanical power to overcome friction can be divided as follows, 30-35% to overcome engine friction [64][65][66]. Advanced lubricating oils can be used to reduce this power loss due to friction. Several studies have been conducted, which focused on the modification of combustion chamber design, which reported that it was possible to attain improved performance [46,[67][68][69].
Exhaust gas recirculation (EGR) is a technique in which some of the exhaust gas is recirculated to the engine intake. EGR is one of the essential ways to reduce NO X emission [70][71][72][73]. EGR recirculates some portion of the exhaust gas into the combustion chamber, thus reducing the availability of oxygen and reducing the peak combustion temperature [74], which helps to reduce NO X emission [75]. Moreover, low temperature combustion (LTC) techniques can help to achieve simultaneous NO X and PM reduction [73,76]. Chadwell and Dingle reported that a reduction of 60% NO X emission was possible to achieve by using 12% EGR [77]. Another study reported that, by using 10% EGR rate, NO X , and smoke were reduced by 36% and 31%, respectively [78]. On the contrary, it is reported that as EGR reduces available oxygen in the chamber, it increases soot formation [79][80][81][82]. Furthermore, Chadwell and Dingle reported an increase of 8.6% BSFC when 12% of EGR is used [77]. Agarwal et al. [74] reported that at low load, the use of EGR slightly increases thermal efficiency and reduces BSFC. This can be attributed to exhaust gas containing higher oxygen and lower CO 2 at low loads. On the other hand, the authors reported higher HC and CO emission when EGR is used. Fathi et al. [83] reported that when EGR is used at homogenous charge compression ignition (HCCI) combustion mode, it results in further reduction of PM and NO X emission, however, increased HC and CO emission is reported too. Multiple split injection strategies can be used to reduce PM emission without compensating for NO X emission [84,85]. Several researchers have reported that multiple/port injections reduce both PM and NO X emission [86][87][88][89][90][91]. However, some of them reported that using these strategies resulted in an increase in BSFC [87,88,90].
The introduction of new combustion techniques can help reduce both NO X and PM emission. One of the techniques that offer a simultaneous reduction of NO X and PM is LTC [92]. There have been several studies that evaluated the performance of different LTC techniques, HCCI, premixed charge compression ignition (PCCI) and reactivity controlled compression ignition (RCCI) combustion [93][94][95][96]. In HCCI mode, a drastic reduction of in-cylinder local temperature slows down chemical reactions that are accountable for NO X formation and reduces soot formation by reducing the presence of the high local equivalence ratio during combustion. Controlling the combustion in HCCI engine is difficult. Thus, PCCI technique evolved, which offers better control of the start of combustion. Simultaneous reduction of NO X and PM emission has been reported in several studies [97][98][99]. However, the major drawbacks of these LTC techniques are an increase in both HC and CO emission. These can be attributed to the reduction of in-cylinder combustion temperatures, and higher oxygen content results in incomplete combustion [92].

In-Combustion Fuel Modification
Modification of fuel properties to achieve improved combustion efficiency and emission reduction can be achieved by several processes blending biofuel with diesel, using fuel additives, etc. Biofuels are considered environmentally friendly, non-toxic, renewable, sustainable, etc. There have been several studies that focused on the performance improvement of the diesel engine while using biofuel. Fuel additives are used for the following reasons: improve handling properties and stability of fuel, improve combustion properties, reduce emissions from combustion, improve fuel economy, etc. [100]. Different techniques to improve fuel properties will be discussed henceforth.

