Alternative fuels from regenerative sources (e.g. BTL) will especially gain importance in the same proportion as the shortage of fossil fuels increases due to the global trend of increasing mobility.
Against the background of European and international legislations, exhaust gas treatment will play a further superior role to fulfill future emission limits - draft of the Commission for Euro V emission limits for passenger cars. In addition to the emission limits, area-wide immission limits for particulate matter (PM10) of 50µg/m³ for the 24 hour mean and 40µg/m³ for the annual mean are in force in Germany and in all the EC member states. Furthermore, a limit for the annual mean NOx immission level of 40µg/m3 will come into force in 2010.
The classic trade-off between particle and NOx emission will dissolve due to the predictably broad application of particulate filters in EURO V passenger car diesel engines and in EURO VI truck diesel engines. Now, in the context of more stringent NOx emission limits and the corresponding engine-internal NOx reduction strategies, new trade-offs come to the fore-unburned Hydrocarbons vs. NOx and fuel consumption vs. NOx. Therefore, the target must be to select an NOx treatment system which operates as selectively and efficiently as possible without putting the efficiency advantage of the diesel engine into question. Observation of acceptable system cost limits and conflict free adjustment of the functionalities of combined exhaust gas treatment solutions of the future, consisting of DPF and DeNOx (SCR, NSC) systems, will be of special importance.
A broader acceptance of directinjection gasoline engines is to be expected, if their significant fuel consumption reduction potential is realised. Catalysts with broad operation range (temperature window), high conversion rates, little ageing susceptibility and sufficient robustness against sulphur contamination are prerequisites for such future success. The research results of homogeneous combustion processes in diesel and gasoline engines are, despite positive findings for the neutralisation of NOx and particulate emissions, also characterised by significantly increased CO and unburnt HC emissions. Current catalyst concepts appear not completely adequate to fulfil future emission limits and need to be improved if the developments in the field of homogeneous combustion achieve a breakthrough.
Liquid mineral fuels from crude oil are expected to play an important role for automotive uses during the next 20 years. Additionally, alternative fuels from regenerative sources (e.g. BTL - biomass to liquid) are going to achieve higher share. These fuels are typically free of sulphur and aromatics and they include a high cetane number. Decreasing availability of fossil fuels, environmental needs such as the reduction of green house effect and a global trend of increasing mobility are reasons for a raised interest in alternative fuels. Furthermore, the implementation of the EC-directive 2003/30/EG prescribes a market share of 5.75 per cent for biogenous fuels in 2010.
Synthetic fuel can be attained from natural gas (Gas to Liquid = GTL), coal (Coal to Liquid = CTL) and biomass (BTL) using the Fischer-Tropsch- Syntheses. Because of its regenerative nature, BTL offers considerable CO2 reduction potential. This kind of fuel is free of sulphur and aromatics while having a high cetane number. It fits well into the existing fuel infrastructure. The quality of synthetic diesel fuel is considered to be independent from its source, i.e. fossil or regenerative. Synthetic fuel does not fulfil the European standard EN 590 for diesel fuel. The tests conducted with engines running on these alternative fuels indicated an average reduction in the emission of particulate matter, CO and unburnt HC by 41, 90 and 46 per cent respectively. The EURO IV PM limit could already be fulfilled with a EURO III application passenger car simply by switching the fuel.
In the future, innovative combustion processes will play an outstanding role in the reduction of the harmful components in the engine-out emissions. Such combustion processes could either avoid expensive and complex exhaust gas treatment systems or reduce the loading of these systems.
In addition to the emission limits, the fuel consumption targets of the automotive industry (conservation of resources, CO2 commitment) must be met. Considering the above mentioned propositions, it is possible to further progress in the engine internal reduction of critical exhaust gas emission components.
The availability of high-quality synthetic fuels, which are virtually free of sulphur as well as aromatic components and have high cetane numbers is of growing importance, as it provides enough scope for the innovations in combustion processes. Alternative fuels from regenerative sources (e.g. BTL) will especially gain importance in the same proportion as the shortage of fossil fuels increases due to the global trend of increasing mobility.
Growing requirements of exhaust gas treatment systems are expected against the background of increasingly stringent European and international exhaust gas emission limits. Thus, in a medium-term perspective, fulfillment of these targets can only be obtained by synchronised progress in the areas of combustion process development and exhaust gas treatment.
Horst Harndorf is a Professor of Piston and Combustion Engines at the University of Rostock. His career graph also includes being the group leader for injection and combustion technology in the central research and advanced engineering department, Robert Bosch GmbH and scientific assistant at the Association for Combustion Engine Research, Frankfurt/Main, supervising the project "acceleration-induced smoke discharge in supercharged diesel engines; causes and remedial action."
