Influence of hydrogenated diesel/H2O2 blend fuel on diesel engine performance and exhaust emission characterization

Comparison of comprehensive output energy at various speed

The output energy (OPE) at various speeds (rpm) is a tool for comparing the comprehensive performance of (mathrmH_2mathrmO_2)/diesel blend fuel with reference diesel (RD). Theoretically, it has measured how much fuel is being disbursed per breakup time to deliver maximum power. Figure 2 illustrates the generator output (kW) of the various test fuels at different engine speeds and various engine torque (6−12.5 Nm).

Figure 2

Comparison of power output efficiency of various quantities of H2O2/diesel fuel blends.

The results revealed that the RD fuel produced higher output at various engine speeds, almost 1–2.5%. However, emulsified H2O2/diesel blends showed lower output efficiency. The reason may be due to the relatively lower calorific value of H2O2/diesel fuel than the RD fuel discussed in our previous study44.

Amongst the (mathrmH_2mathrmO_2)/diesel blend fuel, the 5 wt.% of (mathrmH_2mathrmO_2) showed somewhat higher than higher H2O2/diesel blend content. Such negligible deficiency could be revealed due to their higher combustion efficiency and effective oxygen content in the diesel blend fuel, which is perhaps a good agreement for early combustion efficiency compared to RD diesel. Moreover, in our previous work13,44,60, we have already demonstrated that adding H2O2 in the diesel enhanced the cetane number with thermal conductivity and specific heat. Perhaps the calorific value of the H2O2/diesel blend fuel scarcely lowered because of the lower energy contents of the fuel blends, despite all significances being agreeably within the scope of diesel fuel47,61.

Specific fuel consumption (g/kWh)

The current section of the study investigates engine performance using a convenient parameter of specific fuel consumption (SFC) and a comparison of the RD and H2O2/diesel blend fuel. The tests were conducted under various engine torque (6 12.5 Nm) and speed conditions ranging from 1700 to 3024 rpm. SFC indicates the ratio of fuel consumption rate to brake power output. Figure 3 summarizes the SFC of RD and diesel blend fuels; the results illustrated a decreasing trend as the engine speed increased from 1700 to 3024 rpm.

Figure 3
figure 3

Comparison of specific fuel consumption efficiency of various quantities of H2O2/diesel fuel blends.

Because the test engine’s fuel injection pump was of a customized type, the delivered fuel quantity decreased at the minimum default speed of Yanmar62, such as 1700 rpm. They simulated the breakup comparison rate regarding the RD fuel–air mixing rate and excess oxygen content within the diesel blend fuel. Increasing the engine speed improved system performance while decreasing the SFC of each test fuel.

Nonetheless, the reduced volumetric coherence at higher speeds can reveal an SFC deficiency at speeds above 1700 rpm62. On average, the SFC of the RD test was higher than that of all H2O2/diesel blend fuels. RD fuel’s SFC was 2–5% higher than H2O2/diesel. The test fuel contains 10 and 15 wt.% H2O2, respectively, and the diesel blend showed more promising SFC results than the diesel blend with 5 wt.% H2O2. H2O2 demonstrated a 1.5 to nearly 5.2% reduction in SFC when compared to 5 wt.% H2O2/diesel and RD fuel, respectively. The higher SFC of the RD fuel than all H2O2/diesel blend fuels is attributed to the RD diesel’s slightly higher energy scope. Technically, the heating values of the fuel blends were lower due to the molar volume contents of H2O2 and emulsifier (C14H24O9/C3H6O); thus, consumption was supposed to be increased to achieve slightly more than 11 Nm torque. Despite having relatively lower heating values, all H2O2/diesel blend fuels had lower SFC than RD fuel. The reason for effective SFC is due to the higher cetane value of the H2O2/diesel fuel blend51. When the cetane number of blend fuel rises with increased quantities of H2O2, the temperature and oxygen content in the combustion chamber are in more self-control, promoting thermal cracking and increasing oxidation rates while decreasing unburned HC emissions and specific fuel consumption63. It also suggests that adequate SFC of the H2O2/diesel blends is perhaps found because of the presence of stable high oxygen contents in the diesel blend.

