Here's a nicely integrated genset design:
http://www.greencarcongress.com/2008/09/modeling-a-free.html#more
"The free-piston engine offers a number of benefits, including optimization of the combustion process through variable compression ratios, making multifuel operation possible; reducing frictional losses because of fewer moving parts; and allowing homogeneous charge compression ignition (HCCI) to be more easily achieved. A linear alternator can use the linear piston force without requiring additional mechanical components necessary in a rotary configuration.
http://www.greencarcongress.com/2008/09/modeling-a-free.html#more
"The free-piston engine offers a number of benefits, including optimization of the combustion process through variable compression ratios, making multifuel operation possible; reducing frictional losses because of fewer moving parts; and allowing homogeneous charge compression ignition (HCCI) to be more easily achieved. A linear alternator can use the linear piston force without requiring additional mechanical components necessary in a rotary configuration.