Performance and Emission Characteristics of Biodiesel-Blend in CRDI Diesel Engine – A Review

Energy is the driving force of modern civilization, powering industries, transportation, households, and global economic growth. At present, most of this demand is met through fossil fuels such as coal, petrol, and diesel. However, these resources are depleting rapidly while their excessive use continues to release harmful pollutants including carbon dioxide (CO?), nitrogen oxides (NO?), carbon monoxide (CO), hydrocarbons (HC), and particulate matter (PM). Such emissions contribute to global warming, acid rain, air pollution, and severe health problems. This dual challenge of resource depletion and environmental degradation has created a pressing need for alternative, eco-friendly fuels. Biodiesel has emerged as one of the most promising substitutes for conventional diesel. Derived from renewable sources such as vegetable oils, non-edible oils, animal fats, and waste cooking oil, biodiesel is biodegradable, non-toxic, and capable of reducing harmful emissions. It is produced mainly through transesterification, where oils or fats react with alcohol in the presence of a catalyst to yield fatty acid methyl esters (FAME) and glycerol. Since biodiesel shares many physical and chemical properties with diesel, it can be used in existing engines with little or no modification. However, certain differences—such as higher viscosity, lower calorific value, and greater oxygen content—can influence engine performance, combustion characteristics, and emission levels. Parallel to the development of biodiesel, advancements in diesel engine technology have introduced the Common Rail Direct Injection (CRDI) system, which provides precise control of injection timing, pressure, and quantity through electronically operated injectors. This technology enhances fuel atomization, improves combustion, and reduces emissions, making CRDI engines highly suitable for testing biodiesel blends. Recent studies have examined the use of biodiesel–diesel blends (such as B10, B20, etc.) in CRDI engines to evaluate their effect on performance parameters like Brake Thermal Efficiency (BTE), Brake Specific Fuel Consumption (BSFC), and cylinder pressure, as well as emissions including CO, HC, NO?, and smoke. Results indicate that biodiesel blends generally improve combustion efficiency and reduce CO, HC, and PM emissions due to their inherent oxygen content. However, a notable drawback is the increase in NO? emissions, which is often attributed to higher in-cylinder combustion temperatures. Researchers are addressing this challenge through optimized injection strategies, blend ratios, exhaust gas recirculation (EGR), selective catalytic reduction (SCR), and the incorporation of fuel additives. In addition, nanotechnology and alcohol-based additives are being explored to enhance fuel properties, promote cleaner combustion, and reduce harmful exhaust gases. Feedstock selection also plays a key role in biodiesel performance, with options ranging from edible oils like soybean and palm oil to non-edible oils such as jatropha, karanja, and neem, as well as waste oils and animal fats. Each feedstock influences viscosity, energy content, and oxygen levels differently, leading to varied effects on engine behavior and emissions. Despite its advantages, biodiesel faces challenges such as higher production costs, fuel instability during storage, and cold-weather performance issues. Standardization of fuel properties and long-term engine durability are also concerning. Nevertheless, the integration of biodiesel with CRDI technology, along with the use of additives and improved processing methods, provides a promising path forward. This review consolidates research findings on biodiesel blends in CRDI diesel engines, with emphasis on performance, combustion behavior, and emission characteristics. It further explores the impact of additives, feedstocks, and injection parameters while identifying both the benefits and limitations of biodiesel use. By presenting current knowledge and future prospects, this study aims to support ongoing efforts toward cleaner, more efficient, and sustainable diesel engine technologies.

2. Characteristics of Biodiesel

Biodiesel, produced through the transesterification of vegetable oils, non-edible oils, animal fats, and waste cooking oils into fatty acid methyl esters (FAME), has drawn significant interest as an alternative to petroleum diesel. Its fuel characteristics differ considerably from conventional diesel, and these differences strongly influence engine performance, combustion efficiency, and exhaust emissions. Over the years, researchers have examined these variations in detail. Demirbas (2009) observed that the calorific value of biodiesel is lower than that of petroleum diesel, primarily due to its oxygenated molecular structure. This reduction in heating value leads to slightly higher brake specific fuel consumption when biodiesel is used in diesel engines. Supporting this, Oliveira and Da Silva (2013) reported calorific values in the range of 38–40 MJ/kg for biodiesels derived from different feedstocks, in contrast to about 46 MJ/kg for diesel. Sanjid et al. (2014) reached similar conclusions in their work on palm and jatropha biodiesel, noting that reduced heating value was the major factor behind increased fuel usage. Hoekman et al. (2012) added that although biodiesel has less energy per unit mass, the oxygen content present in the fuel enhances combustion efficiency, offsetting some of the energy loss. In agreement, Graboski and McCormick (1998) also linked biodiesel’s lower energy density to marginal reductions in engine power output.

