1. Introduction
Diesel engines are fundamental to global transportation, powering industries from agriculture to logistics and substantially contributing to economic growth. However, they pose considerable environmental challenges, emitting particulate matter (PM), nitrogen oxide (NO
x), and carbon dioxide (CO
2), significantly contributing to local air pollution and global climate change. These emissions are linked to severe health implications, including respiratory and cardiovascular diseases [
1,
2]. The reliance on fossil fuels exacerbates these issues, as oil and gas extraction and refining processes cause substantial ecological damage, including air and water pollution and soil contamination [
3].
In response to these challenges, the European Union has been enforcing increasingly stringent emission standards to reduce the harmful gases emitted from vehicles. The evolution of Euro standards represents a clear trend toward stricter emission limits and broader pollutant coverage, as shown in
Table 1 for heavy-duty diesel engines. The gradual strengthening of these standards demonstrates the EU’s commitment to reducing vehicle emissions and mitigating their environmental impact. Future regulations are expected to lower allowable emission levels and may include additional pollutants or parameters to address emerging environmental and health concerns.
This regulatory pressure has accelerated the development of cleaner and more efficient diesel engine technologies [
6,
7] and also the exploration of alternative fuels to reduce reliance on fossil fuels and mitigate environmental impacts. Advances in hybrid technologies, including high compression ratios and two-stage turbochargers with intercoolers, have significantly enhanced fuel efficiency [
8]. Additionally, after-treatment systems such as diesel particulate filters (DPFs), diesel oxidation catalysts, and selective catalytic reduction (SCR) have been rigorously studied to decrease atmospheric pollutants [
9,
10,
11]. Innovations in thermal management techniques, such as insulation methods to reduce heat loss, electrically heated catalysts to improve component light-off times, and phase-change materials to stabilize temperature fluctuations, have been crucial in enhancing the efficiency of these technologies [
12].
Recent advances in exhaust gas after-treatment technology have demonstrated that injecting water, alcohol, or hydrogen into the exhaust pipe can effectively burn residual exhaust gases and reduce harmful emissions. For instance, injecting water into the exhaust pipe can reduce NO
x emissions by lowering combustion temperatures and promoting the conversion of NO
x to nitrogen and water vapor. Additionally, water injection helps reduce PM emissions by enhancing the breakdown of soot particles [
13].
Moreover, ethanol–water mixtures have proven effective in reducing NO
x emissions, even at low temperatures when conventional SCR systems are less efficient [
14]. Similarly, butanol significantly boosts NO
x reduction when used in HC-SCR, acting as a reductant and promoter. Its high reactivity, polarity, and diffusivity improve catalytic performance, especially at low and medium temperatures [
15]. In addition, hydrogen, when used with an Ag/Al
2O
3 catalyst, markedly enhances NO
x removal by promoting the partial oxidation of hydrocarbons to form surface intermediates that facilitate SCR. However, the effectiveness of hydrogen varies with the type of catalyst used, showing no positive effect with Pt-based catalysts [
16].
Ongoing research into alternative fuel sources for diesel engines continues to be driven by the need to meet stringent environmental standards. One notable effort involves using biodiesel from natural oils and fats, which provides a renewable and less polluting alternative to conventional diesel [
17,
18]. In addition, synthetic fuels such as dimethyl ether are gaining attention for their potential utility in diesel engines due to their high cetane numbers and oxygen content, which facilitate cleaner combustion processes and considerably lower emissions of soot and NO
x [
19]. Moreover, these synthetic fuels are often derived from renewable energy sources like wind power, solar energy, and hydropower, highlighting their role in promoting energy independence and providing sustainable alternatives to fossil fuels [
20]. Incorporating hydrogen as a fuel supplement in diesel engines also shows the potential to significantly reduce emissions when it is adequately blended with diesel [
21,
22]. These innovative fuel options emphasize the varied approaches being explored to achieve cleaner combustion and a reduced environmental footprint in diesel engine operations.
Similarly, research into using alcohol fuels and hydrogen in gasoline engines reveals significant potential for reducing emissions and improving efficiency. When added to gasoline, ethanol reduces carbon monoxide (CO) and hydrocarbon (HC) emissions due to its high oxygen content, which enhances combustion. Ethanol/gasoline blends promote complete and efficient combustion, particularly under specific operational conditions, effectively reducing exhaust emissions in spark-ignition engines, especially during the cold-start phase and across various engine loads and speeds [
23].
