This is what Wartsila says about efficiency....
The total efficiency of large power plants can be raised by converting part of the waste heat from the exhaust gases into electricity. This requires a steam boiler and steam turbine connected in what is called a “combined cycle”, which can raise the plant’s electrical efficiency to approximately 55%. Another method is to use some of the waste heat from the exhaust gases to produce heat in the form of steam or hot water using a waste gas boiler. This system, called “combined heat and power” (CHP), can raise the plant’s total efficiency to 75 – 90%.
Over the past three decades, the efficiency of Wärtsilä’s most energy-efficient engines has risen from roughly 41% to about 50%. This has been achieved through a variety of methods such as increasing the cylinder pressure, raising the compression ratio, reducing the fuel injection period, optimizing the valve timing, and improving the combustion process. The pace of improvement in efficiency has fallen off somewhat in recent years owing to the increasing restrictions placed on nitrogen oxide emissions. As to engine performance values, the most important priorities in Wärtsilä’s R&D programme include raising efficiency and reducing emissions. The same trend will continue in the years ahead; to maintain its competitive edge, Wärtsilä will need to raise the efficiency of its engines further despite the ever more stringent nitrogen oxide limits.
Raising engine efficiency and lowering emission levels can significantly reduce environmental load because large diesel and gas engines have a long lifecycle, typically 25 – 50 years.