I keep on noticing favorable comments about opposing piston engines, i.e., Junkers and Napier Deltics. If they are so good, then why aren't they used more widely? That is to say, these engines must have a problem or a shortfall. Can anybody say what the problem is? Also, what about their efficiencies?

The list of bad things might include:  they cost more to make, they don't fit as nicely in a car, the pistons "wear funny" compared to vertical ones, they can consume more oil, they're harder to properly lubricate.  

The thermo efficiency shouldn't be much affected by the orientation of the pistons (may be minor effects-globle valve). 

When I worked as a mechanic, I worked on both Fairbanks Morse OP engines and on a converted WWII minesweeper with three of the Deltic engines on board.

I'm sure in their day, these were powerhouse engines, but I found them dirty, overly complex and hard to maintain in top running compared to modern diesel engines.  Both engines suffered liner cracking around the ports, mostly due to poor operational procedures, such as extended times at low load or idle, improper prestart procedures, and lube oil problems.  

We had a competitor underbid us on a major repair of two Fairbanks OP engines, and their mechanics were not familiar with the correct timing procedures between the upper and lower cranks, needless to say a VERY expensive mistake.  We had the advantage of a former US Navy man who had experience with that engine, and knew the manuals were not very clear on all of the repair procedures.

When the Deltic engines were running good they were impressive, but a bit scary to stand next to while underway, lots of moving parts, funny noises, and oil slinging everywhere.

I think now days, when we look at all things we ask of a modern diesel engine, like high horsepower to weight and footprint ratios, low fuel consumption, low emission rates and high reliability, they older designs like complex OP engines, especially two stroke, just can't compare.  But in their day they sure did push the envelope.  I think those engines took real craftsmen and truly dedicated professional operators to assure good performance and reliability, something I don't see much of these days in the engine business.

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