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The best of the worlds of gasoline and diesel engines unified in one? Sounds like illusion and revolution. But it is feasible. Volkswagen is working intensively on trials with such an engine system that could become a reality within the next decade. The name of this new engine technology is CCS. The acronym stands for the combined combustion process and possibly for the engine of the future. It can already be experienced in Touran prototypes.
The crucial aspect of CCS that represents a technological breakthrough is the innovative mixture forming method, in which the principles of gasoline direct injection (TSI) and diesel direct injection (TDI) are in fact merged. Here Volkswagen is exploiting many years of broad experience the brand has acquired in both of these areas, and it is uniting their strengths. The gasoline engine contributes a homogeneous fuel-air mixture and low emissions to the CCS process, while the diesel contributes compression ignition and low fuel consumption.
The CCS process lies midway between the diesel and gasoline principles. On the CCS engine, injection already begins in homogeneous operation while the piston travels upward and compresses the air. Using common rail injectors taken from the diesel engine, the injection volume can be distributed among different cycles as desired and can be precisely metered.
While the piston travels upwards, the fuel and air are compressed and heated – the fuel vaporizes. This produces a largely homogeneous mixture, comparable to that of the TSI engine. Combustion is initiated in homogeneous operation shortly after TDC (“top dead center”), without requiring (and this is analogous to the diesel) an external spark; theoretically the mixture ignites at an infinite number of points simultaneously. Due to the quasi-homogeneous combustion near TDC, both low emissions and – compared to the already economical TDI – even better fuel economy are attained.
The CCS engine operates at a high exhaust gas recirculation rate (EGR rate). The oxygen-poor recirculated exhaust gas makes several contributions here. It ensures that combustion onset is not too early and that no excessively high temperatures or hot spots occur that would allow nitrogen oxides to be generated. In the CCS process with its uniform combustion there are also practically no zones of rich mixture in the combustion chamber. Therefore (in contrast to the diesel) hardly any soot is formed that would otherwise be a shortcoming of high EGR rates.
The first research engine based on a two-liter TDI in the Touran prototype that is currently in an early developmental stage already realizes five percent fuel savings utilizing an optimized fuel compared to a conventional diesel, and it simultaneously reduces NOx and soot emissions significantly.
The basic prerequisite for full implementation of the CCS combustion process is new designer fuels such as SynFuel und SunFuel. They help to achieve homogeneous combustion. SynFuel is obtained synthetically from natural gas and SunFuel from biomass. Both are free of sulfur and aromatic compounds, and this by itself drastically reduces raw emissions. Within certain limits, their compositions and therefore their properties – especially boiling temperatures and cetane ratings in this context – can be freely configured in the design, and the fuels can be reproduced reliably in high quality. In principle, this makes the two fuels ideally suited for use in the CCS engine.
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