Biodiesel as a Diesel Substitute
The use of biodiesel to operate the diesel engine dates back to 1900, when Dr Rudolph, the inventor of the engine, operated it using pure vegetable oil [101]. Afterwards, the use of petroleum diesel fuel popularised as it was readily available, and thus focus shifted from vegetable oils. The petroleum fuel industry is currently facing some problems, such as rapidly decreasing fuel reserves and strict laws enforced by governments to cut down engine emissions. Consequently, researchers have shifted their attention towards the search for an eco-friendly, technically feasible alternative-fuel [102,103]. Biodiesel having similar functional properties can be considered as one of the viable options to replace diesel fuel [104]. Biodiesel is produced from vegetable oil using the "transesterification" process, as is also known as fatty acid methyl ester (FAME) [105,106]. In this process, in order to chemically break the molecule of the raw renewable oil (triglyceride) into methyl or ethyl esters of the renewable oil, alcohol such as methanol or ethanol is used. The process contains three consecutive reversible reactions: conversion of triglycerides to diglycerides, conversion of diglycerides to mono-glycerides, and conversion of glycerides to glycerol. The process yields one ester molecule in each step. Properties of these esters are comparable to that of diesel. Biodiesels are considered renewable, biodegradable, non-flammable, non-explosive, non-toxic, and environment-friendly [107][108][109].
The foremost advantage of using biodiesel is that biodiesel can be used in a diesel engine (up to 20%) without making any engine modification [110,111]. Several researchers reported higher combustion efficiency, which can be attributed to having 10-11% more oxygen [112][113][114]. This extra oxygen of biodiesel allows more carbon molecules to burn, which results in complete fuel combustion. Further, as a result, less CO and HC emissions are emitted when biodiesel is used compared to diesel [115,116]. Biodiesel has a higher cetane than diesel fuel [113,117], which helps to reduce the HC and CO emission. Additionally, biodiesel has better lubrication properties, improving lubrication in fuel pumps and injector units, thus decreasing wear and tear of engine and improving engine efficiency [118,119]. Biodiesel contains a higher flash point than diesel, making it safe for handling, transporta-tion, and storage [120,121]. However, there are several drawbacks of using biodiesel in the internal combustion engine. Biodiesel exhibits 12% lower energy contents, which results in an increase in fuel consumption by almost 2-10% [112,114,[121][122][123]. Due to the use of biodiesel, engine durability problems such as injector cocking, piston ring sticking, filter plugging, etc., can occur [121]. As biodiesel has lower oxygen stability, corrosion of fuel tank, injector, and pipe can occur if biodiesel gets oxidised into fatty acids [124,125]. Due to higher oxygen content, advanced fuel injection timing, and early start of combustion, biodiesel usually emits higher NO X than diesel fuel [112,126,127]. Other factors that can cause NO X emission increase are soot radiation effects, bulk modulus effects, engine control module (ECM)-decision-making effects, prompt NO X formation, changes in fuel composition that affect fuel spray or ignition patterns within the combustion chamber and adiabatic flame temperature, etc. [128]. Some relevant studies on highly researched feedstocks have been presented in Table 3. From the results, we can see that biodiesels' addition generally results in increased BSFC, NO X emission, and reduced BTE, HC CO, and smoke emission. Poor atomisation and lower heating value compared to diesel fuel are the reason behind increased BSFC and decreased BTE [129]. Due to having a lower heating value, biodiesel and its blends release less heat during combustion, and thus to provide the same amount of power needs more fuel to be injected, thus increasing BSFC [130]. On the contrary, a study reported that biodiesel decreased BSFC and increases BTE [131] and CO, HC, and PM emissions and decreases NO X emission [104,[132][133][134][135][136]. Biodiesel is more oxygenative and it causes enhanced corrosion and material degradation [137][138][139].
Another problem with implementing the use of biodiesel is its production cost. The production price mainly depends on feedstock costs [140][141][142]. Currently, the biodiesels that are mostly used are edible in nature, such as soybeans, palm oil, sunflower, safflower, rapeseed, coconut, etc. These are known as first-generation biodiesels. The non-edible biodiesels are known as second-generation biodiesel. The advantages of using second generation biodiesels are that they are not affecting the requirements for human food, which means no food vs. fuel debate. Another promising biodiesel feedstock is algae, which is considered third-generation biodiesel. The advantages of algal biodiesel are that it is possible to get 5-20 k gallons of oil per year per acre, it is biodegradable, and a huge amount of CO 2 is consumed during cultivation. However, the production cost of algal biodiesel is high, and researchers are looking for improved technologies that can cut down the price so that it can be seen as a viable alternative to diesel fuel.

