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Article

The Effect of ZnO and CNT Nanoparticles on the Combustion Characteristics and Emission Performance of a Common Rail Diesel Engine Fueled with Diesel and Biodiesel

by
Vida Jokubynienė
1,2,* and
Stasys Slavinskas
1
1
Agriculture Academy, Vytautas Magnus University, K. Donelaičio Str. 58, 44248 Kaunas, Lithuania
2
Higher Education Institution, Technology Faculty, Bijūnų g. 10, 91223 Klaipėda, Lithuania
*
Author to whom correspondence should be addressed.
Energies 2025, 18(10), 2564; https://doi.org/10.3390/en18102564
Submission received: 14 April 2025 / Revised: 9 May 2025 / Accepted: 13 May 2025 / Published: 15 May 2025
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Abstract

:
This article presents the test results of a turbocharged Common Rail Direct Injection (CRDI) diesel engine operating on diesel fuel and methyl ester biodiesel with nanoparticle additives. The use of nanomaterials has been shown to improve the combustion process. In this study, various nanoparticles, including zinc oxide and carbon plates, were investigated as additives to enhance the combustion performance of selected fuels. The fuel of choice was conventional diesel, and a methyl ester of rapeseed oil called biodiesel. A turbocharged Common Rail Direct Injection (CRDI) diesel engine, model FIAT 192A1000, was used for the experiments. The following engine parameters were measured and recorded: torque (Ms, Nm), fuel consumption (Bd, kg/h), carbon monoxide (CO, ppm), and nitrogen oxides (NOx, ppm). The results show that nanoparticles can improve the combustion performance of the fuels studied in the engine. However, the effect of nanoparticles on engine parameters varied. In summary, the influence of nanoparticles is noticeable: the ID is shorter with diesel fuel with carbon nanotubes at 50 ppm and 100 ppm concentration, the NOx is reduced with zinc oxide and D, and CO is diminished in all load modes when using RME with carbon nanotubes.

