1. Introduction
Increasing fossil fuel prices and severe effects of harmful emissions have encouraged both industry and academia to develop higher efficiency power with lower environmental damage [[1], [2], [3]]. Energy and transportation sector are two main anthropogenic sources that are responsible form greenhouse gas (GHG) emissions worldwide [4,5]. As new emerging countries arise, the consumption of global energy will also increase, thus further increasing the worldwide energy demand and producing more pollutants. Many researchers are, therefore, trying to find clean alternatives fuels while at the same developing advanced combustion system to improve engine's performance, combustion and emission characteristics [6,7].
Declining fossil fuels are another driving force for industries and academia to improve and modify the fuels and engines. In both aspects, the primary concerns that need to be addressed are reducing harmful emissions and dismissing dependency on fossil fuels. Also, the effects of exhaust gas emissionsplay a vital role in recent climate change, thereby imposing the utilization of alternative and renewable fuels, such as biofuel. Yet it is possible to employ pure biofuel in diesel engines, but some obstacles, such as lower cetane number(CN), heating and higher density value, hinder it from switching to conventional diesel. Thus, using blends with biofuels in diesel engines has been introduced.
In the diesel engine field, animal fat, non-edible plant oil, SVO, and waste cooking oil can be utilized as biodiesel. One of the main concerns in biodiesel properties is a tendency to increase NOx emissions. Various studies have predicted the properties of biodiesel (viscosity and density) to investigate biodiesel capability [8,9]. However, with all these benefits, biodiesel could not be extensively applied as a complete substitute fuel for diesel. Numerous researchers have been exploring different areas to find alternative renewable sources of biofuels that are cost-effective, particularly in the production phase due to their higher cost of production [10].
Biofuels are categorized into two types: primary and secondary [11]. Primary biofuels can be directly produced from woods, plants, and crop wastes without modification [12]. Primary biofuels have a minimum process. Primitive technologies for cooking or heating are included in this type. Meanwhile, secondary biofuels are developed from feedstock after modifications with microorganisms or nanomaterials [13]. This type is generally classified into three generations: 1st, 2nd, and 3rd. Some works of literature have introduced the 4th generation of biofuels [14]. The 4th generation biofuels are more complicated to produce. They are developed from genetically modified organisms and synthetic photosynthesis reactions [15]. However, 4th generation biofuels have several challenges with the technical and biosafety aspects, as well as the complexity and diversity of the related regulations, legitimacy concerns, and health and environmental risks [16]. To increase the biofuel's characteristics, recent studies review several approaches on the role of nanoparticles in biofuel production. In addition, the effects of nanoparticles in biodiesel blends on the engine's performance, combustion and emission characteristics have been investigated by researchers extensively [[17], [18], [19]].
Among many available biofuels and modern engine systems, biodiesel and homogeneous charge compression ignition (HCCI) engine can play major roles as renewable fuel and advanced combustion engine [[20], [21], [22]]. Biodiesel has long been studied as a promising biofuel since it can be produced from renewable sources and has the potential to reduce harmful emissions from diesel engine. However, its potential has not yet been extensively studied on advanced engine combustion system such as HCCI. HCCI engine has become a major research interest due to its promising features by combining the advantage of gasoline engine (clean) and diesel engine (efficient) [23,24]. As a result, an HCCI engine can produce an engine performance similar to diesel engine but with emission level comparable or even lower than gasoline engine.
Driven by higher efficiency engine with better fuel economy and low emissions requirement, HCCI is developed as a novel approach for future internal combustion engines. Its major goal is to provide a fuel-efficient engine while simultaneously reducing NOx and PM emissions which are problematic to be solved in conventional diesel engines due to its trade-off [25,26]. In HCCI engine, the combustion occurs at multiple location in the combustion chamberresulting in no flame front, thus reducing the local temperatures [20]. Consequently, the NOx formation is reduced to near-zero level. In addition to ultra-low NOx emissions due to its homogeneous mixture, by maintaining the local fuel-air ratio to be low, the soot or PM formation, normally resulted from heterogeneous flames of diesel engine, can also be avoided. Therefore, the use of after-treatment equipment can be eliminated.
