Total Base Number of Petrol engine

Sai,

The problem can be that the engine is running to hot (maesering soot). Maybe the viscosity is dropped because of leaking fuel in the engine (Viscosity, flashpoit).
Decreasing TBN means also increasing TAN.
De you also have other results (Flashpoint, viscosity, soot, TAN, water)?

TAN is 2.6
flash point is not decreased much, viscosity also same, no presence of water.
is their any thumb rule for TBN depletion in petrol engine

A drop of 8 TBN oil to 3 over 5000 km is pretty steep.
1. Pls Check if petrol adulterated with kerosene, diesel, ethanol etc..
2. Was oil topped up frequently or not at all.
3. What is the grade/brand of engine oil
4. Check source and send for analysis Engine Oil also check for adulteration of the engine oil.

Hussam

We had trial of engine oil for the prolonged drain interval in diesel engines. One of engines had too mach of oil consumption. We have noticed that TBN of motor oil in this engine drops much faster than in other engines which were normal (they had the small oil consumption). We have solved that excessive blow-by of gases from combustion prolongs oxidation of oil and fast dropping TBN. May be this experience will help you.

What oil are you running? What is the capacity? Not all oils are equal, maybe 5000km is too long.

Which method is being used for analysis.Is it by ASTM D 2896B or 4739....

Hi Sai,
Have you changed the oil since the TBN 3 analysis was completed?
If not I think change the oil and check if the smoking decreases, if you have oil changed did the smoking stop? TBN reduction appears significant because something caused the engine to smoke & reduced the TBN.
Is the air filtration system maintained correctly and in good condition? as dirt ingress could cause this,
Regards
Rob S

the engine may be very much old and TBN may be done on ASTM4739