1. Introduction

Currently, the most common alternative fuels for diesel engines are biodiesel, vegetable oils, and others. Some of these alternative fuels can be used in their pure form, and others as additives in blends [1]. The use of alternative fuels in diesel engines can also lead to several negative issues such as increased fuel consumption and NOx emissions, reduced engine power, piston ring deposits, and sometimes cold start problems. These drawbacks can be eliminated by proper combustion control and the use of proper fuel additives [2]. In recent years, nanomaterials (NMs) have become very promising fuel additives for diesel engines. The use of such fuel blends requires proper preparation, i.e., mixing NMs (in powder or liquid form) with diesel or biodiesel fuel and preventing them from settling in the fuel tank or fuel lines. To obtain a homogeneous mixture of NMs and fuel, special preparation and surfactants are needed to ensure the homogeneity and stability of the fuel mixture over an extended period [3].
Nanofuels for diesel engines are produced by blending with a base fuel and various nanoparticles (NPs). The most commonly used fuels are diesel (D) and biodiesel (RME). These base fuels can be blended with pure NPs to enrich the nano fuel blends with different properties to improve engine performance [4].
The most used NPs are aluminum oxide (Al2O3), cerium dioxide (CeO2), copper oxide (CuO), titanium oxide (TiO2), zinc oxide (ZnO), and carbon nanoparticles (CPLs) and nanotubes (CNTs) [5,6,7,8]. Due to their high specific surface area (surface area to volume ratio) and excess surface atomic energy, nanoparticles (NPs) can exhibit unique physical, chemical, thermal, magnetic, electrical, or optical properties that differ significantly from those of bulk materials [9]. As a result, nanoparticles (NPs) can exhibit greater catalytic activity and higher theoretical density than both smaller and larger nanomaterials [10]. Moreover, the synergistic effects between the various doped metal oxides could further enhance this catalytic activity and threshold effect [11].
The emissions and performance of a diesel engine are significantly influenced by the injection process (including injection pressure, torque, and speed) and the combustion process (such as ignition delay, cylinder pressure, cylinder temperature, and exhaust temperature) [12]. Therefore, the use of nanofuels can have a significant effect on these processes. For example, the addition of SNPs may result in a lower ignition delay due to a higher cetane number because ignition delay and cetane number are inversely proportional to each other [13]. NPs affect engine performance depending on the type of diesel engine [14]. An exceedingly small amount of research has been conducted on the Common Rail Direct Injection engine type, and researchers have primarily reported results on single-cylinder conventional diesel engines with a single fuel injection. Evaluating the effects of nanoparticles (NPs) involves numerous parameters, making it challenging to draw accurate conclusions from the data in the reviewed literature. However, a thorough analysis of the available data and experimental results enables us to derive the following general conclusions [15]: The analysis shows an improvement in thermal efficiency (ηth) for all the nanosheets studied [16]. NMs can lead to a more complete combustion of the fuel due to a higher evaporation rate, shorter ignition delay, and higher cylinder temperature [17] This effect is enhanced by NMs with higher catalytic activity [18] or better secondary atomization [19].
Basha J. and Anand R. conducted an experimental study on a single-cylinder constant-speed diesel engine to find the effects of carbon nanotube (CNT) and Jatropha methyl ester (JME) emulsion fuels. The experimental results showed that the braking thermal efficiency of CNT blended JME emulsion fuels was significantly increased compared to pure JME and pure JME emulsion fuels. At full load, the brake thermal efficiency of the JME fuel was 24.80%, while that of the JME2S5W and JME2S5W100CNT fuels was 26.34% and 28.45%, respectively, with a reduction of harmful pollutants (e.g., NOx and fumes) in the exhaust gas [20].
The specific fuel consumption (ge) decreased with the addition of NMs. In analyses of the literature, this is mostly seen for higher catalytic activity [21] and secondary injection [22] of NMs, as well as for nanoparticles with a higher cetane number [23]. Chen A. conducted a scientific analysis on the impact of carbon nanotube (CNT) nanoparticles and diesel fuel blends on the combustion characteristics, performance, and emissions of a four-stroke single-cylinder diesel engine. The study revealed a reduction in brake-specific fuel consumption by up to 19.8% and an increase in brake thermal efficiency by 18.8%. The combustion analysis indicated that CNT blends with diesel could be further explored as additives to diesel fuels, as they significantly enhance NOx emissions during combustion. However, the stability of these blends must be addressed before their full potential can be realized [24].
In general, NOx emissions can be increased by NMs that cause higher heat release, cylinder pressure, and cylinder temperature. Base diesel blends at different CNT concentrations (25, 50, and 100 ppm) can reduce NOx emissions by 4.48%, but other emissions are elevated, such as CO, CO2, and HC [24]. These effects are a consequence of the catalytic effect of NMs [25], which accelerates the combustion process [26]. Conversely, NOx emissions can be decreased if the heat release effect of nanomaterials (NMs) is sufficiently strong [27]. Additionally, some researchers have proposed that the reduction in NOx may be attributed to the catalytic activity of certain NMs, which can convert NOx into O2 and N2 during combustion [28].
Adding NMs reduces CO emissions, although negative effects can also be seen. CO reductions are generally observed for NMs with higher catalytic activity, which act as an oxidation catalyst, converting CO to CO2 during the combustion process, and for NMs that promote better secondary injection [29]. Some researchers have suggested that CO emissions may also increase; for example, if the NMs contain oxygen functional groups above the carbon quantum dots, CO emissions may increase [30].
The substitution of diesel and biodiesel by CNT requires a detailed study of the combustion characteristics and emissions of the diesel engine due to the properties of the nanomaterials, as well as an assessment of the engine operating conditions and the effect of the individual mixtures of CNT on the engine’s combustion parameters and emissions.
To date, numerous studies have explored the impact of nanofuel applications on various characteristics of diesel engines, particularly emissions and engine performance (such as specific fuel consumption and effective power). The reviewed literature indicates a growing interest in using nanomaterials (NMs) in diesel and biodiesel fuels. However, there is a scarcity of studies on common rail (CRDI) diesel engines, with most research focusing on single-cylinder engines. Typically, studies on the effects of nanomaterials pertain to the performance and emissions of internal combustion engines. Nonetheless, the practical application of nanoscale fuels poses challenges related to technical issues, dosage, stability, and the performance of the diesel particulate filter (DPF). This paper aims to analyze the effects of NMs, such as carbon nanotubes (CNTs) and zinc oxide (ZnO), and their concentrations on engine operating modes and CO and NOx emissions in a CRDI engine. These NMs possess excellent properties that make them suitable as fuel additives to enhance diesel engine performance and reduce harmful emissions.

2. Materials and Methods

2.1. Material Used

In this study, conventional diesel fuel (D), meeting the requirements of EN 590:2022 [31], and rapeseed oil fatty acid methyl ester biodiesel (RME), meeting the requirements of EN 14214 [32], were used as base fuels (BFs). The diesel fuel was obtained from “Orlean Lietuva” (Mažeikiai, Lithuania), while the biodiesel was obtained from “Rapsoila” Ltd. (Mažeikiai, Lithuania). Both fuels were used as received, without any additional preparation. The main physicochemical properties of the investigated fuels provided by the supplier are listed in Table 1.
Several nanoparticles were selected for use as additives to improve combustion properties. Carbon nanotubes (CNTs) and zinc oxide (ZnO) were purchased from Sigma Aldrich (Burlington, MA, USA) and used as received. CNT and ZnO nanomaterials are powder-like. The main characteristics of the investigated nanomaterials are presented in [33]. Sorbitanmonooleat eth-oxylith (SPAN80), obtained from Sigma Aldrich, was used to stabilize the dispersions of nanoparticles in fuels.

2.2. Preparation of Fuel Samples

The 50, 100, and 150 ppm (by wt.) concentrations of nanoparticles in both fuels were selected; the nanoparticles, which are powder-like, were weighed. Concentrations of 50, 100, and 150 ppm (by wt.) of SPAN80 were added to the mixture to stabilize the dispersions.
Fuel samples loaded with nanoparticles were prepared using a magnetic stirrer and ultrasound. The preparation scheme is shown in Figure 1.
  • First step: the weighted mixtures were stirred on a magnetic stirrer at 20 °C and 3000 min−1 for 20 min.
  • The second step: in an ultrasonication bath, these were mixed at 60 °C for 30 min.
The visual appearance of the prepared fuel samples has been presented in previous articles [33]. The addition of the other nanoparticles tested did not change the color of the fuel. The prepared nano-dispersions were stable for at least 5 h, which was sufficient for the engine performance for experimentation.

