Special advantages of U-cylinder uniflow scavenged engines are that only a little amount of the fresh charge flows out through the exhaust port because of their one-way scavenging paths, and that the hydrocarbon concentration in the exhaust gas is considerably low. On these engines, however, we cannot expect, like other two-cycle engines, the perfect scavenging quality which four-cycle engines can normally achieve. In the case of U-cylinder uniflow scavenged engines the hydrocarbon concentration of the exhaust gas becomes naturally higher with increasing delivery ratio(SAE Paper 850181).The author has developed a U-cylinder uniflow engine in which the front of the fresh scavenging medium consists of air layer. The running performance of this engine was, however, not stable. The author therefore developed another engine in which one of the cylinders performed Schnuerle scavenging, and its through-flow mixture of scavenging and combustion gases were introduced into an adjacent where cylinder it burned again with fresh secondary air. It has been found on this engine that the hydrocarbon concentration of the exhaust gas was suitably low regardless of the delivery ratio.