Abstract
This paper reviews the technical and economic feasibility of Dimethyl Ether (DME) as a future fuel for compression ignition engines. Compression-ignition engines are pivotal in the transportation sector as result of their high thermal efficiency in contrast to spark ignition engines. This high efficiency resulting in low levels out GHG emissions, ultimately favourable for all parties. DME can produced cleanly via the conversion of several feedstocks such as renewably sourced biomass. Although the bulk of research has focused on the practicality of DME for use in medium or heavy-duty engines, this paper provides an overview of the feasibility of DME as a candidate fuel for environmentally-friendly compression-ignition engine regardless of size or application. DME’s shares many properties with the traditional diesel engine, such as its low ignition temperature and high cetane number. Another advantage is the particulate-matter free combustion achieved in engines due to the simple chemical structure and high oxygen content. However, in spite of these advantages, the adoption of DME hasn’t excelled due to different physical properties compared to diesel fuels, such as low values for viscosity and lubrication. The low lubrication value becomes an issue during fuel injection as slight modification of the standard system is required to prevent corrosion. Fuel injection is performed through both conventional mechanical and low-pressure common rail systems. DME is easily applied to compression-ignition engines due to its spray characteristics, despite property differences such as easier evaporation and lower density. Ultimately, the negligible levels of particulate matter produced by DME provides sufficient justification for its consideration as an alternate fuel for use in compression-ignition engines. Research has indicated that this reduction in PM production is coupled with on-par vehicle performance, hence strengthening the case for DME. Even with the advent of diesel particulate filter technology in OECD markets, problems relating to operation, maintenance and cost issues have been regularly cited as inhibiting uniform long-term PM removal.

Introduction
“Every industrial revolution brings along a learning revolution”. This famous quote from Alexander De Croo is echoing in the background as mankind face an enormous task to save planet earth from inhabitability. Since the illustrious industrial revolution, humans have emitted 1,540 billion tonnes of carbon dioxide gas. Some may refer to the fact our planet has coping mechanisms for combating excess CO2 concentrations, however we are currently emitting this gas at least a hundred times faster than it’s being eliminated.
The Paris agreement in 2015 was a promising start, as this transition to a more sustainable future is impossible without financial support from governments. A considerable proportion of these increased emissions can be attributed to the increased number of motorised vehicles on the road, where it is estimated over a billion passage cars travel the roads of the world today. These vehicles are traditionally powered by petrol and diesel, both derived from fossil fuels. Recent trends indicate a desire for electrifying vehicles, however questions linger over the background manufacturing emissions for these vehicles and whether the change is capable of achieving the desired effect. Electric vehicles are often unpopular due to the required stoppages for those who frequently travel longer distances, hence a search for an alternative liquid-based fuel could be a superior solution of reducing emissions.
Dimethyl Ether has long been proposed as a synthetically produced alternative to diesel for use in specially designed compression ignition diesel engines. DME is a clean, colourless gas that is easy to liquefy and transport. It also has the potential to replace coal in the role of electricity production, which could also reduce emissions significantly as Ireland’s coal burning-factory in Moneypoint reportedly provided 21.5% of Ireland’s electricity demand in April 2016. DME is largely produced from synthesis gas, produced from natural gas, coal yet more importantly biomass. Biomass has the potential to become the world’s largest and most sustainable energy source and will be very much in demand. The geographical flexibility of production for DME is another major benefit as production plants don’t need to be established near sources of crude oil for example, but any place where bio-based feedstocks or natural gas can be found. DME’s lack of carbon-to-carbon bonds can virtually eliminate particulate emissions and potentially negate the need for costly diesel particulate filters.
Motivations for seeking alternative fuels to dirty fossil fuels varies from country to country. Several are fearful of the scarcity of available reserves, however with further exploration this shouldn’t be at the top of the list. Others, such as Ireland, have cause to invest in Wind Energy to enable them to be self-sufficient, and not relying on imports that although steady now, may not be in years to come. The final motive, and in my opinion, the one Governments genuinely have the least concern for is the increasing risk of global warming that is no longer just a mere looming presence in the background.