Despite its potential, HCCI engine faces great challenges to be widely used and commercially produced. The challenges include controlling its ignition and combustion phasing, reducing its high HC and CO emissions and solving its cold start problem [[27], [28], [29]]. Every approach to develop HCCI will focus on ensuring that the fuel will ignite automatically at the right time leading to lower emissions and greater fuel efficiency. Since HCCI engine works based on lean premixed combustion at low temperature, the conversion of CO into CO2which normally occur above 1500 K is difficult to be achieved. Therefore, some CO emissions could be produced. Moreover, because HCCI operates based on auto ignition from the mixture of fuel and oxygen, the combustion occurs simultaneously at multiple location in the cylinder. As a result, knocking may occur and excessive engine noise may be prevalent due to large pressure rises from prompt combustion. To solve the problem mentioned above, fuel is one of important parameter. The use of biodiesel in HCCI engine can achieve that goal.
Therefore, this paper aims to provide a review of HCCI engines with biodiesel, so that the prospects and challenges found in the previous studies could be specifically understood. Numerous kinds of literature were selected and reviewed by investigating relevant articles. Database of article from prominent journals were searched and selected using the combination of keywords: homogeneous charge compression ignition, HCCI, biodiesel, biofuel, review, experimental, simulation, diesel, performance, emission. This paper employs the progress and development of HCCI engines by reviewing numerous recent studies of a HCCI engines with biodiesel. The review discusses systematically about the development of biofuel, the promising of biodiesel, HCCI engine development, the deployment of biodiesel in HCCI engines and the significant findings of studies from five last year publications. Then, each of the discussions are summarized into the prospects and challenges aspect as a direction for further study. Although extensive studies have been discussed on HHCI engines with biodiesel in specific experiment, less studies exist which review HCCI (Table 1) with biodiesel with comprehensive discussion in both aspect; fuel and engine. It is important to make continuity discussion from fuel aspect to the engine development and the outcome result, such as engine's performance, combustion and emission characteristics. Fig. 1 shows the outline of the present review and assists in a better understanding of it.
Table 1. Review articles on HCCI in five last year publications.
| Author(s) | Year | Review Articles on HCCI (Title of Article) | Main Discussion | Subject |
|---|---|---|---|---|
| Padey et al. [30] | 2018 | A review of combustion control strategies in diesel HCCI engines |
R&D on the fuel mixture process (stable combustion):
|
|
| Alias et al. [31] | 2019 | A review of hydrogen addition in an HCCI engine fuelled with biofuels |
Ignition control strategies:
|
|
| Chaudhari and Deshmukh [32] | 2019 | Challenges in charge preparation and combustion in homogeneous charge compression ignition engines with biodiesel: A review |
|
|
| Saiteja and Ashok [14] | 2021 | A critical insight review on homogeneous charge compression ignition engine characteristics powered by biofuels |
|
|
| Verma et al. [33] | 2021 | Performance characteristic of HCCI engine for different fuels | The increasing performance characteristics of HCCI engine for various fuels and additives |
|
| Duan et al. [34] | 2021 | A review of controlling strategies of the ignition timing and combustion phase in homogeneous charge compression ignition (HCCI) engine | Effective techniques and controlling strategies used in the HCCI engine |
|
| Verma et al. [35] | 2021 | Emissions from homogeneous charge compression ignition (HCCI) engine using different fuels: a review |
|
|
| Rao and Sharma [36] | 2021 | Prospects of hydrogen in HCCI engines - A review |
|
|
| Minh et al. [37] | 2022 | A review of internal combustion engines powered by renewable energy based on ethanol fuel and HCCI technology |
|
|
| Chauhan et al. [38] | 2022 | State of the art in low-temperature combustion Technologies: HCCI, PCCI, and RCCI |
Comparative analysis of LTC techniques (HCCI, RCCI, PCCI):
|
|
Fig. 1. Schematic representation of a present review article.2. Classification of biofuel
Feedstock plays an important role in biofuel production as it accounts for 75% of the whole production cost [39]. While many are concerned with the high cost and dilemma of food vs fuel of biodiesel derived from edible oils, low-cost biodiesel from non-edible sources such as agriculture wastes is a more convincing choice [[40], [41], [42]]. This is known as the second-generation biofuels. Moreover, the opportunity to produce biodiesel from algae is also attracting global interest in the last 10 years [[43], [44], [45]]. This type of biodiesel is known as the third generation of biofuels. The three classifications of biofuels along with their feedstocks sources and products are shown in Fig. 2.
