Long-term, alternative fuels from biogenic raw materials will gain major significance on the diesel market. While it is mainly biodiesel that plays any part today, promising alternatives in the form of second and third-generation synthetic fuels are waiting in the wings. This will have consequences for the development of diesel engines.
The market for diesel fuels is astir: In the long term, fossil energy sources will be ruled out – either on account of their limited availability or because of the global warming they produce. Although batteries or fuel cells could provide alternatives one day, they will not be able to replace liquid fuels on any major scale over the next few decades because they have still not been perfected. "This is why reducing CO2 by 2020 through new propulsion energy will mainly be achieved on the basis of liquid or gaseous fuels from biomass", says Heike Puschmann, development engineer for diesel combustion processes at IAV.
Three Generations of Alternative Fuels
Experts divide the alternative fuels into three generations: Typical examples of the first generation are vegetable oil, biodiesel (e.g. rape methyl ester, RME) and hydrotreated vegetable oil (HVO). These are produced using only part of the plant, giving them a relatively low energy yield. They also compete with food crops. "Second-generation fuels use the whole plant", Puschmann explains. "Here, any biomass can provide the base material – with straw, for instance, being used for producing BtL (biomass to liquid) diesel fuel." With the alternative diesel fuels of the second generation, however, natural gas (gas-to-liquid, GtL) or coal (coal-to-liquid, CtL) can also serve as the raw material ("multi source fuel") although these do not help to cut greenhouse-gas emissions because they come from fossil energy sources. Within certain limits, the synthesis processes allow the resultant fuel molecules to be varied in structure and size.
A third generation of alternative fuels also exists: As their base material, they use algae that convert CO2 and solar energy directly into the fuel precursors by photosynthesis. "Here, there is no need for farmland, fertilizers, pesticides and heavy machinery, and they do not compete with food production", Puschmann reports. "Although these approaches are still only at the beginning of their development, they are highly promising."
Admixture of Biodiesel Varies from Country to Country
As yet, BtL does not make any appreciable contribution on account of the high investments involved, the long lead times in constructing the necessary plants and higher price. To obtain a premium fuel, no more than five percent GtL is currently added to conventional diesel. At present, biodiesel is the only alternative diesel fuel of any major significance on the world market – depending on country, one to a maximum of seven percent of it is added to commercial-grade diesel fuel.
This level has no repercussions for diesel engines: "Within these limits, vehicles require no adjustments, and the resultant fuel blend also meets the DIN EN 590 standard for diesel fuel", says Puschmann. "In a number of countries, though, special fuels are available with a share of up to 30 percent biodiesel – and in some cases they even come in the form of pure biodiesel." But there are only a few engines that can cope with this because it leads to problems in calibrating combustion and controlling exhaust-gas aftertreatment.
If the alternative fuels continue to increase their market share, the manufacturers will have to modify their diesel engines. "Adding higher quantities of first-generation biogenic fuels is a major challenge for manufacturers because the chemical composition of these fuels differs considerably from that of conventional diesel", says Puschmann, describing the problem. "This influences combustion behavior, demands the use of new materials in the fuel system and affects the life of components in the exhaust-gas aftertreatment system."
Properties Can Be Tailored
In principle, using second-generation alternative diesel fuels does not cause any problems of this type. They leave the synthesis process in extremely pure form and a certain amount of freedom is provided in defining their composition – posing the question as to the physical and chemical properties a future "ideal" diesel fuel should have. "This can only be answered after further research and development work", Puschmann says. "This is where we must not only find the optimum fuel but also adapt the engine concepts to it."