The fuel that is used holds immense potential to reduce CO2 emissions. As crude oil becomes increasingly scarce, research into alternatives and fuel additives gains in importance. By 2012, Audi will reduce the CO2 levels of its models by about 20 percent. As part of the company’s research, Audi is working intensively in this area and is also accelerating this in a partnership with the Shell oil company.

Modern fuels will play a decisive role in future reductions in CO2 emissions. Besides ethanol and natural gas, these primarily include the next-generation fuels that are produced synthetically from biomass or natural gas. These specially tailored fuels improve the combustion processes inside the engine and thus allow a significantly improved energy balance.

Ethanol

Ethanol is a classic among biogenic fuels that offers an alternative for use in gasoline engines. Bioethanol, a fuel that is already on the market, is a readily available alternative to gasoline. The commercially available fuel with the designation E 85 contains 85 percent ethanol and 15 percent gasoline. The total CO2 level in the vehicle as a whole is around 75 percent better than that of conventional fuel produced from crude oil. Today, bioethanol is produced from energy-providing plants such as wheat, corn, and sugar cane by means of alcoholic fermentation. For the future, however, a method is emerging in which the waste from these plants can be used to produce a second-generation form of bioethanol.

The use of regenerative fuel, when considering the entire process, can lead to significant savings. During growth, the plants convert CO2 from the air into biomass. This regenerative energy can be deducted when considering car emissions.

Audi has explored all the alternatives in the fuels sector and has developed an ethanol concept that can be employed immediately in all markets. The use of regenerative biological fuels improves final CO2 balance by up to 75 percent.

CNG (compressed natural gas)

Another interesting approach from an environmental perspective involves the use of prepared biogas, since whole plants as well as agricultural waste can be used to produce this fuel. Biogas therefore performs very well with regard to energy efficiency per area as well with regard to the environmental and CO2 balance. The Audi A5 2.0 T-CNG, Audi’s concept vehicle in the natural gas segment, is uncompromising and nonetheless achieves important technological progress. New generations of lightweight pressure tanks and pressure regulators, plus the outstanding technical basis of the new TFSI engine generation and the package benefits of the new A4 and A5 generations make it possible to produce a concept that meets the exacting demands of Audi customers. With the new A5 2.0 T-CNG, Audi has also demonstrated that the company already possesses technical competence in the field of natural gas and biogas, and that it can set new standards with systematic concepts and innovative technologies.

Synthetic fuels

Bioethanol and gas are accompanied by a host of new types - synthetically produced fuels from gas (Gas to Liquid, or GTL), biomass (Biomass to Liquid, or BTL) or coal (Coal to Liquid, or CTL). These specially tailored fuels improve the combustion process inside the engine and therefore allow a significantly improved emission balance.

Sunfuel (from biomass)

Future second-generation BTL fuels are superior in all of the critical points mentioned. They do not compete with food production, they have a high yield for a given area and they have a very high potential for reducing CO2 emissions - approx. 90 percent. BTL fuels are specially ‘tailored’ during productions. This means that absolutely no modifications need to be made to the cars that they are to power. They are perfectly adapted to the combustion process.

Today, an Audi A4 TDI running on a conventional mineral oil diesel emits 149 g/km (240 grams per mile) of CO2. Running on SunDiesel, it emits a mere 22 g/km CO2 (35 grams per mile), and this figure is likely to be reduced further when the process is running on an industrial scale.

Synfuel (from gas or coal)

So-called SynFuels are synthetic fuels that are already used in small quantities today. These are also known under the terms Gas to Liquid (GTL) and Coal to Liquid (CTL).

The great benefit of GTL fuels is that they are free of sulphur and aromatics. This means that in a combustion engine, emissions are greatly reduced, especially particulates and sulphur compounds, which have a negative effect on exhaust gas after treatment. Depending on the observation method, the potential for reduction is up to 80 percent. In field trials, drastic reductions in carbon monoxide and hydrocarbon emissions were measured with unchanged engine tuning. GTL fuels can be mixed with mineral oil diesel. Their benefits become significant once a proportion of 50 percent has been exceeded. If the engine is optimally configured for GTL fuel, there is a further reduction of nitric oxides by about 50 percent. The direct reduction in CO2 emissions is about 10 percent.

Hydrogen

The most common element in the universe, H, can also be produced as a biogenous fuel on the basis of biomass, similar to BTL. Hydrogen may represent a CO2-neutral route to the future.

Hydrogen can be used as an energy source in the car in two ways: it can be burned in a piston engine in the traditional way, or it can be used in a fuel cell. There are many arguments in favour of using hydrogen in a fuel cell. The efficiency of this system is significantly greater, and the drive system works extremely quietly and produces absolutely no pollutants.

Apart from the problem of technical maturity, therefore, there are good grounds for using hydrogen in a fuel cell. Audi began testing the potential of fuel cell technology as early as 2004 in its A2H2 concept car.

The brand is continuing its work in this field as part of the California Fuel Cell Partnership, in which major carmakers and various other partners are involved. Today, fuel tank technology and filling station infrastructure present questions that have yet to be answered.

Hydrogen, which is extremely light and quickly volatising, also represents a major challenge with regard to logistics, but Audi will nonetheless continue to work toward the development and testing of hydrogen-powered fuel cell cars.