In the medium and long term, cogeneration plants in Germany will play an important role in the transition towards a cleaner and more efficient electricity and heat generation, compared to the conventional uncoupled power plants. For most of the currently used CHP (Combined Heat and Power) units, which operate with a lean-burn process, the NOx emissions limit represents an obstacle to increasing the electrical efficiency. Therefore, the lean burn process has become less attractive because of stricter future NOx emissions limit. In this context, the stoichiometric combustion process with a three-way catalytic converter provides a solution. However, the present study shows that lean burn operation still has potential due to direct water injection into the combustion chamber.
This work includes an experimental investigation of the impact of different injection parameters (beginning of injection timing, injection pressure difference and water-to-fuel ratio) on the effectiveness of direct water injection regarding the improvement of the trade-off between engine efficiency and NOx emissions. For the execution of the experimental investigations, a series-production CHP-engine was equipped with a direct injection system consisting of a high-pressure unit, a high-pressure pipe and a GDI-injector. For the injector integration, the cylinder head was machined sidewise (close to the exhaust gas valve). Furthermore, 3D CFD simulations of the injection process allowed gaining a deeper insight into the complex spray-flow interaction, wall film formation and evaporation at different injection timings. For the 3D CFD simulations, the spray model used was tuned with help of spray pictures, taken on the spray test bed.
Water injection at the beginning of the intake stroke (330 °CA BFTDC) reduces NOx emissions most effectively. Moreover, it causes the least engine efficiency loss. The increase of the injection pressure difference (between 20 and 80 bar) leads to a significant increase of the engine efficiency. However, it has a secondary effect on the NOx emissions reduction. The lowest NOx emissions are reached with an injection pressure difference of 60 bar. The combination of direct water injection (at the beginning of the intake stroke, injection pressure difference of 60 bar) with earlier combustion phasings enables an increase in the engine efficiency and a simultaneous decrease in NOx emissions without loss in engine performance. Main drawbacks of water injection are longer combustion duration and higher CO and HC emissions. In addition, the lower exhaust gas temperature causes a deterioration of the conversion of the HC molecules in the oxidation catalyst and reduces the heat recovery efficiency of the CHP-system.