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Review

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

by
S. M. Ashrafur Rahman
1,*,
I. M. Rizwanul Fattah
2,
Hwai Chyuan Ong
2 and
M. F. M. A. Zamri
3
1
Biofuel Engine Research Facility, Queensland University of Technology, Brisbane, QLD 4000, Australia
2
School of Information, Systems and Modelling, Faculty of Engineering and Information Technology, University of Technology Sydney, Ultimo, NSW 2007, Australia
3
Institute of Sustainable Energy, Universiti Tenaga Nasional, Jalan IKRAM-UNITEN, Kajang 43000, Selangor, Malaysia
*
Author to whom correspondence should be addressed.
Energies 2021, 14(6), 1766; https://doi.org/10.3390/en14061766
Submission received: 14 February 2021 / Revised: 15 March 2021 / Accepted: 17 March 2021 / Published: 22 March 2021
(This article belongs to the Special Issue Alternative Fuels, Energy, and Environment: Recent Advances and Challenges)
Review Reports Versions Notes

Abstract

:
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.

1. 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 also can cause morbidity in people who have respiratory or circulatory complications [4]. NOX formation is a complex mechanism, which can be divided into three parts, thermal, prompt and fuel. At first, the high combustion temperature breaks the triple bonds of “Nitrogen molecules.” Then, these nitrogen molecules dissociate into their atomic states and produce NOX while reacting with oxygen. The development of free radical in the flame front of hydrocarbon flames leads to rapid production of NOx [8]. During combustion of fuel, oxygen reacts with nitrogen bound in the fuel and forms NOX. Irritation of the lungs, lowering respiratory infection resistance, oedema, bronchitis, and pneumonia—these are the problems caused by NOX emission [4]. Exposure to heavy metals causes adverse health effects, including toxicity, severe respiratory, and cardiovascular problems and shorten life expectancy [9,10].
The acute effect of polycyclic aromatic hydrocarbon (PAH) on human health depends upon several factors, such as concentration, extent, the process of exposure, etc. [11]. Exposure to PAH may result in nausea, diarrhoea, vomiting, skin irritation, etc. [12]. Long time exposure increases the chances of lung, skin, bladder, and gastrointestinal cancers [13,14]. The most prominent aldehydes have carcinogenic effects, which are harmful to human health. Aldehyde over-exposure results in sore throat, nausea, headache, irritation of eyes, nose, skin, and throat, and difficulty in breathing and can cause chronic diseases at higher concentrations. Studies have shown that haematological, immune, neurological, and reproductive systems are each affected by N2O emissions [15]. Furthermore, human exposure to benzene may cause adverse health effects and diseases, including cancer and aplastic anaemia [16]. Toluene is a respiratory irritant that can affect the central nervous system. Inhalation of high levels of toluene vapours for a short period may cause headache, drowsiness, visual changes, nausea, dizziness, muscle spasm, and loss of coordination. Long time exposure to high-level toluene may result in attention and concentration and motor performance deficits [17]. To reduce the health effects of the combustion of diesel in internal combustion engines, regulators have imposed more and more stringent regulations on manufacturers.
The increased effect of global warming, limited efficiency of diesel engines and stringent anti-pollution laws (especially NOX and PM emissions) enforced by the governments have generated a spur to develop efficient engines with the acceptable emission level [18,19]. This development can be divided into three categories:
  • 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.

2. 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].

3. 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 CO2 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 CO2 emission due to advanced injection timing, Nwafor [33] reported that advanced injection timing produced the lowest CO2 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 NOX 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 NOX 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 NOX 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 NOX 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 NOX 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 NOX emission [75]. Moreover, low temperature combustion (LTC) techniques can help to achieve simultaneous NOX and PM reduction [73,76]. Chadwell and Dingle reported that a reduction of 60% NOX emission was possible to achieve by using 12% EGR [77]. Another study reported that, by using 10% EGR rate, NOX, 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 CO2 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 NOX emission, however, increased HC and CO emission is reported too. Multiple split injection strategies can be used to reduce PM emission without compensating for NOX emission [84,85]. Several researchers have reported that multiple/port injections reduce both PM and NOX 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 NOX and PM emission. One of the techniques that offer a simultaneous reduction of NOX 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 NOX 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 NOX 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].

