During his first Diesel engine tests, Diesel also used illuminating gas as fuel, and managed to build functional designs, both with and without pilot injection. According to Diesel, neither a coal dust producing industry existed, nor was fine, high quality coal dust commercially available in the late 1890s. This is the reason why the Diesel engine was never designed or planned as a coal-dust engine. Only in December 1899, Diesel tested a coal-dust prototype, which used external mixture formation and liquid fuel pilot injection. This engine proved to be functional, but suffered from piston ring failure after only very few minutes due to coal dust deposition.