During stratified-charge combustion operation of a direct-injection spark ignition engine, at the cylinder compression stroke, a tumble is generated which flows between a spark plug electrode and a piston crown surface toward an injector. The fixture includes a gun barrel mount including an annular enclosure with first and second axial ends, and a sample platform. The individual add-on pieces are optimally matched to one another and have, due to their designs, a low weight and a low requirement of finishing work.