The combustion engine is not yet dead

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Life cycle greenhouse gas emissions for different passenger transport modes. Ill

Life cycle greenhouse gas emissions for different passenger transport modes. Illustration: Empa

A lot has happened in 2021: In January, the Swiss government adopted the "Long-Term Climate Strategy for Switzerland" with the goal of net zero by 2050. Over the course of the same year, Empa published a series of research papers on the internal combustion engine. But how does this fit together? Aren’t combustion engines yesterday’s technology?

The familiar engine hum of the 20th century will become quieter in the coming years. "The switch to electric drives will happen gradually," says Christian Bach, head of the Automotive Powertrain Technologies Laboratory at Empa. "But in some applications, it will be difficult or even impossible to do without combustion engines."

Even the venerable Deutz AG in Cologne is saying goodbye to the combustion engine as its main business model. The company was founded in 1872 by Nikolaus August Otto, counted Gottlieb Daimler and Wilhelm Maybach among its employees, and built the world’s first four-stroke engine in 1876. Now Frank Hiller, the CEO of Deutz, plans to generate half of the company’s revenues from hydrogen engines and electric drive systems as early as 2031, according to a report in Germany’s "Manager Magazin". The money for the conversion is to be generated through traditional engine manufacturing. Hiller expects that combustion engines will be needed for a longer time, for example in construction and harvesting machinery, according to the magazine. This would give the company some time to master the conversion.

Hence research on internal combustion engines is focused on the coming two to three decades. The goal is to emit as few greenhouse gases as possible. On the one hand, this involves CO2 emissions from engines, and on the other, methane, a particularly potent greenhouse gas. Methane generated from green electricity is one of the key fuels for the mobility of the future. But even this climate-neutral methane must not be allowed to escape unburned from the exhaust, otherwise little would be gained in terms of the greenhouse effect.

We therefore need a new generation of fuel-efficient engines. At the same time, we must keep a close eye on the composition of exhaust gases so that as little climate damage as possible occurs in areas where combustion engines are, for the time being, hard to replace. On the next two pages you will learn how Empa is tackling this issue.

Research in time with the engine - the internal combustion engine on new paths

Emissions under real driving conditions can be measured using a technique called remote sensing. The measuring system records the concentration of pollutants in the exhaust gas of passing vehicles from the roadside. During a measurement, the vehicles pass through a light barrier generated by ultraviolet and infrared sources. By absorbing light of different wavelengths, a spectrometer determines the concentration of various pollutants. The measurement system is thus fixed on site and determines the real-world emissions of passing vehicles in a snapshot under the conditions prevailing at the measurement site.

In collaboration with the Federal Roads Office (Astra), Empa evaluated two measuring devices that are already on the market. The long-term goal is to identify vehicles with defective exhaust or engine management systems in flowing traffic, have the exhaust values verified in the laboratory and request the owner of the vehicles to repair them.

To reduce greenhouse gases, internal combustion engines could in future be powered by biofuels such as vegetable oil or by synfuels such as polyoxymethylene dimethyl ether (OME). OME can be produced from green hydrogen and CO2 from ambient air, for example. It can be used in self-igniting engines (i.e., diesel engines).

However, it is still unclear what kind of soot particles are produced from these alternative fuels and how they can be removed from the exhaust gas. In a laboratory at ETH Zurich, a single-cylinder engine was operated with vegetable oil, OME as well as various fuel mixtures. researchers collected the resulting soot in particle filters and burned the filter under controlled conditions. The result: OME produced far fewer soot particles, but they were difficult to ignite and could therefore only be removed from the filters at higher temperatures.

Just like OME, methane can be produced from green hydrogen and ambient CO2, making it a suitable climate-neutral synfuel for long-haul trucks. But there is a problem: Unburned methane is difficult to oxidize in a catalytic converter. It then slips through the exhaust tract into the atmosphere, causing a greenhouse effect that is 30- to 80-times greater than that of CO2, depending on how you look at it. This would reduce the ecological benefit of methane-powered trucks. researchers conducted tests with truck engines in "rich" operation (with surplus fuel), in "lean" operation (with surplus air) and in lambda-one operation (methane and oxygen in a perfect combustion ratio). This corresponds to a truck that drives uphill or downhill at a constant speed.

A model experiment was conducted to investigate the chemical processes in the catalytic converter that are required to ensure that unburned methane is destroyed as much as possible - and hence does not produce an undesirable greenhouse effect. As a result, the team came up with a catalytic concept that significantly reduces methane emissions. This allows gas engines to meet the stricter requirements of the next emissions standard (Euro 7).

The Comprex supercharger, like the turbocharger, is a Swiss invention. The Comprex uses pressure waves in direct contact of the exhaust gas with fresh air for supercharging, while a turbocharger couples two flow machines (a turbine and a compressor).

In the 1980s, Comprex superchargers were used in diesel passenger cars by Opel and Mazda. But the supercharger had drawbacks: During cold starts, it was difficult to build up the pressure wave process, and temperature-related effects during load changes led to higher emissions and efficiency problems.

Meanwhile, engineers from the Swiss company Antrova AG have further developed the Comprex supercharger: Supported by an electric motor, it works smoothly in all conditions, and a new design of the so-called cell rotor completely solves the difficulties caused by temperature changes. researchers, in collaboration with a commercial vehicle manufacturer and the Comprex manufacturer, have built a natural gas engine with such a "Comprex 2.0" supercharger and have been able to demonstrate that the new Comprex design works perfectly well in cold start conditions as well as under warm and dynamic operation. In contrast to its turbo counterpart, the engine delivers enormously high torque practically from idle speed, which on the one hand improves drivability and, in combination with so-called Miller operation and an adjustment of the transmission ratio, helps save fuel.

At the same time, the catalytic converter warms up six times faster than in a turbocharged engine, which ensures better exhaust gas values. Finally, the Comprex enables a high engine braking effect - truck drivers would have to use the mechanical brakes much less frequently.

In recent years, researchers have developed a fully variable electrohydraulic valve control system called FlexWork, which can be used for internal combustion engines and other thermal/pneumatic machines. Fully variable means that the valve lift as well as the opening and closing timing can be freely adjusted - even from one cycle to the next. When used on an internal combustion engine, this flexibility provides new degrees of freedom for optimization. For example, the load on gasoline engines can be adjusted without a throttle valve and from one cycle to the next, the full load can be optimized and the engine can be adapted to different fuels "by software".

The Empa team has built the valve train on a gasoline engine and is now exploring the possibilities offered by this new technology. One variant of load control is cylinder deactivation. This means that, in the partial load range, individual cylinders are operated at high load, while others are switched off completely by keeping all valves closed. However, the sudden transition from an operation with all cylinders to an operation with some cylinders shut down would result in undesirable torque peaks, so the transition must be smooth. In systems on the market today without a fully variable valve timing, such cylinder deactivation is triggered by ignition interventions that greatly reduce efficiency. With Empa’s fully variable valve control system individual cylinders can be shut down without any loss of efficiency.

Just as it is possible to shut down cylinders completely, it is also possible to fire them less frequently. This turns a four-stroke operation into an eightor twelve-stroke operation. Compared with a throttled four-stroke engine, such an engine operates much more efficiently.

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