4. 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.

4.1. 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, transportation, 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 NOX than diesel fuel [112,126,127]. Other factors that can cause NOX emission increase are soot radiation effects, bulk modulus effects, engine control module (ECM)-decision-making effects, prompt NOX 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, NOX 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 NOX 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 CO2 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.

4.2. 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.

4.2.1. 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 FeCl3 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 metal-based 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 metal-based 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].

4.2.2. 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 NOX 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 (FeCl3) 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 NOX, SO2 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 NOx 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.

5. 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 NOX into CO2, H2O, and N2. 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 SO2. This can result in increased total PM emission. Furthermore, diesel oxidation catalysts have negligible/no effect on the reduction of NOX emission [226,229].
SCR is one of the proven technologies that can reduce NOX 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 NOX, HC, CO, and CO2 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 NOX emission.
DPF is an effective option to reduce PM. DPF uses a regeneration process to convert the entrapped elemental carbon portion of PM to CO2 by letting it through elevated exhaust temperatures. DPF can be used alongside SCR to reduce both NOX 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.

6. 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:
  • Retardation of injection timing can reduce NOX emissions, whereas advancement reduces EGT, BSFC, HC, and CO emissions, and smoke opacity. However, the advancement of injection timing results in an increase in NOX emission. Contrary, some researchers reported an increase in PM emission and BSFC and reduced BTE when injection timing is advanced and an increase of NOX 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.
  • 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 NOX and CO emission with an increase in injection pressure.
  • An increase in compression ratio reduces BSFC, EGT, CO emission, and smoke opacity and improves BTE; however, it increases NOX and HC emission.
  • LTC techniques and EGR can reduce NOX and PM emission simultaneously; however, they generally increase HC and CO emission.
  • Multiple or split injection strategies also reduce PM and NOX 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 NOX compared to diesel fuel. Biodiesel lacks oxidation stability. If stored for a prolonged time, stability deteriorates rapidly. Furthermore, biodiesel production cost is still higher.
  • 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 NOX 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 NOX emission, and inadequate lubricating behaviours.
  • Cetane improvers reduce NOX 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 NOX 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.

Author Contributions

Conceptualisation, S.M.A.R.; methodology, S.M.A.R.; formal analysis, S.M.A.R. and M.F.M.A.Z.; investigation S.M.A.R. and I.M.R.F.; resources, M.F.M.A.Z., I.M.R.F., and H.C.O.; data curation, S.M.A.R.; writing—original draft preparation, S.M.A.R.; writing—review and editing, S.M.A.R. and I.M.R.F.; supervision, H.C.O. All authors have read and agreed to the published version of the manuscript.

Funding

No funding received.

Acknowledgments

The authors would also like to acknowledge the Research Development Fund of the School of Information, Systems and Modelling, Faculty of Engineering and Information Technology, University of Technology, Sydney, Australia.

Conflicts of Interest

The authors declare no conflict of interest.