Fig. 2. The classification of biofuels based on generation.2.1. 1st generation biofuel
The first-generation biofuels are produced from edible sources such as palm oil and soybean oil. During the late 90s, soybean oil was primarily used in the US, being the only oil that is sufficiently available to the national demand [46]. Yet, production cost to produce this edible vegetable oil was expensive. As a result, it was only used in cases of serious scarcity of petroleum diesel fuel. To be commercial visible, it is important to reduce the cost of the feedstock. In addition to soybean oil, jatopha has also been produced commercially as an emerging biodiesel feedstock [[47], [48], [49]].
In general, vegetable oils are increasingly popular as a non-toxic renewable alternative diesel fuel [50]. The esters from vegetable are known as biodiesel. Many researchers have pointed out that biodiesel's properties from vegetable oils bear a close resemblance to those of diesel fuel. As a result, it can be widely used in diesel engine with little or no alteration. The essential advantages of biodiesel in internal combustion engines are; higher cetane number than conventional diesel fuel, no aromatics, and contains 10%–11% oxygen by weight. As a result, these will decrease the emissions of CO, HC, and PM compared to diesel fuel [12]. The problem of first generation of biodiesel has always been the dilemma with human food supply, leading to an increase in the feedstock prices. One possible way to make it more affordable is to use feedstocks that are less expensive from non-edible sources. Conversion of waste fat and oil into higher-generation biodiesel against first-generation biodiesel produced from edible resources is well aligned with the important principles of sustainable development [51].
2.2. 2nd generation biofuel
Second generation biofuels are produced from non-edible feedstocks [52]. They are made by means of advanced technological processes in order to replace the necessity of producing biofuels from edible sources [53]. Second generation biofuels are projected to overcome the restrictions of the previous generation biofuels while at the same time decreasing net carbon emission, increasing energy efficiency and reducing energy dependency.
The utilization of farming and wood waste is believed to reduce the cost of biofuel production with regards to the cost of first-generation biofuels from edible sources [54]. Agricultural residues such as maize, wheat, barley, rice and rye can be utilized to produce bioethanol [55,56]. Used cooking oils, greases, and animal fats are good sources for economical biodiesel. Many countries are concerned with the issue of increasing food prices if biodiesel is produced from edible sources. Hence, second-generation biofuels are developed, and its feedstock examples are shown in Table 2.
Table 2. Second generation biofuel feedstock from various sources.
| Farm & Wood Waste | Barn, Citrus waste, Corn stover, Green waste, Industrial waste, Sugarcane bagasse, sawdust, Wheatstraw, Waste ricestraw, Wood, Wood chips, etc |
|---|---|
| Vegetable Oils | Calophyllum Inophyllum, Corn oil, Castor bean, Cottonseed, Jatropha, Palm, Pogamia Pinnata, Rapeseed, Soybean, Sunflower, etc |
| Organic Waste | Food waste, Municipal solid waste, Olive pulp, Recycled cooking oil, Wastewater from pulp and paper industry, etc |
Trees offer higher heating values than crops from farming for biofuel production [57,58]. The forestry industry produces great amounts of sawdust, wood chips and other wood wastes that can be used as biofuel feedstock [59,60]. In addition, trees can be fully grown in negligible land, decreasing rivalry for space with food crops. In the present day, only an insignificant percentage of liquid biofuels are from wood or forest residues.
