Impact of Intake Port Water Injection Timing on Mitigating the Tradeoff between Performance and NO _x Emissions for a Naturally Aspired Micro-CHP-Engine

Small natural gas cogeneration engines usually operate with lean mixture and late combustion phasing to comply with NO_x emission standards, leading to significant losses in engine efficiency. Owing to water evaporation heat and high specific heat capacity of the water vapor, leads the water injection to cooling the combustion chamber charge, which enables earlier combustion phasing, higher compression ratio and thus higher engine efficiency. Therefore, water injection enables mitigating the tradeoff between NO_x emissions and engine performance, without loss in engine efficiency. The intake port injection represents, because of the low required injection pressure and the simple injector integration, a cost-effective way to introduce water into the engine. Hence, the purpose of this work is to adapt the intake port water injection timing to the charge mixture flow conditions in the intake port.

For this purpose, a variation of the injection timing was experimentally carried out on the engine test bench. The combustion phasing and the NO_x emissions were kept constant. At the same time, the engine power, engine efficiency, temperatures in the intake port and mass flow rates were recorded. To gain a deeper insight into the complex processes in the intake port, such as spray-flow interaction, wallfilm formation and evaporation, 3D CFD simulations were conducted for three selected cases with 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.

Experimental investigations have shown that an injection close to the intake valve opening allows an increase in engine performance at constant NO_x emissions without loss of engine efficiency. In fact, early injection allows effective cooling of the intake port mixture and allows part of the injected water to enter the combustion chamber as liquid and to vaporize there. A disadvantage of an injection close to the intake port opening, shown by the 3D CFD simulations, is that the water vapor is distributed homogeneously in the combustion chamber with a slightly higher concentration in the prechamber area, which could lead to a significant slowdown of the combustion speed, if the injected amount of water is increased.