Application of a Novel Laser-Optic Method for Characterizing Various Water Injection Technologies for Modern Gasoline Engines 2021-01-5025

In the constant development process of modern gasoline engines, the focus is on reducing fuel consumption and pollutant emissions, as well as increasing efficiency. A promising approach is the application of stoichiometric operation in the entire characteristic map. In conventional operation, thermal limitations arise for the components in the exhaust system at high speeds and high specific outputs. At present, additional fuel is injected as a countermeasure because the enthalpy of vaporization leads to a reduction in exhaust gas temperature. This can be also achieved with water injection, together with the reduction of fuel consumption and emissions. The mixture formation with water is of special importance to achieve sufficient cooling and is, therefore, visualized by means of optical measuring technology.

A novel laser-optic measuring technique for the detection of liquid water in the combustion chamber is developed on a theoretical basis and implemented in practice on a single-cylinder optical engine. Furthermore, a corresponding evaluation method is used to ensure the comparability of different measurements and account for statistical fluctuations. To evaluate the informative value of the optical measurements, the measuring procedure is validated by means of operation with isooctane and a large number of tests.

Three different water injection concepts are optically measured: a low-pressure injection in the plenum chamber, a low-pressure injection in the intake pipe, and a high-pressure injection of a premixed fuel/water mixture via the central injector. All three concepts are optically characterized at a reference point. The results show high differences comparing the high-pressure fuel/water mixture injection with both low-pressure injections. Low-pressure injection leads to large water droplets, which evaporate slowly and show high wall contact. The atomization due to the high injection pressure leads to a higher evaporation rate and reduces liquid water residuals on the walls of the combustion chamber.