Low-cost sources such as used oils from restaurant have also been widely used recently [[61], [62], [63]]. Waste cooking oils are abundant in most part of the world. In fact, the biodiesel from used cooking oil has been produced commercially in some countries. However, producing produce fuel-grade biodiesel from these used cooking oils is far more difficult than that of first-generation biodiesel such as from palm oil. This is because used cooking oils encompass high level of free fatty acids [64,65]. Therefore, second generation biofuels have not yet been produced commercially on a large scale due to lack of technological innovation and infrastructure requirements.
2.3. 3rd generation biofuel
With more than 70% of earth's surface is covered by water [66], algae as energy crops are proposed as the next generation biofuels due to their capability to grow in sea water and on non-arable land. Moreover, algae can produce oil within merely 3–5 days, making it readily available to be harvested on daily basis while a crop cycle may take the production ranging from three months to three years. On 1-ha wasteland, oil from algae can be produced over 10–100 times more than any other source of oil-crops [67].
In addition to that rapid growth rate, it is important to note that algae could solve the dilemma of using edible oil as fuel due to their uncomplicated requirement to be produced. Algae can grow in sea water environment and non-potable water on wastelands in which hardly anything can grow. Algae also only need light, sugars, CO2, N, P, and K to yield great amounts of lipids, proteins and carbohydrates resulting in biofuels and useful chemicals in short time.
In contaminated water, algae absorb the urea produced by animals while at the same time release more O2 for the atmosphere by absorbing CO2, decreasing emissions level on the air [68]. Oil derived from algae can be converted into wide variety of biofuels by means of liquefaction, pyrolysis, gasification, extraction and transesterification, fermentation, and anaerobic digestion[69,70]. It is, therefore, algae would genuinely be viable alternative of huge potential for the future world energy demands. To understand the feasibility of different feedstocks as biodiesel resources, Table 3 provides a comparison of the oil content that can be extracted from various feedstocks.
Table 3. Oil content for biodiesel production from three generation biofuels [[71], [72], [73], [74], [75]].
| 1st Generation biodiesel | Oil content (%) |
2nd Generation biodiesel |
Oil content (%) |
3rd Generation biodiesel |
Oil content (%) |
|---|---|---|---|---|---|
| Sunflower oil | 25–35 | Jatropha oil | 30–40 | Micro algae | 30–70 |
| Soybean oil | 15–20 | Stillingia oil | 44.15 | ||
| Rapeseed oil | 38–46 | Karanja oil | 27–39 | ||
| Peanut oil | 45–55 | Neem oil | 20–30 | ||
| Olive oil | 45–70 | Castor oil | 45–50 | ||
| Canola oil | 40–45 | Rubber seed oil | 53.74–68.35 | ||
| Palm oil | 30–60 | ||||
| Coconut oil | 63–65 | ||||
| Linseed oil | 40–44 |
While algae, crop wastes, perennial grasses, wood and wood waste are still on their ways to become commercial fuels, research on the fourth generation are being conducted globally. However, in general, vegetable oils that are extracted from oil seeds can be used as biodiesel. Despite its established technology and potential supply, the use of vegetable oils has been a debatable issue over the past decade as it is competing with human food consumption. It is a great dilemma for mankind when it comes to replacing crops used for human consumption. It can cause a rise in food-grade oils prices. Moreover, the demand of biodiesel is far more than the plant oils could provide. The supply of actual feedstock cannot fulfil current the potential market. The search for plantation could eventually lead to unhealthy land competition and extensive cultivated fields. As a result, it may damage the ecosystem and create problem associated with biological diversity.
Most current research on microalgae today attempt to develop optimized organisms that are high in productivity and value. This is done by applying metabolic engineering and genetic methods in order to obtain their optimum processing capabilities [25,31]. The scarcity of fossil fuel and the urgency to decrease the dependency of foreign oil have led an increase in the production of biodiesel. One possible way for biodiesel to be a feasible alternative fueldepends on its competitive price.
