Full throttle into the energy transition

Pistons for eco-friendly gas-engine power plants booming

Combustion engines powered by natural gas will play a decisive role in the climate-friendly energy mix of the future. They are the heart of ­power plants that are operated in a cogeneration system. ­Rheinmetall ­Automotive is supplying the pistons of the large engines for the most modern of these, the German K.I.E.L. coastal power plant.

When Wolfgang Hartmann is asked about his contribution to combating carbon dioxide emissions, he points to a huge engine piston that is packed and ready for delivery at the outgoing goods department of the Neckarsulm plant. Lifting it would hardly be possible even with both hands, because the component weighs around 87 kilograms and measures 31 centimeters in diameter. By comparison, a piston for passenger car engines weighs just 200 grams. “This is a steel piston for large natural gas engines running in cogeneration power plants,” explains Hartmann, Head of Large Bore Pistons at Rheinmetall Automotive. At first glance, his response to the question may seem surprising, but it does make sense. In the current debate about green electricity from renewable sources such as wind, solar and hydro power, it is often forgotten that the environment-friendly generation of heat energy for heating systems and hot water supply also offers plenty of climate protection potential. In addition, the supply of electrical energy must also be ensured in times when alternative sources barely provide any electricity, for example when there is no wind on a cloudy winter’s day.

This makes modern gas-engine power plants, which are operated in combined heat and power generation, an essential part of the energy transition. The trick: the energy released by the combustion engines during operation is used twice. On the one hand, the crankshaft of the engines drives a generator that supplies electricity; on the other hand, the combustion heat is used to heat a water reservoir, which is then used to supply hot water and to heat buildings in the district heating network. 

World’s most modern plant in Kiel

One of the world’s most modern power plants of this type is the coastal power plant K.I.E.L. (Kiels Intelligente Energie Lösung = Kiel’s Intelligent Energy Solution). The overall system consists of 20 gas engines in four blocks, an electrode boiler and a heat accumulator. The engines of the modular system can be run up to a rated output of 190 megawatts in less than five minutes. At the same time, a heat output of 192 megawatts is generated during operation – enough to supply the 70,000 customers of Kiel’s municipal utility with environment-friendly electricity and heat energy via the district heating network. Compared to the previous coal-fired power plant, nitrogen oxide emissions from the K.I.E.L. plant are reduced by up to 70 percent, and no sulfur is released during the combustion of natural gas.

The units used are 20-cylinder Jenbacher J920 FleXtra engines, each 8.4 meters long, 3.2 meters wide, 3.5 meters high and weighing 91 metric tons. In addition, there is a turbocharger module and generator and hence one complete unit is 16.8 meters long and weighs 176 metric tons. Rheinmetall Automotive is equipping the engines with pistons and a matching set of rings. Like the overall system, they are designed for particularly high efficiency. The extremely good energy conversion of the engines during combustion creates the basis for the plant’s high overall efficiency of 90 percent, half of which is thermal and half electrical. Compared with the separate generation of heat by gas boiler and electricity in the current EU mix, up to 7,800 metric tons of carbon dioxide can be saved annually per engine-generator unit, which, when extrapolated to the 20 units of the K.I.E.L., add up to 156,000 metric tons less carbon dioxide. 

High pressures open up efficiency advantages

One component of the high efficiency of the engines used in Kiel is their combustion control. Unlike diesel engines, in which the air-fuel mixture ignites automatically due to the pressure in the combustion chamber, gas engines generally operate according to the spark-ignition principle – they require an external ignition source such as a spark plug to initiate combustion in the cylinder. During engine operation, the combustion and ignition parameters are set in such a way that maximum efficiency and thus energy exploitation of the fuel are achieved. This, however, increases the risk of knocking during combustion. Experts define knocking as uncontrolled combustion, resulting in pressure peaks that can damage engine components such as bearings, valves, cylinder heads and pistons. “The secret is to operate the engine as close as possible to the knocking threshold, but not to exceed it. To do this, combustion is set so that knocking occurs briefly, then the parameters are quickly readjusted,” says Hartmann.

The efficiency of the system is 90 percent – half of it electrical, the other half thermal

Pressure peaks can still occur, but they are much less harmful than with permanent knocking combustion and do not lead to any component damage if they are taken into account in the design of the engine components. For efficiency reasons, in the case of the power plant engines there is an additional engineering consideration, they are designed for particularly high ignition pressures of up to 250 bar, and the pressure spraying that can occur briefly during head control of the engine is correspondingly pronounced. “The ignition pressure level presented piston developers with a design challenge. After a detailed potential assessment, we decided on a two-part piston made of forged steel. The lower and upper halves of the component are bolted together,” says Hartmann, explaining the design of the piston.