2.3. Physical Properties

The densities and kinematic viscosities of the explored nanoparticle-loaded fuels were established using an Anton Paar Stabinger viscometer, SVM 3000 (Anton Paar, Graz, Austria). Three measurements were performed for each sample. However, there was no significant difference between nanoparticle-free and nanoparticle-loaded fuels. Therefore, it was decided that the concentration of nanoparticles and surfactants was too small to change the viscosity or density of the base fuel [33].

2.4. Experimental Engine Tests Evaluation

Experimental engine tests were performed at the Engineering faculty of Vytautas Magnus University (VDU). A turbocharged Common Rail Direct Injection (CRDI) diesel engine FIAT 192A1000 (FIAT, Turin, Italy) was imploded for the experiments, the basic design features and operational parameters of which are listed in Table 2.
The principal layout of the test rig, the equipment, the instruments used for the experiments, and the tests carried out followed methodology described previously [34].
Engine performance was recorded for diesel (D), biodiesel (RME), and nanoparticle components at various concentrations. The following engine parameters were measured and recorded: torque Ms (Nm), fuel consumption Bd (kg/h), engine air consumption Go (kg/h), intake manifold pressure, carbon monoxide CO (ppm), and nitrogen oxides NOx (ppm). The study was carried out comparatively at equal loads and 2500 rpm. To improve the accuracy of the experimental evaluation, generalized indicator diagrams were used, which consist of 100 consecutive engine cycles.
A high-speed AVL indicating system, which consisted of the AVL angle encoder 365C and high-performance pressure transducer GU24D connected to the AVL microIFEM piezoelectric amplifier and signal acquisition platform IndiModul 622, was used for the recording, acquisition, and processing of crank-angle gas pressure signals in the first cylinder. An example of the measured in-cylinder pressure diagram, the calculated heat release rate, is shown in Figure 2. The injector control signal was used to determine the start of injection.
The engine torque was measured with an electric dynamometer KS-56-4 with a definition rate of ±1 Nm. An air mass flow into the cylinders was measured with an AVL air mass flow meter, and fuel mass consumption was recorded with an AVL dynamic fuel balance 733S flex-fuel system (AVL, Graz, Austria).
Pilot injection ignition delay (ID1) was defined as the period in degrees of crank angle from the start of the first (pilot) injection to the start of combustion. The start of combustion was considered the crank angle at which the heat release rate curve crosses the zero line and changes its value from negative to positive. Accordingly, the main injection ignition delay (ID2) was defined as the period in degrees of crank angle from the start of the second (main) injection to the start of combustion. The start of combustion was considered the crank angle at which the heat release rate curve crosses the zero line and changes its value from negative to positive. Thus, MBF90 is an end-of-combustion in terms of the TDC of the burnt fuel (in degrees), since the remaining 10% of unburnt fuel no longer affects the heat release. The duration of combustion was calculated as the period, in degrees of crankshaft rotation angle, from the onset of ignition of the main fuel portion to the MBF90 position.

3. Results

3.1. Combustion Parameters

This chapter focuses on the combustion characteristics of a diesel engine concerning the crank angle under different load conditions. Fuel parameters such as ignition delay periods, MBF50, combustion duration, thermal efficiency of the test, and main fuel portions’ ignition delay periods are evaluated to assess the influence of nanoparticles on the combustion process.