2.1 Density and Viscosity

The physical properties of density and viscosity have received considerable attention. Alptekin and Canakci (2008) showed that biodiesel blends are denser and more viscous than conventional diesel, which can negatively affect fuel spray and atomization. Lapuerta et al. (2008) confirmed this by demonstrating that higher viscosity results in larger droplet formation during injection, ultimately influencing the combustion process. Wan Ghazali et al. (2015) reported that increased viscosity contributed to poor cold-starting characteristics in diesel engines. Similarly, Silitonga et al. (2013) highlighted that viscosity issues could be mitigated by blending biodiesel with diesel to achieve better atomization. Arbab et al. (2013) further emphasized that modern injection technologies, particularly CRDI systems, can counterbalance these shortcomings through precise control of injection pressure. Fig.1 represents the density values of biodiesel blends.

Fig.1 Density values of biodiesel blends

Fig.2 Kinematic viscosity values of biodiesel blends

2.2 Cetane number

With respect to cetane number, biodiesel generally performs better than diesel. Knothe (2005) noted that higher cetane numbers shorten ignition delay and promote smoother combustion. Ryan and Knothe (2003) found typical cetane numbers for biodiesels to be in the range of 50–65, compared to 45–55 for petroleum diesel. Rizwanul Fattah et al. (2013) showed that higher cetane ratings improve ignition quality but may also contribute to elevated NOx emissions due to higher peak combustion temperatures. Sharma and Singh (2009) similarly observed that while cetane-rich biodiesel enhances engine startability and combustion stability, the associated NOx rise remains a challenge. Lapuerta et al. (2008) supported these findings, presenting experimental evidence of reduced ignition delay when biodiesel blends were used. Fig.2 represents the kinematic viscosity values of biodiesel blends.

2.3 Flash point and Cold flow

Other fuel characteristics such as flash point and cold flow properties are also important. Pinto et al. (2005) reported that biodiesel generally has a much higher flash point than diesel—often above 120?—making it safer to store and transport. Graboski and McCormick (1998) echoed this advantage, highlighting biodiesel’s reduced flammability risk during handling. However, its poor cold flow properties are a limitation. Das and Agarwal (2001) showed that biodiesel tends to crystallize at low temperatures, which can cause operational problems in cold climates. Jain and Sharma (2010) found that biodiesel usually exhibits higher cloud and pour points than diesel, restricting its suitability in winter conditions. Worgetter and Prankl (1996) emphasized that biodiesels with higher saturated fatty acid content, such as palm-based biodiesel, are more vulnerable to gelling at low temperatures. To counter this, Sanjid et al. (2014) recommended blending strategies to improve cold-weather performance.

2.4 Iodine value

The iodine value, a measure of unsaturation, is another property that has been extensively studied. Jain and Sharma (2010) reported that biodiesels with higher iodine values, such as soybean oil-based biodiesel, are more prone to oxidation, making them less stable during storage. Worgetter and Prankl (1996) confirmed this by showing that fuels with lower iodine values, like coconut biodiesel, are much more stable. Dunn and Knothe (2003) further demonstrated that high levels of unsaturation accelerate oxidative degradation. Likewise, Anwar et al. (2009) confirmed that biodiesels with high iodine numbers require antioxidants to maintain stability. Silitonga et al. (2013) also concluded that iodine value remains one of the most critical parameters for assessing biodiesel quality and long-term usability.

2.5 Metal contamination

Metal contamination in biodiesel has also been reported to influence fuel quality. Dunn and Knothe (2003) demonstrated that copper contamination, even in trace amounts, accelerates oxidation and reduces storage life. Anwar et al. (2009) observed similar issues, finding that metal ions catalyze degradation and lead to gum and deposit formation. Arbab et al. (2013) stressed the importance of monitoring metal content during biodiesel production. Silitonga et al. (2013) likewise highlighted that metals negatively affect stability, while Jain and Sharma (2010) suggested antioxidant treatments as a preventive measure. In summary, biodiesel offers several desirable characteristics such as higher cetane number, oxygen content, lubricity, and flash point, all of which improve combustion and handling safety. At the same time, limitations such as reduced calorific value, high viscosity, poor cold flow behavior, and low oxidative stability present practical challenges. Researchers agree that these issues can be addressed through blending, additive use, and the application of advanced injection technologies such as CRDI systems, making biodiesel a viable step toward sustainable diesel engine operation. Table.1 represents the properties of biodiesel.

