Engine efficiency

Engine efficiency

The efficiency of an engine is the ratio of the engine’s power output to the energy in the fuel fed into the engine. Hence high engine efficiency is fundamental to low fuel consumption and to savings in costs and emissions. Sulphur and carbon dioxide emissions, for example, are directly proportional to fuel consumption and to the content of carbon and sulphur in the fuel. Large diesel and gas engines typically run at very high efficiencies and for this reason they also display the lowest specific SO2 and CO2 emission levels when comparing different engine types running on the same fuel quality.


  Development of the shaft efficiency of Wärtsilä  

The shaft efficiency of Wärtsilä diesel and gas engines is in the range of 42 – 50% depending on the engine type. Unlike gas turbines, reciprocating internal combustion engines achieve high efficiency over a broad load range; moreover, their high efficiency and power output remain virtually unchanged over a wide range of intake air temperatures. This feature, unique to reciprocating engines, is important with respect to both marine vessels and onshore power plants. In ships, the engine’s load range varies between 30% and 85% for most of the time, while the intake air temperature varies considerably due to the time of year and geographical location of the vessel. Where onshore power plants with reciprocating engines are concerned, the need to overrate the engine due to a higher intake air temperature is minimal. Again, compared to a large gas turbine, a multi-engine installation offers the advantage of being able to run at optimal efficiency simply by choosing the right number of engines for the required load.

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.

*This page has been reviewed by KPMG as a part of its assurance of Wärtsilä's sustainability reporting for 2008. Please refer to Assurance Report for a full description of the conclusions and the scope and nature of assurance offered.