Smoke density (SD)

The exhaust smoke density, also called multiple particulate matter (PM), relates to unburnt hydrocarbons (HxYx), NOx, and SOx and has proven to be a critical issue for diesel fuel. Therefore, since the last decay, developed countries have made rigorous policies to restrict light-grade diesel (EURO II and III) usage in public automobiles. Yet the PM, particularly HxYx and NOx, are still challenging in European countries due to the freezing environment10,12. Even though public transport uses high-speed diesel (EURO V and VI) followed by advanced technology like in-cylinder and advanced hybrid oxidation catalysts with catalytic filters system.

Thus, this section investigated a comprehensive assessment of the engine performance on the SD of different H2O2/diesel blend fuels. The SD analysis was carried out using an AVL smoke meter during the test running condition with variable torque (6−12.5 nm) followed by different engine speeds ranging from ~ 1700 to 3600 rpm. The SD results can be seen in Fig. 4; the SD comparison of H2O2/diesel blends with RD fuel showed a decreasing tendency as the engine speed increased from 1700 to 3600 rpm.

Figure 4
figure 4

Comparison of exhaust smoke density of various quantities of H2O2/diesel fuel blends.

It has also been noticed that the SD followed a similar trend level to Fig. 4 decreases for each H2O2/diesel blend than the RD. However, the drought of SD showed a significant drop in all H2O2/diesel blend fuel, about 10–25% reductions. The reduction of SD level was probably revealed due to excess oxygen content, which has also been attributed to better mixing of intake air and fuel and an increase in the OH radical molar mass contents in the combustion chamber38,51,64. Usually, the components of diesel fuel exhibit an intense interaction capability with oxygen. Furthermore, the stability of diesel/H2O2 is higher, secondary combustion is reduced, and combustion performance is enhanced.

Moreover, our previous studies have demonstrated that the emulsifier used in H2O2 and diesel prevented the phase rift between diesel and H2O2, as seen in Fig. 444. Therefore, H2O2 likely invariably reduces soot and PM emissions in diesel. Also, it could be the consequence of rapid fuel breaking up due to the distinct engagement of oxygen content in the fuel combustion chamber, probably more related to smoke density. The highest SD reduction was obtained by 15 wt. of H2O2 diesel blend fuel at maximum load conditions is 26% (see Fig. 5).

Figure 5
figure 5

Comparison of percentage reduction in exhaust smoke density (SD) for various quantities of H2O2/diesel fuel blends.

Also, Fig. 5 shows the lower peak value attained by the 5 wt.% of H2O2 in the diesel blend fuel at a load speed of 2900 rpm is about 12%. The smoke density is further decreased with the 10 wt.% H2O2 addition in diesel blend because of excess oxygen content. Thus, it presumably revealed the molar volume difference between the agar/acetone (C14H24O9/C3H6O) and the (mathrmH_2mathrmO_2) added diesel blend fuel, which might reveal the direct relevancy of SD and particulate matters (HxYx + Nox) to each other. Particulate reduction will most likely be due to a good agreement in the combination of acetone and H2O2 in diesel fuel, which may act as an oxidizing agent to keep the combustion chamber clean. In addition, the SD is reduced for the H2O2/diesel blends because of the higher molar mass contents of hydrogen in the emulsifier. Thus, it can also be combusted practically as SD-free under a specific combustion environment65. Ashok and Saravan51 also reported similar observations for the (mathrmH_2mathrmO_2) added emulsified fuel because of the presence of excess oxygen. Nevertheless, based on the supplementary molar mass of C14H24O9/C3H6O as an emulsifier accumulation with H2O2. Hypothetically, the least emulsified fuel probably has a significant role in reducing SD.