3. Biodiesel as promising biofuel
Biodiesel is beneficial to reduce life cycle of CO2, thus reducing the risk of global warming. It is true that carbon dioxide is released into the environment when biodiesel is used, but plants used for biodiesel production consume carbon dioxide from the atmosphere in their cycle. Also, since various feedstocks can be used to make biodiesel. Biodiesel producers can choose the cheapest available feedstock, thus reducing the fluctuations of biodiesel's price on the market [76].
As a renewable biofuel that can be produced domestically, biodiesel offers several benefits to numerous interests, including economy and environment sectors. It is originally intended to reduce the use of petroleum diesel and improve the emissions from diesel engines. Biodiesel is normally produced from vegetable oils, waste cooking oils and animal fats. However, these sources are limited, thus hampering its mass production. On the other hand, biodiesel from edible sources can affect global food markets and threaten food security. As a result, non-edible biodiesel has attracted more attention such as microalgae.
3.1. Production of biodiesel
A chemical reaction is involved in the biodiesel process. Transesterification is the chemical reaction which transforms vegetable or animal fat to biodiesel [77]. The biodiesel industry is therefore a chemical industry. It is produced using alcohol, such as methanol or ethanol to chemically bind to vegetable oil or animal fat [78]. Biodiesel and glycerine are produced as the final products, with biodiesel being at the top layer with relatively lighter colour, while glycerine sunk to the bottom with darker colour. It is important to remember that a commercial biodiesel available in the market should comply with the ASTM standard D6751 [[79], [80], [81]]. Compliance testing such as ASTM may be costly, particularly for small biodiesel producers, but it is the most reliable way of ensuring the consumers of its biodiesel quality. Numerous study investigated the optimization of operation parameter to produce biodiesel [82,83].
In latest trends, biodiesel production is complicated process that involves nonlinear relationships between the input and output data with multivariable. Due to the complexity and nonlinearity, ML or AI technology, such as ANN were introduced by numerous researchers [84,85]. The tools are required for handle, design, control, optimization, and monitoring. Data-driven ANN have demonstrated superior predictive capability compared to conventional methods using reaction temperature, reaction time, calcination temperature, pressure, and flow rate as input variables and FAME content, viscosity, composition, quantity, CN, and density stand as output variables.
3.1.1. Transesterification
Transesterification (alcoholysis) is a straightforward process of reacting long and branched chain triglycerides (oils and fats) with an alcohol (catalyst) in order to produce mono-ester (biodiesel) and alcohol (glycerol). It comprises of three reversible reactions. Triglycerides are transformed to diglycerides which then converted into monoglycerides. The glycerides then become glycerols and each step results in an ester molecule whose properties are similar to diesel fuel. Its simplified transesterification chemical reaction is shown below:
Oils/fats (triglycerides) + alcohol ⇌ biodiesel (top layer) + glycerol (low layer).
Although esters in the form of biodiesel are the desired products of the transesterification process, the recovery of glycerine is also essential for several industrial applications. In conventional biodiesel production, alcohol is used as an acyl acceptor that produces glycerol as a by-product. Thus, the increase in global biodiesel production will lead to an excessive production of glycerol [86]. Consequently, the production of biodiesel becomes not economical feasible. This leads researchers to find alternative ways to address this problem. One solution is to replace alcohol with different solvents so that a new profitable by-product could be formulated. Scientists have, for example, strongly encouraged the development and use of methyl acetate to produce biodiesel without glycerol being formed as the by-product [87,88]. In this novel method, instead of glycerol being produced, a by-product called as triacetin is produced [89,90]. It is commonly used in the medical and cosmetic products. In fact, it can be used as a fuel additive to enhance engine performance.
3.1.2. Oils sources
In terms of its oil sources, biodiesel products can be produced from almost any source from vegetable oil and animal fats. All these sources contain triglycerides. Biodiesel can therefore be made from soybean oil, canola oil, beef tallow, and pork lard. Also, exotic oils such as walnut or avocado oil can be great sources for biodiesel production. Even used cooking and waste oil can be used to produce biodiesel, but they come with some challenges. It is because cooking and waster oil usually have contaminants that need to be filtered before the oil is turned into biodiesel.
