I am finding it difficult to select a topic for my masters thesis (my advisor is unwilling to offer suggestions on the matter). I intend to do thesis in the field of internal combustion engines. To get some good ideas about finding a good topic I want to study some recent PhD theses in internal combustion engines but I don't know where to find them. Also I would like to know about modern research trends in that field.
In general, doctoral theses do not receive much recognition outside of a university, let alone on an international scale. The reason for this is that nowadays the thesis is nowhere near as widely circulated as the papers that can be extracted from the thesis.
Moreover, if you want to find out about good thesis topics, look up what's going on in the scientific literature of the area you're interested in—in this case, internal combustion engines. Find the most highly-respected journals, and see if they've published recent review articles in different areas related to your topic. That might give you a sense of where recent work has been, and provide you with a good body of reading material to work from.
Many disciplines have prizes and awards for the best thesis in a given year (or longer period). For instance, in our field there is a well-recognized one called Householder award. There are many more if you restrict to the national level, often funded by privates or industries.
If you look for past winners, you can be fairly sure that they are outstanding theses. (After that, getting your hands on the actual document might be more difficult, though).
Unfortunately I am not familiar enough with Engineering to suggest you a good one in your topic; you may try to add "thesis award" to your google searches, but sometimes they are tricky to find without knowing the exact names.
My answer complements the other answers.
While writing my dissertation proposal, I felt the need to look at some other published dissertations in my area (~HCI/privacy). I realized that one of my committee members, who dissertated very recently (2007) has an incredibly relevant dissertation in my area and also won the best dissertation award from her alma mater.
So, that is one strategy. In general, look at the best works in your field and find out if any happens to be the primary work of a PhD student. Chances are, that work will be incorporated into his/her dissertation and if you look at their CV's, often, you will find that they won a "Best Dissertation" award or something similar to that.