Personally, unless you can do multiple categories that will show the same product, I feel you are risking causing people to not find items once you go any further than Steam/Diesel/Electric.
I have no idea how the data base you use works, but if I were to use a simple table in Excel as an example, such a table could support multiple categorization by having columns for each category type, populating with a "1" for any line item where that category is applicable. Set up the Excel filters to only show entries with a "1" for any given category and you get a full subset of of items in that category.
I know the web site does not use Excel, but the sort of multiple categorization this method would allow would be useful, rather than a simple list of items if you do start narrowing it down too much once past Steam/Diesel/Electric.
Another option would be to do something similar to what computer or technology stores do (see Best Buy, Staples, TigerDirect, etc). Have a bunch of characteristics to the left that have check boxes next to options (scale/traditional, price ranges, etc). Each time a box is checked, the full list gets narrowed down (when it works right). Those being much larger operations, I don't know if that type of function can easily be implemented on your site.
After re-thinking my response I'm realizing I'm veering into the world of filtering with the last 3 paragraphs. I probably should have stopped after typing the first paragraph.
-Dave