Lower-alcohol fuels, such as methanol and ethanol, are promising alternative fuels for internal combustion engines due to their high-octane ratings, which enhance engine performance and efficiency [
24]. Ethanol can be blended with gasoline in various proportions without significant engine modifications, offering flexibility. However, the challenges associated with their use include lower energy densities compared to gasoline, their hygroscopic nature leading to water absorption and potential phase separation, and the risk of corrosion in fuel systems [
25,
26]. Moreover, methanol’s toxicity requires careful handling [
27,
28]. Despite these obstacles, the potential environmental benefits drive ongoing research and development, emphasizing the role of these technologies in reducing greenhouse gas (GHG) emissions and strengthening energy autonomy [
29].
Research also indicates that incorporating even small amounts of hydrogen into gasoline engines can enhance combustion efficiency, increase engine torque, and lower emissions [
30]. Hydrogen’s high diffusivity and flame speed improve combustion, leading to more efficient fuel usage and lower emissions. Furthermore, combining exhaust gas recirculation (EGR) with hydrogen supplementation has been shown to enhance performance by increasing EGR rates, effectively reducing NO
x emissions without compromising engine efficiency [
31]. These initiatives highlight the varied strategies for achieving cleaner combustion and reducing the environmental impact of engine operations, underscoring the need for solutions tailored to specific engine types.
As part of the broader quest for sustainability, the historical development of alcohol fuels like methanol and ethanol has been a crucial aspect of the journey toward sustainable energy solutions. These fuels were first explored seriously during the oil crises of the 1970s, when the need to reduce dependence on finite petroleum reserves became apparent. Methanol and ethanol, derived from biomass and fermentable crops, emerged as viable alternatives due to their renewable nature and potential for cleaner combustion [
32,
33]. The late 20th century saw accelerated research into these biofuels, spurred by advancements in production technologies that enhanced both their economic viability and environmental benefits [
34,
35].
In this context, it is also essential to acknowledge the historical use of alcohol fuels in multi-fuel military vehicles. Previous studies have shown that using methanol fuel in a rotary stratified charge engine allows for smooth operation, despite prolonged fuel injection periods due to nozzle sizing requirements for conventional fuel [
36]. Additionally, the use of ethanol fuel in two-stroke outboard engines has demonstrated high torque characteristics at high speeds and faster combustion than other fuels [
37]. These examples show the versatility of alcohol fuels in military applications, improving operational flexibility and reducing reliance on traditional fossil fuels.
Building upon foundational research into methanol and ethanol, researchers now focus on higher alcohols like n-butanol, n-pentanol, and n-octanol as more effective alternatives. Recent studies highlight n-butanol’s potential in diesel blends, which enhance fuel efficiency and significantly reduce emissions such as soot, although they present challenges like increased NO
x emissions [
38]. These blends also lower polycyclic aromatic hydrocarbon emissions, improving environmental compatibility [
39]. Adding alcohols such as butanol and pentanol in high concentrations can lower the cold filter plugging point of the fuel, thereby enhancing performance in cold environments and reducing fuel gelation and filter clogging [
40]. Higher alcohols offer greater energy content and cetane numbers, enhancing compatibility with diesel engines and improving performance. Moreover, they address issues like phase separation and volatility, thanks to their increased molecular weights and reduced volatility [
41,
42,
43].
Moreover, n-octanol blends notably decrease PM and NO
x emissions, enhancing brake thermal efficiency (BTE) and reducing fuel consumption [
44,
45]. The oxygen content of these alcohols promotes more complete combustion, which is crucial for minimizing smoke and particulate emissions. Additionally, their seamless blending with diesel offers a practical transition strategy toward more sustainable fuel systems, requiring minimal modifications to existing engine designs or fuel distribution infrastructures [
46,
47].
However, integrating higher alcohols into diesel fuel presents several challenges despite their benefits. Previous studies underscore the need for further research to optimize combustion processes and address concerns related to the production costs and availability of raw materials [
48,
49]. While higher alcohols can mitigate emissions like PM and NO
x, their use may increase other emissions, notably of aldehydes such as formaldehyde and acetaldehyde. This increase largely depends on the type of higher alcohol employed and the specific combustion conditions, including temperature and engine operational parameters [
50,
51].
