There are many small business units in and around the country which have a pair of 16 cylinder (Ship engines) inside them, they are all heavily soundproofed and run on natural gas, these things are dotted around at intermediate points between main subs on the grid, they have one running at all times as a trickle into the grid, when the demand increases they can kick in the second generator to give some capacity. They don't tell you about these computer controlled, extremely efficient things which have the "Perfect Burn" 17.5:1 air/gas... They only seem to paint the grim picture, it's as though they're trying to send us back to the 70's.
I don't doubt that they're efficient compared to car engines but a piston engine will never get close to a fossil fuel fired steam turbine or gas turbine power station due to the amount of waste heat that is a necessary by-product of the process. Unless you use the heat in a CHP type project you have something that's useful as a standby due to the quick startup times, but not something that can compete on cost or "green-ness" with the larger scale generating stations.
Even domestic and commercial CHP systems installed in buildings where the waste heat is used are of only marginally greater efficiency compared to large scale electricity generation, which means that a small increment in the efficiency of the latter, or the addition of some renewable generation to the grid, and such systems become environmentally unfriendly.
When I was at school I did a week's work experience at a film studios. I couldn't help but notice that every evening the engineers would remove everything from the shelves above their benches and place them in the floor.
I came in one morning to find my cup had dropped off the shelf and smashed. One of the guys took be down into the basement where there were 2 rather great ship engines thrumming away. Apparently they were installed to provide a suitable power supply for the lighting in the early days and never got replaced by grid power. The vibrations they sent through the building were to blame for the demise of my cup.

Kevin