Comparison of emission characteristics of H2O2/diesel blend fuel at maximum speed of engine (with and without the load)

This study evaluated exhaust emissions with the Lancom 4, a portable gas analyzer at a full-speed (2989 rpm) diesel engine with and without the load. This analyzer meets the requirements of the US EPA CTM 034 reference method with a maximum deviation of the detection limit within 2 ppm for exhaust gases and unburned hydrocarbons from 0.1 ppm66. Figures 6, 7, 8 and 9 summarizes that the average exhaust emission from the exhaust stream of diesel generators has been thoroughly studied. The results summarize the reduced average exhaust emissions concentrations of the loaded and unloaded generator at maximum power (see Fig. 3) after inserting 5–15 wt.% of H2O2 in the blended fuel.

Figure 6
figure 6

Comparison of carbon-based emissions at unloaded (UL) and full-loaded (FL) diesel engine.

Figure 7
figure 7

Comparison of sulfur dioxide (SO2) and unburned hydrocarbon emissions at unloaded (UL) and full-loaded (FL) diesel engine.

Figure 8
figure 8

Comparison of nitrogen-based emissions at unloaded (UL) and loaded (FL) diesel engine.

Figure 9
figure 9

Comparison of unburned hydrocarbon (CxHy) based emissions at unloaded (UL) and loaded (FL) diesel engine.

Figure 6a and b summarize the emission results of CO and CO2 from loaded and unloaded generator exhaust, respectively. The test results (see Fig. 6a and b) of the unloaded engine revealed reference diesel (RD) CO emissions of 565 ppm and 706.25 ppm loaded, compared to 437.5 ppm and 525.4 ppm (loaded). It is generally known that diesel fuel requires more oxygen to be burned, so in the case of a fully loaded diesel engine, combustion requires a greater amount of air intake to be drowned out by each intake stroke, regardless of the position of the throttle. The air is then compressed and heated before diesel fuel is fed into the cylinder. When fuel is exposed to a higher amount of hot air, it rapidly burns. This results in a higher concentration of COx and NOx exhaust gases in the loaded engine compared to the unloaded engine. The three best diesel/H2O2 blend emulsions were evaluated in order to reduce CO content.

The UL generator shows that the 5 wt.% of H2O2 in the diesel blend represents a 22.5–25% reduction. The 10 wt.% of the diesel blend reduced CO emissions to 348.5 ppm, a 38% reduction, and the 34.8% reduction of CO emissions represents the 15 wt.% of H2O2 in the diesel blend. The 10 wt.% of H2O2 in the diesel blend shows the greatest reduction in CO emissions of the UL generator. Theoretically, the air/fuel equality ratio is defined as the difference between the definite air/fuel ratio and the stoichiometric air/diesel ratio in the compression chamber of a diesel engine58. In contrast, in the case of H2O2/diesel, the unstable peroxide likely provides some of the oxygen needed for the diesel to be ignited early, reducing the need for additional air in the compression chamber. Technically speaking, if the required amount of oxygen is present, then the UL diesel engines run on the leaner side of stoichiometry, CO emissions are very low in the case of an additional molar volume of peroxide in the compression chamber.

However, the loaded generator has CO emissions of about 400.7 ppm, which is 13% higher than the unloaded generator but smaller than the loaded and unloaded generators of RD fuel. It is suggested that in the case of a fully loaded diesel engine, the diesel requires more oxygen, and the probably unstable oxygen present in the diesel emulsion is probably not enough for the ignition. Therefore, the compression chamber takes in more air, and thus the contents of the CO emissions are higher than in the UL-loaded diesel engine. Nevertheless, the 15 wt.% of the diesel blends also show lower CO contents in the loaded and unloaded generator than the reference diesel but are a little higher than the 5 and 10 wt.% of the diesel blend composition, respectively.

