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Why is the actual capacity of automotive fuel tanks bigger than nominal capacity?

“It seems like the fuel tank is filled up more than the rated capacity!!!” “It’s never been like that!!!”
Driver may have all experienced something like these. Especially when filling up the car, drivers sometimes doubt if the right amount of fuel is filled for these reasons. So it does when the amount of fuel remained and filled exceeds the capacity of a fuel tank officially indicated by automakers. However if the difference is merely 5~10 liters, this is natural. It’s because it was originally designed to be larger than the nominal capacity of a fuel tank specified in the user’s guide.


Therefore if the phenomenon above happens, do not panic and check the actual difference from the nominal capacity.


1. Nominal capacity of a fuel tank (nominal capacity)
① “Nominal capacity” is designed for passenger cars to drive *about 600 km at the speed of 80~100 km/hr on the motorways.
Nominal capacity varies depending on car models and an engine displacement because it is designed considering fuel efficiency and the weight of car body.
* About 600 km driving if it is converted into a distance on the assumption that a driver can drive 5~6 hours a day at the speed of 100 km without physical fatigue (on a basis of fueling once a day).
② It was designed with spare capacity to allow a driver to drive to the next highway service area* (the average distance between service areas is about 50~60 km) and it is about 10% of the capacity of a fuel tank.​


2. Why is the actual capacity larger than the nominal capacity?
If the nominal capacity of a fuel tank is 65ℓ, the actual capacity is about 75ℓ. It’s because automobile manufacturers made the fuel tank by securing spare capacity corresponding to 10~15% of the nominal capacity1. The reason is as follows:


① It is intended to prevent VOC from being leaked in the event of volume expansion caused by an air temperature rise. If the fuel tank is filled up, the fuel is likely to overflow as the internal temperature rises to push up internal pressure.
② It is also intended to secure room for expansion to prevent the leakage of fuel when the car is parked on a slant after its fuel tank is filled up. This room is called “spare capacity for expansion.”


(Note) ¹ Maintain the reference amount of fueling LPG vehicles’ fuel tanks (85%)
LPG expands if its temperature in liquid state is raised. Therefore in case of filling the container with LPG, it is regulated to have the container’s temperature maintained below 40℃ to keep liquid LPG from exceeding 85% of container content (90% in case of a storage tank).

ACEA Standards
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