Powerful, clean and economical

To reduce exhaust emissions further still but also to enhance the cost-effectiveness and pure fun of driving, the combustion process of vehicle engines is continually being developed. Another significant challenge is the question of energy sources for future fuels and drive systems.

Synthetic fuels, known as SynFuels, offer the potential to meet all these challenges. Thanks to the special manufacturing process, synthetic fuels are in principle free of sulphur and of aromatics, which can lead to notable emission reductions. As well as this, SynFuels can be ideally matched to new engine demands, which then enables a further lowering of emissions while at the same time increasing efficiency. The fuel quality, which is extremely high and can be very reliably reproduced, is not dependent on the primary energy used (natural gas, coal, or biomass) and this allows a significant step to be taken towards securing a constant source of supply (see Figure 1). Moreover, CO2 emissions are almost completely avoided with the use of biomass for the manufacture of SunDiesel® (a second-generation synthetic biofuel especially designed for diesel engines).

Figure 1: The raw material is broken down into synthesis gas in a first process step, which is then converted into synthetic diesel n the downstream fuel synthesis. The synthesis gas can be produced from a whole range of quite different regenerative or fossil raw materials, such as wood, straw, natural gas, or coal. Volkswagen supports in particular the use of CO2-neutral biomass such as wood, straw, and energy plants for the manufacture of BtL, which we call SunDiesel®, because it is a second-generation biofuel.
BtL: Biomass to Liquid
GtL: Gas to Liquid
CtL: Coal to Liquid
 

Emission advantages of synthetic fuels

When an engine combusts fuel, the sulphur compounds and aromatics encourage the formation of soot particles and nitrogen oxides. As a result, the automotive industry has been working for decades to reduce these constituents in fuel. In Europe, low-sulphur fuels (<10 mg/kg) have by now become established, but synthetic fuels go a crucial step further. They are not only completely sulphur free, but also free of aromatic compounds.

By using synthetic fuel made from natural gas (SynFuel or GtL – Gas-to-Liquid) or biomass (SunDiesel® or BtL – Biomass-to-Liquid) in a fifth generation Golf fitted with a standard TDI® engine, it proved possible for the soot emissions in the New European Driving Cycle (NEDC) to be reduced by almost 30 percent (see Figure 2), and without any changes needing to be made to engine hardware or software. Further emission reductions can be achieved by adaptation of the electronic engine management control to accommodate the slightly lower density of the fuel, and this is already allowing the engines of today to cut NOx exhaust gas levels by more than 30 percent in the NEDC.

Figure 2: Emission reduction potential of synthetic fuel from biomass (SunDiesel®) in comparison with low-sulphur European filling station diesel. The measurements were taken in the New European Driving Cycle (NEDC) using a standard Golf with a 2.0 litre TDI engine. Three engine applications were tested: 1) standard data state; 2 and 3) data state adapted to the lower density of the SunDiesel®, with the aim of maximum reduction of NOx or soot emissions.

The fact that synthetic fuels are free of aromatics, however, consequently means that the density is lower in comparison with conventional commercial diesel which contains some 20 percent aromatics (785 kg/m³ instead of 830 kg/m³). As a result, the density of SynFuel lies outside the European specification for diesel fuel (EN590), although SynFuel is available as an admixture in diesel. For this reason, Shell V Power, for example, contains a GtL proportion of up to five percent. In large urban areas with major air quality problems, such as Bangkok or Athens, adding Shell GtL to diesel has reduced the problems of summer smog since 2002 and 2003. This means that in countries with poor diesel quality, GtL can help directly in improving the quality of the air, and without the need for expensive and elaborate changes to the vehicles or infrastructure.

Volkswagen’s new generation of CCS® engines
Achieving a significant reduction in emissions from diesel engines calls for intensive further development of the diesel engine combustion process and the use of homogenisation effects - the ideal mixing of evaporated fuel and air in the combustion chamber of the engine. In this situation, areas in the combustion chamber with insufficient oxygen which lead to soot formation, and local peak temperatures responsible for the NOx emissions, are avoided or reduced.

Therefore, measures taken inside the engine simultaneously reduce NOx and soot emissions. To achieve this, further joint development of engine and fuel is especially worthwhile as the diesel combustion process in particular has great potential to offer in reducing emissions when engine and fuel are matched to one another. SynFuel is making this approach universally possible for the first time. The CCS® engine generation currently under examination by Volkswagen Group Research combines the low emissions of a petrol engine with the low fuel consumption of a diesel unit. The heart of the system lies in the use of homogenisation effects which can be achieved by applying both engine and fuel measures, and specifically changing the fuel properties is a decisive way to optimise homogenisation in the diesel engine. One such fuel that has been optimised to the CCS® combustion process is “CCS®-kerosene”, but even with conventional diesel fuel the CCS® combustion process achieves significant emission reductions, as Figure 3 makes clear.

Figure 3: Emission reducing potential of the CCS® engine with diesel fuel and CCS® kerosene in comparison to the standard diesel engine at a constant load point of 2000 rev/min and 90 Nm (output of about 18 kW).

Figure 3 shows the emission reduction potential of the CCS® combustion process at an average constant load point. The red point corresponds to the emissions from a standard TDI® engine. Thanks to the CCS® engine, which in terms of hardware and engine control is designed in such a way that good homogenisation of the fuel-air mixture is achieved, both the nitrogen oxide and the particle emissions are substantially reduced when using diesel fuel (blue curve). Further emission reduction to zero can be attained by optimisation of the fuel. The most significant features of CCS® kerosene are its low boiling point and the reduced cetane rating, as well as being free of sulphur and aromatics, resulting in very good homogenisation with extremely low emission values (see Figure 4).

Figure 4: Properties of CCS®-kerosene