Nomenclature

ARTEMISAssessment and Reliability of Transport Emission Models and Inventory Systems cycle
BHAButylated hydroxyanisole
BHTButylated hydroxytoluene
BMEPBrake Mean Effective Pressure
BSFCBrake Specific Fuel Consumption
BTEBrake Thermal Efficiency
EHNCyclohexyl nitrate
CHRCumulative heat release rate
CICompression ignition
DEEDiethyl Ether
DIDirect Ignition
DOCDiesel oxidation catalyst
DPFDiesel Particulate Filter
DPPDN,N′-diphenyl-p-phenylenediamine
DTBPDi-tert-butyl peroxide
EGRExhaust gas recirculation
EGTExhaust Gas Temperature
EHN2-ethylhexyl nitrate
FAMEFatty acid methyl ester
HCHydrocarbon
HCCIHomogenous charge compression ignition
IDIIndirect injection
IPInduction period
IPCCIntergovernmental Panel on Climate Change
LTCLow temperature combustion
MEE2-methoxyethyl ether
NEDCNew European Driving Cycle
NOXOxides of Nitrogen
ODAOctylated/butylated diphenylamine
PAHPolycyclic aromatic hydrocarbons
PCCIPremixed charge compression ignition
PCIPremixed compression ignition
PMParticulate matter
RCCIReactivity controlled compression ignition
SCRSelective Catalytic Reduction
SOFSoluble Organic Fraction
TBHQtert-Butylhydroquinone
TDCtop dead centre
TDITurbocharged direct injection

