Empa researchers use catalyst to combat cold starts
Researchers came up with the idea of using a catalytic converter to preheat the gasoline engine. A cold start of a gasoline engine produces far more particulate matter and other pollutants than while driving, because a cold catalytic converter is far less efficient at cleaning exhaust gases.
A catalytic converter could do the trick, because the current internal combustion engines are repeatedly criticized. At first, it was sooty diesels, which could then be "helped" with particulate filters. Then, again in the case of diesel, harmful nitrogen oxides came into focus, which were (supposedly) brought under control with complicated exhaust aftertreatment systems - or not, as the diesel scandal showed. What is often overlooked in the diesel debate: Gasoline engines also contribute to particulate pollution in cities. Especially where many engines are started cold. Around 90 percent of all pollutants are produced in the first minute after a modern gasoline engine is cold-started.
Solution for the cold start problem
In other words, the first 500 meters of driving pollute the air just as much as the next 5,000 kilometers if you were to drive that far non-stop. To further improve air quality, therefore, automotive catalytic converters are needed that warm up as quickly as possible - or, even better, efficiently clean the exhaust gas as soon as the first engine revolution after starting. Potis Dimopoulos Eggenschwiler, a specialist in exhaust gas aftertreatment at Empa's engine laboratory, has been researching a solution to the cold-start problem, which severely pollutes the air, especially in cities and when outside temperatures are cold, for just under two years with support from the Swiss National Science Foundation (SNSF) and the Federal Office for the Environment (FOEN).
And the high cold-start emissions affect not only millions of gasoline engines, but also hybrid models. In these phases, the combustion engine and, above all, the catalytic converter cool down again, in some cases below the optimum operating temperature. If the control technology then switches back to the gasoline engine, exhaust gases containing pollutants flow through the (cooled) catalytic converter again, almost like after a cold start.
To ensure that the catalytic converter can be heated to 300 degrees Celsius by the car's power supply with as little energy as possible, even before the engine starts, it has to be small and conduct heat as well as possible. Dimopoulos Eggenschwiler and his team have developed an open-pored structure with a special coating that can be heated up by a small microwave transmitter within ten seconds - similar to a microwave oven at home. Back in 2012, the Empa researcher developed a particularly efficient catalytic converter - a ceramic casting of a polyurethane foam that swirls the exhaust gases better and generates less backpressure than a catalytic converter with a conventional, honeycomb structure.
Ceramics from the 3D printer
The "foam cat" now gave rise to the next idea: a geometric lattice structure made of thin ceramic struts that manages with a smaller precious metal coating but still efficiently cleans the exhaust gas swirling inside it. "First, we looked for an optimal structure on the computer," says Dimopoulos Eggenschwiler. "A structure that heats up quickly, accelerates chemical reactions and at the same time obstructs the flow as little as possible."
Then the task was to reproduce the structure in ceramics. Specialists at the "Scuola universitaria professionale della Svizzera italiana" (SUPSI) in Lugano produced the lattice designed on the computer using stereolithography, a kind of 3D printing from liquids and UV light. The Empa researchers then coated the ceramic with silicon carbide, zirconium oxide and aluminum oxide - and the active catalyst substances platinum, rhodium and palladium. EngiCer SA, a company based in Ticino, is taking over production of the first small batches and is in a position to expand its capacity if there is greater demand. The Swiss catalyst manufacturer HUG Engineering AG is also involved.
The geometric ceramic structure of the test catalyst designed on the computer. Specialists at Empa coated it with the active layer and tested the cleaning effect in an artificial exhaust gas stream.
What is probably the world's first exhaust gas catalyst from the 3D printer fulfilled all expectations in the practical test: In the artificially generated exhaust gas stream of Empa's model gas reactor, the new polyhedron geometry cleaned the pollutants even better than the foam catalytic converter from 2012. After initial laboratory tests with small model catalytic converters were successful, a follow-up project is now in preparation: A full-size 3D catalytic converter will be installed in a prototype vehicle and then tested on the test bench and on the road.
The next step is to integrate microwave heating. "It is important that we do not heat the entire ceramic structure," explains Dimopoulos Eggenschwiler. "We want to concentrate the microwaves generated with valuable battery power only on the first part of the catalyst. If the first chemical reactions run, the rest of the catalyst heats up very quickly." One to two kilowatts of power for ten to 20 seconds could easily be diverted from the car battery, says the exhaust specialist. "That should be enough." Once the engine is running, the exhaust gas and chemical reactions in the catalytic converter itself provide enough heat to keep it warm. Then the microwave can be turned off. Cold-start emissions could thus soon be history.