3.1.1. Ignition Delay

The start of the combustion process of the fuel mixture depends on when the fuel is injected into the combustion chamber, as well as on the combustion emissions and fuel consumption. Precise injection timing is essential to optimize engine performance. The start of injection is expressed in terms of the angle of the engine crankshaft to the piston top endpoint TDC, which opens the injector and injects the fuel into the combustion chamber. The quality of the fuel mixture also depends on the start of injection. The start of injection, therefore, has a significant impact on combustion emissions. Proper control of the ignition delay period allows for improved combustion efficiency, reduced emissions, and optimized fuel consumption.
The results for ID with D (Figure 3a) show that the ID of the pilot fuel portion was shorter for the fuel with nanoparticles in all cases tested, Figure 3b shows that the ID of the main fuel portion was not significantly affected by the nanoparticle addition. The pilot fuel portion ID (Figure 3a) is shorter by 2° with DCNT50 and DCNT100 in all load conditions. The ID of the main fuel portion (Figure 3b) is shorter only at maximum load and with DCNT50 and DCNT100 fuels, where the combustion is advanced but not significantly. At low load, a slight increase in ID is observed. The earlier ignition of the pilot fuel portion could have been caused by the catalytic effect of CNT. As can be seen, increasing nanoparticle concentration did not have a positive effect on the ID lifetime duration. An increase in ID was observed when their concentration was increased to 150 ppm. It can be concluded that for DCNT50 and DCNT100, the pilot portion of the ignition was advanced in all engine modes, while the ID period of the main injection showed a slight change.
Figure 4a shows the effect of CNT additives in RME fuels on the pilot portion ignition delay (ID) varies with load. At low loads, the ID was longer, while at medium loads, the influence was minimal. At high loads (IMEP = 14 bar), the ID was shorter by approximately 11% (2.2 degrees). The influence of nanoparticles on the ID of the main portion (Figure 4b) was little noticeable in all cases studied. The ignition delay period of the fuel is a key factor in determining the maximum heat release rate during combustion in an engine. A longer ID enhances fuel–air mixing, forming a ready mixture for combustion, which burns rapidly once the auto-ignition temperature is reached, releasing a significant amount of energy at once.
Najafi, G. found that biodiesel and biodiesel with a nano-additive have a higher heat release rate in the diffusion phase of combustion and a shorter combustion time compared to pure fuels. The ignition delay of biodiesel with CNT120 was reduced by 8.98% compared to the neat fuel [35].
Figure 5 shows the effect of ZnO nanoparticles on the duration of the ignition delay period of diesel fuel. As seen, the ignition delay of pilot fuel with ZnO was shorter by 3.9%, 7.2%, and 5.6% on average at low, medium, and high loads. Meanwhile, the main fuel portion’s ignition delay was shorter by only 1.1%, 2.4%, and 3.2%, respectively, at low, medium, and high loads.
Figure 6a shows that the addition of ZnO nanoparticles to RME fuel extended the ignition delay period of the pilot fuel at low loads (IMEP = 6 bar) by 3.9%, 8.1%, and 9.0% respectively at 50 ppm, 100 ppm, and 150 ppm ZnO nanoparticle concentration in the fuel.
At medium loads (IMEP = 10 bar), the influence of nanoparticles was smaller. The pilot dose ignition delay increased by 2.8%, 4.7%, and 5.9% respectively at 50 ppm, 100 ppm, and 150 ppm ZnO nanoparticle concentration in the RME. At high loads, the influence of the nanoparticles was even smaller. The ignition delay increased by only 1.3%, 3.7%, and 3.9%. A later pilot fuel portion ignition delay can cause a higher gas temperature in cylinder at main dose injection and, in turn, lead to a shorter main dose ignition delay. Fuel with nanoparticles (Figure 6b) showed an increase in ignition delay compared to fuel without nanoparticles only under medium load conditions. In the case of BZnO50 fuel (Figure 6b), it was found that the ID at low load regimes was shorter by an average of ~2.8%.

3.1.2. Mass Fraction of Burned Fuel

The combustion of the fuel–air mixture is crucial for analyzing the performance of a diesel engine, as it is related to specific fuel consumption, thermal efficiency, and engine stability. The combustion angle of the MBF 50 is the basis for heat release and affects the energy performance of the fuel in the cylinder. The fuels analyzed are D and RME base fuels with various nanoparticles, the concentrations of which are given in the methodological section. The crankshaft angle corresponding to MBF50 (50%) of the fuel burnt reflects the fuel energy conversion. The earlier the 50% fuel burn relative to the TDC, the lower the heat loss during the expansion stroke and the higher the thermal efficiency. Figure 7, Figure 8, Figure 9 and Figure 10 show the MBF50 position and the variation of the combustion duration when the engine is operated at IMEP 6, 10, and 14 bar.
Figure 7 shows the influence of carbon nanoparticles in diesel fuel on the MBF50 position. As seen, 50 ppm and 100 ppm nanoparticle concentrations in the fuel brought the MBF50 closer to TDC; however, this change reached only 1 CAD.
A higher particle concentration (150 ppm) did not show an effect at a low load, while at medium and high loads, the MBF50 position was respectively 3.4% and 4.7% further TDC. However, the combustion duration at all load conditions was 3.5–2.5% lower with nanoparticles in the fuel (Figure 7b).
The impact of carbon nanoparticles in biodiesel on MBF50 position is shown in Figure 8. As can be seen, with all concentrations (50 ppm, 100 ppm, and 150 ppm) of nanoparticles in the fuel, MBF50 approached TDC; however, this change reached up to 1 CAD.
It is evident that in low-load mode, the greatest advance was achieved with a 100 ppm nanoparticle concentration—3.15% closer to TDC—in medium-load mode with 50 ppm—4.47% closer to TDC. In full load mode, the impact of nanoparticles on MBF50 was minimal. The combustion duration with nanoparticles was shorter across all engine loads: at low loads—2.2%, at medium—3.3%, and at high loads—3.8% (Figure 8b).
Figure 9 shows the impact of zinc oxide nanoparticle concentration in diesel fuel on the MBF50 positions. As seen, a 50 ppm concentration of nanoparticles brought the MBF50 closer to TDC, but this change reached up to 0.8 CAD. Higher particle concentration did not show any effect at low load mode (IMEP = 6 bar) and high load mode (IMEP = 14 bar). The combustion duration at all loads was shorter for fuels with nanoparticles (Figure 9b). The shortest combustion duration was observed with 50 ppm and 100 ppm particle concentrations at all load modes. The greatest impact is seen with a 50 ppm particle concentration when the engine operating at IMEP = 10 and 14 bar loads: respectively −2.73% and −2.1%.
Figure 10 shows the influence of zinc oxide particle concentration in biodiesel on the MBT50 position and combustion duration. As can be seen, the addition of 50 ppm and 100 ppm particles pushed the MBT50 position further from the TDC at low load, although the main fuel portion ignition delay period was shorter in these cases. The addition of nanoparticles in this load mode did not affect the combustion duration. At the medium load mode (IMEP = 10 bar), the MBF50 position approached the TDC by an average of 2.4%, and the combustion duration decreased by 3.0%, independent of ZnO particle concentration. At the high load mode, adding ZnO particles did not have a noticeable effect on either the MBF50 position or combustion duration.
In the presence of carbon particles in fuels, the shortening of the combustion can be attributed to factors such as the catalytic effect of nanoparticles. It is thought that nanoparticles can provide additional oxygen, making the injected fuel burn completely during combustion.

