Diesel oil (distillate oil) is the traditional fuel for diesel engines. The fuel does not need any fuel treatment, separators or heating prior to operation. This is still the most important fuel for power plants, especially for small generating units.

Heavy fuel oils (HFO) are residual refinery products that come in varying qualities depending on the refinery processes. HFO can be characterised as having quite high viscosity and high sulphur content.

Heavy residues are even heavier refinery products than HFO. They have very high viscosities and set high demands on engine technology and fuel system design.

Emulsified fuels are very heavy hydrocarbon fractions, either naturally occurring as bitumen or refinery products as heavy residues, having been treated with water in a homogenisation process. The fuels may be handled almost in the same way as HFO.

Crude oil qualities come according to the oil wells and generally, after the initial treatment at the oil fields, the oils can be used as fuels in Wartsila diesel engines.

Vegetable based bio oils are agricultural products, such as olive oil, palm oil, soy bean oil that may be used as such in diesel engines.

Biodiesel (B100), a transesterified bio oil derived from vegetable oil or animal fat, that in many aspects is similar in quality to ordinary diesel oil that it may be used for diesel engines wherever diesel oil is used.