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Growing pressures
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01/03/2007
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How viable is it to produce biofuels from plants? Ian Adcock looks at a crop of new technologies...
Predictably, the European Union’s proposals to limit car exhaust emissions across the board to 120g CO2/km by 2012 bought a furious response from the European Automobile Manufacturers Association (ACEA). It made the point that of EU CO2 emissions, 11.5% are created by passenger cars; on a global basis passenger cars account for 5% of which Europe contributes 1.5%.
On the face of it these figures are low, but the call for vehicle manufacturers to do even more came shortly after the Stern report appeared in the UK in 2006 and the influential Intergovernmental Panel on Climate Change published its report. This determined that global warming is with us and that carbon dioxide levels have increased from about 280ppm in pre-industrial times to 379ppm in 2005.
Clearly the solution does not lie entirely within the motor industry. However, politicians worldwide see its products as easy targets when it comes to legislating against emissions.
Europe’s position is further complicated by a predicted imbalance between petrol and diesel production by 2030, driven by a growth in trucks and buses, by which time dependency on oil imports will increase from 82% to 93%.
Add oil security and the rapidly growing demands of emerging automotive nations like India and China, never mind a doubling of oil prices since 2003, and there is clearly a need for manufacturers to develop alternative powertrains.
The question is what? Fuel cells, if they ever happen, are unlikely to become a commercial reality for another 25 years at least while battery-powered cars are no nearer reality than they were a decade or more ago. Hydrogen is a possibility but has issues over storage in the vehicle and the supply chain.
This leaves us with what we have today – petrol and diesel engines. Technological advances mean both are considerably more efficient and cleaner than they were even a decade ago. Advances will continue to make both petrol and diesel engines the most cost-effective means of propulsion, but they still rely on mineral oil.
Surely the time has come to start weaning ourselves off mineral oil and onto an alternative?
The Swedes have already started. Last year nearly 37,000 ethanol-powered cars were registered, serviced by some 600 filling stations, numbers that are growing year on year.
In the USA there are about 1,000 ethanol fuel stations and Ford alone sold nearly 250,000 flexible fuel vehicles, mainly pick-ups, in 2006.
Even President George W Bush leapt on the ethanol bandwagon in his 2006 State of the Union address when he proposed increasing the use of biofuels to 35 billion gallons by 2017, although an increase in the American car parc is likely to negate any potential savings this would make.
Given the Swedish sales figures it’s not surprising the domestic duo of Saab and Volvo are among those manufacturers at the forefront of technology developments in this sector. Ford is up there too – not surprisingly since Henry Ford engineered the Model T to run on ethanol, which he saw as the fuel of the future, not petrol – and most European OEMs have multi-fuel programmes.
At the Geneva Salon last year Saab unveiled what it claims is the first production engine capable of running on pure bioethanol (E100) fuel.
Based on Saab’s two-litre four-cylinder engine it produces 224kW with a peak torque of 400Nm. Boost pressure and compression ratio have been increased while the engine management system has been recalibrated and revised componentry fitted.
The main changes to flexible fuel vehicles include hardened valves and seats, engine management system, widened fuel hose and bigger injectors.
First generation biofuels use themostsugar/starch/oil-rich parts of crop (wheat ears, rape seed etc); second generation uses all of the plant.
In the case of the petrol substitute, the biofuel is usually bioethanol, and is the same end fuel regardless of being first or second generation; the only difference is the efficiency of production of fuel from the crop. Using higher percentages of ethanol does require enhanced vehicle technology.
With diesel, the first and second generation fuels are very different. Again, firstgeneration uses the food content (usually vegetable oils in this case), while second generation uses the rest of the plant, but in this case the end products are different. Biodiesel is a fatty acid methyl ester, somewhat different to mineral diesel, and this cancause problems. Second generation (either biomass to liquid or hydrogenation) produces a conventional hydrocarbon, much the same asnormal diesel, though typically of a higher quality. However, this is amore complex process and requires refinery-type levels of investment, whereas biodiesel can be done on an almost ‘DIY’ basis, though with major quality control concerns.
Biodiesel at higher concentrations brings a raft of potential problems, whereas the second-generation product is a direct diesel subsitute, and technically much preferred. However, second generation is not yet commercially available.
According to Dr-Ing Wolfgang Steiger, head of powertrain research activities at Volkswagen: “The quantitative biomass potential is virtually limitless: the constant plant growth around the globe exceeds the demand for primary energy by several multiples. Second generation biofuels, therefore, could replace a significant portion of conventional fuels. The existing acreage already today allows substituting biomass for 25% of our primary energy needs.”
VW is working to develop this so-called SunFuel with Shell, Choren Industries and Iogen to develop viable industrialisation and distribution of the fuel.
While these new fuels can be used in blends of up to 10% in current VW engines, thereby having an immediate affect on CO2 emissions, Steiger sees the development of this fuel having a direct impact on the fundamental design of petrol and diesel engines.
“High quality synthetic fuels can also stimulate the development of new engine combustion processes, like the Combined Combustion System [Volkswagen’s name for homogeneous charge compression ignition], in which the advantages of petrol and diesel engines are combined. We anticipate several advantages, such as reduced overall emissions levels, lower consumption and an associated substantial reduction in CO2 emissions,” says Steiger.
Petrol engines have developed from a pure homogeneous combustion process based on different port injection systems to a stratified air/fuel mixture with direct injection in the part-load range. Diesels, with higher injection pressures and delayed injection timings, have shifted from stratification to greater homogenisation. During this period turbocharging has now become standard on diesels and is increasingly used on smaller petrol engines as manufacturers downsize, while the differences in compression ratios between diesel and petrol are narrowed as those for diesel are reduced and petrol increased.
Although early/homogeneous autoignition can result in reduced NOx and improved particulate combustion with improved efficiency in both petrol and diesel engines it is difficult to maintain this combustion process as the load increases.
Steiger says the limiting factor in both instances is fuel quality, so the Volkswagen Research Laboratory is taking the unusual step of simultaneously developing a CCS engine based on known diesel engine hardware and the fuel specification.
Results so far from an experimental two-litre CCS engine are encouraging, proving that it can be run mainly in the homogeneous/early mode even during the NEDC’s high load cycle. NOx levels are significantly below production engines even when they momentarily increase under acceleration.
However, given the current state of technology it isn’t possible to achieve homogenisation across the engine’s operating range, even using the latest fuels.
This can be achieved by changing the operating temperature and the air/fuel ratio in one cycle. To achieve this VW is experimenting with a common rail system that can vary the injection(s), combined with an electronic camless valvetrain, even an oil-free cylinder head.
It is clear from the work VW and others are doing that biofuels have a significant role to play in the future. The challenge will not be just for the motor industry to develop affordable technologies but for oil companies to come up with efficient methods of producing sustainable, environmentally neutral crops that can be harvested and converted into biofuels and governments to implement taxation strategies that will encourage consumers to reduce their dependence on mineral oils.
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Author
Ian Adcock
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