3D Printing opens the way for developing exciting new engine designs. At the University of Canterbury we have been working on both fully polymer and part polymer Stirling engines. Also, an ethanol fueled titanium internal combustion engine is being developed.
Many different part polymer engines have been made. See here to see a selection of early designs.
We are currently working on STEMgines which are kits of non-3D printed parts and the ###.stl files for printing the rest. The intention is for these to be used by enthusiasts and schools for STEM education. With these kits the students can make a fully functioning engine that runs on just hot water.
A fully 3D printed polymer working Stirling engine has been made and could be the first fully 3D printed thermal engine to be made that did not require any post machining. View it here.