What Does TBN Mean And Why Does It Matter For Ship Propulsion?
You might recall from high school chemistry lessons that ‘an acid plus a base equals a salt plus water’ a well known saying to many thousands of students; the TBN of a marine lubricant actually means Total Base Number.
A higher total base number means that a crank case or marine engine oil is more alkaline in nature due to the additives package with which the base oil is blended. As an aside, this high TBN additive package would likely damage the exhaust catalyst in a modern car or van…
Why Is This A Bit Unique To Shipping?
In short, despite the proliferation of press releases about use of biofuels and liquified natural gas in dual fuel trunk piston engines, many ship engines continue to run on HFO or heavy fuel oil.
HFO is known also as residual fuel, a helpful name pointer to the nature of this heavy black treacle-like fuel that is left behind after cracking by distillation has taken off fractions like DERV, petrol, LPG, kerosene and so on.
Residual fuels are cheap, that is the bottom line of why they are still the fuel of choice for the maritime industry with cargo vessels, bulk carriers, and tankers.
In the case of military naval vessels this seems to be less common with the use of gas turbine engines or high/medium speed diesel engine gensets from brands like MTU, Scania, or Wartsila using lighter distillates, marine diesel, or kerosene derived jet fuel.
The Acid Problem
Heavy fuel oil often contains a fairly high amount of sulphur (or sulfur) which, when combusted in the cylinders of a ship’s engine, leads to production of sulphuric acid, this damaging by-product of combustion needs a ‘base’ (another name for an alkali) to neutralise this acid.
This is the simple reason why trunk piston engine lubricating oil and engine oil for crosshead 2 stroke and 4 stroke engines on ships will often have a relatively high TBN.
It plays the role of an alkaline neutraliser in engines running on HFO fuel as well as acting as a cylinder lubricant to stop ring sticking on the liners.
Because of the huge size and specifically the stroke length of the cylinders, marine engine oils for these large powerplants from Bergen, MaK, MAN Energy Solutions, and Daihatsu, the cylinder oil is generally fairly viscous so that it stays in place on cylinders that can be 2 to 3 metres in height.
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