The four-stroke spark-ignition (SI) internal combustion engine has good fuel economy and high power/weight ratio, making it very suitable for small aircraft. Normally, four-stroke aviation SI engines are fuelled with gasoline. Using kerosene can improve the system safety; However, a four-stroke SI engine fuelled with kerosene suffers from a small indicated mean effective pressure (IMEP) because of the knock limit of kerosene. In this study, water injection is investigated as a method of extending the knock limitation and improving IMEP of a four-stroke SI engine fuelled with kerosene. First, a Rotax 914 engine is retrofitted. Two port-fuel-injection systems supplied with kerosene and water are developed. Then, the combustion characteristics with water injection are studied. The effects of water injection on the in-cylinder pressure and heat-release rate are analysed. Additionally, the extent of knock suppression due to water injection under various engine speeds is evaluated. The results indicate that the knock limit of the four-stroke SI engine with kerosene is extended significantly via water injection. The measured IMEP is improved by 25-28% under different engine speeds. Thus, the requirement for ordinary cruise operation is satisfied. Furthermore, the security is enhanced with water injection.