How Is Fuel Burnt Inside A Marine Engine?

Efficient combustion of fuel oil inside the engine cylinder is an important aspect for the overall engine efficiency. The process of fuel combustion depends on many sub-divided processes, which together contribute to the engine efficiency.

A good combustion of fuel oil can easily be identified from the clear exhaust of the engine, the total output power and the exhaust temperature. Any variation in these parameters indicates that there is some problem in the fuel combustion.

General indications of improper combustion are uneven running of the engine, variation in engine speed and a knocking sound from cylinders or fuel system.

In this article, we will describe the various sub-divided processes and how they collectively contribute to the whole process of a fuel combustion.

To easily understand the fuel combustion process, it is divided into four main parts. These four aspects not only play an integral role in the combustion process but also represent four different processes that contribute equally to the main process.

The main parts of fuel combustion process are :

• Correct Viscosity
• Atomization
• Penetration
• Turbulence.

Controlling the viscosity of the fuel oil is the first step towards efficient combustion. Viscosity,or resistance to flow, needs to be kept in proper limits to ensure correct atomization at the fuel injector. Viscosity can be reduced by increasing the temperature of the fuel oil. Exact viscosity can be adjusted using viscosity regulator and fuel oil heaters. Fuel oil viscosity is ideally adjusted at 15 cst at 50 degree Celsius before atomization.

Atomization is a process of breaking down of the fuel into minute droplets with the help of fuel injector. Fuel oil is pressurised through tiny holes in the injector, which splits the fuel into small droplets . These atomized droplets have a higher surface to mass ratio which helps in good heat transfer from hot compressed air to the oil droplets, resulting in rapid evaporation.

The size of the droplets depends on the size of the holes and the pressure difference between fuel pump discharge and compressed air inside the cylinder. This means that the sizes of droplets will vary at different areas inside the cylinder.

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It can be referred as the distance the atomized particles travel from the injector into the combustion space of the cylinder, before the actual mixing, evaporation and ignition takes place.

Penetration depends on the size of the atomized particles, their velocities and condition inside the combustion chamber.

Special care is taken in order to spread the particles evenly inside the combustion space without touching the internal surfaces, before the actual combustion occurs. The spray pattern is decided by the number, size and position of the atomizer holes.

Turbulence is the movement given to the mixture of compressed air and fuel oil droplets inside the cylinder. This is particularly done to increase the mixing capability of fuel and air and also to facilitate rapid combustion. Turbulence is a kind of swirl that is provided to the air on its entry at the scavenge ports. Fuel spray pattern along with the turbulence provide an ideal ground for efficient combustion. Swirl can be generated with the help of design and shape of the piston crown. Turbulence is extremely important for rapid combustion of heavy fuel in diesel engines.

Diesel Engines, 3rd edition by A.J Wharton

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