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Literature Review
1 Dimethyl Ether is an organic compound with the chemical formula CH3OCH3 (C2H6O). It has long been associated with applications such as propellants in aerosol cans and cooking fuels, due to its ability to be absorbed into the troposphere, the lowest level of the earth’s atmosphere. DME is primarily produced by converting hydrocarbons, predominantly sourced from natural gas, to synthesis gas. This gas is then converted into methanol in the presence of a catalyst, with subsequent methanol dehydration resulting in the production of DME. Estimates indicate that in the region of 5-9 million tons are produced every year via this method. Economists will be attracted to the feasibility of the onshore DME production which can be contributed to the high levels of animal waste, as opposed to the more expensive alternatives of offshore developments.
For consistency, a light-duty vehicle for this report is any vehicle with a mass of 4,500kg or less. In theory DME can be used to power vehicle of any size, however it is more suited for use in heavy duty transport vehicles. DME is often proposed as an alternative for diesel, not petrol. This is often attributed to the fact that only moderate modifications are required to convert a diesel engine to burn DME. Diesel is traditionally associated with heavy-loads, as it provides a far higher torque than a comparable petrol counterpart. This higher torque arises from the desire for a higher compressing ratio needed for compression ignition, in the absence of a spark plug. The issue with DME comes in the form that its energy density is lower than that of diesel, which results in larger fuel tanks being required to achieve the same range of travel. It will be interesting to see the adaption of industrial trucks to facilitate such a large volume change, when their sole purpose is the transportation of goods where space is at a premium. This is coupled with DME being a poor lubricant for the moving parts of the injection system. The addition of miniscule doses of lubricant additive has the potential to give rise to particulate matter emissions that might re-introduce the need for exhaust PM filters in order to meet the most recent stringent limits.
The stoichiometric AFR (Air-Fuel Ratio) of Dimethyl Ether fuel is roughly 9, compared to 14.6 for diesel. This means that complete combustion of one kilogram of DME requires less air compared to one kilogram of diesel fuel. It should be noted however that one kilogram of diesel produces more energy than a single kilogram of DME. The cetane number of DME, which indicates the ignition properties of diesel fuel relative to cetane as a standard, is between 55 and 60 which makes it very suitable for a diesel cycle engine. This will aid in reducing engine noise when contrasted to a conventional diesel engine, particularly at start-up.
Dimethyl Ether, naturally, isn’t the first proposed alternative fuel for compression ignition engines. Various studies have also been carried out into the suitability of hydrogen, biodiesel and alcohol-diesel mixtures. The use of cleanly generated hydrogen in combination with sequestered CO2, to synthesize methanol and then DME, as a means of energy storage and as a pathway to renewable fuels for transport for spark ignition and diesel engines appears technically feasible but does not currently make economic sense, with today’s fossil fuel prices and carbon tax levels. Instead, employing methanol dehydration is more favourable.
(Park and Lee, 2014) investigated the applicability of the transition to DME. They discovered although DME has lower viscosity, lubrication and a lower LHV (Lower Heating Value) compared to conventional diesel, though these characteristics have been improved through research and can be optimising through blending with other petroleum fuels. This amalgamation has been developed to address the disadvantages of DME such as reducing surface wear, along with improving combustion and emission characteristics. They acknowledge the on-going field-testing in Asia and Scandinavia, which so far have been successful in furthering the reputation of the fuel as feasible. If infrastructure is implemented to enable to supply and demand of DME over the coming years, compatible vehicles have the potential to become the most environmentally friendly alternative energy cars available.
Multinational companies such as the Mitsubishi Corporation and The Volvo Group have focused their efforts on the promotion of DME and have heavily invested into production plants. The below image from Toyo Engineering gives a step-by-step guide to their two-reaction production process of DME. Methanol is initially produced from synthesis gas from both reactions, and then following the removal of product water DME is produced by methanol dehydration reaction. This indirect route enables the reaction process to be optimised to a greater extent as separate operation conditions can be employed for each step. All reaction are exothermic, meaning that energy is released during the process.

