Effect of intake manifold water injection on a natural gas spark ignition engine: an experimental study

Design and development of gas CHP (combined heat and power) engines are strongly influenced by the progressively more severe European NO_x emissions normative. Water injection represents a promising approach to reduce these emissions while attaining high engine efficiency. In this work, the effect of intake manifold water injection on combustion parameters and performance of a single-cylinder naturally aspirated natural gas spark ignition engine is presented. First, the most appropriate injector was selected, using a spray test bed. Subsequently, engine experiments at constant indicated mean effective pressure (IMEP) and engine speed were conducted with water-fuel ratios of 0.1 to 0.3. IMEP was kept constant at about 6.3 bar by adjusting both air-fuel ratio and spark timing. A NO_x reduction of 0.2 g/kWhi (15 %) for a constant ISFC of about 204 g/kWhi was achieved. In the low NO_x regime, water injection allows for an improvement of the NO_x-ISFC trade-off, while leading to poor fuel consumption at same NO_x in the high efficiency regime. Furthermore, water injection implies a reduction of intake mixture temperature, lengthened burning delay and combustion duration and a moderate increase of combustion instability.