**5. Conclusions**

In this chapter, the feasibility of knock suppression using water injection for an SI engine fuelled with kerosene is investigated. First, a 3D numerical model is established in AVL Fire software. The influences of water injection timing, initial droplet diameter, and water injection quantity are analyzed. Then, an engine experiment is performed to evaluate the limit of IMEP improvement.

The results indicate that water injection can suppress the knock of kerosene effectively. The water injection timing should be advanced and far away from TDC because the evaporation of water droplets needs time. Accordingly, a lower pressure rise rate can be obtained, and the phase angle of the peak pressure can be postponed. The autoignition time enlarges, the flame propagation speed drops, and the intensity of the detonation decreases, due to a reduction of in-cylinder temperature. Meanwhile, a better result of knock suppression can be obtained with a smaller initial water droplet diameter, owing to a larger evaporation surface area of droplets. With the increase of the water injection amount, the reduction of in-cylinder temperature increases. The allowable throttle opening and injected kerosene amount also increase, resulting in a significant increase of the IMEP. The experimental results show that the maximum IMEP is improved from about 8 to 10 bar with an engine speed of 5500 r/ min. However, an excessive water injection will lead to a decrease of the IMEP. To evaluate the effect of water injection comprehensively, an optimization design of the water injection system and the combustion chamber geometry is required. Meanwhile, more experimental work should be carried out, especially for the practical operation conditions of an unmanned aerial vehicle.
