Biodiesel, as a replacement for diesel, is easily made from renewable biological sources, such as vegetable oil and animal fats. Chemically, biodiesel is referred to monoalkyl-esters of long-chain fatty acids, and to a variety of ester-based oxygenated fuels. It is well known that transportation almost depends entirely on fossil fuel, particularly petroleum-based fuels such as gasoline, diesel fuel, liquefied petroleum gas (LPG), and natural gas (NG). An alternative fuel to mineral diesel must be technically feasible, economically competitive, environmentally acceptable, and easily available. The current alternative diesel fuel can be termed biodiesel. It can be used in diesel engines with little or no modification [5,6].

The global warming may be reduced by using alternative fuel—biodiesel is considered as a promising fuel. The best potential future energy source in the transportation sector is biodiesel that mainly emitted carbon monoxide (CO), carbon dioxide (CO₂), oxides of nitrogen (NO_x), sulfur oxides (SO_x), and smoke. The combustion of biodiesel alone provides a high reduction in total unburned hydrocarbons (HCs) and polycyclic aromatic hydrocarbons (PAHs) [7]. Biodiesel further provides significant reductions in particulates and carbon monoxide over mineral diesel fuel. Biodiesel provides slight increases or decreases in NO_x depending on the engine family and testing procedures that follow [8]. Currently, global warming caused by CO₂ is the main climatic problem in the world; therefore, environmental protection is important to ensure a better and safer future. Because biodiesel is made from renewable sources, it presents a convenient way to provide fuel while protecting the environment from unwanted emissions. Biodiesel is an ecological and nonhazardous fuel with low emission values; therefore, it is environmentally useful. Using biodiesel as an alternative fuel is a way to minimize global air pollution, and in particular, it reduces the emission levels that are potential or probable threats to human health [9].

Biodiesel properties are one of the noteworthy issues that restrict the use of biodiesel fuel. It has a direct effect on the main engine elements and parameters, such as fuel handling systems and performance [10,11]. Neat biodiesel contains no petroleum, but it is traditionally blended with mineral diesel to create a biodiesel blend, typically at 20% (by volume), or less. Pure biodiesel is biodegradable, nontoxic, and essentially free of sulfur and aromatics. Biodiesel is the only alternative fuel for compression ignition (CI) that has fully completed the health effects testing requirements of the 1990 Clean Air Act Amendments. Biodiesel, produced to industry and meet the specifications of ASTM D6751, is legally registered with the Environmental Protection Agency as a legal motor fuel for sale and distribution [12].

In the diesel engine, air is compressed with a compression ratio typically between 12 and 24 [13]. This higher compression promotes more efficient fuel combustion due to the longer effective expansion stroke. Furthermore, diesel engines run lean overall, except at full power, with air–fuel ratios as high as 65:1 [13]. Engine speed and power are controlled by changing the amount of fuel in each injection instead of throttling the intake air, which gives diesel engines high thermal efficiency. Due to their lower fuel consumption and CO and HC emissions, diesel engines are used in many trucks and almost all railroad engines and ships, and the power sources use diesel engines for different industrial applications.

In the transportation sector, diesel engine is widely used, due to its simple arrangement design, higher reliability, more power delivered at lower fuel consumption, and higher thermal efficiency [14]. In general, the diesel engine utilization for different applications depends on the engine speed, where engines with speed of 1200 rpm and above are used for transportation. These engines are equipped with pump and small electrical generators. Meanwhile, the medium speed engines from 300 to 1200 rpm are mostly used in large electrical generators, ships, large compressors, and pumps.

The biodegradability of biodiesel has been proposed as a solution to waste problems. Biodegradable fuels, such as biodiesel, have an expanding range of potential applications and are environmentally friendly. Therefore, there is growing interest in degradable diesel fuels that degrade more rapidly than the conventional petroleum fuels. Biodiesel is nontoxic and degrades faster than mineral diesel due to its higher oxygen content (typically 11%) [26,27]. Furthermore, the high oxygen content of the biodiesel fuel improves the combustion efficiency due to the increase of the homogeneity of oxygen with the fuel during combustion. Therefore, the combustion efficiency of biodiesel is higher than that of mineral diesel [28,29]. Biodiesel has good lubricant properties compared to mineral diesel oil, in particular very low-sulfhur diesel. This is crucial in reducing wear on the engine parts and the injection system [30,31].

