The BTE, brake specific energy consumption (BSEC), and BSFC are given by the following equations.

Here, P_b and m˙f denote brake power and fuel flow rate, respectively. Figure 2 shows a comparative analysis of the BTE across different brake mean effective pressure (BMEP) values for a diesel engine fueled with pure diesel (D100) and blends containing 20 vol% 1-butanol (D80B20), 1-pentanol (D80P20), and 1-octanol (D80O20), respectively. The data illustrate that the addition of alcohols to diesel generally improves the BTE across the range of BMEP values tested. D80B20 demonstrated a significant increase in efficiency, particularly at elevated BMEP values. This is primarily due to the higher oxygen content and favorable volatility of 1-butanol. The inclusion of 1-butanol in the diesel blend introduced more oxygen into the combustion chamber, facilitating a more complete combustion process and enhancing the BTE. Furthermore, the favorable volatility of 1-butanol contributed to a better air-fuel mixture, further improving the combustion efficiency and increasing the BTE. D80P20 exhibited a slight improvement over D100, suggesting that 1-pentanol also contributed positively to the combustion process, albeit not as significantly as 1-butanol. D80O20, despite outperforming pure diesel at the highest BMEP, exhibited the least improvement or deterioration.

Figure 2: 
Variations of BTE against BMEP

Table 2 shows that 1-octanol has a more favorable cetane value and lower self-ignition temperature than other alcohol components considered in this study. A higher cetane number signifies a shorter ignition delay, leading to more controlled combustion and potentially lower emissions of pollutants such as NO_x and particulate matter. Moreover, the lower self-ignition temperature of 1-octanol under-scores its suitability for blending with diesel fuel. This characteristic is particularly relevant under cold-start conditions, where lower self-ignition temperatures can significantly enhance engine performance and reduce emissions. However, the long carbon chain of 1-octanol, while beneficial for energy content and phase compatibility with diesel, may pose challenges in terms of complete combustion. The molecular structure of 1-octanol can lead to a more complex combustion process, potentially resulting in incomplete combustion under certain conditions.

The physical implications of these results are significant. They indicated that blending alcohols with diesel can enhance the combustion efficiency of diesel engines, leading to better utilization of the energy content of the fuel. This improvement in efficiency could translate into reduced fuel consumption and greenhouse gas emissions, thereby contributing to a more sustainable transportation sector. The higher BTE at elevated BMEPs imply that these blends are particularly effective under high-load conditions, which are common in commercial and industrial applications.

The relationship between air-fuel ratio (AFR) and BMEP in the test engine is shown in Figure 3. For D100, the AFR decreased with increasing BMEP, which is a common trend in diesel engines, because of the increased fueling required to meet the higher load demand. The three alcohol blends generally exhibited higher AFR at lower BMEP levels, with D80B20 maintaining the highest AFR across the entire BMEP range. D80P20 showed a marginal increase in the AFR compared with D100, whereas D80O20 presented the lowest AFR among the blends, particularly at higher BMEP values. This reduction in the AFR for D80O20 may suggest less efficient combustion, possibly because of the heavier molecular structure and lower volatility of 1-octanol compared with those of 1-butanol and 1-pentanol. The physical implications of these findings extend to the efficiency and environmental impacts of diesel engines. Higher AFRs in alcohol–diesel blends could lead to improved combustion efficiency, as previously discussed in the BTE analysis. Additionally, higher AFRs are typically associated with lower exhaust gas temperatures, which can mitigate the challenges of managing the higher temperatures observed in alcohol blends. However, if the AFR is excessively high, it can lead to incomplete combustion.

