By Dan Shapley
Today's generation of lawn mowers are far different from the smoke-coughing, shoulder-straining rip cord machines of old. Little engines like those, which the Environmental Protection Agency recently vowed to improve further, are among the biggest producers of pollution in the suburban environment. But looking back on the bad old days, things are much improved. Pollution controls, machinery technology and other improvements have made them lighter, quieter, safer - and cleaner.