Flexible output

The power plant engine by Innio Jenbacher is committed to longevity, simple installation and maintenance. It consists of standardized modules: Generator, engine and turbocharger. The engine concept ensures short downtimes; the complete engine unit can be exchanged as a module. The ease of maintenance is also apparent in the segmented camshaft. Each engine can provide 10.4 megawatts electrical power in less than 5 minutes when required. State-of-the-art model-based control technology ensures low emissions when starting the engine. Together with the integrated exhaust gas aftertreatment systems, it also minimizes nitrogen oxide emissions.

“In addition, the piston crown was adapted to gas operation and the specific requirements of the engine,” says Hartmann. The engine developers first calculated its shape by means of a simulation and then tested the piston on the test bench. This is because the optimum crown design plays a decisive role in determining whether combustion takes place homogeneously in the combustion chamber. “It is absolutely essential to avoid so-called hot spots, that is particularly hot areas on the component walls where fuel ignites uncontrollably and causes knocking. 

Abundant flexibility

A special feature of the plant in Kiel is its high degree of flexibility, which allows the operating modes to be adapted to the specific electricity and heat requirements. In wintry weather, for example, the power plant delivers the required thermal energy and electricity to customers in Kiel. Excess electrical energy for which there is no local customer is sold on the electricity market. If good wind conditions in winter generate a lot of electricity from renewable sources, the capacities of K.I.E.L. can be reduced. Additional “green” electricity from the grid then heats the water for district heating in the electrode boiler. And in summer, when clouds and calm winds threaten to cause short-term bottlenecks in the supply of solar and wind energy, the K.I.E.L. intervenes at short notice, stabilizes the electricity grid and stores excess energy in the heat storage tank.

Idea ignites precedent

Other cities and municipalities have also recognized that innovative gas-engine power plants offer an environment-friendly and efficient alternative to traditional, frequently still coal-fired power plants. “We are experiencing an ever-increasing demand for pistons for power plant gas engines,” says Hartmann. For example, a new gas engine power plant is currently being built in Cologne. “Of course with pistons from Rheinmetall Automotive,” he adds. 

Wolfgang Hartmann, Head of Large Bore Pistons at Rheinmetall Automotive

“Trend towards gas ­engines not limited to energy sector”

Large-bore pistons from Rheinmetall Automotive, what sizes and weights are we talking about?

The smallest piston we supply has a diameter of 145 millimeters and weighs around 5 kilograms, the largest measures 640 millimeters and weighs 500 kg, or half a metric ton. 

In what areas are the engines used?

The applications are many and varied: among other things, in power generation plants. Another application is in shipbuilding. Here, the engines are used on the one hand as the main propulsion unit and, on the other, as an auxiliary unit for power generation. On oil platforms, they are used for on-site energy supply. They also power large mining vehicles, excavators, and locomotives. 

Apart from their size, how do ­pistons for large engines differ from those for passenger cars?

The functional requirements on pistons for large engines are significantly higher, both in terms of running time and fuel compatibility. In the passenger car engine segment, the specifications for service life are around 200,000 kilo-meters or 2,000 to 3,000 operating hours. In the case of steel diesel pistons for commercial vehicles, the -demands are in the direction of 800,000 to one million kilometers or 10,000 hours. Pistons for large engines must be designed for a service life of between 60,000 and 90,000 hours or ten years. This is a big difference that requires considerable design engineering effort. The other point is the fuel properties. In passenger cars, fuel properties are clearly defined by the standards for gasoline, premium and diesel. This is different for large engines: there is virtually no fuel that is not used in large engines. Accordingly, the pistons must also be able to tolerate all fuels, from gas to heavy fuel oil.

Is there a technology transfer from large to vehicle engine pistons?

Especially in the development of steel pistons, in which the large engine sector was a pioneer, there is a technology transfer. This applies, for example, to the machining of pistons in production.

How does the growing demand for gas engines manifest itself in your company?

The trend towards gas engines is not limited to the energy sector. There are also more and more projects of this kind in the marine sector, for example in the construction of new cruise liners. I think that gas engines will establish themselves there in the coming years. In some cases, locomotives are already being equipped with gas engines. We are noticing the greater interest in gas engines, among other things, in a rising demand for corresponding pistons – in percentage terms, their share of our total production continues to increase.

What has been the most spectacular project to date for which Rheinmetall Automotive has supplied large-bore pistons?

Perhaps the most unusual and largest project to date was a gas-engine power plant in Jordan near the capital Amman. With an output of 573 megawatts, it is considered the world’s largest combustion-engine power plant. The overall system consists of 38 engines, each with 18 cylinders. The pistons in question have a diameter of 500 millimeters, and we supplied a total of 684 of them.