I saw pictures of a prototypical diesel aircraft engine a few years ago
that was a 4-cylinder water-cooled boxer engine.  It was intended for the
European market, where avgas is hideously expensive.  It would run on jet
fuel.

The cylinders and heads were machined as one solid piece -- no chance of
head gasket leaks, but for overhauls each head and cylinder bank would
have to be replaced as a unit.

             _ _
  __ _ _   _| | |  | David M. Brodbeck (N8SRE)              Ypsilanti, MI
 / _` | | | | | |  +-----------------------------------------------------
| (_| | |_| | | | @ cyberspace.org
 \__, |\__,_|_|_| "BSD is for people who like UNIX.  Linux is for people
 |___/             who hate Microsoft." -- Brendan Howes