Figure 3: 
Variations of AFR against BMEP

Figure 4 shows the relationship between EGT and BMEP. The trend across the EGT data for all fuel types showed an increase in temperature with higher BMEP values, which was expected because higher engine loads typically lead to higher combustion temperatures. D80B20 exhibited a notable trend—although slightly lower temperatures than those of D100 were observed at lower BMEP values, it exceeded the D100 temperature at the highest BMEP. This pattern may be attributed to the higher AFR and oxygen content of the blend, which cools the incylinder gas at lower loads and facilitated more complete combustion at heavy loads. D80P20 generally followed a similar trend to that of D100, suggesting that 1-pentanol had a lesser impact on altering the combustion temperatures than 1-butanol. In contrast, D80O20 consistently exhibited higher EGTs than D100 across all BMEP levels. This indicates the different combustion characteristics of 1-octanol, which could be attributed to its longer carbon chain length affecting its burning rate and heat release profile. The physical significance of the observed EGT variations among different blends was correlated with the previously discussed BTE and AFR results. Higher EGTs in conjunction with improved BTE, as observed for D80B20, suggest that alcohol blending may optimize the combustion process, thereby extracting more energy from the same amount of fuel. However, the elevated EGTs, particularly for D80O20, indicate that adapting an engine cooling system to handle higher temperatures may be required to prevent potential engine knocking or damage.

Figure 4: 
Variations of EGT against BMEP

The impact of alcohol additives on energy efficiencies is illustrated in Figure 5; it presents a comparison of the energy consumption across the different fuel blends to generate 1 kWh of power. The BSEC values decreased with increasing BMEP for all fuels, which is a typical behavior in diesel engines, as higher load conditions allow for more efficient fuel usage. D80B20 exhibited a consistent improvement over D100 across the entire BMEP range, indicating that the addition of 1-butanol enhances the energy efficiency of the engine. This finding aligns with previous observations, in which D80B20 demonstrated a superior BTE and maintained a higher AFR, suggesting a more complete combustion process. Comparatively, D80P20 showed a slight improvement over D100 in BSEC, particularly at higher BMEP values. This could be due to the intermediate chain length of 1-pentanol, which may offer a balance between the improved combustion efficiency and volatility. In contrast, the D80O20 blend exhibits BSEC values that are generally larger than those of D100, with a slight improvement at the highest BMEP. This may reflect the less favorable combustion properties of 1-octanol owing to its longer carbon chain, which could affect its combustion rate and energy release during the combustion process. The reduced BSEC indicates that alcohol–diesel blends, especially those with 1-butanol, could lower the overall energy demand for the same engine output, leading to potential reductions in fuel consumption and operating costs.

Figure 5: 
Variations of BSEC against BMEP

Figure 6 shows the BSFC data of the test engine for various BMEP values. BSFC is a critical metric that indicates the efficiency of the engine in converting fuel into work, with lower values representing higher efficiency. Across the BMEP range, it is observed that the BSFC decreases as the BMEP increases for all fuel types, which is an expected trend because the engine operates more efficiently at higher loads. The D80B20 blend consistently shows a slight increase in BSFC at lower BMEP levels compared to D100 but demonstrates a notable decrease at the highest BMEP, indicating better fuel efficiency under higher load conditions. This can be attributed to the higher oxygen content in 1-butanol, which contributes to more complete combustion, as supported by the previously analyzed increase in BTE and AFR for this blend. For D80P20 and D80O20, the BSFC values were generally higher than those for D100, particularly at lower BMEPs, suggesting that the engine does not convert these alcohol blends into work as efficiently as it does with pure diesel. However, as the engine load increases, the gap in the BSFC between the alcohol blends and D100 narrows, and the efficiency of D80B20 eventually surpasses that of D100. This could indicate the different combustion characteristics of alcohols, with 1-butanol providing a more favorable combustion profile in the context of this engine setup. Moreover, the previously conducted EGT analysis showed that D80O20 had the highest exhaust temperatures across the board, which may be correlated with less efficient combustion under these conditions, as higher EGTs can sometimes be a sign of energy not being fully utilized in the power stroke and instead being lost as heat in the exhaust.