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Table
Table 1. Euro emission standards for passenger and heavy-duty vehicles.
Table 1. Euro emission standards for passenger and heavy-duty vehicles.
Euro StandardsPassenger Car
Nitrogen oxides (NOX)Total hydrocarbon +NOx)
THC+NOX		Particulate Matter (PM)Particle Number (PN)
mg/kmmg/kmmg/km#/km
Euro 1-970140-
Euro 2-70080–100-
Euro 350056050-
Euro 425030025-
Euro 5a1802305-
Euro 5b18023056 × 1011
Euro 68017056 × 1011
Euro StandardsHeavy-Duty Vehicles
NOXTHCPMPN
g/kWhg/kWhmg/kWh#/kWh
Euro 181.23360-
Euro 2a71.1250-
Euro 2b71.1150-
Euro 350.66100-
Euro 43.50.4620-
Euro 520.4620-
Euro 60.40.13106 × 1011
Table
Table 2. Emission standards followed in Australia.
Table 2. Emission standards followed in Australia.
CategoryGross Vehicle Mass2002/032006/072007/082010/112013/162017/18
Passenger Vehicles
≤3.5 tEuro 2Euro 4Euro 5Euro 6
>3.5 tEuro 3Euro 4
Buses
Light≤3.5 tEuro 2Euro 4Euro 5Euro 6
3.5–5 tEuro 3Euro 4Euro 5
Heavy>5 tEuro 3Euro 4Euro 5
Goods Vehicles (trucks)
Light≤3.5 tEuro 2Euro 4Euro 5Euro 6
Medium3.5–12 tEuro 3Euro 4Euro 5
Heavy>12 tEuro 3Euro 4Euro 5
Table
Table 3. Recent studies on engine performance, emission and combustion characteristics of highly researched feedstocks.
Table 3. Recent studies on engine performance, emission and combustion characteristics of highly researched feedstocks.
Biodiesel FeedstockRef.Fuel Blends UsedTest ConditionsCombustion CharacteristicsPerformance CharacteristicsEmission Characteristics
PressureHeat Release RateBSFCBTENOXHCCOSmoke/PM
Palm[143]B1015 operating
points in engine map from NEDC and ARTEMIS cycle		−1%
to +3% (pmax)		−5% to +7%--−6% to +4%--−50% to +70% (smoke)
[144]B10, B201000 rpm to 2400 rpm at full-load condition--+3.81% to +7.13%−3.04% to
−5.9%		+4.81% to +8.03%−10.23% to
−13.27%		−2.45% to
−4.79%		−26.34% to −27.45% (smoke opacity)
Jatropha[145]B20, B40, B60, B80 and B100At 75% of engine load and different speeds−10% to
−27% (BMEP)		−4% to −11%-−10% to
−33%		+10% to +47%-+2% to +16%−4% to −22% (smoke)
[146]B10, B201200 rpm to 2400 rpm at 100% load--+1.81% to +3.1%-+3% to +6%−3.84%
and
−10.25%		−16% to
−25%		-
Soybean[147]B20varying load (0 to 12 kg) at 1500 rpm--↑↓----
[148]B20, B40, B100varying loads in brake power (0, 1.1, 2.2, 3.3, and 4.4 kW) at 1500 rpm--+4.2% to +14.65%−2.61% to −8.07%+7.5% to +23.81%−15% to
−38.4%		−11.36% to
−41.7%		−20.5% to
−48.23% (smoke opacity)
Canola[149]B5, B10, B15 and B20Varying load (4.8, 3.6, 2.4, and 1.2 bar BMEP) at 2200 rpm-↓ (with increase in biodiesel percentage)Max +6.56% (for B20)Min
−4.2% (for B20)		Max +8.9% (for B20)Max +30.3% (for B20)Max
−32% (for B20)		Max −53% (for B20)
[150]B20, B50, B100At seven different speeds, 800 (idle speed) –1000–1200–1400–1600–1800–2000 rpm↓ (pmax)-↑↓----
Waste cooking oil[151]B10, B20, B30Varying load at 1500 rpm--↑↓↑↓↓↓ (smoke opacity)
[152]B100Varying loads (0–100%) at 1500 rpm↑(pmax)↑ (CHR)↑↓↓↓↑-
↑ = increased; ↓ = reduced; NEDC: New European Driving Cycle; ARTEMIS: Assessment and Reliability of Transport Emission Models and Inventory Systems cycle; pmax: maximum pressure; BMEP: Brake mean effective pressure; CHR: Cumulative heat release rate.
Table
Table 4. Effect of additives on fuel properties.