Addition of Additives to the Fuel
In recent times, chemical additives are used to improve the performance of automotive fuel. There are several types of fuel additives for diesel engine, such as cetane improvers, combustion improvers, oxygenates, ignition improver, antioxidants, etc.

Effect of Additives on Fuel Properties
The introduction of additives has a significant effect on fuel properties [153][154][155]. The effect of various additives on fuel properties is shown in Table 4. Çaynak et al. [156] studied the effect of the addition of manganese additives on the properties of pomace oil methyl ester. The authors reported that 12 µmol/Ll reduces viscosity by approximately 20%, reduces the freezing point by 15 • C and flash point by 7 • C. The authors also reported that the addition of additive results in an increase in vapour pressure-which means the additive dosage increases fuel volatility. In another study effect of the addition of Mn and Ni-based additives were studied [157], where the authors reported that both the additives reduced flash point, pour point and viscosity. Guru et al. reported that an increase in Mg-based additive concentrations from 0 to 16 mmol/l reduced freezing point, viscosity, and flash point [158]. Kannan et al. [159] reported that the addition of FeCl 3 slightly reduced flash point and improved heating value and cetane number. Yasin et al. [160] reported that the addition of methanol reduces viscosity and density and increases cetane number. From Table 4, it can be seen that kinematic viscosity is reduced up to 49% by oxygenated additives [161], up to 25% by antioxidants [162], 18% by metalbased additives [157] and up to 10% by cetane improvers [163]. Significant reduction of density was achieved by using oxygenated additives, such as methanol (reduction up to 30%) [160]. From Table 4, it can be seen that antioxidants and metal-based additives slightly increased the heating value [8,164,165]. Oxidation stability is significantly increased by the use of antioxidants [162,166,167]. Flash point was reduced when oxygenated, and metalbased additives were used [157,158,160,[168][169][170][171][172] and increased when antioxidants were used [8,166,167]. From Table 4, it can be seen that the cetane number of fuel is significantly increased when antioxidants, oxygenated, metal-based additives, and cetane improvers are used [158,160,162,163,165,[173][174][175].

Effect of Additives on Engine Performance and Emission
Oxygenates can be used to improve the combustion process by increasing the fuel's oxygen content [169,180]. Alcohols, ethers, and esters are common oxygenates. Many researchers have reported that the addition of ethanol significantly reduces emission [181][182][183]. n-butanol and diethyl ether also exhibit similar performance as oxygenated additive [184][185][186]. Biodiesel as well can be used as an oxygenated additive, and many studies have shown that the use of biodiesel reduces PM emission [128,187,188]. Oxygenated additives can also decrease ignition temperature [189,190]. However, there are some disadvantages of using oxygenated additives, such as high heat of vapourisation, low cetane number, high auto-ignition temperature, an increase of NO X emission, and inadequate lubricating behaviours, etc. [191][192][193][194]. An et al. studied the effect of ethanol edition with waste cooking oil biodiesel and reported that, at light load, the addition of ethanol reduces peak cylinder pressure [195]. Chen et al. observed that the use of ethanol as fuel additives increases both the maximum heat release rate and peak cylinder pressure [196]. The authors implied that the increases were attributed to prolonged ignition delay and a faster rate of evaporation of ethanol in the premixed combustion phase.
One of the major problems of biodiesel is its oxidation stability. When biodiesel is stored for a long time, the oxidation stability reduces rapidly. Degradation by oxidation yields products that may compromise fuel properties, impair fuel quality and engine performance. In Europe, standardisation and fuel quality assurance are crucial factors for biodiesel market acceptance, and storage stability is one of the main quality criteria. Antioxidants can be added to avoid oxidation and prolong the shelf life of biodiesel [154,197,198]. Ryu reported that tert-butylhydroquinone (TBHQ) exhibited improved oxidation stability of biodiesel and a significant reduction in PM emission [199]. Tang et al. [200] reported that antioxidants enhanced oxidation stability. However, researchers have demonstrated that the use of antioxidants results in an increase in CO and HC emission [162,166,201,202].
Metal-based combustion catalysts are another type of fuel additives that can be used to modify the combustion process and achieve fuel savings. Kannan et al. studied the effect of metal-based additives (FeCl 3 ) on combustion characteristics [159]. The authors reported that the addition of additives increases maximum cylinder gas pressure, which was due to advanced injection timing and increased ignition pressure. Moreover, the addition of additives increased the maximum heat release rate, which is attributed to the higher accumulation of fuel and early injection. Valentine et al. [203] reported that by using a bimetallic catalyst, 5-7% fuel economy, and 10-25% PM emission reduction were achieved. May and Hirs [204] reported a 50% reduction in PM emission due to the use of bimetallic catalysts.
Cetane improvers can be used to improve the cetane number of diesel fuel. Cetane improvers ensure uniform and early injection and prevent premature combustion and excessive pressure increase in the combustion cycle. It also ensures smoother combustion and efficient burning of fuel. There are several types of cetane improvers that can be used, such as peroxides, nitrites, nitrates, nitroso-carbamates, thio-aldhydes, tetra-azoles, etc. Alkyl nitrates are the most commonly used, and 2-Ethylhexyl nitrate is being considered the most prominent cetane improving additive [205]. Altiparmak [206] reported that increasing cetane number results in a reduction of NO X , SO 2 and CO emission. Tat [207] reported that the use of EHN improved cetane number by 24% and reduced ignition delay by 10%. Li et al. [163] reported a 3.87-12.9% reduction of NO x and 11.76-38.24% reduction of smoke when cetane improvers were used due to reduction in temperature and hypoxia [208]. Vallinayagam et al. [209] reported that ignition promoting additives reduced the burning rate as well as the ignition delay.
The effect of various additives on engine performance and a diesel engine's emissions is shown in Table 5. From the table, it can be seen that most of the additives help reduce NOx emission significantly.