Nevertheless, the loaded generators show almost 10–20% higher emissions than the unloaded generator in all the fuel tests. The loaded generator required more power and more fuel and air intake to be combusted, thus consequently higher the rate of CO emission. It is probably because the higher molar mass of oxygen in the diesel blend composition and higher contents of CO in the reference diesel emissions are in good agreement due to the air intake inside in-cylinder combustion. Moreover, the tendency H2O2 is entirely reactive, flaring once it has ideal environments like ignition in a closed chamber. Thus, it reacts independently and does not need any oxidizer, helping the diesel for an early and clean combustion process. But the higher amount of H2O2 in the blend yet contributes to reducing the contents of CO. Gribi et al.67 also found that H2O2 has individual combustion characteristics. They have reported that H2O2 can be used as a fuel or an oxidizer when reacting with other fuels, particularly in combustion chambers. Thus, it assumes the dual nature of H2O2 and explores its potential benefits in clean combustion technology.

Figure 6c and d also shows the H2O2 impact on reducing CO2 parts of the unloaded and loaded diesel generator’s exhaust stream. Although reference diesel had a very low CO2 emission (1.2%), the 5% (mathrmH_2mathrmO_2) fuel blend slightly increased the CO2 emission to 1.75%. Similar results are also observed in loaded generators, and the CO2 emission exhibits higher content but is lower than the RD diesel emission, either loaded or unloaded generator. However, the values of CO2 emissions for higher percentages are quite like RD (1.2%), and the effect is not significantly evident on CO2 emission. Al-lwayzy et al.69 and Scragg et al.70 observed a small decrease in the proportion of CO2 in the exhaust produced by emulsion fuel containing microalgae in comparison to fuel made from biodiesel. According to Koc and Abdullah71, higher oxygen atom levels in the fuel mixture as a result of higher water concentrations may explain why emulsified diesel fuel has higher CO2 levels. The Koc and Abdullah71 justification could be a good agreement in the case of 5% of H2O2 in the diesel fuel blends, but the increment gap is not large with the RD diesel emission of CO2. Nevertheless, more experimentation is needed to explain the impact of a 5% H2O2/diesel blend on CO2 emissions.

Furthermore, the 10 wt.% of H2O2 in the fuel blend revealed CO2 contents emission totaled 0.95 percent in unloaded and 1.09 in loaded generator, representing an overall reduction of 19–21 percent from RD. Ashok and Saravanan51 observed similar results with diesel/ H2O2 and David and Reader56 (CH4/ H2O2) in their studies with H2O2 blended fuels, which showed a reduction of about 16.5%. However, the 15 wt.% H2O2 diesel blend slightly reduced the exhaust component of carbon dioxide in the loaded and unloaded generator.

Comparisons of particulate matters emissions

The results of a portable gas analyzer at a full-speed (2989 rpm) diesel engine unloaded and load generator showed the influence of (mathrmH_2mathrmO_2) on the fuel blends’ emission of sulfur dioxide (SO2). Figure 7 shows the comparison of SO2 emissions. The RD fuel shows higher SO2 emissions of 16 ppm and 20 ppm of the unloaded and loaded generator than all H2O2/diesel blends.

The H2O2/diesel blend also positively impacted the concentrations of SO2 exhaust stream as measured in an unloaded and loaded diesel generator. The reduced exhaust concentrations of SO2 are due to the substantial oxidizing property of H2O2 in the blended fuel. The 5 wt.% of H2O2 in the diesel blend shows a significant reduction of SO2 to 11 ppm in unloaded and 13.2 ppm in loaded generator exhaust, nearly 31.5% and 34% lower than RD diesel. Similarly, SO2 emissions from the 10 wt.% of H2O2 were at 13.4 ppm, and 14.35 ppm revealed the 15 wt.% H2O2 in the blended fuel. The 15 wt.% of H2O2 in the diesel blend slightly reduced SO2 (14.35 ppm) emissions.