Table 4. Effect of additives on fuel properties.
Additives TypeRef.Additives UsedCharacteristics Properties
Kinematic ViscosityDensityHeating ValueFlash PointOxidation StabilityCetane Number
Oxygenated[161]DEE5, 8, 10, 15, 20, and 25%↓49%↓3.7%↓8% ↑35%
[160]Methanol5%↓ slight↓30% ↓61% ↑18%
[168]Ethanol5%↓9 to 10% avg↓slight↓up to 1.4%↓13.5%
n-butanol
DEE
[169]n-butanol5%↓12.5%↓ up to 1.6%↓1.4%↓26%
DEE
[170]DEE10% and 15% ↓2.7%↓~1%↓12.8%
[176]DEE2, 4, 6, and 8%↓26%↓1%↓2%↓28% ↑4%
Pentanol10 and 20%
butanol
Antioxidant[167]BHA2000 ppm↑ slight↑ slight↓ slight↑ 1%↑31% (BHA), ↑ up to 85% (BHT)
BHT
[162]BHA500, 750, and 1000 ppm↓ up to 25%↓0.7% avg 6.9 h (Base fuel), increased to 24.8, 11, 38.7, and 9.8 h, respectively↑up to 24%
BHT
TBHQ
[8]DPPD0.15%↑2%↑ slight↑ slight↑16%
[166]BHA2000 ppm↑1%↑1%↓0.5%↑1.3%↑ 64 to 110%
BHT
TBHQ
[177]TBHQ300, 600, and 1000 mg/kg IP increased up to 10.2 h (initial IP was 4.9 h)
[178]BHA500, 1000, and 2000 ppm↑2% ↓1.8%↑11%
BHT↑3.8%
Cetane Improver[163]EHN0.3% (EHN, CHN)
3% (MEE)		↓3 to 10% ↑8 to 40%
CHN
MEE
Metal-based[157]Mn8 & 12 μmol/L↓up to 18% ↓up to 10%
Ni
[158]Mn13.5, 27.1, 54.2, and 94.9 μmol/L↓5% ↓5% ↑5%
[179]CeO250, 100, 200, 500 ppm ↑ up to 38% (IP)
[171]Mn8 and 16 μmol/L↓ up to15% ↓ up to 15%
Mg
[164]TiO280 mg/L↑ 6%↑ slight↑ slight↑41.7%
[165]CeO220 ppm↓2.8%↓3%↑5.8% ↑~1%
[173]Iron (ii, iii) Oxide nanoparticles25 and 50 ppm ↑1%↑1.5%↑15% ↑5.5%
Mixed[174]Alumina, Ethanol and Iso-propanol5%↓9%↓ slight↓1% ↑24%
↑ = increased; ↓ = reduced; DI: Direct injection; IDI: Indirect injection; DTBP: Di-tert-butyl peroxide; DEE: Diethyl ether; BHA: Butylated hydroxyanisole; BHT: Butylated hydroxytoluene; TBHQ: tert-Butylhydroquinone; DPPD: N,N′-diphenyl-p-phenylenediamine; EHN: 2-ethylhexyl nitrate; CHN: Cyclohexyl nitrate; MEE: 2-methoxyethyl ether; ODA: Octylated/butylated diphenylamine; IP: Induction period.
Table
Table 5. Effect of various additives on engine performance and emission.
Table 5. Effect of various additives on engine performance and emission.
Additive TypeAdditive UsedEngine DescriptionFuel ConsumptionRegulated EmissionUnregulated Emission (Smoke)Ref.
COHCNOXPM
Metal-based additivesPt-Ce4–8 ppm5.9L, EGR equipped↓5 to 7% ↓10 to 25% [203]
Mg-based<500 ppmNot Given -↓20%-↓70%-[210]
CeO2 Single cylinder, naturally aspirated, water-cooled, rated speed 1500 RPM ↓ 50%↓ 23.5% ↓14.5%[211]
20 ppmSingle cylinder, 5.2 kW, naturally aspirated, 4 stroke, water-cooled, DI, constant speed↓16.3% ↓25% [165]
Mn-based8–16 µmol/LSingle cylinder, Swept Volume 395 cm3, CR 18:1, Max Speed 3600 RPM↓2 to 3%↓6 to 11%-↑10%-↓29%[171]
Mg-based↓1 to 2%↓3 to 8%---↓17%
Ferrous Picrate1:3200Single cylinder, DI,
CR 19.9:1, max power 3.5 kW		↓2%----↓6 to 26%[212]
TiO280 mg/LSingle cylinder, naturally aspirated, DI and water-cooled, Rated power 3.8 kW↓21%↓25%↓18%↑32% [164]
Iron (ii, iii) Oxide nanoparticles25, 50 ppmSingle cylinder, water-cooled, CR 17.5:1, Max power 5.2 kW↓9%↓48 to 52%↓↑ ↓[173]
Oxygenated AdditivesETBE5–15%Four cylinder, Euro 4, DI↓ 1 to 2%--↑ -[213]
Diglyme
Ethanol5%Single cylinder, DI,
CR 17.7:1, Max power 7.7 kW		-↓14 to 42%-↓7.5 to 13%--[168]
n-butanol
DEE
n-butanol5–10%Four cylinder, CR 21.1,
Turbocharged, Rated power 65 kW		↓2 to 6%↓11 to 30%↑ 28.4 to 52%↓8 to 12%-↓17 to 38%[169]
DEE