3.2. Change in Thermal Efficiency

Thermal efficiency is commonly referred to as fuel conversion efficiency, which evaluates the engine’s ability to convert the thermal energy of fuel into useful work.
As seen in Figure 11, the addition of carbon nanoparticles in diesel fuel slightly increased the thermal efficiency coefficient only at low loads. At medium loads, the nanoparticle additive had practically no impact on thermal efficiency, while at high loads, the effect was negative, meaning that the thermal efficiency coefficient obtained when using fuel with nanoparticle additives was lower than when using fuel without additives. These changes may have been influenced by combustion process changes, as reflected by MBF50 position and combustion duration.
Figure 11a shows that the highest BTE is achieved with fuel containing carbon particles at a concentration of 150 ppm: at low-load mode (IMEP = 6 bar)—0.388, medium (IMEP = 10 bar)—0.417, and maximum load (IMEP = 14 bar)—0.432. Chen A., studying the effect of CNT on diesel fuel with 50 ppm, showed results where the BTE in all load modes increased by about 18.8%. Increasing the carbon content in the CNT mixture also positively affects BTE [24]. These higher values may be associated with increased evaporation rates and better air-fuel mixing, as well as the larger surface area-to-volume ratio, where complete combustion occurs.
Figure 11b shows that at low engine loads, the BTE was lower with fuel containing zinc oxide particles than with pure diesel fuel. At medium and high loads, the addition of ZnO nanoparticles also reduced the BTE compared to diesel. Increasing the particle concentration in the fuel increased the BTE, and at a particle concentration of 150 ppm, the BTE at these conditions was slightly higher than that of pure diesel fuel. Seela C. and others noted that BTE is 2–3% higher when 50–100 ppm ZnO is added compared to diesel fuel [36].
Figure 12a shows that the addition of carbon nanoparticles reduces the BTE of RME fuel; it was on average 3–3.5% lower compared to pure RME. A slightly smaller reduction in thermal efficiency was obtained when the engine was running on RME with the addition of 100 ppm CNT nanoparticles. Sandeep K and others found that a 50 ppm concentration of carbon particles influences the brake thermal efficiency of biodiesel fuel, which is 2.24% higher compared to RME, but lower by 1.68% compared to diesel [37]. A similar effect was observed with zinc oxide nanoparticles added (Figure 12b). The use of RME fuel with ZnO nanoparticle concentrations of 50 and 100 ppm reduced thermal efficiency by 1.0%, 1.5%, and 1.8% at low, medium, and high loads, respectively. Increasing particle concentration to 150 ppm resulted in a decrease in thermal efficiency of 2.0%, 2.8%, and 3.3%, respectively. This shows that although particle content is important for BTE, it does not show significantly better effects.
Zinc oxide particles at all concentrations had a slightly greater impact on BTE than carbon particles, which could be due to the catalytic effect of metal oxide nanoparticles. The zinc oxide surface is characterized by high energy, with many surface atoms abundant in such high-energy surfaces, which improves the combustion process of fuels. The lower BTE compared to other researchers could be because the combustion process in CR engines is more efficient and smoother because of the high-pressure injection and multiple injection stages. This ensures better fuel and air mixing, lower emissions, and more consistent engine performance.

