Re: looper? cross flow?
The terms are used to describe the way the engine's exhaust / intake charge is cycled. Both are two stroke methods, but the cross flow is a bit more "conventional" at least to me.
In the cross flow design, the intake charge comes in on one side of the cylinder, and runs across the top of the piston, pushing the exhaust out. It "scavenges" the cylinder by it's "across the top" motion. Cross flow engines will most often (or always) have a domed piston with an abrupt drop off on one side of the dome.
Loopers, use a loop charge in the intake / exhaust exchange. It's "looped" because the intake charge comes from several openings in the skirt of the piston, about 90 degrees apart from each other in three locations. The intake charge loops up to the head and then down to the exhaust port. The exhaust port is on the last of the 90 degree areas. Loop charged engines usually have a flat piston top and are the current standard in large displacement 2 cycle engines.
Both have strengths and weaknesses in performance, weight, and fuel efficiency.
Here's a great overview of the two types, with illustrations.
http://www.maxrules.com/fixtheory2.html
Best,
JamesK
