Fuel oil is a hydrocarbon consisting of hydrogen and carbon, together with other elements most of which are unwanted.
Hydrogen has a higher calorific value than carbon, therefore, more heat may be obtained from fuels containing higher Hydrogen/Carbon ratios.
The lower specific gravity of hydrogen than carbon allows a rough rule of thumb to be; the higher the Specific Gravity, the lower the Calorific Value (and quality) of the fuel. The presence of impurities clouds the issue slightly
For efficient combustion an ignition source and sufficient oxygen need be present to completely oxidise the Hydrogen to water vapour and the carbon to carbon-dioxide.
The combustion is required to occur in a short period of time in an internal combustion engine, there are five essential requirements to ensure this;
Correct Air/fuel ratio
There must be sufficient oxygen to burn not only the hydrogen and oxygen present but also any other combustibles, such as sulphur. To be effective and efficient all the fuel must be burnt in the cylinder i.e. all the hydrogen must be burnt to water and all the carbon must be burnt to carbon dioxide. As the time for combustion is short excess air must be supplied to increase the possibility of the fuel being in close proximity to the oxygen molecules. The correct maintenance of the scavenge system including turbocharger suction filters is therefore essential.
To ensure that the fuel breaks down into its constituent elements as quickly as possible it is atomised, which means it is injected into the cylinder under pressure through a small orifice (high surface area/volume ratio allowing rapid oxidation ).
Atomised fuel made up of fine droplets does not penetrate well into the cylinder combustion space, mixing with the air is promoted by giving the a swirling motion.
As the fuel burns it creates a pressure wave which acts against the piston. If the injection is too late, the piston is travelling down the liner. The pressure wave created by ignition moves rapidly down to meet the piston causes excessive shock loading on the top of the crown (this is the characteristic ‘Diesel knock’ of engines when started from cold).Less power is derived as the correct pressure does not act on the piston during the early stages of the stroke.
If the injection is too early then very high temperatures and high peak pressures can be generated caused by the rapid combustion period occurring when the space available is very small. This can lead to increased engine efficiency but also to overloading of the bearings, particularly the top end bearings.
The diesel engine is a compression ignition engine , this means that the ignition of the fuel is reliant on the temperatures generated by the compression of the combustion air.
The compression ratio is set at the design stage to give the correct temperature. However, loss of compression, say by a leaky exhaust valve or piston rings can lead to a late timing of ignition. A similar effect can occur if the cylinder parts are not kept at the correct temperature.