Post-Combustion Treatment Considerations
There are numerous post-treatment processes available, such as exhaust after-treatment, Selective Catalytic Reduction (SCR), Diesel Particulate Filter, etc. Exhaust after-treatment involves a different process that treats the exhaust before it is released into the environment. A catalytic converter converts products of incomplete combustion (HC and CO) and NO X into CO 2 , H 2 O, and N 2 . Diesel Particulate Filter (DPF) removes PM from exhaust gas; Lu. Ku and Liao [225] reported that the introduction of DPF resulted in a reduction of PM weight by 92.5%.
Catalytic converters are used to convert harmful toxic pollutants present in the engine exhaust using redox reaction (oxidation or reduction) into less harmful pollutants. The most common catalytic converter used in the diesel engine is the diesel oxidation catalyst (DOC), which is used to promote of oxidation of pollutants such as HC, CO, and organic fraction of diesel particulates (SOF). Zhang et al. [226] reported a 21-38% and 8-16% reduction of HC and CO emissions, respectively, due to the introduction of DOC. The use of DOC can reduce the odour of diesel exhaust. PM consists of mainly three major fractions (solid particles, SOF, and sulphates). DOC oxidises the SOF of the PM, and thus some literature reported PM reduction due to the use of DOC [226][227][228]. Zhu et al. [227] reported that particulate mass concentration was reduced by 16-34%, and the particle number concentration was reduced by 15-38% when DOC was used. Similar results were reported by other researchers. Shah et al. [228] reported 20-65% PM reduction, whereas Zhang et al. [226] reported 10-28% particulate number reduction and 5-27% particulate mass reduction. Conversely, the use of DOC can increase sulphates by oxidising SO 2 . This can result in increased total PM emission. Furthermore, diesel oxidation catalysts have negligible/no effect on the reduction of NO X emission [226,229].
SCR is one of the proven technologies that can reduce NO X emission by converting it to nitrogen by using an outside agent like ammonia. However, as ammonia is corrosive and toxic, urea is usually used. Loganathan and Chandrasekaran [230] reported that the use of SCR reduces NO X , HC, CO, and CO 2 emission by 69-81%, 43-58%, 90-100%, and 80-84%, respectively. However, there are some disadvantages of using SCR, such as higher capital and operating costs, a large volume of catalyst requirement, strict monitoring of exhaust temperatures to avoid the formation of excessive NO X emission.
DPF is an effective option to reduce PM. DPF uses a regeneration process to convert the entrapped elemental carbon portion of PM to CO 2 by letting it through elevated exhaust temperatures. DPF can be used alongside SCR to reduce both NO X and PM emissions. Additionally, it can be used with a DOC to reduce SOF portions, which can ensure 90% total PM reduction. The disadvantages of DPF are its high cost and maintaining a threshold exhaust temperature to ensure regeneration.

