Originally Posted By: A_Harman
Opposed piston engines are 2-strokes, so they have higher power density than 4-strokes. Historically, OP diesels were very fuel-efficient, achieving thermal efficiencies of 40% back in the 1940's. Combined crankshaft strokes are very long, giving a lot of expansion to the combustion gases. The biggest problem to the OP is that since it doesn't have a cylinder head, fuel must be injected from the cylinder walls, which leads to problems with spray impingement on the piston rims and cylinder walls, and poor distribution of fuel in the cylinder if only one injector is used. Two or more injectors could be used, but that adds cost.
But is this an advantage over a two stroke with a conventional head?
Can't I just build a single piston in cylinder two stroke and not need to carry all the extra reciprocating mass of a second crank (worst case) and rods, pistons, etc? I could use a turbo to blow the cylinder full of air and displace any exhaust that doesn't escape on its own via its own heat and expansion.
Seems like unnecessary and wasteful complication compared to using what would be wasted exhaust energy to provide forced induction.