to fig3 c as a pair of opposed pistons move through their respective tc locations their adjacent end surfaces form a combustion chamber 202 having a shape that operatively interacts with swirl and squish flows of pressurized charge air to generate one or more tumble flows of air in the combustion chamber preferably the combustion chamber 202 has an elongated bilaterally symmetrical shape referenced to an injector axis 203 and a pair of injector ports 205 located on the injector axis 203 opposing spray patterns of fuel 208 209 injected through injection ports in the cylinder 50 into the combustion chamber 202 via the injectors 68 and 70 which are opposed diametrically along the axis 203 the fuel spray patterns 208 209 travel in opposing directions along the axis 203 and meet in the central portion 211 of the combustion chamber where they mix with a largely spherical flow field of swirling charge air the tumble flows result from interactions of swirl and squish as described in u s patent application ser nos 13 066 589 13 843 686 and 14 117 831 the tumble flows circulate around the injector axis 203 and act jointly with the swirl in the combustion chamber central portion 211 to create a spherical flow field as injection continues the swirling mixture of air and fuel is increasingly compressed in the combustion chamber when the mixture reaches an ignition temperature fuel ignites in the combustion chamber in some instances both fuel spray patterns 208 209 are constituted of low reactivity fuel in other instances one of the spray patterns is constituted of low.