Ethanol2.5, 5%Single cylinder, air-cooled, Rated power 4.4 kW, CR 16.5:1↓4 to 7%↓13 to 17% ↓1 to 5% ↓6 to 15%[214]
DEE5–25%Single cylinder, DI, naturally aspirated, CR 18, rated power 3.7 kW-↓↑↓-↓[161]
8, 16, 24%Ricardo/Cussons ‘Hydra’ single cylinder, DI, naturally aspirated, CR 19.8 ↓↑↓-↓[215]
10, 15%Single cylinder, DI ↓↓↓ [216]
1–3%Single cylinder, DI, CR 16.5:1 ↓33%↓38%↓80% [217]
10, 15%Single cylinder, 553 cc, CR 16:1, Rated Speed 1500 RPM ↓[170]
Ethanol5, 10%Six cylinder, DI, Turbocharged, CR 18:1 ↓↑↓ (slight) ↓[218]
n-butanol8, 16%
butanol10 and 20%single cylinder, naturally aspirated, four-stroke, direct injection, 296cc↑2 to 4% ↓ [219]
pentanol↑2 to 7%
methanol10%four cylinder 1.9 TDI CR 19.5↑2 to 13%↓7 to 22%↑4 to 18%↑2 to 8%↓13 to 44.5% (soot) [220]
AntioxidantBHA500–1000 ppmDI, turbocharged, CR 19.8, Euro III standard↓4 to 10%↑20% (up to)-↓1 to 5%--[162]
BHT
TBHQ
BHA2000 ppmFour cylinder, IDI, turbocharged, CR 21.1↓marginal--↓2 to 5%--[201]
BHT
TBHQ
DPPD0.15% (m)Four cylinder, 2.5 L, CR 21.1, max power 55 kW, radiator cooling↓1 to 3%↑↑↓16% (maximum)--[8]
BHA2000 ppmFour cylinder, IDI, turbocharged, CR 21.1↓marginal-↑10 to 22%↓1 to 3%--[166]
BHT
TBHQ
ODA1%Four cylinder, IDI, CR 21, max power 39 kW-↓↓↓22%--[221]
p-phenylenediamin0.025% (m)Single cylinder, DI, CR 17.5-↑↑↓43%--[202]
L-ascorbic acid0.010, 0.020, 0.030 and 0.040% (m)Single cylinder, CR 17.5:1↑4%↓48%↓29.75↓23% ↓28.6%[222]
BHA500, 1000 and 2000 ppmSingle cylinder, 4-stroke, direct injection, air-cooled, rated power 4.4 kW, rated speed 1500 RPM↓1.6%↑15%↑10%↓11% ↑11.8%[178]
BHT↓1%116%↑11%↓9% ↑17.5%
Cetane ImproverDTBP0.5, 1, 1.5, 2, 2.5 & 3%Single cylinder, rated speed 1500 RPM, rated power 7.5kW, CR 17.5:1-↓25%-↓3 to 5%--[223]
EHN0.3~3%DI, Naturally aspirated, CR 19---↓4 to 13%-↓11 to 38% (Smoke)[163]
CHN
MEE
EHN10%Single cylinder, rated speed 1500 RPM, rated power 4.4kW, CR 17.5:1↓↓↓↓ [224]
↑ = increased; ↓ = reduced; DI: Direct injection; IDI: Indirect injection; TDI: Turbocharged direct injection; DTBP: Di-tert-butyl peroxide; DEE: Diethyl ether; BHA: Butylated hydroxyanisole; BHT: Butylated hydroxytoluene; TBHQ: tert-Butylhydroquinone; DPPD: N,N′-diphenyl-p-phenylenediamine; EHN: 2-ethylhexyl nitrate; CHN: Cyclohexyl nitrate; MEE: 2-methoxyethyl ether; ODA: Octylated/butylated diphenylamine; DTBP: Di-tert-butyl peroxide; CR: Compression ratio; ETBE: Di-tert-butyl peroxide.
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Rahman, S.M.A.; Rizwanul Fattah, I.M.; Ong, H.C.; Zamri, M.F.M.A. State-of-the-Art of Strategies to Reduce Exhaust Emissions from Diesel Engine Vehicles. Energies 2021, 14, 1766. https://doi.org/10.3390/en14061766

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Rahman SMA, Rizwanul Fattah IM, Ong HC, Zamri MFMA. State-of-the-Art of Strategies to Reduce Exhaust Emissions from Diesel Engine Vehicles. Energies. 2021; 14(6):1766. https://doi.org/10.3390/en14061766

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Rahman, S. M. Ashrafur, I. M. Rizwanul Fattah, Hwai Chyuan Ong, and M. F. M. A. Zamri. 2021. "State-of-the-Art of Strategies to Reduce Exhaust Emissions from Diesel Engine Vehicles" Energies 14, no. 6: 1766. https://doi.org/10.3390/en14061766

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