Table.1 Properties of Biodiesel

3. Performance, Emission and Combustion Characteristics of Biodiesel blends in CRDI Diesel engine

3.1 Brake Specific Fuel Consumption (BSFC)

Brake Specific Fuel Consumption (BSFC) reflects the efficiency of an engine in converting fuel into useful power. Biodiesel blends usually record slightly higher BSFC values compared to diesel, mainly due to their lower calorific value. Demirbas (2009) earlier highlighted this drawback, but recent research shows that CRDI technology can reduce the penalty. For example, Radhakrishnan et al. (2022) studied lemongrass biodiesel blends and found that higher injection pressure in CRDI engines reduced BSFC by nearly 29% compared to conventional systems. Likewise, Selvan et al. (2021) investigated waste frying oil methyl ester (WFME) with ZnO nanoparticle additives and reported improved energy efficiency and reduced BSFC. In another study, Muthukumaran et al. (2023) optimized sesame biodiesel blends in CRDI engines and reported a minimum BSFC of 0.3705 kg/kWh at B20 under 550 bar injection pressure with 7% EGR. These findings suggest that although biodiesel naturally increases BSFC, the advanced atomization and multi-injection strategies of CRDI engines help minimize the increase, especially at low-to-moderate blend ratios.

3.2 Brake Thermal Efficiency (BTE)

Brake Thermal Efficiency (BTE) measures how effectively the engine converts the chemical energy of fuel into mechanical energy. Biodiesel blends, when optimized, can achieve comparable or higher BTE than diesel. Graboski and McCormick (1998) first emphasized the effect of oxygenated fuel in enhancing combustion efficiency, and recent works support this claim. Radhakrishnan et al. (2022) showed that lemongrass biodiesel blends in CRDI engines achieved up to 26% improvement in BTE due to superior atomization at high injection pressures. Similarly, Selvan et al. (2021) found that WFME with ZnO nanoparticle additives produced a peak BTE of nearly 32% under optimized conditions. Muthukumaran et al. (2023) reported that sesame biodiesel at B20 achieved about 28.2% BTE when combined with high injection pressure and moderate EGR. These studies confirm that CRDI’s precise control of injection timing and pressure allows biodiesel blends to perform efficiently, making them strong contenders for replacing fossil diesel.

3.3 Carbon Monoxide (CO) Emissions

Carbon monoxide (CO) emissions result from incomplete fuel combustion. Since biodiesel contains inherent oxygen, it generally produces lower CO emissions compared to diesel. Lapuerta et al. (2008) demonstrated reductions in CO when using biodiesel blends, a trend confirmed by recent studies. Radhakrishnan et al. (2022) reported that lemongrass biodiesel blends in CRDI engines achieved up to 11% CO reduction. Muthukumaran et al. (2023) observed even lower values, reporting 0.0319% vol CO at optimized operating conditions with sesame biodiesel. Similarly, Sharma et al. (2022) tested thumba methyl ester (TME) blends and found that B20 reduced CO emissions by nearly 12% compared to diesel. These results show that biodiesel blends are effective in reducing toxic CO emissions, especially under CRDI injection strategies that promote complete fuel oxidation.

3.4 Nitrogen Oxides (NOx) Emissions

Nitrogen oxides (NOx) are among the most concerning emissions due to their role in air pollution and health hazards. While biodiesel blends reduce CO and HC, they often increase NOx emissions because of higher in-cylinder combustion temperatures. Graboski and McCormick (1998) first noted this trend, and later researchers confirmed it. Radhakrishnan et al. (2022) observed a slight NOx increase with lemongrass biodiesel blends in CRDI engines. However, recent works also highlight mitigation strategies. Rajasekar et al. (2022) studied Prosopis juliflora biodiesel with dimethyl carbonate as an oxygenated additive and employed EGR, achieving a 32% reduction in NOx compared to standard biodiesel blends. Muthukumaran et al. (2023) also recorded reduced NOx levels (~447 ppm) at optimized injection timing and EGR conditions with sesame biodiesel. These findings suggest that while NOx emissions remain a drawback, CRDI’s flexibility and after-treatment strategies like EGR and SCR can effectively control them.

