The rapidly burning fuel in internal combustion engine produces pressures to push the pistons

Medium speed generators use a better grade diesel than those used in the main engine

The fuel is made suitable for burning by a process of settling, filtration, purification and heating

Combustion of Fuel Oil

One of the most easily available energy sources to power engines on ships is the fuel oil. When fuel oil is burnt in air, heat is released. This heat can be used to drive internal combustion engines or to heat up water in boilers to form steam. The process of burning fuel in air is called combustion. The release of this heat energy can be slow or can be extremely rapid.

Fuel oil at normal room temperature is viscous and not easily ignited. The fuel oil, also called Bunker C oil, must be heated to around 98 degree Celsius or more before it can be used for combustion. The Bunker C oil also contains solid particles that can choke up the fine nozzles of the burner equipment. These are removed by passing through a series of removal methods like settling, filtering and finally purification.

On ships, when fuel oil is sprayed as a fine mist or atomized in the boiler burners, it is able to burn at a relatively slow rate as a burning flame.

When fuel is sprayed into the cylinders of diesel engines, the fuel burn in such a rapid rate that explosions occur. Fortunately, these explosions are internally within the space of the cylinder liner, piston crown and the cylinder head, protected from persons outside. These engines are called internal combustion engines.

However fast or slow the combustion rate, it is still a chemical reaction between carbon, hydrogen, sulphur and oxygen that releases heat. Fuel oil is basically a hydrocarbon that contains carbon and hydrogen atoms. The reaction when fuel oil is burnt in air can be written as follows:

C + O2 = CO2
2CO + O2 = 2CO
2H2 + O2 = 2H2O

Fuel oil tends to contain sulphur as an impurity. The reaction between sulphur and oxygen is shown below. The water formed in the above reaction also reacts with the sulphur products as follows:

S + O2 = SO2
2S + 3O2 = 2SO3
SO3 + H2O = H2SO4
SO2 + H2O = H2SO3

The by-products of the combustion of sulphur with air are sulphuric acid and sulphurous acid. These are corrosive to the equipment and the surfaces inside the engine, the boiler furnace, or the exhaust trunking. However, sulphur corrosion, also called low temperature corrosion, can be minimized by maintaining the exhaust temperature above 120 degree Celsius, i.e. above the dew point when the water in the exhaust gas is able to condense back to liquid.

Air consists of 77% Nitrogen and 23% Oxygen by mass. For a particular design of combustion equipment, the theoretical amount of oxygen multiplied by 100/23 will give the theoretical amount of air required for combustion.

In boilers, besides maintaining the burner atomizers, air diffuser vanes, fuel temperature, forced draft fan, fuel pump and other hardware in good condition, there are some adjustments on the fuel-air ratio that can produce a good flame for the most efficient operation.

By measuring the percentage of Oxygen or Carbon Dioxide in the exhaust gas, we should be able to tell whether the combustion is complete or not.

The lower the Oxygen content in the exhaust gas, the better the combustion is. It means that the Oxygen has been fully utilized for burning. It also means that the fuel-air ratio is set properly. Too much excess air is not good because the heat generated is being lost through the exhaust trunking instead of being made used of in heating the water to produce steam. Too little Oxygen content may also give rise to unburnt fuel. That too will cause problems of soot formation and inefficiency in the boiler.

Boilers are able to achieve good combustions. Oxygen content lower than 5% can be achieved. These exhaust gases with low oxygen content can be used for inerting oil tanks in crude oil tankers.

Internal combustion engines tend to have a lot of excess air because of the rapid process of exhaust stroke, air intake stroke, compression, and mixing of the combustible fuel-air mixture. Furthermore, the combustion is meant to provide the gas expansion power to drive the pistons, and not so much to produce heat.