3.3. Change in Emissions

3.3.1. Change in NOx Emissions

In Figure 13, the change in NOx emissions depending on the load for all tested fuels is illustrated. Nitrogen oxides form by the oxidation of atmospheric oxygen at elevated temperatures in the engine cylinder. It can be seen from the figure that the NOx emissions increased with the load boost for all tested fuels. This occurs because, as the load increases, the temperature in the combustion chamber rises since NOx formation is highly dependent on temperature [38].
In Figure 13a, it is evident that for an engine operating at medium (IMEP = 10 bar) and high (IMEP = 14 bar) loads, NOx emissions were on average 10% higher when using fuels with a CNT additive. The reason could be more intensive combustion, as indicated by a shorter combustion duration (Figure 7b). At low-load conditions (IMEP = 6 bar), the impact of the CNT additive on NOx emissions was minimal. According to Chen A, who researched a single-cylinder direct-injection diesel engine, 50 ppm carbon particle concentration in fuels reduced NOx by 4.48% [24].
When using zinc oxide nanoparticles (Figure 13b), no significant changes in NOx emissions are observed compared to those of pure diesel fuel. Reduction in nitrogen oxide emission is noticeable only at low load conditions regardless of particle concentration in the fuels. According to Seela et al., fuels with 50 ppm ZnO reduced NOx emissions compared to all tested fuels, including diesel [34]. Among nanoparticles, higher NOx emissions were seen in the case of CNT, which could be related to the hydrogen content in the fuels that promote NOx formation.
Figure 14a shows that the addition of carbon nanoparticles to RME reduced nitrogen oxide emissions in almost all engine operating modes. At low engine load, NOx emissions were reduced by 25.1% on average. The highest reductions were obtained with RME fuel with a CNT concentration of 100 ppm: 28.0% at low load, 13.2% at medium load, and 6.8% at maximum load. Further increase of CNT particle concentration up to 150 ppm increased NOx emission. Sharma S., et al. present further data that the addition of CNT100 nanoparticles to biofuels increased NOx emission by 15.11% at full load conditions [16].
Figure 14b shows the effect of the concentration of ZnO nanoparticles in RME fuel on NOx emissions. At low engine load, the addition of 50 ppm ZnO particles reduced NOx emissions by 24.9%. Increasing the nanoparticle content further reduced NOx emissions and the RME fuel with 150 ppm ZnO additive resulted in 32.9% lower NOx emissions compared to pure RME engine operation. At medium load, the 50 ppm additive reduced NOx emissions by 9.3% and the 100 ppm additive by 13.4. A further increase to 150 ppm reduced NOx emissions by only 6.8%. At high load, the highest NOx reduction was obtained with a 50 ppm ZnO particle concentration in the RME fuel. It was 12.6%. When the engine was running on RME fuel with 100 ppm and 150 ppm particle additive, the NOx reduction was lower, 7.8% and 4.6%, respectively. NOx emissions may increase in cases where NMs cause higher heat release, cylinder pressure, and cylinder temperature. This effect is a consequence of the catalytic effect of NMs which accelerates the combustion process [26].
The reduction in NOx emissions can be attributed to those NMs with a sufficiently strong heat release effect [27]. This can be seen for all the RME fuels studied, with shorter MBF50 and combustion times. In fuel blends containing ZnO nanoparticles, NOx formation is reduced by the nanoparticles. The metal oxide nanoparticles function as an oxygen barrier, which reduces the oxygen content and thus reduces NOx emissions.

3.3.2. Change in CO Emission

The trend of change is seen in Figure 15a, where carbon oxides increase in the maximum load using carbon particles with a 50 ppm concentration. In the medium load (IMEP = 10 bar), the lowest CO amount is at 100 ppm, while in the maximum load (IMEP = 14 bar), with 50 ppm. The influence of zinc oxide particles on CO (Figure 15b) shows a significant increase at low loads with 150 ppm and maximum loads with only 50 ppm concentrations. The reduction in CO emissions with CNTs is possible due to a shorter ID. The influence of zinc oxide particles on CO emission reduction is seen only in medium load modes.
CO emissions can be higher due to the catalytic effect of NMs. To reduce CO emissions, both carbon particles and zinc oxides have an influence when the load is low (IMEP = 6 bar) and medium (IMEP = 10 bar). However, a negative effect can also be seen at maximum load (IMEP = 14 bar) with CNT and ZnO. CO reduction can only occur with those NMs whose catalytic activity is greater, acting as an oxidizing catalyst, converting CO to CO2 during the combustion process, and which NMs stimulate better secondary atomization and ID advance.
In Figure 16a, there is a notable change trend: CO reduction is influenced by carbon particles with all concentrations at all load modes. The best result is seen at maximum load (IMEP = 14 bar) when using a 50 ppm concentration—a 17.8% reduction, at low load (IMEP = 6 bar) with 150 ppm—26.13%. The best result is when using 50 ppm: the CO amount decreases by −16.12%. The addition of CNT nanoparticles reduces CO emissions compared to diesel, as said by Sharma S., and etc. [16]. Najafi G., also noted a CO reduction of up to 6% when using carbon particles with concentrations of 40 ppm, 80 ppm, and 120 ppm [35].
In Figure 16b, it is seen that at low load modes, CO decreases as particle concentrations change, while at medium and maximum loads, the CO amount is greater. The influence of zinc oxide particles on CO changes is uneven across all load modes. The most notable change is observed at a concentration of 150 ppm. At medium (IMEP = 10 bar) and maximum (IMEP = 14 bar) load, with 100 ppm, it is a larger amount; CO is reduced by 5.1% and 4.56%, respectively. R. Gavhane studied biodiesel with ZnO nanoparticles and said that CO emissions decreased correspondingly up to 41.08%. The results and conclusions confirm that ZnO nanoparticles in biodiesel improve the functioning and combustion of the DI and reduce the number of emitted pollutants [39].
CO emissions may be lower due to the catalytic activity of NMs the reduction in CO emissions is seen with the addition of CNT at all load conditions, and ZnO only at low load. Although an increase in CO can be seen at medium and maximum loads with ZnO. The CO emission may be due to poor mixing, sometimes an oily mixture, and incomplete combustion. In this study, it was found that the addition of CNT nanoparticles to the base fuels D and RME had a positive effect on CO emissions.

