I am very familiar with your particular problem. An exposed dirt or gravel crawlspace is prone to higher humidities, which is associated with mildew and mold. Along with the foul odors, an exposed crawlspace can host insects and rodents, cause mold allergies, and even permit radioactive radon gas to enter your home.
The best solution for this situation is to seal off the entire area by pumping in a concrete slab. This sounds difficult, if not impossible on an existing home, but the process is surprisingly simple and clean. Of course, if any ground water seepage problems exist, they should be addressed, in some cases a complete draintile and sump pump systems and in others foundation cracks sealed by injection. After that, install a vapour barrier over the exposed ground and pump in 3 - 4 inches of concrete. The pumping is done by what is called small line P-gravel trailer pumps, which use 3.5 inch flexible rubber lines to access the crawl via a vent window, side basement window, garage, or even heat registers in the floor above. The solid concrete floor will seal the area from water, smells and all the other nasties permanently and provide extra clean, dry storage. The method of putting in only a vapor barrier most often is
I own a company called "A Better Crawlspace" in the Chicago area and deal every day with situations like yours. Our website is www.crawlspaces.com.
Good luck, and please call for any further answers. Greg Mitchell