Whilst there is much current discussion about the use of Liquid Natural Gas (LNG) as a fuel for ships, liquid fossil fuel oils, based on either distillate or residue from petroleum refining appear set to remain the predominate fuel for the foreseeable future.
Residual fuel is a complex hydrocarbon blend of residue and cutter stock, mixed to achieve the desired viscosity. Marine diesel oils are distillates containing some residue, and although clear and bright, the higher quality marine fuels are somewhat misnamed as simply 'distillates'. This is because, with refiners striving to extract more value from a barrel of crude oil, much gas oil is now produced from various downstream cracking processes rather than just distillation. All these fuels result in emissions to air that affect the environment when burnt in a diesel engine or boiler.
In a typical marine diesel engine, fuel is directly injected into the cylinders and in a period known as the ignition delay undergoes both physical and chemical changes before combustion starts. The fuel forms an atomised spray, which has to intimately mix with the charge air in the cylinder and at the desired point within the engine cycle be ready to oxidise in order for spontaneous ignition. This latter condition is determined not only by engine related parameters such as the timing of injection and conditions of temperature and pressure within the cylinder, but also the composition of the fuel. Residual fuels in particular will contain complex heavyweight hydrocarbon molecules known as asphaltenes that do not readily 'crack' into a state ready for combustion. The Cetane Number of a distillate fuel and the Calculated Carbon Aromaticity Index (CCAI) of a residual fuel indicate the propensity for ignition delay. Both of these parameters have limit values in ISO 8217.
As combustion commences there is a rapid rise in the pressure and temperature followed by a rapid fall and then a further release of heat within the cylinder until the exhaust valve opens.
Above: The stages of fuel combustion in a diesel engine