Imagine a fuel that can be made out of almost any organic waste material, is non-toxic, burns cleanly and will power a diesel engine without appreciable loss of output. About a decade ago, that fuel was about to be launched into the American truck market, while Europe was expected to follow. The name was dimethyl ether C2H6O), or DME. Back then, it was a fuel of tomorrow. Today, it’s almost forgotten.
DME can be either a fossil fuel, or a non-fossil fuel, as it can be made from coal, natural gas (methane) or biowastes. Volvo was attracted to DME because it could be produced from the so-called ‘black liquor’ waste from timber-mills.
The fuel could be produced on a relatively small scale: almost a home-made fuel. Truck fleets serving sawmills or waste treatment plants could be powered by local fuel effectively made from rubbish. Using biomass waste reduced CO2 output by 90%. Volvo was also keen to convert America’s plentiful reserves of natural gas into the liquid fuel, arguing that using DME in conventional diesel engines, as a diesel substitute, was far more convenient and less costly than introducing new gas-fuelled vehicles. DME’s high cetane number means it can be ignited by compression, and its clean-burning qualities meant that a particulate filter was not required to meet then-current emissions standards.
A BACKWARDS STEP
But it didn’t happen. “That programme did not make it to the market. While the key technological learnings still exist, we do not have DME in our current or future product forecasts,” confirms Volvo Trucks North America spokesperson, Kyle Zimmerman. “There were several factors at play. Most notably, it was non-availability of DME which made it not feasible to continue down that path. The ability to build micro-refineries existed, but production at scale was not something that was going to happen. As a result, cost of the DME was too high and availability too low.”
Transport Engineer has also spoken to an engineer who worked for a supplier of fuel-injection systems to Volvo, who confessed that colleagues working on the project referred to the fuel as “Dear me!” It lacked the lubricity and film strength needed to withstand the pressures generated by modern fuel-injection systems and protect pumps, needles and seats from catastrophic wear, he reported.
A decade later, it seems that, in Europe, the heavy truck industry has steered down a one-way street towards electrification. The conventional wisdom is that the vast majority of heavy vehicles will be battery-electric, with hydrogen fuel cell electric occupying a niche in the premium long-haul market.
A NEW LEASE OF LIFE
But both EV and hydrogen solutions are currently ruinously expensive – and the construction of suitable infrastructure lags way behind government plans for the phase-out of internal combustion engines from the sector. That potentially leaves the door open to look again at DME as a means of giving the diesel a fresh lease of life.
In 2021 research in Switzerland’s Empa Automotive Powertrains Laboratory, led by Patrik Soltic, involved taking an FPT Cursor 11 engine – as found in the IVECO Stralis – and modifying it to run on DME, rather than diesel. Major modifications were made to suit the common-rail fuel system to DME: fuel delivery and return pressures have to be carefully managed to prevent the fuel from vaporising. Flow rates had to almost double over those of the standard diesel system because of the high compressibility and low volumetric energy content of the fuel. The fuel return from the rail and injectors had to be modified accordingly. The poor lubricity of DME meant the high-pressure common-rail fuel pump had to be lubricated externally, although using additives in the fuel could be an alternative. New DME-compatible seal materials – Teflon or PTFE – were also required.
Within the combustion chamber, DME’s behaviour is very similar to diesel’s, even though it is introduced at lower pressures. And its combustion produces very little in the way of particulate material in terms of mass or numbers, as the DME molecule contains an oxygen atom binding the hydrogen and carbon atoms. However, revised injector nozzles to pass more fuel at lower pressures and piston bowl profiles to raise compression were still required, along with a new turbocharger to cope with the high EGR rates.
The researchers experimented with new valve and seat materials but, in practice, the differential wear rate of them versus standard components was negligible. However, an oil with a DME-compatible additive package was required: the research project used a Motorex 0W-20 formulation.
The other major change to the engine itself is an electrically driven exhaust gas recirculation system. Where such systems are used on diesel engines, the amount of gas circulated is ultimately controlled by turbocharger pressures. The DME engine’s electrically driven volumetric pump allows independent control of EGR, ensuring precise and effective control of NOx output in all conditions without excessive particle production. The inherent low-soot nature of the fuel also allows later injection at lower pressures than are required for the clean combustion of diesel, again reducing NOx production.
ENVIRONMENTAL WINS
In terms of greenhouse gases, DME can be produced from either fossil or renewable (biowaste) materials. But, in combustion, its high ratio of hydrogen to carbon means that less CO2 is produced than when diesel is used as a fuel. Even if a fossil-based DME fuel is used, well-to-wheel CO2 emissions of 10-12% over comparable diesel are recorded.
The very low production of particulate matter allows the exhaust aftertreatment system’s layout to be revised. In a conventional diesel system, the first aftertreatment unit the exhaust gas passes through after the catalyst is a particulate filter: temperature of the PM filter must be maximised to allow accumulated carbon to be burned off before the filter clogs. This is not an issue on the DME engine. Instead, the hot exhaust stream can pass directly through the SCR unit, enabling it to perform more effectively even at the slightly lower temperatures generated by DME combustion.
A PM filter is fitted close to the aftertreatment system’s tailpipe. It catches residual ash from the combusion engine lubrication oil.The need to periodically perform a forced regeneration of the PM filter is also eliminated. Indeed, researchers reported that the engine could comfortably meet Euro VI limits for PM mass and numbers without a filter fitted at all!
The researchers suggested that production of renewably sourced DME could be ramped up – and retrofit kits offered to repower existing trucks on the fuel. Carrying the technology forward is dependent upon two factors. One is the availability of DME fuel, ideally from renewable sources, in sufficient quantities and at suitable geographic locations. The other factor is persuading fuel-injection system manufacturers to produce a new generation of pumps and injectors suitable for use with DME. But it is quite possible that as the major players in this field are concentrating their R&D efforts on electric vehicles, this one-time ‘fuel of the future’ may never have its day.