Biodiesel has lower carbon and hydrogen contents compared to diesel fuel, resulting in about a 10% lower mass energy content, and also biodiesel has a higher viscosity and higher density [32,33]. These properties may result in high NO_x emissions, lower engine efficiency, injector coking, and engine compatibility. The high viscosity and low volatility of biodiesel fuel will cause problems in fuel pumping and spray characteristics as well as cause poor combustion in diesel engines. The inefficient mixing of fuel with air contributes to incomplete combustion. Biodiesel has a higher cloud point (CP) and pour point compared to diesel. Therefore, biodiesel fuels are plagued by the growth and agglomeration of paraffin wax crystals when ambient temperatures fall below the fuel's CP, which may cause start-up problems, such as filter clogging in cold climates [34]. While the CP of mineral diesel is reported as −16°C, typically biodiesel has a CP higher than 0°C, and because of that its use is limited to ambient temperatures around freezing [35].

ASTM has established standard specifications for biodiesel fuel blend stocks (B100) for middle distillate fuels, called ASTM D6751 [62], as well as for biodiesel blends of B6 to B20 in mineral diesel, called ASTM D7467 [63]. Blends of B5 and below are permitted under the standard specifications for No. 2 diesel fuel, ASTM D975 [64]. To date, the CEN has only established standard specifications for B100, called EN 14214 [65], but not for mid-level blends, such as B20. The European standard specifications for conventional No. 2 diesel fuel (EN 590) permit blends of B7 and below; and deliberations are underway to allow an increase to B10 [66]. Appendix A provides a side-by-side listing of specifications for biodiesel blend stock (B100; ASTM and CEN) and mid-level biodiesel blends (B6–B20; ASTM only). For each specification, both the limits and the methods are shown [11].

Various factors contributing to the cost of biodiesel include raw material, other reactants, nature of purification, its storage, and so on. However, the main factor determining the cost of biodiesel production is the feedstock, which is about 80% of the total operating cost. Therefore, a great economic advantage could be achieved simply by using more economical feedstock, such as waste fats and oils [17,23]. Although the use of renewable energy as an alternative energy source is growing rapidly, it provides only 10% of the world's primary energy consumption in 2010 [68]. It is rather surprising that even in a country such as Malaysia, where biomass can be obtained easily, the use of renewable energy is still very low.

The high oil yield of feedstock is necessary for ensuring large production scale in cheap prices. In terms of the production cost, palm oil stands out as the least expensive oil to be produced per tonne compared with other major vegetable oils as shown in Fig. 18.5 [37]. For instance, the production cost of rapeseed oil in Canada and Europe is more than double the price compared to palm oil. This may be rationalized as oil palm being perennial (it grows every year without annual sowing) and may be considered a low energy–input crop. A major cost of production in the biodiesel market involves about 75% of raw material cost and the remaining 25% for the other manufacturing and production costs as shown in Fig. 18.6 [69]. Therefore, selecting the cheapest feedstock, such as palm oil–based fuel, is vital to ensure low production cost of biodiesel.

The typical GHG emission saving for the main feedstock of biodiesel is shown in Fig. 18.7 [70]. The life cycle analysis (LCA) conducted on various biodiesel reveals that palm oil–based biodiesel can reduce GHG emission by 62% as compared to soybean oil (40%), rapeseed oil (45%), and sunflower oil (58%) [70]. Palm biodiesel has a higher cetane index compared to all types of biodiesel derived from vegetable. Its oxidative stability is four times higher than biodiesel from soybean and it has higher lubricity than Diesel.

Biodiesel blends are being considered to replace pure mineral diesel in many applications. Biodiesel has combustion characteristics similar to diesel and its blends with diesel has a shorter ignition delay, higher ignition temperature and pressure, as well as peak heat release as compared to the diesel fuel [100,101]. Moreover, the engine power output and brake power efficiency are found to be equivalent to diesel fuel. Biodiesel can be used in its pure form (B100), but more commonly it blends with the conventional diesel fuel. Blends are designated as BXX, where XX indicates the percentage of biodiesel by volume. B0 is conventional diesel. B5 is 5% biodiesel, 95% conventional. B20 is 20% biodiesel, 80% conventional, and so on.

Biodiesel can be blended in any proportion with the diesel fuel, and it can be used in most conventional internal combustion diesel engines with little or no modification [102,103]. It is likely that this value of blend ratio will increase as the biodiesel fuel quality improves and appropriate additives are developed. It has been well proven that the presence of higher amount of saturated components increases the CP and pour point of biodiesel. Table 18.3 presents the effect of blending biodiesel from different sources with mineral diesel on various fuel properties that change with the increasing biodiesel ratio in the blend.

Palm biodiesel has higher level of saturated FAME resulting in higher cetane number and higher oxidation stability [104]. Unfortunately, such a biodiesel does not possess good cold flow properties and it is not suitable for the production of winter-grade diesel fuels. Blending sufficient amounts of ordinary diesel can improve the cold flow properties of blended diesel fuel [105].

