The free piston also eliminates the leverage concerns with rods and pistons. The calculated efficiency in the EPA docs from 5 years ago was 58%.
http://www.sunpower.com/lib/sitefile...ns/Doc0083.pdf
http://www.freepistonpower.com/fp3.aspx
Free piston engines can also be configured to be linear alternators, which if they ever get to that range of efficiency could be the combination for a battery electric car with a free piston engine replenishing the battery or driving the wheels directly, with a regenerative hydraulic braking system that worked on the non driven axle.
That would create an existing platform that could easily be transformed into a pure electric, hydraulically regenerated vehicle when battery technology gets to the point where range and cost-life expectancy, and safety issues are resolved.
regards
Mech