Micro-explosion of Fuel Blends in Low Carbon Diesel Engines: Experimental and Modelling Study

The transport sector accounts for a significant part of carbon emissions worldwide and in the UK about 20% of CO2 emissions are attributed to road transportation. Consequently, the need to mitigate the greenhouse effect of CO2 and reduce vehicle exhaust emissions has provided the driving force for developing cleaner more efficient vehicle powertrains and environmentally friendly fuels. Reducing consumption of petroleum-derived fuels has become one of the top priorities in the 21st century. As substitutes of the internal combustion engine have yet to overcome technical challenges to attain significant utilisation in the transport sector, the compression ignition diesel engine remains a very attractive powertrain option due to its high thermal efficiency. The International Energy Agency estimates that biofuels can grow to as much as 30% of the world's road transport fuel mix by 2050. Such fuels will include biodiesel and synthetic diesel fuels. In the same frame, alcohols such as bio-ethanol produced from non-food sources with reduced production costs and low CO2 emissions have been proposed as alternative fuels for direct blending with diesel, biodiesel or synthetic diesel. According to Shell, our industrial partner, ethanol made from Brazilian sugar cane produces around 70% lower CO2 emissions from production to use compared to gasoline. Therefore, the potential of ethanol-diesel blends (e-diesel) as alternative fuel for low carbon advanced diesel engines of today and tomorrow has become very important. The use of bio-derived fuel blends such as e-diesel also offers the benefit of compatibility with existing infrastructure.

On the other hand, as the complexity of the properties of fuel blends increases, new phenomena that affect engine performance occur in the engine combustion chamber. Thus, improved scientific understanding is essential to overcome potential issues and/or gain potential benefits. One key phenomenon is the micro-explosion of multi-component fuels that is exceedingly possible to occur during spray atomisation and combustion in the case of fuel blends with difference of physical properties among the different fuels in the mixture. The micro-explosion of a miscible multi-component fuel droplet is due to the difference of volatility and boiling point among the different components. For an immiscible multi-component fuel droplet (emulsion droplet as routinely termed), the likelihood of micro-explosion will considerably increase if the lower-boiling-point component cannot dissolve in the mixture and disperse as micro-droplets inside the fuel droplet, such as in the case of e-diesel as the volume fraction of bioethanol increases.

Micro-explosion in diesel engines has potentially significant implications on engine performance, combustion and emissions. The phenomenon offers a unique potential in optimising charge preparation in spray combustion systems, making the demands and design of fuel atomisation devices potentially more flexible.

However, despite the potentially significant impact, fundamental understanding of the micro-explosion of fuel blends in diesel engines is still lacking. In the present study, we propose for the first time to carry out frontier research using both experimental and modelling techniques to investigate systematically the micro-explosion phenomenon and its effects on spray atomisation and combustion and emissions under realistic diesel engine conditions. The proposed research exploits the creativity that is highly likely to occur at the active interfaces and with the close collaboration between the experimental and modelling researchers with support from Shell which is one of the world's biggest distributors of biofuels. The research outcomes will be disseminated at top international conferences and journals. Development of this science base is vital for the UK to lead the world in advanced technologies of clean, efficient engines and sustainable low CO2 fuels today and tomorrow.

The proposed experimental and modelling research is directed towards the design and development of fuels and engine technologies that will lead to clean and efficient low carbon internal combustion engines. The major deliverables of the project are: (i) new understanding of the complicated micro-explosion phenomena occurring in engines fuelled with alternative fuels that are being introduced in the fuel market blended with conventional fossil fuels such as alcohol-diesel blends, and (ii) development of modelling and experimental methodologies to study combustion related processes of this type of fuel blends in IC engines.

Understanding of the behaviour of multi-component fuels under micro-explosion conditions will increase the number of choices in bio-derived fuels (e.g. alcohols and synthetic diesel type fuels) for efficient clean engines in order to reduce CO2 and other emissions, thus meeting the relevant emissions legislation and contributing to the Department for Transport Carbon Reduction strategy.

Greater knowledge of micro-explosion phenomena in engines will have significant practical/technical implications in the design of fuels and fuel injection devices and processes. These scientific and technical advances are vital to control and optimise fuel spray atomisation and achieve more efficient and cleaner combustion in future advanced engines. Subsequently the outcomes of the research will lead to societal and environmental benefits. In addition, they will yield economic benefits by means of improved fuel efficiency and they will result in increased energy security by reducing the petroleum-derived fuel demands and hence the dependence on such fuels.

The project partners expect to generate know-how relating to fuel and engine system design and operation and hence the key stakeholders in this proposal will be the fuel and engine manufacturers who will use the results to develop their next generation products to meet carbon reduction, fuel economy and stringent exhaust emissions targets. With public dissemination occurring throughout the project lifetime, fuel producers, engine technology designers, and other potential end-users will be exposed to the outputs with little delay. This will enhance the probability of alternative applications being identified. It will also allow policy makers to make informed decisions based on good science.

Finally the impact of the research on the careers of the researchers employed on the project will be of great importance. The project will ensure the provision of high quality training to the researchers who will gain knowledge and advanced skills that they will be able subsequently to use to make a significant contribution in a research and development environment in either academia or industry.