I made this a while back in August 2010. It's the first stud-less framed Lego vacuum engine I know of. It has a chain driven valve gear, 2 pistons and is single acting. I will probably post if there is enough interest in it. I though it not too interesting because of its weak performance, but hey, at least it is not ridiculously fast that you can't see it working!
I did not take any pictures of it at the time, because I was using a video camera, so the pictures below are stills from the video.
NOTE: These instructions do not include the valve train, but they include the valve (shown in RED). This is due to the fact that Lego Digital Desinger doesn't include the parts needed for it. I will do my best to explain how to build the valvegear, and installation of the chain.
Video in action: