upper port 29 is uncovered and as the lower port 32 is covered with flow being directed from the cylinder 12 into the crankcase 20 crankcase pressure is nearing its maximum value at this time and the cylinder blowdown will tend to increase this value either the lower port should be covered before the cylinder pressure falls below that in the crankcase or the check valve 33 should be inserted in the bypass port which will prevent flow out of the crankcase both intake and exhaust ports 22 24 should be timed to open at about the same crank angle and at a crank angle for which the crankcase pressure exceeds cylinder pressure continued piston motion then permits cylinder scavenging in the normal manner piston motion during compression depending upon operating conditions may cause the crankcase pressure to fall below that in the cylinder if so reverse flow through both transfer and bypass passages 23 28 will occur in the conventionally scavenged engine gas will be forced from the cylinder into the exhaust system this can result in significant cooling of the exhaust gas and reduce the effectiveness of the catalytic converter especially during a cold start an important function of the bypass flow passage 28 is to direct this flow of cold gas into the crankcase 14 after closure.