4. Conclusions

Experimental studies were carried out on a CRDI diesel engine to determine the influence of nanoparticles (CNT and ZnO) on the combustion properties of diesel fuels and RME. The nanoparticles were present in equal concentrations (50, 100 and 150 ppm) in diesel with CNT and ZnO (DCNT50, DCNT100, DCNT150 and DZnO50, DZnO100, DZnO150) and biodiesel with CNT and ZnO (BCNT50, BCNT100, BCNT150 and BZnO50, BZnO100, BZnO150) at 2500 rpm.
The results show that the ID of the pilot fuel portion of the carbon nanoparticle diesel fuel was shorter at all cases studied compared to pure diesel fuel. The effect of the CNT additive in RME fuels on the pilot portion ignition varied with the load. At low loads, the ID was longer, while at medium loads, the influence was minimal; at high loads, the ID was shorter meanwhile, ZnO addition to RME prolonged the pilot portion ID. The ID value increased with increasing concentration of ZnO nanoparticles in the fuel. CNT and ZnO additives tend to reduce pilot portion ID in diesel, enhancing ignition readiness due to the enhancement of thermal conductivity and catalytic activity. ZnO in RME has an adverse effect, prolonging ignition delay due to potential reactivity suppression.
Meanwhile, the influence of nanoparticles on the ignition of the main fuel portion is negligible. This is related to the peculiarities of the combustion process with split fuel injection. The combustion of the ignited fuel portion increases the temperature of the combustion chamber charge, so the ID of the main portion is very short, and the effect of nanoparticles is small.
The addition of carbon nanoparticles to diesel fuel brought the MBF50 closer to TDC; however, this change reached only 1 CAD. The combustion duration at all load conditions was 3.5–2.5% lower with nanoparticles in the fuel. Adding carbon nanoparticles to biodiesel also brought MBF50 minimally closer to TDC. The combustion duration with nanoparticles was shorter across all engine loads.
Adding zinc oxide nanoparticles to diesel fuel at a concentration of 50 ppm brought the MBF50 position closer to TDC, but this change was only 0.8 CAD. Higher particle concentration did not have any further effect on the position of the MBF50. The combustion duration at all loads was shorter for diesel fuels with nanoparticles. The effect of ZnO nanoparticles in biofuel on the MBF50 position and combustion duration was obtained only at medium load conditions. The MBF50 position approached the TDC by an average of 2.4%, and the combustion duration decreased by 3.0%, independent of ZnO particle concentration. Carbon and ZnO nanoparticles improve combustion phasing and duration slightly, especially under medium load conditions, supporting better combustion stability and efficiency.
The addition of carbon nanoparticles in diesel fuel slightly increased the thermal efficiency only at low loads. At medium loads, the nanoparticle additive had no impact on thermal efficiency, while at high loads, the effect was negative. The addition of carbon nanoparticles to RME reduced the BTE by an average of 3.5% compared to pure RME. Diesel fuels with ZnO nanoparticles showed a slightly lower thermal efficiency compared to fuels without additives. The addition of ZnO particles in RME fuels showed a similar effect on thermal efficiency. While CNT shows marginal BTE improvement at low load in diesel, both CNT and ZnO generally reduce thermal efficiency in RME applications due to possible heat transfer inefficiencies.
The addition of CNT particles to diesel fuel increased NOx emissions by an average of 10%, while the addition of ZnO particles had no significant effect on NOx emissions. The addition of carbon nanoparticles to RME fuels reduced NOx emissions by 25.1%, 5.9%, and 1.5% on average at low, medium, and high engine loads, respectively. The influence of ZnO particles was even higher. Here, the NOx reduction was 28.8%, 9.8%, and 8.3%, respectively. CNT increases NOx in diesel due to higher combustion temperatures; however, both CNT and ZnO reduce NOx significantly in RME—ZnO being more effective, possibly due to catalytic and thermal regulation effects.
The addition of CNT in both diesel and RME reduced CO emissions, especially in the case of RME. The effect of the ZnO particle additive was minimal in most cases. CNT effectively reduces CO emissions by promoting more complete combustion. ZnO has little to no impact on CO levels, indicating a limited effect on oxidation processes.