Numerous experimental studies were conducted to investigate the effect of blending palm oil biodiesel percentage on engine performance and emission. The specific consumption is directly related to the heat value of a fuel [106]. The higher the heat value of fuel, the lower the specific consumption [107,108]. Biodiesel has low heating value (10% lower than diesel) on a weight basis because of the presence of a substantial amount of oxygen in the fuel. As a result, in order to maintain the same brake power output, the brake-specific fuel consumption (BSFC) of palm biodiesel blends would be increased to compensate for the reduced chemical energy in the fuel [109]. Furthermore, the higher BSFC for the biodiesel compared to reference diesel is largely attributed to their higher fuel densities [110]. It is noted that the BSFC is the ratio between the mass of fuel consumption and brake effective power [111]. For a certain volume, as it is calculated on a weight basis, obviously higher densities of fuel will result in higher values for BSFC [112,113]. At all loads; biodiesel and its blends with commercial diesel shows higher specific consumption since these fuels have lower heat value compared to diesel [114,115]. Literature reviewed indicates that commercial diesel fuel shows the lowest specific fuel consumption [102]. It can also be observed that increasing the percentage of biodiesel in the mixture with commercial diesel causes an increase in specific consumption [74,107].

Engine power and brake thermal efficiency for blended biodiesel–diesel fuel is found to be nearly equal to that of diesel [102,108], and the ignition delay is noted to decrease as the palm oil blend percentage becomes higher [110]. Thermal efficiency is the ratio between the power output and the energy introduced through fuel injection, the latter being the product of the injected fuel mass flow rate and the lower heating value. Since it is usual to use the brake power for determining the thermal efficiency in experimental engine studies, the efficiency obtained is really a brake-specific efficiency. This parameter is more appropriate than fuel consumption to compare the performance of different fuels, besides their heating value [116]. In most studies [74,108,115] the brake thermal efficiency is lower for biodiesel than mineral diesel throughout the engine speed range due to the lower calorific values of the blended fuels. However, the brake thermal efficiency for biodiesel is found to be nearly equal to diesel in other experimental works [102,103]. On the other hand, [117] conclude that the lower blends of biodiesel increase the brake thermal efficiency and reduce the fuel consumption compared to diesel. This is a consequence of the tested palm oil biodiesel properties, which have a higher cetane number and lower viscosity value compared to the diesel fuel sample. These contradictory results may be attributed to fuel properties, such as density, viscosity, composition, biodiesel production method, as well as test condition. However, it is difficult to determine the effect of any single fuel characteristics alone on engine performance since many of the characteristics are interrelated.

Biodiesel fuel is biodegradable and nontoxic, and its use provides a reduction of many harmful exhaust emissions. A nearly complete absence of (SO_x) emissions, particulate, and soot, and reduction in unburned HC emissions can be achieved [118]. Life cycle studies have shown that biodiesel contains substantially more energy than what is required for its production and also it significantly reduces net CO₂ emissions. However, unmodified engines using biodiesel blends typically emit higher levels of NO_x [83,119]. The “biodiesel NO_x effect” has been, and continues to be, a subject of a great deal of scientific research where the consensus for the exact reason(s) for this increase has not yet been reached.

The engine emissions trends with increasing biodiesel content are plotted in Fig. 18.8 which show the compilation of data from a 2002 EPA study of a large number of pre-1998 model diesel engines for fuel blends from B0 to B100 [120]. From this figure, it is obvious that the increasing blend fractions of biodiesel generally result in dramatic decreases in particulate matter (PM), CO, and unburned HC, where these reductions are frequently accompanied by significant increases in NO_x. For engines using B100, there is a 10% increase in NO_x. Of even greater concern is the observation that these NO_x increases appear to be more, and not less, dramatic in most modern diesel engines.

Several studies have been conducted to investigate the effect of the blended palm oil biodiesel–diesel fuel on exhaust emissions as compared to diesel. Most of the studies reviewed show that increasing the content of palm oil biodiesel in the blend can reduce the engine emissions except for NO_x, which increases. Increasing palm oil biodiesel in the blend is efficient to reduce PM and PAHs [106], similarly, CO, HC, and CO₂ emissions are decreased [110,117]. Contrarily, most of the previous literature [107] have shown that CO₂ emissions of palm oil biodiesel and its blends are higher compared to the commercial diesel fuel. Emission of NO_x is increased when the engine is fueled with palm oil biodiesel and its blends with commercial diesel [107,110]. However, Wariwan et al. [117] show that NO_x emission is decreased when the content of palm biodiesel increases in the blend, which is in contrast with those generally found in previous studies.

The majority of the studies reviewed have found sharp reductions in exhaust emissions with palm biodiesel as compared to diesel fuel. The more accepted reasons in the reduction of emissions, particularly CO, CO₂, HCs, SO₂, particulates, and smoke can be attributed to the presence of sufficient oxygen in biodiesel. Oxygen content and cetane number can also be varied based on the biodiesel feedstock and therefore the emission characteristics are different for different biodiesels [29]. As mentioned previously, biodiesel contains oxygen while diesel has no oxygen content. The increased amount of oxygen in the fuel-rich combustion zone is believed to ensure more complete combustion and thereby to ensure that exhaust emissions are reduced [121].