Conclusions
The article's primary focus is the in-combustion fuel modification section, where a thorough analysis was provided-pre-and post-combustion modifications discussed with less emphasis. From the foregoing review of the literature, the following can be reported:

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Retardation of injection timing can reduce NO X emissions, whereas advancement reduces EGT, BSFC, HC, and CO emissions, and smoke opacity. However, the advancement of injection timing results in an increase in NO X emission. Contrary, some researchers reported an increase in PM emission and BSFC and reduced BTE when injection timing is advanced and an increase of NO X emission when injection timing is retarded. Furthermore, some study reported that any change of injection timing, advanced or retarded, results in an increase of BSFC and reduction of BTE.

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Increasing the injection pressure improves BTE and reduces BSFC. However, some researchers had reported an increase of BSFC when the injection pressure was reduced or increased. An increase in injection pressure reduces CO, HC emission and particulate number concentration. Contrary, some researchers reported an increase in NO X and CO emission with an increase in injection pressure.

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An increase in compression ratio reduces BSFC, EGT, CO emission, and smoke opacity and improves BTE; however, it increases NO X and HC emission. • LTC techniques and EGR can reduce NO X and PM emission simultaneously; however, they generally increase HC and CO emission. • Multiple or split injection strategies also reduce PM and NO X emission but increases BSFC. • Biodiesel can be used with diesel fuel, as it has better lubricity, higher flash point, emits less CO, HC, and PM emission. However, they reduce efficiency and increases fuel consumption and also emit higher NO X compared to diesel fuel. Biodiesel lacks oxidation stability. If stored for a prolonged time, stability deteriorates rapidly. Furthermore, biodiesel production cost is still higher.

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The oxidation stability of biodiesel can be improved by using antioxidant but will result in an increase in CO and HC emission. • Algal biodiesel can solve some of the problems of first-generation biodiesels, such as the food vs. fuel debate. There are a lot of researches going on which aims to find an economical production process. • Metal-based additives improve fuel economy; reduce HC, CO, and smoke emission, on the other hand, increase NO X emission. • Oxygenated additives improve combustion by increasing oxygen contents. The use of additives increases the maximum heat release rate and in-cylinder pressure. In contrast, these additives have some disadvantages: the high heat of vapourisation, low cetane number, high auto-ignition temperature, an increase of NO X emission, and inadequate lubricating behaviours. • Cetane improvers reduce NO X emission significantly; however, from the literature reviewed, there is a lack of studies, which focused on the effect of these additives on PM emission. • DOC can reduce HC, CO emissions, and SOF. However, it has little/no effect on NO X emission and sometimes can increase PM emission by producing more sulphates.
There are a robust debate and widespread demand for shifting away from combustion engines due to the harmful emissions. The use of hybrid and electric vehicles is getting popular day by day. Thus, it is imperative to continue research on combustion engines to improve their performance and limit their emission levels to a minimum. Shifting from petroleum fuels to bio-sustainable fuels is one of the options. Biodiesel is considered one of the popular alternatives. Biodiesel has some disadvantages, which can be eliminated by using additives. Thus, research on suitable additives should also be carried out. Algal biodiesel and biodiesel from waste products (such as waste cooking oil) can solve sustainability-related problems. However, extensive research should be carried out to find a way to reduce the production cost to make it economically feasible, minimise by-product generation and improve biodiesel yield. Further research on engine modification can pave the way for the construction of engine suitable for pure biodiesel utilisation.