Figure 6: 
Variations of BSFC against BMEP

The analysis of NO_x emissions is crucial because NO_x is a significant air pollutant with implications for both environmental pollution and public health. Figure 7 shows the relationship between NO_x emissions and the BMEP. Although the NO_x emissions increased with increasing BMEP levels across all fuel types, it was notably altered by D80B20. This blend achieved a significant reduction in NO_x emissions, which was attributed to the enhanced combustion facilitated by 1-butanol. Specifically, the higher latent heat of vaporization of 1-butanol leads to cooler combustion temperatures, while its increased oxygen content supports complete combustion at these reduced peak temperatures, effectively lowering NO_x formation [17][18].

Figure 7: 
Variations of NO_x against BMEP

Conversely, D80P20 and D80O20 showed varying effects on NO_x emissions. Although D80P20 resulted in lower NO_x emissions at lower BMEP values, it exceeded the NO_x levels of D100 at higher BMEP values. D80O20’s NO_x emissions remained closer to those of D100 throughout the BMEP range. These variations suggest that the combustion characteristics of 1-pentanol and 1-octanol, especially under high-load conditions, may not be as conducive to reducing NO_x emissions as those of 1-butanol. These findings underscore the effectiveness of alcohol–diesel blends, particularly D80B20, in reducing the NO_x emissions that play a significant role in causing smog, acid rain, and respiratory issues. Given its reduced NO_x emissions, D80B20 is a promising alternative fuel for reducing environmental harm.

Figure 8 depicts the carbon monoxide (CO) emissions of the test engine with varying BMEP values. CO is a product of incomplete combustion and a significant pollutant that contributes to environmental and health issues. The data indicate that the CO emissions for D100 increased as BMEP increased, which could be due to the richer fuel mixture required at higher loads, leading to incomplete combustion.

Figure 8: 
Variations of CO against BMEP

D80B20 showed a reduction in CO emissions than D100 at lower BMEP values; although the emissions increased slightly at higher BMEPs, it remained below those of D100. This suggests that D80B20 may facilitate more complete combustion at lower loads, potentially owing to the better oxygenation and flame propagation characteristics of 1-butanol. For D80P20, the CO emissions were generally lower than those of D100. This implies that 1-pentanol, while providing some benefits in terms of emissions at higher loads, may not be as effective as 1-butanol at reducing CO emissions under lower load conditions. However, D80O20 exhibited a different trend, with the CO emissions rising significantly at higher BMEPs, exceeding those of D100. This could be indicative of the combustion characteristics of 1-octanol, which may lead to a poorer combustion efficiency or slower oxidation of CO to CO₂ at higher engine loads.

Smoke opacity is an indicator of particulate emissions and is a concern because of its impact on air quality and human health. Figure 9 shows the smoke opacity measurements from the test engine across a range of BMEP values. The data show that for D100, smoke opacity increased significantly with BMEP. This is a typical characteristic because higher loads often lead to richer combustion and increased particulate formation. In contrast, D80B20 and D80P20 demonstrate a substantial reduction in smoke opacity across the entire BMEP range, suggesting that these alcohols contribute to cleaner combustion with fewer particulate emissions. At high BMEP values, D80B20 maintained the lowest smoke opacity, indicating its potential to reduce visible smoke emissions. On the contrary, D80O20 exhibited an increased smoke opacity at higher BMEP values, although it performed better than the D100 blend. This could be attributed to the less favorable combustion properties of 1-octanol, which may lead to incomplete combustion and higher particulate formation under certain conditions. Considering previous analyses of BTE, AFR, EGT, NO_x, and CO, the trends for smoke opacity aligned with the expected outcomes. The lower smoke opacity of D80B20 and D80P20 correlated with their generally higher BTE and AFR values, indicating more efficient combustion. However, the elevated EGT and NO_x emissions for D80O20 corresponded with the increased smoke opacity, supporting the notion that the combustion process for this blend may not be as complete or clean, particularly at higher loads.

Figure 9: 
Variations of smoke opacity against BMEP