Nevertheless, the unloaded and loaded generator shows considerably lower emissions of SO2 than RD diesel fuel. These reductions were observed due to the overall lower sulfur content of diesel fuel and no sulfur contents in H2O2. Ashok & Saravanan51 and David & Reader56 observed similar findings in their studies with H2O2 blended fuels.

The Lancom 4 portable gas analyzer has also computerized the results of nitrous oxide (NOx), nitric oxide (NO), and nitrogen dioxide (NO2) of exhaust emissions of reference diesel and H2O2/diesel fuel blends, and Fig. 8 summarizes the results of the comparison.

The primary mechanism causing the reduction in exhaust emissions looks like the decrease in the temperature of the combustion products as a result of vaporization of the liquid water and subsequent dilution of the gas-phase species. NOx results found positive impacts on concentrations of NO2 and nitrous oxide (NO) in the diesel fuel exhaust streams, either unloaded or loaded generators. Figure 8 illustrates an overall reduction comparison in nitrogen dioxide and nitrous oxide emissions due to the solid oxidizing capacity of H2O2 as it decomposes in the combustion chamber to oxygen and water.

Water produced during this reaction absorbed heat which, in turn, slightly decreased the temperature in the combustion chamber. This reduction in a temperature limited the production of NO2 and NO. Although reference diesel has very low emissions of NOx (12 ppm), the 5 and 10 wt.% of H2O2 in the diesel blend decreased its formation to 9 and 5.8 ppm, respectively. The reduction of NOx formation caused by the combination of higher cetane number and water content reduces the diesel engine’s temperature13,43,51. Similar results are also observed in loaded generator emissions. The significant reduction of nitrogen-based emissions of blend fuel on unloaded or loaded generators might be a possibility of rapid vaporization and disassociation of H2O2 into hydroxyl radicles. In addition, it can also be interpreted that the H2O2 has become strenuously unstable and highly active in the combustion chamber, consequently oxidizing the NO and NO2 in the exhaust. Kasper et al.68 also investigated the significance of H2O2 on the decomposition and reduction of nitrogen-based emissions; they have experimentally demonstrated that NOx can be oxidized to NO and NO2 in the gas phase by OH radicals generated by the thermal decomposition of H2O2. Similar results were also observed by Saravanan et al.72. and Ashok & Saravanan51, in their studies of H2O2-diesel blends, found an overall reduction of about 18.5%.

Figure 8 also shows the results of Nox, and it was noticed the, the 5 wt.% of H2O2 in the diesel blend shows higher NOx contents in UL and FL generater emission. It has been found that a 5% H2O2 diesel blend doesn’t make a big difference in reducing NOx and CO2. This is likely because there isn’t as much H2O2 in the diesel, but it does produce less heat (see Fig. 10) than RD diesel, regardless of whether the UL or FL generator. Although the temperature reduction is 2–20% in the case of 5% H2O2/diesel blend fuel exhaust, this gap is probably not enough to overcome the reduction of NOx and CO2. On the other hand, higher the concentrations (10–15%) of H2O2 in the diesel blend shows significant reduction in the NOx in UL and FL generator emissions. It seems that a higher water content level in the diesel blend reduces the temperature of the combustion chamber, resulting in a lower NOx concentration.

Figure 10
figure 10

Comparison of exhaust temperature and excess air intake at unloaded (UL) and loaded (FL) diesel engine.

Typically, the combustion temperature, oxygen concentration, and the retention time of the combustion product in the combustion zone are often the most prevalent variables determining the amount of NOx generated. The high temperature inside the cylinder caused by the high compression ratio encourages NOx emission, and the RD results show good agreement with the experimental results74. The local adiabatic flame temperature is reduced by the heat of vaporization and sensible heating of water, which also reduces NOx generation. Therefore, the higher the concentration of H2O2 in the diesel blend, the greater the reduction in NOx75. Scrage70 and Koc71 reported similar results, which increased the water and oxygen contents while decreasing NOx and CO2, but the CO2 reduction is not yet significant. Perhaps it might be overcome in the case of the alteration of the engine.