Note that non-modified vegetable oil sometimes referred to as SVO or WVO are not considered as biodiesel. A number of studies have used SVO in diesel engines with different degrees of success as a result of its high viscosity and low volatility properties. Therefore, biodiesel is generally chosen over SVO and WVO because the esterification involves in the production biodiesel converts the oil and fat into compounds comparable to petrol diesel fuel. In fact, U.S. engine manufacturers prohibits the use of SVO and WVO.
3.1.3. Alcohol
Regarding the use of alcohol for biodiesel production, various type of alcohols can be used including methanol, ethanol, propanol and butanol. The esterification yield does not depend on the type of alcohol used. It is selected based on the issue of cost and efficiency. In this regard, methanol is extensively used due to its affordability.
In most cases, methanol is used for their transesterification processes, making biodiesel merely 90% renewable as it is mainly composed by methyl esters. Biodiesel produced using methanol is known as the FAME as its fatty acid ester are derived by transesterification process with methanol. Methanol is the most commonly used alcohol to produce biodiesel. Note that skin contact with methanol and breathing its vapours can result in severe problems. It is highly toxic, and it can cause blindness or even death with just small amount being swallowed. However, since methanol is normally cheaper than ethanol, it is used in the production of biodiesel more often.
The use of bioethanol, by the contrary, produce a 100% renewable fuel, being composed by ethyl esters. Thus, biodiesel from bioethanol would consequently decrease CO2 emissions more significant than biodiesel from methanol. However, few studies have been documented about the effect of biodiesel from ethanol on the performance and emissions of diesel engine with respect to biodiesel from methanol. Note that since ethanol is a type of alcohol used in alcoholic beverages, it is not harmful in small amounts. However, due to the tax regulations for alcoholic drinks, the use of ethanol is susceptible to very difficult strict government rules.
3.1.4. Catalyst
A catalyst is required for the chemical reaction used to produce biodiesel. Typically, a catalyst is a chemical introduced to the reaction to accelerate the reaction. Since the catalyst in the reaction is not absorbed, it will be left in a certain way at the end. The compound catalyzing the reaction is known as methoxide. Methoxide can be produced by dissolving sodium hydroxide or potassium hydroxine in methanol. Note that major manufacturers purchase a solution of sodium methoxide in methanol as it is relatively safer to use.
3.2. Benefits of biodiesel for engines
Biodiesel may not be able to entirely substitute diesel fuel from fossil fuel, yet it can assist the achievement of a balanced energy usage. One of the benefits of using biodiesel is that it can be used in the existing engines without major modification. Certain changes, however, should be made for old vehicles whose fuel lines contain natural rubber. They should be changed since biodiesel will cause rubber fuel lines to crack. On the other hand, for modern car equipped with DPF, a dilution of fuel or oil may occur in the fuel system.
Another advantage of using biodiesel is its superior lubricity. It is known that diesel engines depend on how good the fuel provide lubrication to the fuel injection system. Therefore, the use of diesel-biodiesel blends can improve their overall's lubricity. Also, since nowadays diesel fuel contains less sulphur, its lubricity can be deteriorated as the compounds that used to provide lubrication have disappeared. This could shorten the lifespan of engine's fuel injection system [91].
3.3. Challenges with biodiesel
Two important factors should be taken into consideration when using biodiesel in internal combustion engines. Biodiesel is known for its high viscosity and low volatilities. These two properties can cause long-term problems on engine performance. Higher viscosity affects the fuel droplet size, fuel penetration in the cylinder and causes poor atomization qualities which thus affecting the combustion quality [92].
Much effort has been done to improve engine performance, combustion and emission behaviours using biodiesel. The diesel-biodiesel blends can reduce the PM, CO, THC, SO2 and PAH emissions from diesel engines. NOx emissions, however, were found to be higher with biodiesel use [93]. High level NOx emissions of biodiesel can be a huge challenge when more stringent emission regulations are imposed in the coming years. To reduce NOx emissions, EGR is frequently employed. However, the use of EGR also leads to PM. To solve the trade-off relationship between NOx and PM, emulsified biodiesel is often used. It is oxygenated and water-containing solvent additives to reduce both NOx and PM and can be used without engine modifications with little impact on engine performance. However, fuel emulsions with high water contents can severely affect the injection systems due to their low lubricity.
