Each time the fuel-air mixture is ignited in petrol and diesel engines a small portion of the gases flows from the combustion chamber into the crankcase as leakage current. For the engines to function optimally, it is necessary to dissipate these so-called blow-by gases and separate the engine oil droplets contained within them. This reduces the pollutant emissions and oil consumption of the engines. For modern, higher-compression engines, researchers have developed new and effective separation systems that undercut the more stringent emission limits for oil mist.

Downsizing is the current task of modern combustion engines: smaller, higher-compression engines achieve high outputs out of less cylinder capacity. As a result, significantly smaller oil particles are to be separated from the blow-by gas in the crankcase ventilation. Depending on the available installation space, passive separation systems, such as simple impact separators, can also be used in vehicles for crankcase ventilation in combination with, for example, fleece materials for enlarging the surface of the baffles or cyclones. Actively driven separators, such as disc stack centrifuges, are also already being used in the commercial vehicle sector. The challenge for the developers was to design separation systems, in particular for very fine oil droplets, that have a better separation performance than the simply constructed passive systems. In addition, the new system should be inexpensive to produce and need as little energy as possible for the drive.

The treatment of the blow-by gases largely prevents oil droplets dripping into the intake system. These would soil engine components such as intercoolers and valves and impair the function of catalytic converters. Oil droplets settling on the blades of the turbocharger are decreasing its efficiency. The power density of the internal combustion engine decreases, the efficiency drops and the fuel demand increases.

Very fine droplets are partly created in highly turbocharged engines as a result of the high mean pressures during combustion. This is where the passive oil mist separation systems used today, such as impactor-fleece separators which are often integrated into cylinder head covers, reach their limits. This is why new, active separation concepts need to be developed for modern, highly-charged engines. The currently available active systems such as disc stack centrifuges, with which even oil droplets smaller than 0.5 μm are separated with high efficiency, have until now been limited to use in the commercial vehicle sector owing to their space requirements.

For this reason, researchers from the ElringKlinger AG component manufacturing company and the Institute of Mechanical Process Engineering (IMVT) at Stuttgart University developed two physically different, active separation concepts: a centrifuge and a wet scrubber. These are more efficient than existing systems and take up less space. It is intended that they can be used in the future for a wide variety of applications. While the centrifuge reliably separates fine droplets with a droplet diameter d50 < 0.5 μm due to the high speeds that can be generated either by a hydraulic drive or an electric motor, the wet scrubber is used for applications where only a small pressure loss may be generated with simultaneously limited installation space. Both concepts promise high separation efficiency and thus a significant reduction of the oil droplet entry into the intake system of gasoline or diesel engines.

At IMVT, the researchers in particular studied the flow and pressure behaviour of the separation units since this affects how much energy the separation process requires. For the design of the separation units and atomisation elements for the new separation system, they developed mathematical-physical models as well as simulation methods.

At ElringKlinger, the focus was on how such systems can be manufactured in a mass production-oriented and cost-effective manner. In particular, they were looking at how the design could be further developed for its space-optimised use in cars. Intermediate stages were tested on engine test rigs for the further development and optimisation. The researchers investigated the separation concepts with a test aerosol. The particle size distributions measured before and after the separation process enable them to evaluate the separation performance under different operating conditions.