How efficient are fossil fuels when burned?

Answer 1

Typical thermal efficiency for electrical generators in the industry is around 33% for coal and oil-fired plants, and up to 50% for combined-cycle gas-fired plants.

The Otto engine (internal combustion four stroke engine often found in motor vehicles) has a thermal efficiency (on the test bed) of about 25 - 40% depending on the compression ratio- this can be improved somewhat by turbocharging. A Diesel engine is less efficient at the same compression as an Otto engine but the increased compression offered by diesel engines make them 30 - 35% more thermally efficient than (not turbocharged) Otto engines.

These vehicle engine efficiencies are drastically reduced when in practical situations of varying loads (i.e. driving on the roads).