This relatively straightforward production process has lead Brisbane based company “DME Fuels” planning on constructing Australia’s first commercial-scale production plant by 2020. They plan on not only using DME to operate as a diesel-replacement fuel but to also be blended with liquefied petroleum gas (LGM) for use in passenger vehicles at significantly reduced emissions. The company is being financially supported by bodies such as the Australian Renewable Energy Agency, something that highlights without lack of Government Incentives these expensive projects often mail to materialise further than the initial design stage. “DME Fuels” estimate the cost of their renewable products to be upwards of three times more expensive per gigajoule than current fuels but are confident this can be offset that cost by selling renewably produced ethanol in bulk, in conjunction with the downwards trends in the overall cost of renewable power production and government tax reliefs.
Although determined that the implementation of DME is technically feasible, financial support is required for any project to develop. The Ministry of Industry, Energy and Tourism in Iceland 6 carried out an economic feasibility study report into a DME project in Iceland sponsored by the previously referred to Mitsubishi Corporation. In the report, they factor in that DME fuel is considered carbon neutral, as it is produced from the recovered CO2 gas and water. This means that not only will extra emissions stop being released into the atmosphere, but the atmospheric concentration levels of CO2 will in fact be reduced. This Icelandic report claims that the DME production cost after deduction of carbon is nearly equivalent to tax added diesel retail price for land transport in Iceland, assuming negligible plant infrastructure costs. Iceland have placed emphasis on DME as a fuel for their considerable marine operations, however the cost of marine diesel oil is significantly lower than DME as there are less taxes on such oil compared to passenger diesel. For this reason, extra support mechanisms would be required from the government to provide incentives for purchase to not only contribute to enhancing renewable energy, but also reducing the emission of existing CO2 in the country.
The report concludes by stating that the project is considered to be feasible subject to a strong and dedicated support rendered by the Icelandic government. They refer to countries where government support mechanisms have led to successful renewable energy projects in the past, such as the feed in tariff for solar energy in Germany and a bioethanol subsidy in the United States. It is also insisted that further detailed studies and research are required in the field before a conclusive decision could be reached. These studies should include the cost investigation into a DME transportation network, fuelling stations, the modification of diesel engines, a finance plan, and the realistic reduction of applicable taxes with the support of the Icelandic Government.

Discussion
It is the opinion of myself and many others, that the numerous investigations of DME as an alternate fuel source have highlighted its huge potential for use in compression-ignition engines in the automotive sector. Although questionable the feasibility of implementing DME in light-duty vehicle applications as result of the expensive additives and other factors, the prospect of it replacing Diesel in the near-future for industrial purposes should definitely not be neglected.
The Biofuel Obligation Scheme in Ireland requires all suppliers to ensure that at least 8.695% of the motor fuel they supply to the market is reduced from renewably sources. DME offers huge promise to fulfil this obligation, as not only can it be sourced form biomass, it can also be mixed with local diesel. Power outputs of the blended fuel are found to be similar to the unblended, but smoke emissions are 30% lower. 22 Similar schemes appear imminent worldwide, so a DME/Diesel mixture could undoubtably satisfy these needs.
The Irish vehicle market is a comprehensive indication on how government incentives can drive a major transition in the country. Tax reliefs were offered on diesel vehicles in an attempt to reduce annual CO2 emissions, and from 2007 the rapid decline is clearly evident. This demonstrates fairly conclusively that public opinion can be easily swayed on a matter of financial incentives available for them.
Another significant advantage of DME is the minimal modifications required to a diesel compression-ignition engine for effective operation. The addition of a pressurized fuel delivery system is the only notable difference, meaning that R;D costs could be kept to a minimum in this department.
My view is that collaboration is required between corporations for significant progress to me made in the implementation. Although reasonable to continuing researching potential alternate sources such as Compressed Natural Gas (CNG), Liquefied Petroleum Gas (LPG) and Liquified Hydrogen fuels, serious resources should be dedicated towards DME.

Conclusions
The clear favourable results obtained by the Mitsubishi Group and other establishments highlight their reasoning for the switch clearly. However, no transition to cleaner fuels will ultimately be made without large economic support from respective governments. Much is made of Government support mechanisms towards electric vehicles in developed countries, however with high-levels of emissions in the manufacturing stages of their batteries, they aren’t perfect. Dimethyl Ether has the potential to serve as an ideal gateway until further development is made into electric vehicle performance.
It has been shown that the Well-to-Wheel energy efficiency and CO2 emissions for DME produced from fossil fuels such as gas, are at similar levels to existing vehicles. This is partly due to the fact that diesel has the highest Well-to-Wheel energy efficiency of current fuels. DME produced renewably, through biomass for example, would be a clear winner in terms of its emissions. Future optimisation of fuel injection equipment, and lessening the effects of problems such as the low lubricity, density and corrosiveness levels, will allow for large-scale production of DME-fuelled compression-ignition engines when sufficient levels become available in the market.
Overall, DME has been found to be a very promising alternative fuel for compression-ignition engines, capable of providing high thermal efficiency, low combustion noise, low NOx levels and soot-free combustion; it thus merits further research and development before a final decision is taken on its potential as a mass production fuel for the automotive market.

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