However, most of the researchers have reported slight increase of NO_x emissions for biodiesel. It is quite obvious that with biodiesel, due to the improved combustion, the temperature in the combustion chamber can be expected to be higher, and higher amount of oxygen is also present which leads to the formation of higher quantity of NO_x in palm biodiesel-fueled engines. The most commonly accepted justification for this behavior lies in the higher cetane number of the palm biodiesel that reduces the ignition delay which can increase NO_x emissions [122].

A key property of biodiesel currently limiting its application to blends of 20% or less is its relatively poor low-temperature properties. Petroleum diesel fuels are plagued by the growth and agglomeration of paraffin wax crystals when ambient temperatures fall below the fuel's CP. These solid crystals may cause start-up problems, such as filter clogging, when ambient temperatures drop to around −10 to −15°C [82]. Meanwhile, the CP of mineral diesel is reported to be −16°C, with biodiesel typically having a CP of around 0°C, thereby limiting its use to ambient temperatures above freezing point [85,123].

Although biodiesel has become more attractive as an alternative fuel for diesel engines, its commercial usage is still limited to B5 in Malaysia and B20 in many other countries. It can be concluded that developing fuel additives will become an indispensable tool to make biodiesel fuel economically viable as well as to use cleaner fuels. The additive technical specifications not only cover a wide range of subjects, but also most subjects are interdependent. This makes the expertise behind the additive technology indispensable in the global trade of fuels. It is likely that, as energy sources become cleaner and renewable, we might find ourselves facing issues that are quite difficult to overcome, and biodiesel additives may become an indispensable tool worldwide.

Many published research has used different additives as a component of the biodiesel or blends of biodiesel fuels to improve either the combustion properties or the emission of a CI engine. The additives used by different researchers can be classified into chemical additives, solid metallic additives, and commercial additives. Some chemical additives such as methanol can be produced from coal- or petrol-based fuels with low cost production, but it has very limited solubility in the diesel fuel. On the other hand, chemical additive such as ethanol is a biomass-based renewable fuel, which can be produced from vegetable materials, such as corn, sugarcane, sugar beets, sorghum, barley, and cassava, and it has higher miscibility with diesel fuel [124,125]. Ethanol is a low-cost oxygenate with high oxygen content (about 35%) that has been used in biodiesel–ethanol blends. It is reported [126] that the ethanol–diesel–biodiesel fuel blends are stable well below subzero temperature and have equal or superior fuel properties to mineral diesel fuel. Ethanol and methanol, as well as products derived from these alcohols, such as ethers, are under consideration or in use as alternative fuels or as an additive biodiesel fuel. Methanol offers very low particulate emissions, but the problems are their toxicity, low energy density, low cetane number, high aldehyde emissions, and harmful influence on materials used in engine production. Ethanol seems to be the best candidate as a sole fuel as a component of either gasoline or diesel oil [127]. Until now, ethanol has been recognized only as a component of gasoline and not as a component of diesel oils. The properties of ethanol enable it to be applied as a component of diesel oil. The potential of oxygenates as a means of achieving zero net CO₂ renewable fuel has resulted in considerable interest in the production and application of ethanol. In many countries such as the United States of America, Canada, Australia, Brazil, South Africa, Denmark, Sweden, and others ethanol programs are realized. The research on ethanol programs is directed to identify factors that could influence engine performance and exhaust emissions. The understanding of these factors is necessary for the interpretation of the test results.

The fuel type has a direct influence on the engine cycle-to-cycle variations that are associated with power and efficiency losses, in addition to fluctuations in engine speed, torque, and work. Furthermore, the variations in cylinder pressure are correlated with the variations in the brake torque which directly influence engine operation [13]. In general, the fuel additives have a different chemical composition compared to diesel and biodiesel fuel and it may develop a cycle-to-cycle variation when it exceeds certain limits [118]. These variations might lead to both a reduction of engine output power and higher emissions; so it is necessary to develop effective control strategies for optimum additive ratio through gaining a better understanding of the various factors that affect the overall combustion process.

The selection of additives for the biodiesel fuel depends on economic feasibility, toxicity, fuel blending property, additive solubility, flash point of the blend, viscosity of the blend, solubility of water in the resultant blend, and water partitioning of the additive. At present, concern about environmental regulations has been the major reason to look for alternative fuels. The use of biodiesel has presented a promising alternative in the world. It is not only a renewable energy source, but it can also reduce the dependence on imported oil and support agricultural subsidies in certain regions. The growing interest in this renewable fuel can be illustrated by the number of articles published and patents registered in this area during the past few decades.