Author Contributions

Conceptualization, S.S. and V.J.; methodology, S.S. and V.J.; software, S.S.; validation, S.S. and V.J.; formal analysis, V.J.; investigation, S.S. and V.J.; resources, S.S.; data curation, S.S.; writing—original draft preparation, V.J.; writing—review and editing, S.S.; visualization, S.S. and V.J.; supervision, S.S.; project administration, S.S.; funding acquisition, S.S. and V.J.; All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The flow scheme of nanoparticle-loaded fuel preparation.
Figure 1. The flow scheme of nanoparticle-loaded fuel preparation.
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Figure 2. The flow determination scheme of pilot fuel ignition delay duration and main fuel portion ignition delay duration.
Figure 2. The flow determination scheme of pilot fuel ignition delay duration and main fuel portion ignition delay duration.
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Figure 3. Influence of the CNT nanoparticles content in the D on pilot fuel ignition delay duration (a) and main fuel portion ignition delay duration (b).
Figure 3. Influence of the CNT nanoparticles content in the D on pilot fuel ignition delay duration (a) and main fuel portion ignition delay duration (b).
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Figure 4. Influence of the CNT nanoparticles content in the RME on pilot fuel ignition delay duration (a) and main fuel portion ignition delay duration (b).
Figure 4. Influence of the CNT nanoparticles content in the RME on pilot fuel ignition delay duration (a) and main fuel portion ignition delay duration (b).
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Figure 5. Influence of the ZnO nanoparticles content in the D on pilot fuel ignition delay duration (a) and main fuel portion ignition delay duration (b).
Figure 5. Influence of the ZnO nanoparticles content in the D on pilot fuel ignition delay duration (a) and main fuel portion ignition delay duration (b).
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Figure 6. Influence of the ZnO nanoparticles content in the RME on pilot fuel ignition delay duration (a) and main fuel portion ignition delay duration (b).
Figure 6. Influence of the ZnO nanoparticles content in the RME on pilot fuel ignition delay duration (a) and main fuel portion ignition delay duration (b).
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Figure 7. Influence of the CNT nanoparticles content in the D on MBF50 (a) and combustion duration (b).
Figure 7. Influence of the CNT nanoparticles content in the D on MBF50 (a) and combustion duration (b).
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Figure 8. Influence of the CNT nanoparticles content in the RME on MBF50 (a) and combustion duration (b).
Figure 8. Influence of the CNT nanoparticles content in the RME on MBF50 (a) and combustion duration (b).
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Figure 9. Influence of the ZnO nanoparticles content in the D on MBF50 (a) and combustion duration (b).
Figure 9. Influence of the ZnO nanoparticles content in the D on MBF50 (a) and combustion duration (b).
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Figure 10. Influence of the ZnO nanoparticles content in the RME on MBF50 (a) and combustion duration (b).
Figure 10. Influence of the ZnO nanoparticles content in the RME on MBF50 (a) and combustion duration (b).
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Figure 11. Influence of the nanoparticles content in the D on thermal efficiency: (a) CNT, (b) ZnO.
Figure 11. Influence of the nanoparticles content in the D on thermal efficiency: (a) CNT, (b) ZnO.
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Figure 12. Influence of the nanoparticles content in the RME on thermal efficiency: (a) CNT, (b) ZnO.
Figure 12. Influence of the nanoparticles content in the RME on thermal efficiency: (a) CNT, (b) ZnO.
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Figure 13. Influence of the nanoparticles content in the D on NOx emission: (a) CNT, (b) ZnO.
Figure 13. Influence of the nanoparticles content in the D on NOx emission: (a) CNT, (b) ZnO.
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Figure 14. Influence of the nanoparticles content in the RME on NOx emission: (a) CNT, (b) ZnO.
Figure 14. Influence of the nanoparticles content in the RME on NOx emission: (a) CNT, (b) ZnO.
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Figure 15. Influence of the nanoparticles content in the D on CO emission: (a) CNT, (b) ZnO.
Figure 15. Influence of the nanoparticles content in the D on CO emission: (a) CNT, (b) ZnO.
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Figure 16. Influence of the nanoparticles content in the RME on CO emission: (a) CNT, (b) ZnO.
Figure 16. Influence of the nanoparticles content in the RME on CO emission: (a) CNT, (b) ZnO.
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Table 1. Properties of diesel and biodiesel fuels.
Table 1. Properties of diesel and biodiesel fuels.
PropertyStandardValue
Diesel Fuel
Kinematic viscosity@40 °C, cStEN ISO 31402.88
Density@15 °C, kg/m3EN ISO 3675835
Cetane numberEN ISO 426455.5
Sulfur, mg/kgEN ISO 208466.5
Content of polycyclic aromatic carbohydrates, %EN 129161.1
Biodiesel
Kinematic viscosity@40 °C, cStEN ISO 31404.47
Density@15 °C, kg/m3EN ISO 3675883
Cetane numberEN ISO 515654.3
Sulfur, mg/kgEN ISO 20846<3
Acid number, mg KOH/gEN 141040.16
Table 2. Basic design parameters of the engine.
Table 2. Basic design parameters of the engine.
Name of ParametersData
Engine typeFour-cylinder, in-line, turbocharged, JTD
Fuel injection systemCommon rail, direct injection (CRDI)
Engine displacement1910 cm3
Bore × Stroke82 × 90.4
Compression ratio18.0:1
Rated power85 kW (115 HP)
Maximal torque255 Nm (EEC), at 2000 rpm
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Jokubynienė, V.; Slavinskas, S. The Effect of ZnO and CNT Nanoparticles on the Combustion Characteristics and Emission Performance of a Common Rail Diesel Engine Fueled with Diesel and Biodiesel. Energies 2025, 18, 2564. https://doi.org/10.3390/en18102564

AMA Style

Jokubynienė V, Slavinskas S. The Effect of ZnO and CNT Nanoparticles on the Combustion Characteristics and Emission Performance of a Common Rail Diesel Engine Fueled with Diesel and Biodiesel. Energies. 2025; 18(10):2564. https://doi.org/10.3390/en18102564

Chicago/Turabian Style

Jokubynienė, Vida, and Stasys Slavinskas. 2025. "The Effect of ZnO and CNT Nanoparticles on the Combustion Characteristics and Emission Performance of a Common Rail Diesel Engine Fueled with Diesel and Biodiesel" Energies 18, no. 10: 2564. https://doi.org/10.3390/en18102564

APA Style

Jokubynienė, V., & Slavinskas, S. (2025). The Effect of ZnO and CNT Nanoparticles on the Combustion Characteristics and Emission Performance of a Common Rail Diesel Engine Fueled with Diesel and Biodiesel. Energies, 18(10), 2564. https://doi.org/10.3390/en18102564

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