One of 3ders better articles, this engine is about 1/4 the weight of a normal metal engine. They use carbon fiber filled PEEK; it would be amazing to be able to 3D print a car engine. Even if you needed steel sleeves, and other metal parts.
http://www.3ders.org//articles/20151208-polimotor-2-implements-3d-printed-fuel-intake-runner-plastic-race-car-engine.html
How would you solve the layer height issue? Otherwise for the cylinder and piston you’d have to size it all the way through polish.
@Andrew_Hodel the cylinders are typically lined with sleeves. The first engine that they did was something like 150lbs (68kg) - and the top half of the cylinder chamber was sleeved with steel, as well as a cap in the head.
And threaded for the spark plug? The 3d printed part can hold the valves down on compression? Is it a sealed part to hold oil for the cam?
Unfortunately, a lot of the specifics of the engine are shrouded in secrecy.