High Octane Number Fuels in Advanced Combustion Modes for Sustainable Transportation

The research community recently proposed a low-temperature combustion (LTC) concept that can simultaneously reduce Nitrogen Oxides (NOx) and soot emissions while maintaining high engine efficiency. Given that diesel fuel is prone to preignition with early injection, gasoline-like fuel with high octane number is utilised to provide sufficient ignition delay and extend the load range. Understanding the influence of high-octane fuel on ignition delay is a key parameter to achieve higher loads in LTC. However, understanding is still lacking on the combustion characteristics of high-octane fuel under LTC in real engines, and the effect of fuel spray–piston interaction is not fully understood. Despite the extended load limits offered by high-octane fuels, they require energy-intensive production during the refinery processes, a condition that raises an issue with well-to-wheel carbon dioxide (CO2). To mitigate the issue of CO2 emission, research proposed methanol as a high-octane renewable fuel.
This thesis focuses on assessing the impact of higher-octane number fuels in LTC under a low load condition. To achieve this objective, the work was divided into two parts. The first part was devoted to evaluating the required ignition delay of high-octane fuels and explaining the effect of fuel spray–piston interactions. In this work, the fuels were evaluated under similar operating conditions in a light-duty multi-cylinder engine. The experimental results revealed a linear correlation between octane number and required ignition delay for lower octane fuels. However, an exponential correlation was observed for higher number octane fuel because of the fuel spray–piston geometry interaction.
The second part aimed to evaluate the effect of injection strategies and air dilution on methanol combustion in a heavy-duty engine. A comparison was performed between methanol and isooctane (primary reference fuel, PRF100) under injection timing sweep. Methanol was then compared at two intake pressures. Later, double and triple injection strategies with different mass proportions and dwells were performed on methanol under the partially premixed combustion. Additionally, numerical simulations were used to interpret the experimental results. The results revealed that the φ-stratification of methanol is less sensitive to the injection timing compared to that of PRF100. Soot emission was always low for methanol and insensitive to injection timing, compare to PRF100. When the intake pressure was increased, the mixture became globally lean, resulting in a lower NOx and unburned hydrocarbon (UHC) but a minor penalty on carbon monoxide (CO) emission. The gross indicated efficiency of methanol was improved at the later injection timing for the boosted case.
Subsequently, when compared with single injection, the double injection strategy with lower pilot mass and shorter pilot-main dwell showed an effective strategy to simultaneously reduce UHC and CO emissions and increase engine efficiency at the expense of a minor rise in NOx emission. Interestingly, the results demonstrated that the triple injection strategy was capable of achieving similar engine efficiency as the single and double injection strategies. Although a minor rise in CO emission occurred, the triple injection strategy demonstrated its potential to significantly decrease NOx and UHC emissions compared to other strategies.

Growing populations have led to an increasing demand on transportation for people and goods. For decades, internal combustion engines have played an important role in the transportation sector to develop the society, but their widespread utilisation has contributed to massive global energy consumption and pollutant emissions. For instance, an estimated 24% of CO2 emissions in the world come from the transportation sector. This scenario creates a dual challenge, which is to satisfy the growing transportation needs by keeping the energy demands and risks of climate change at the minimum level.
Historically, the high torque output and fuel efficiency of diesel engines make them a very attractive power source for the transportation sector. However, these engines come with high NOx and soot emissions, which cause a wide variety of environmental and health impacts. For instance, long-term exposure to NOx and soot can potentially decrease the lung function and increase the risk of damaging the respiratory system. Moreover, NOx contributes to acid deposition and nutrient enrichment problems, which can adversely affect both land and aquatic life. As exhaust emissions are harmful to humans and the environment, the European Commission was motivated to implement the emissions legislation Euro I in 1992 regulating NOx, particulate matter, carbon monoxide and hydrocarbons. Since then, the emissions regulations have been increasingly stringent to reduce progressively the negative impact of diesel engines.
To meet stringent emissions legislations, the research community has proposed new combustion strategies based on low-temperature combustion (LTC), which has the potential to achieve a simultaneous reduction in NOx and soot emissions while reducing energy demands through improved efficiency. Typically, gasoline-like fuels with high ignition resistance are utilised in diesel engines to achieve LTC. However, to also meet future legislation on CO2 emissions, researchers have been working towards renewable fuels, such as methanol. A combination of methanol and LTC strategy can be a future solution to develop clean and sustainable combustion engines.
The goal of this thesis is to explain the relation between fuels with high ignition resistance and the fuel injection process while using LTC. This knowledge was extended by implementing a fully renewable fuel. To reveal the underlying phenomenon, a combination of engine experiments and computer simulations was employed.
The results showed that increasing ignition resistance among the fuels requires increasingly earlier fuel injection. The combustion chamber shape interacts with the spray of the fuel injection, and this study shows how these interactions can be exploited to lower emissions and improve energy use. The use of methanol fuel combined with LTC and triple injection removes soot emissions all together, and a favourable compromise between very low amounts of regulated emissions and low energy consumption can be reached. The results provide engine manufacturers with a preliminary platform for strategies with methanol fuel to meet current and future emissions legislations.