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01/04/2008
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Economic and environmental pressures are driving a move to non-fossil fuels, says
Andrew Noakes, but there is still no dominant technology in the field
 Fuelling cars from renewable sources might be the latest preoccupation of environmental scientists, politicians and powertrain engineering teams, but the idea is far from new. A century ago Ford's Model T was designed to run on a wide range of gasoline/ethanol blends, so farmers could grow their own fuel. Nikolaus Otto ran prototype spark-ignition engines on ethanol, while Rudolf Diesel's compression ignition engine, originally designed to run on coal dust, was successfully demonstrated on peanut oil, castor oil and even marine animal oils. The 20th century may have been dominated by mineral oils, but it those alternative fuels from decades ago – with maybe one or two newcomers – are likely to be the fuels of the future.
Mineral diesel oil can be replaced, at least in part, by biodiesel made from rapeseed oil (in Europe) or soybean oil (in the US). Biodiesel is made by combining the vegetable oil with an alcohol (commonly methanol) in the presence a base catalyst (often sodium hydroxide or potassium hydroxide). The process, called 'transesterification', produces a hydrocarbon chain of similar length to mineral diesel oil, attached to a –OH group. Glycerine is produced as a by-product and has applications in lubricants and the cosmetics industry. Its value can help offset biodiesel's production costs, although increasing production of biodiesel is already saturating the glycerine market in Europe.
Because biodiesel has built-in oxygen, combustion is improved and emissions of HC, CO and particulates are reduced. The fuel contains virtually no sulphur impurities, so emissions of sulphur oxides are also low. Biodiesel has good lubricity and a high cetane number, but contains about 10% less energy than regular diesel. It can be used at low concentrations in unmodified engines without problems, although the warranty coverage provided by OEMs varies: some approve only the 5% mix (B5) commonly seen in France, while others allow higher concentrations such B20 (popular in the US) or even B100. Straight biodiesel requires slightly retarded injection timing to avoid rough running and is better suited to warmer climates as it is more viscous than mineral diesel fuel, though problems can be avoided if a pre-heating unit is installed.
The arguments against biodiesel are more concerned with feedstocks and manufacturing processes than with its use in vehicles. Already research is developing techniques for second generation feedstocks such as cellulosic biomass – wood, tall grasses and forestry residues. Technology company Choren, in partnership with Volkswagen, Daimler and Shell, is building a BTL (biomass to liquid) plant that will convert these cellulosic materials using a thermochemical process into a synthetic diesel it calls SunDiesel. Research investment is also being ploughed into microalgae, which can be grown on poor soil or in contaminated water and ultimately turned into vegetable oil. Shell is establishing a pilot plant in Hawaii to grow marine algae for biofuels.
Meanwhile oil prices continue to rise, almost doubling in the past two years, and biofuels are becoming more profitable. Land-owners in Indonesia have cleared swathes of established forests to turn over to production of palm oil, releasing huge amounts of CO2 into the atmosphere in the process and, from a 'carbon balance' point of view at least, defeating the object of moving away from mineral diesel oil. New legislation looks likely to ban the import into the EU of feedstocks from crops grown in deforested areas.
Debate also rages over the strategic, economic and environmental benefits of bioethanol, which are largely dependent on the feedstocks that provide the sugars from which it is fermented. In Brazil, which has been at the forefront of ethanol production and use since the 1970s thanks to its government's 'Proálcool' programme, ethanol is made from sugar beet, a relatively simple process with a low energy overhead. In the US, state and federal incentives encourage mid-west farmers to grow corn for bioethanol production, but extracting the sugars from corn is a tougher task which uses large amounts of energy and water – and that limits the end product's environmental pedigree.
Performance and tailpipe emissions are ethanol's key strengths. Because it is an alcohol, it has a hydrocarbon chain linked to a hydroxide (-OH) group. The presence of oxygen in the fuel encourages complete combustion, reducing harmful emissions of carbon monoxide, unburnt hydrocarbons and particulates. Although, weight for weight, ethanol contains only about two-thirds the energy of gasoline, it has a higher octane number which allows an optimised engine to operate at a higher compression ratio without detonation, improving thermal efficiency. It also has a higher latent heat of vaporisation – more than twice that of gasoline – so injecting ethanol into the intake airstream causes a greater cooling effect, a denser intake charge and higher volumetric efficiency.
Blended with gasoline at low concentrations, ethanol can be used in unmodified spark-ignition engines. Performance does not change noticeably, but emissions of benzene, 1,3-butadiene, toluene and xylene all fall. In the US around 30% of gasoline is E10, 10% ethanol and 90% gasoline, but in Europe fuel quality regulations restrict ethanol content to just 5%. A recent move by two of the biggest European OEMs, Volkswagen and Daimler, to extend warranty coverage to vehicles using E10 blends in Europe may be the start of higher levels of ethanol use in the EU.
Higher concentrations of ethanol demand changes to fuel system components, as natural rubber, metals such as aluminium and brass, and man-made materials including nylon and PVC are all at risk from prolonged exposure to ethanol. Higher proportions of ethanol also raise the fuel's vapour pressure, demanding recalibration of engine ignition and fuelling controls to take account of the more volatile fuel.
