Untersuchung der Auswirkungen neuer Technologiekombinationen auf den ottomotorischen Arbeitsprozess zur weiteren Wirkungsgradsteigerung von zukünftigen Pkw-Antriebsstrangkonfigurationen

Vehicle development must contend with the demands of ambitious CO2 fleet targets and future legislation with stringent emissions standards. More efficient gasoline engines will help achieve these targets. In theory, the higher the compression ratio, the more efficient a gasoline engine is. In reality, increasing the compression ratio in the lower partial load may improve efficiency. But it will also substantially reduce knock resistance at higher loads. Under real driving conditions, high mean pressures tend to occur especially with downsizing gasoline engines combined with a high vehicle weight. This setup promotes engine knock and requires a reduction in the ignition angle. The resulting late center-of-combustion positions do not optimize efficiency. High knock resistance is therefore essential for future ultraefficient downsizing combustion processes.

This scientific work investigates and evaluates new technology combinations that make gasoline engines more efficient. These researches also consider exhaust emissions. The largely experimental analyses focus on the Miller combustion process, direct high-pressure fuel injection, and water injection technology. Calculations, simulations and optical high-speed photography supplement these analyses. The experimental tests are performed with special single-cylinder engines and four-cylinder engine technology prototypes.

This work produced the following key findings:

- Increasingly earlier closing times of the intake valves steadily increase knock resistance. Combustion stability is reduced clearly where there is no suitable charge movement. Therefore operation at higher loads is no longer possible in some cases.

- The intake-cam, intake-port and piston geometries significantly affect the charge movement. Insufficient maximum intake cam lift can decisively disrupt tumble flow formation. The interplay of intake-port and piston geometries results in complex flow structures. These have a substantial impact on combustion behavior. Increased knock resistance can overcompensate losses caused by relatively low intake-port flow rates.

- Specific selective cooling, in combination with a far lower coolant volumetric flow, does not decrease knock resistance with the Miller combustion process.

- Multiple fuel injection optimized for the Miller combustion process further increased efficiency and smooth running. It also resulted in low particulate emission levels. Increasing fuel pressure from 350 bar to 500 bar does not fundamentally change thermodynamic combustion. This fuel pressure increase can further reduce particulate emissions provided an optimal injection strategy and certain boundary conditions exist.

- Compared with port water injection, direct water injection delivers much better efficiency and smooth running with the Miller combustion process. This technology theoretically offers even greater potential because of the incomplete water vaporization up to the ignition point.

- Timing separation for fuel and water injection is recommended for low-particulate combustion. While fewer nanoparticles are visible with low quantities of water, larger quantities of water promote particulate formation and increase HC emissions.