Transport: The cold start dilemma of cars with hybrid drives

Almost a quarter of a million cars with hybrid drives were newly registered in Germany last year – 83.7 percent more than in the same period last year. And the strong demand for this form of drive, fueled by tax incentives and a growing range of models, continues to be very large. Because these cars can drive through the city purely electrically for a few kilometers, so that they use significantly less fuel in everyday life. This applies all the more to the so-called plug-in hybrids – part-time streamers, in which the battery can be recharged at a socket. They move up to 50 kilometers electrically through the city. The internal combustion engine does not start here until the car is accelerated on the highway or country road.

The problem with this is that the engine makes a cold start every time, at high engine speed and engine load – quite differently than was previously the case. Can the exhaust gas cleaning keep up? Are the catalysts that we have been using since the 1980s suitable for such cases? Viola Papetti and Panayotis Dimopoulos Eggenschwiler from Empa, the Federal materials testing and research institute, have recalculated this with a specially developed mathematical model. And they give recommendations on how catalysts can be preheated in the future in order to function optimally.

Only a warm Kat is a good Kat

During a cold start, the engine blows hot combustion gases into the cold catalytic converter. This has to warm up successively in order to develop its chemical cleaning effect. As long as it is cold, carbon monoxide (CO), nitrogen oxides (NOx) and unburned hydrocarbons escape unhindered to the outside air. The good emission values ​​of modern Euro 6 vehicles are only achieved with a warm catalytic converter. However, the differences between warm and cold are drastic: in the first three minutes after a cold start, a vehicle emits more pollutants than a 1000 km journey with a warm engine.

For their model calculations, the researchers from Switzerland chose a typical catalytic converter for a 2.0-liter petrol engine. With the help of specially developed programs based on the open source software platform OpenFOAM they calculated what the hot exhaust gases called the ceramic honeycombs of the catalytic converter and the catalytic cleaning layer "Washcoat", Warm up. At first, the cat only gets through the hot gases "Warmgeföhnt", then the heat gradually penetrates the ceramic and the sheet metal casing of the catalyst. A little later, the first chemical reactions begin in the front part of the catalytic converter: the pollutants are initially chemically broken down on the washcoat. This provides additional heat, which is sufficient to heat up the remaining catalyst.

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