Another solution to reduce NOx is by using alcohol additives. Alcohol has higher vaporization heat which can result in a cooling thus reducing NO formation. Ethanol was previously investigated but due to its low solubility in diesel, as well as low lubricity, CN, energy density and heating value, butanol is more preferred. Both ethanol and butanol can be produced by fermentation of ABE from biomass feedstock. The final products are acetone (22–33%), butanol (62–74) and ethanol (1–6%), being butanol the major component [94].
In general, the addition of butanol into diesel-biodiesel blends in compression-ignition engines have reduced the soot emissions and increased the HC emissions. In terms of fuel consumption, CO and NOx emissions, however, contradictory results were reported. While some studies showed that butanol addition in diesel-biodiesel blends increased the engine fuel consumption, others showed the opposite trend. This conflicting result may be resulted from many factors such as fuel type, butanol percentage, engine specification and operating conditions.
Ibrahim [95] investigated and compared the effects of butanol-diesel-biodiesel blends in which butanol was added into B0, B100 and B50 on diesel engine performance, combustion characteristics, NO emissions and engine stability. The results showed that the optimum blends were the B50 where the maximum engine thermal efficiency increased by 6.5% and the lowest engine brake specific fuel consumption decreased by 5% compared to the diesel fuel. The NO emission increased slightly with the use of oxygenated fuel and as the engine load increased, NO emission increased significantly. The combustion duration also increased with increasing the engine load. It was concluded that changing fuel type had negligible effect on the combustion start timing and engine stability.
Few studies can be found on the use of butanol/vegetable oil/diesel fuel blends in diesel engines. Furthermore, studies concerning the use of butanol and non-edible biodiesels feedstock such as microalgae are also limited. Since microalgae can synthesize high amount of lipids, it is considered as the source of renewable biodiesel that can meet the global demand for transport fuels. Tüccar et al. [96] used microalgae biodiesel, butanol and diesel blends to investigate its performance and emissions on diesel engine. The results showed that even though butanol addition reduced the torque and brake power slightly, it improved the CO, NOx and smoke opacity emissions.
Considering its problems when used in conventional diesel engines, biodiesel can be implemented in advanced combustion engine such as HCCI. HCCI operates on a premixed charge of fuel and air as in gasoline engine, yet the combustion occurs due to compression as in diesel engine. Therefore, both PM and NOx emissions could be reduced simultaneously while maintaining the efficiency of diesel engines.
In addition, economic aspect become also the main challenge in biodiesel production. Reduction of cost production can be achieved through improving productivity of the technologies to increase yield, reducing capital investment cost and reducing the feedstock price [10,97]. However, the overall cost-benefit ratio of employing biofuels is significantly higher [98]. Biofuels have the potential to become less expensive in the future, given their rising demand. To reduce biofuel cost, the fuel characteristics must also be upgraded, which can be accomplished by introducing nanoparticles [17].
The economic assessment is critical decision for commercial-scale biodiesel production from feedstock. The total investment cost (fixed and running capital investment) to produce biodiesel vary depending on a number of factors, such as the technology selected, type of catalyst, the plant capacity, raw material type and feedstock price. However, the feedstock price is the most significant factor in biodiesel production, which covers about 80% of the total biodiesel production cost [99]. Besides, the catalyst price is also influenced biodiesel production. Moreover, the utilization of low-cost materials, such as biomass waste-derived catalysts would minimize biodiesel production and support the sustainability ecosystem.
4. HCCI engine
Despite being considered as relatively new combustion concept, the HCCI may have existed as long as the presence of conventional SI and CI engines. However, the development of HCCI was not explored further until stringent emissions was imposed in the mid-1990s to reduce both NOx and PM. This marks the beginning and massive research in HCCI combustion. Table 4 compare the characteristics of SI, CI and HCCI engines and the comparison is illustrated in Fig. 3.