Hydrocarbon emissions from diesel engine exhaust are also essential pollutants. The (mathrmH_2mathrmO_2)/diesel blends also demonstrated constructive impacts on the total hydrocarbon content of the diesel generator’s exhaust stream. Figure 9 shows comparisons of the overall reduction in the concentration of unburned hydrocarbons (CxHy) due to the considerable oxidizing property of H2O2.

The RD fuel shows higher CxHy content emissions in unloaded and loaded generator exhaust. The diesel blend fuel has a 5wt.% of H2O2/diesel blend and didn’t reveal CxHy content in the unloaded and loaded generator exhaust. However, the 10 wt.% of H2O2 in the diesel blend slightly increased the production of unburned hydrocarbons. The 15% H2O2 in the blend is lower than RD diesel and the 10 wt.% H2O2/diesel blend. The lower level of unburned hydrcarbons is most likely due to the formation of acetone peroxide prior to the solution being mixed with the diesel. which most likely acts as a strong oxidizing reagent in the combustion chamber, and once diesel is ignited, this acts as a cleaning tool along with water vapors to overcome the unburned hydrocarbon reduction in the higher concentration of H2O2 in the diesel blend.

Moreover, the results revealed that, as the concentration of H2O2 in the blends increased, unstable oxygen contents improved due to the peroxiding nature of H2O2, although viscosity, density, and high heat value decreased slightly47,73. In general, higher density and lower viscosity lead to higher flow; thus, these findings suggested that the lower viscosity of diesel/H2O2 blend fuel could succeed in lowering fuel injection with an early ignition time47,73, which could result in a good agreement in the reduction of unburned hydrocarbons and NOx. Furthermore, the higher molar ratio of the peroxide group resulted in a drop in the viscosity of each stable blend compared to RD and a lower concentration of H2O2 in fuel blends. It also suggested that the 70% water content of H2O2 formed water droplets inside the diesel, and these droplets mixed well due to the polysaccharide polymer in the H2O2/diesel blend.

Nevertheless, unburned hydrocarbon emissions were well below those from pure diesel fuel. In terms of particulate matter (PM) emissions, the presence of water during the intensive formation of soot particles appears to significantly reduce and enhance burnout by increasing the concentration of oxidation species such as OH73.

Figure 10 compares the exhaust temperature of the unloaded and loaded generator at maximum power output. The exhaust temperature of RD fuel shows a higher temperature than all H2O2/diesel blend fuels either the generator is unloaded r or loaded at maximum power output. The higher exhaust temperature of RD fuel was revealed due to the higher heat of evaporation and delayed combustion process of lean diesel. However, all H2O2/diesel blend shows almost 20–41% lower exhaust temperature of the loaded generator.

Due to the higher cetane number H2O2, it has a lower latent heat of evaporation than diesel. The ignition delay for H2O2/diesel fuel diminishes, resulting in a low exhaust temperature13,44,72. In addition, during typical engine running, the coolant absorbs the majority of the heat. The H2O2 also has water particles, which interact with the coolant and absorb more heat, decreasing or controlling the exhaust emission temperatures51. The peak engine temperature constantly boosts NOx generation. Including H2O2 in the diesel blend raises the cetane rating, which precedes a reduction in ignition latency. This decreased ignition delay reduces the amount of fuel accumulated before to combustion and lowers the initial combustion rates, lowering the peak temperature and thus lowering NOx generation. Reducing NOx, COx, and CxHy in exhaust emission is a significant agreement to justify the temperature reduction60,73. Figure 10 also compares air intake amounts during the combustion process. Compared to RD diesel, the H2O2/diesel blend fuel shows lesser air intake in the combustion process, probably due to the availability of required oxygen in the combustion chamber.