Lube TBN - Depletion Rate
Cylinder-oil TBN depletion rate based on fuel sulphur content and feed rate - the basis for selecting 40/70/100/140 BN grades.
Sulphur in fuel
0.875g S/kWh
Required BN
3.8mg KOH/g
BN grade
40 BN (LSMGO / 0.10 % S)
Formula, assumptions, and limits
$$BN_{req} = \frac{m_S \cdot 2 \cdot M_{KOH}}{M_S \cdot \dot m_{oil}}$$
Stoichiometry: 1 mol S → 2 mol H₂SO₄ → neutralised by 2 mol KOH (56 g/mol). Actual BN selection also accounts for margin and acid condensation efficiency (~30 % for 2-stroke).
Symbol legend
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
| $m_S$ | Sulphur mass per kWh | g/kWh | fuel analysis × SFC |
| $\dot m_{oil}$ | Cylinder-oil feed rate | g/kWh | engine setting |
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