3.5 Hydrocarbon (HC) Emissions

Hydrocarbon (HC) emissions arise from unburned fuel escaping combustion. Biodiesel blends consistently demonstrate lower HC levels because of their oxygen-rich composition. Hoekman et al. (2012) observed this trend in earlier work, while Radhakrishnan et al. (2022) confirmed that lemongrass biodiesel blends reduced HC by about 10% in CRDI systems. Selvan et al. (2021) also showed that WFME with ZnO nanoparticle additives lowered HC emissions significantly, reporting around 29.6 ppm under optimized operating points. Similarly, Muthukumaran et al. (2023) achieved a very low HC level of 13 ppm with B20 sesame biodiesel blends in CRDI engines. These results clearly indicate that CRDI systems, through precise fuel atomization and multiple injections, enhance combustion completeness and substantially reduce HC emissions when biodiesel blends are used.

3.6 Smoke Density (Particulate Matter)

Smoke density, which is closely associated with particulate matter (PM), is a major concern in diesel exhaust. Biodiesel blends reduce smoke emissions significantly due to their oxygen content and absence of sulfur. Lapuerta et al. (2008) reported noticeable reductions in smoke opacity with biodiesel, and more recent studies have strengthened these observations. Radhakrishnan et al. (2022) showed that lemongrass biodiesel blends reduced smoke density by over 10% when high-pressure split injection strategies were used in CRDI engines. Selvan et al. (2021) found that WFME biodiesel with nanoparticle additives yielded smoke levels as low as 69.7% opacity. Similar reductions have been documented by Muthukumaran et al. (2023) with sesame biodiesel blends. These results suggest that biodiesel blends in CRDI systems are particularly effective in reducing particulate emissions, making them attractive for meeting stringent emission regulations. Table.2 represents the performance and emission results for various test conditions.

Table 2. Performance and Emission Results for Various Test Conditions

4. Effect of Additives and Nanotechnology on Biodiesel Blends

The performance and emission behavior of biodiesel can be improved by using suitable additives and nanomaterials. These materials enhance fuel properties such as ignition quality, viscosity, stability, and combustion efficiency. In recent years, the addition of metal-based nanoparticles and alcohols has become a promising approach for achieving better engine performance and lower emissions when biodiesel is used in Common Rail Direct Injection (CRDI) diesel engines.

4.1 Alcohol Additives

Alcohols such as methanol, ethanol, and butanol are widely used as oxygenated additives in biodiesel blends. Their main advantages are improved combustion efficiency and reduced exhaust emissions. Alcohols increase the oxygen concentration in the fuel, resulting in better oxidation of carbon monoxide and unburned hydrocarbons. Among them, ethanol and butanol are more suitable for diesel engines due to their higher energy content and miscibility with biodiesel. Raman et al. (2020) observed that the inclusion of 5–10% ethanol in biodiesel–diesel blends enhanced the brake thermal efficiency (BTE) and reduced smoke and CO emissions. However, the NOx level slightly increased due to higher combustion temperatures. Butanol-based additives provide better cold flow properties and reduce knocking problems because of their higher molecular weight and lower vapor pressure. Researchers also noted that using alcohol blends improves atomization and shortens ignition delay, which helps achieve smoother engine operation in CRDI systems.

6.2 Metallic Additives

Metal-based additives such as cerium oxide (CeO₂), zinc oxide (ZnO), aluminum oxide (Al₂O₃), and titanium dioxide (TiO₂) are commonly used to improve the oxidation rate during combustion. These metal oxides act as catalysts and promote complete fuel burning, which lowers the number of unburned hydrocarbons and soot. Selvan et al. (2021) reported that a ZnO nanoparticle additive mixed with waste frying oil biodiesel improved the BTE by nearly 8% and reduced CO and HC emissions. Similarly, cerium oxide nanoparticles help lower the ignition temperature of soot, thus reducing particulate matter in exhaust gases. The concentration of nanoparticles should be optimized carefully. Excessive dosage may lead to fuel filter clogging or injector wear. Most studies recommend a nanoparticle dosage in the range of 20–80 ppm for stable operation and effective emission reduction. The combination of nanoparticles and CRDI injection technology provides a strong advantage by promoting better atomization and faster combustion. Table 3 represents the summary of additives and their effects on biodiesel performance and emissions.

Table 3. Summary of Additives and Their Effects on Biodiesel Performance and Emissions