Table 4. Comparison between SI, CI and HCCI engine.
| Parameter | SI Engine | CI Engine | HCCI Engine |
|---|---|---|---|
| Combustion control | Spark plug | Injector | N/A |
| Ignition | Spark ignition | Compression ignition | Compression ignition |
| Mixture | Premixed | Non-premixed | Premixed |
| Burning rate control | Speed of flame propagation | Time of fuel vaporization and mixing | Chemical kinetics |
| Emission | NOx, ↑ CO2 | ↑ PM, NOx | ↑ HC, CO |
| ↓ CO2 | ↓ NOx, PM, CO2 |
Fig. 3. Comparison HCCI, SI and CI engines.Due to the absence of direct control mechanism to initiate its combustion, HCCI engine suffers from the difficulty to control its combustion phasing, thus lowering its operating range [100,101]. At low loads, misfire as well as high HC and CO emissions may occur due to incomplete combustion. At high loads, soot, NOx and knocking may be present resulting from the rich mixtures and increased in-cylinder temperature. Therefore, it is important to control the combustion phasing of HCCI engine.
Several methods are found in the literature to indirectly control its combustion phasing such as the use of preheater [102,103] and EGR [104,105]. Heating the intake air can effectively control the combustion phasing. As a result, too early or too late autoignition of HCCI engine can be avoided. The use of preheater will also improve the fuel evaporation. Other approaches to reducing the burning rate include high air-fuel ratios utilization, water injection and atomization. The limited HCCI operating range can also be overcome by finding the appropriate fuel and blends that can fulfil its engine's requirement. In general, a combination of several approaches is sometimes required simultaneously to successfully control HCCI combustion phasing. HCCI is considered as a flexfuel engine meaning that it can use a variety of fuels. Since the use of biodiesel is more prominent to be used in diesel engines, this review article focuses on the application of HCCI combustion in diesel engines.
4.1. HCCI in diesel engines
This section is devoted to discussing the strategies to achieve HCCI combustion in diesel engine. To prepare the homogeneous mixture of HCCI engine, several studies have used a port injection system to allow the charge to be evenly mixed. However, in order to obtain the HCCI combustion, heating the intake air were also required. This is done to control its combustion phasing. Although these two basic processes have enabled the HCCI engine to produce ultra-low PM and NOx emissions, achieving autoignition of a homogeneous mixture is still considerably challenging. Two important factors must be addressed in HCCI engine. First is how to form a homogeneous mixture as well as improve its homogeneity and second is how to ignite such mixture and control the combustion phasing.
4.2. Pioneering research in HCCI diesel engines
In diesel engines, some early 2-stroke and 4-stroke engines run on the concept of premixed mixtures using early injection onto the preheated chamber. In the 1940s, PAW developed a small 2-stroke diesel model airplane fuelled with kerose, oil and ether blends [106]. The fuel blends flow into the intake through carburettor, forming a premixed of air and fuel. To fire the engine, the compression screw was used to set the engine to a higher compression ratio. Once the engine has started, the compression was unscrewed to obtain maximum power. The engine could produce 0.06 bhp to 1.2 bhp at speeds from 10,000 rpm to over 20,000 rpm using the concept of HCCI combustion.
4.3. Fundamentals of HCCI diesel combustion
The combustion of homogeneous mixture in HCCI engine fuelled with diesel fuel is characterised by the two-stage heat release region as shown in Fig. 4[[107], [108], [109]]. A significant amount of heat is produced in the first stage. After this, heat release is absent, and this region known as NTC. Despite the absence of heat release, the temperature increases significantly to the second stage which is the main heat release region. This is where the engine performance is affected. When the fuel is injected early in the compression stroke, the of autoignition chemistry leads to the first peak of heat release. The combustion phasing of this first stage heat release is controlled by charge temperature. Once the homogeneous mixture is formed, the autoignition will then occur regardless the start of injection if no other engine parameter is changed.
