sentences:  The blower also assists in scavenging spent combustion gasses at the end of the power stroke.All Series 71 engines use uniflow scavenging, in which a gear-driven Roots blower mounted to the exterior of the engine provides intake air through cored passages in the engine block and ports in the cylinder walls at slightly greater than atmospheric pressure.The engine exhausts through pushrod-operated poppet valves in the cylinder head(s), with either two or four valves per cylinder.