h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Compression ignition engines are theoretically able to burn even powdered coal dust!.
In practise the larger engines, especially in marine and power applications, burn the cheaper fuels. The economies of scale mean that quite sophisticated fuel treatement systems and exhaust gas emission control is possible and affordable.

Heavy Fuel oils are the residual oils from oil refining blended with the low quality distillates to achieve particular specifications. Most commonly they are described by their kinematic viscosity at 50 degree C.

Large engines burn heavy fuel oils, orimulsion and fuel water emulsions.

One of the most important features of burning the fuel is correct atomisation. As the fuel is injected it creates a dispersion of droplets.
If the droplets are too large (the viscosity is too high), they take too long to burn and can accumulate on the combustion chambers surfaces. They also do not mix efficiently with the air.
If the droplets are too small (the viscosity is too low) then a non-dispersive spray is formed which also does not mix efficiently with the air.

Viscosity is a critical parameter in obtaining the optimum spray pattern. For large diesel engines, turbines burning intermediate fuels or burners in steam raising plant, the viscosity is optimised by heating the fuel.

In burner applications this has been done by monitoring the temperature of the fuel and adjusting the heating as necessary. This is not efficient because fuel quality change mean tht the optimum temperature can vary. To track the optimum temperature it is necessary to take regular samples for laboratory analysis. Today, new instrumentation is overcoming this problem so that burners can, like engines, use feedback control to continuously adjust the fuel temperature. For engines, digital viscometers provide a measue of the fuel viscosity at the operating temperature. This information is used to control the fuel heaters. They also record a variety of different fuel quality parameters which are important in analysing engine performance.

Heavy fuel oils do not have a cetane rating. A guide to fuel quality is obtained from the density and viscosity and calculated as the "Ignition Index". Also of value is the specific energy which is calculated from the density, viscosity and ash content.