In recent years, the internal combustion engine has been the subject of debate mainly concerning its environmental impact. Despite all the discussion it becomes clear day by day that combustion engines will continue to occupy their dominant role over the following decades, especially in the mid- and large-size power spectrum ranges and retain a large share of the market in the smaller-size segment of their application. In this context, in the present paper, a novel engine kinematic mechanism is introduced, which converts rotary to reciprocating motion, and aims to become a potential replacement for the traditional crankshaft mechanism of piston engines. Following a description of the fundamental principles of the new design, we detail the main problems with the application of the new design in the first prototype SI engine and the actions and improvements implemented to overcome them. The actual measurement data from basic engine performance parameters are provided and evaluated, leading to conclusions and decisions for further action which should be implemented in the next improvement steps. Overall, the new SI engine, implementing the novel kinematic mechanism, seems to be quite promising especially in hybrid automotive applications, a fact that encourages the implementation of further improvement plans.