Higher alcohol use
In Brazil E20/E25 blends are common and engines are tuned for a compromise 22% ethanol blend, but in the US and Europe the focus is on the E85 blends introduced in the 1990s for a new generation of flexible fuel vehicles. There are now more than six million FFVs in use worldwide, two-thirds of them in the US. In Europe the biggest adopters of FFVs are the Swedes, spurred on by a plan to end the country's dependency on foreign oil by 2020, in part by using Sweden's huge areas of woodland to produce raw materials for biofuel production. Saab and Volvo have FFVs in their ranges and there is even one available from supercar manufacturer Koenigsegg.
All these FFVs automatically detect changes in fuel composition and adjust fuelling and ignition timing to suit, allowing them to run on E85, pure gasoline or any combination in between. Naturally aspirated FFVs adapt to the higher octane rating of ethanol by advancing their ignition timing, giving a potential power increase of up to 5%, while Saab's turbocharged flex-fuel engines are able to go a step further. The BioPower engines take full advantage of the fuel's higher knock limit by increasing boost pressure, so raising the dynamic compression ratio and liberating, Saab claims, up to 20% more power.
All of this sounds promising, but the practicalities of bioethanol production still threaten its claim as the environmental fuel of the future. Though reform of the EU's Common Agricultural Policy in the 1990s allowed energy crops to be grown on set-aside land, energy crops may compete with food crops as demand increases. Alternative techniques – including the production of ethanol from biomass waste such as crop stubble – are currently expensive. In addition, a recent study published in Science has suggested that conversion of land to grow energy crops leads to emissions of greenhouse gases so great that they dwarf the savings made by the cleaner fuel.
Lotus Engineering has recently announced research which might ultimately provide the solution to the problem of atmospheric CO2 and at the same time provide an efficient, renewable fuel: synthetic methanol. At the Geneva show in March, Lotus unveiled a 'tri-fuel' technology demonstrator, a modified Exige that can run on any mixture of gasoline, bioethanol and methanol. Bioethanol, says Lotus, is unsustainable in the long term but a necessary stepping stone towards more environmentally friendly fuels. It believes methanol holds the key to a sustainable future.
Methanol was trialled in California in the 1980s, but without making a major impact. It is most commonly used to make MTBE, an octane-boosting oxygenate additive for gasoline, and in the production of biodiesel.
Using methanol to power a gasoline vehicle does not require major changes, although it is even more corrosive than ethanol, so fuel system materials must be chosen with care. It has a higher octane rating than ethanol and a higher latent heat of vaporisation, leading to good thermal and volumetric efficiencies. The potential it has to generate extra power is well known: methanol has been widely used as a race fuel for decades, and the tri-fuel Exige proves the point by recording faster acceleration times and a higher top speed on methanol than on gasoline.
According to Lotus, mass-production of synthetic methanol could be achieved using electrochemical techniques that combine oxygen, hydrogen and carbon. The carbon and oxygen would be taken from the atmosphere either by growing crops or by using large-scale CO2 extraction facilities – already being researched by the CO2 Capture Project, funded by eight major energy companies including BP, Chevron and Shell. Hydrogen is the most abundant element of all, but not easy to source in its pure form. Currently it is reformed from natural gas – not a sustainable option – but in the long term it would be sourced from the electrolysis of water using renewable sources for the electricity required. As Lotus points out, strategies for the economic, sustainable production of hydrogen are already being developed.
In the automotive sector the highest profile hydrogen supporters are Honda and BMW. At the Los Angeles Auto Show in November 2007, Honda unveiled its production-ready FCX Clarity hydrogen fuel-cell car, which combines a new, smaller hydrogen fuel cell with an efficient lithium-ion battery pack and an electric drive motor. Electrical power is generated by the reaction of hydrogen and atmospheric oxygen in the fuel cell stack, and used to propel the vehicle. The battery pack stores energy captured by regenerative braking and supplements the fuel cell where necessary.
At the same show Honda announced plans to begin limited production of FCX Clarity by the summer of 2008, the cars being leased to customers in southern California for up to three years.
BMW, meanwhile, is using hydrogen in a very different way. Beginning in 2006, BMW built 100 of its Hydrogen 7 cars alongside conventional 7-Series models at its Dingolfing assembly plant. The Hydrogen 7 carries liquid hydrogen in a super-insulated tank behind the rear seats, and burns it in a dual-fuel version of BMW's 6.0 litre V12 gasoline engine. The standard gasoline engine generates 445bhp (327kW) and 442lb ft (599Nm); in the dual-fuel version those figures drop to 260bhp (191kW) and 287lb ft (390Nm) thanks to the significant intake volume occupied by the gaseous fuel, and the lack of charge cooling through vaporisation. At full load the engine runs at stoichiometric air/fuel ratios and NOx emissions are handled by a conventional three-way exhaust catalyst, but at part load the very wide range of hydrogen ignition temperatures allow reliable operation on a very lean mixture, keeping combustion temperatures low and almost eliminating NOx. The only other harmful emissions are caused by tiny amounts of lubricating oil being drawn into the cylinder and burned off. Otherwise hydrogen is a clean and hugely abundant fuel.
There are problems, however. The twin troubles with hydrogen in liquid form are the energy consumed in making gaseous hydrogen then cooling and compressing it, and the lack of an effective infrastructure to deliver it. Biofuels can be handled very much like conventional gasoline and diesel fuel, but hydrogen is very different – and requires significant safety precautions due to its high flammability.
All of these alternative fuels show promise, and all of them have been demonstrated to work well under the bonnet. The challenges that remain are largely in the manufacture and distribution of those fuels, rather than their use – and those are questions for the chemists, the environmentalists and the politicians.
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Author
Andrew Noakes
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