fig2 which center lines are coplanar with the center line 8 in other words the center lines of the cylindrical recesses forming the combustion space 10 and the center lines of the recesses forming the combustion space 11 are located in the same plane with the center line 8 which is the plane of the drawing in fig2 this plane is at right angles to the plane in which the center lines of the cylindrical recesses forming the combustion spaces 12 and 13 are located in which plane is also located the center line 8 of the shaft 7 the combustion spaces 10 to 13 are thus arranged in rotation symmetrical manner around the center line 8 see also fig6 and these combustion spaces may have identical shapes the plane 14 fig2 constitutes a plane of symmetry for the combustion spaces the two cylindrical recesses forming together the combustion space 10 have wet cylinder linings 15 and 16 which are coaxial with the center lines of the respective cylindrical recesses of the combustion space 10 these center lines which as stated above intersect one another in the plane 14 are at an angle of 140 u00b0 to one another opening towards.