I think the term crossfow/ no cross flow is a bit confusing to some. People tend to think that the air in an engine flows smoothly from the intake to the exhaust, and that having the air seeming do a 180 and travel back in the same direction would be bad for efficiency, when the air isnt really flowing at all. When what really is happening is that the air is being sucked into a cylinder, then the vlaves close, end of flow. their is no air flow once the valve is shut, theres no where for it to go. then the pistons compresses the air/fuel mix, the spark plug ignites it causing the mix to explode, which increases the pressure in the closed cylinder, forcing the piston down. Once the power stroke is done, the air, which is still compressed needs to escape, which it does when the exhaust valve opens, not so much because the piston is forcing it out in the exhaust cycle, but because pressurised gas will move to a lower pressure area to equalise pressure with the surrounding enviroment. So the air doesnt really flow, it get sealed in a container,compressed and exploded, and allowed to vent to a lower pressure zone through the exhaust. So I guess what i'm saing is that air flows through an engine, like water flows down a river - if the river was a line of buckets, and you emptied the water from one bucket to the next, and then the next etc... (Sure theres valve overlap, but i'm not going into that, maybe soemone else can)