The use of various alcohols in diesel engines also presents issues due to their lower cetane numbers compared to conventional diesel fuel. This can lead to increased NO
x emissions while reducing PM emissions when methanol or ethanol is added to the fuel. Addressing these issues requires the implementation of variable engine systems, such as adjustable compression ratios, precise fuel injection technology, and variable valve timing (VVT). Changing the compression ratio can significantly impact engine performance and exhaust emissions, especially when using fuels with different cetane numbers. Additionally, applying VVT and variable valve actuation systems optimizes the air–fuel mixture, improving combustion efficiency and effectively balancing PM emission reduction and NO
x emission control [
52]. Furthermore, the tribological optimization of engine parts can reduce mechanical losses, thereby lowering fuel consumption and decreasing exhaust gas emissions. This approach enhances the durability and efficiency of engine components, contributing to overall emission reduction [
53].
Similarly, the addition of n-octanol to diesel fuel affects the combustion characteristics, particularly the peak in-cylinder pressure, which can lead to increased engine noise and vibration. Studies show that n-octanol/diesel blends result in higher peak pressures and heat release rates compared to pure diesel, due to longer ignition delays and enhanced premixed combustion, which is facilitated by n-octanol’s lower cetane number and higher oxygen content [
54,
55]. For instance, a 30% n-octanol blend in diesel produced higher in-cylinder pressure peaks and heat release rates, especially at advanced injection timings. This suggests that n-octanol can increase combustion intensity and, potentially, noise and vibration. However, the increased oxygen content and improved combustion efficiency of n-octanol blends can contribute to more complete combustion and smoother engine operation, potentially offsetting some of the increased noise and vibration.
To address these complexities comprehensively, a thorough evaluation of alternative fuels through various experimental cases is required, due to the diversity of diesel engine designs and operating conditions. By systematically studying the effects of n-octanol addition across various engine loads and speeds, this research provides valuable insights into the practical implications of using higher alcohols in real-world scenarios. Furthermore, this study contributes to the body of knowledge by offering a comparative analysis of n-octanol’s effects on diesel engines, filling the gaps left by previous research. It highlights the potential of n-octanol as a sustainable alternative fuel that is capable of significantly reducing harmful emissions while maintaining or enhancing engine performance. It aligns with global efforts to reduce GHG emissions and improve air quality. The findings of this research could inform future fuel formulation and engine design, aiding in the transition toward more environmentally friendly and efficient transportation solutions. The remainder of this paper is structured as follows.
Section 2 describes the experimental setup and methodology used in this study.
Section 3 presents the results and discusses the effects of n-octanol/diesel blends on engine performance and emission characteristics.
Section 4 concludes the paper by summarizing the key findings and suggesting directions for future research.
4. Conclusions
This study has thoroughly evaluated the effects of n-octanol/diesel fuel blends on a compression ignition engine’s performance and emission characteristics. The findings reveal significant emission reductions and efficiency improvements, underscoring the potential of n-octanol as a sustainable alternative fuel despite some challenges in emission behaviors, particularly regarding NOx emissions.
Across various engine loads and speeds, the experiments consistently exhibited reductions in CO, HC, and smoke with the addition of n-octanol, along with enhancements in BTE. It was found that increases in the n-octanol ratio in the fuel blend led to higher NOx emissions relative to lower n-octanol concentrations; however, these levels remained substantially below those emitted by pure diesel. The properties of n-octanol, such as its oxygen content and the cooling effects from its higher latent heat of vaporization, contribute positively, albeit complicatedly, to emission control.
The results suggest that a 30% blend of n-octanol (D70O30) offers a balanced improvement in both performance and emissions. This blend maintains competitive brake torque and power while significantly enhancing BTE and reducing harmful emissions, presenting an optimal compromise between performance benefits and environmental impact.
The significance of these results is underscored by global carbon neutrality goals. N-octanol, when potentially derived from renewable sources, reduces critical pollutants and boosts engine efficiency, aligning closely with efforts to decrease fossil fuel reliance and lessen transport’s environmental impact. Positioned as a promising candidate for more sustainable and environmentally friendly internal combustion engine technologies, n-octanol shows significant potential. Future research should focus on expanding its scope to include multi-cylinder engines and a wider range of engine speeds and loads. This broader investigation will provide a more comprehensive understanding of n-octanol’s effects under varied operating conditions. Additionally, refining higher-octanol blends to balance performance benefits and minimize potential increases in NOx emissions remains crucial. Innovations in fuel formulation and engine design are expected to address these challenges, enhancing the viability of high-octanol blends for widespread use. By incorporating these elements, future studies will further solidify n-octanol’s role in advancing sustainable transportation solutions.