Why is the windshield washer fluid tank so strange?

I think I might have posted on this before but cannot find it now.

The Prius washer fluid tank is strange. It has a cylindrical region that extends almost an inch below the top of the tank. So it isn't possible, or at least easy, to completely fill the tank. Pour in the top and once the fluid reaches the bottom of the cylinder it only fills the cylinder, leaving a huge void at the top of the tank around the cylinder. I keep forgetting about this, correctly estimate the volume needed to fill the whole tank and it overflows slightly because the whole tank isn't used.

Why is the tank like that??? All I can think of is that it is to prevent sloshing from blowing the cap off when it is completely full.

Can anybody think of a reason a small hole couldn't be drilled through that cylindrical area, so that the air could drain out of (most) of that void, allowing the tank to fill (almost) completely.

 

On mine when it is filled to the very top the water line on the side of the tank is about 1 to 1.5 cm lower than the water level in the inlet. It is a pretty large empty volume on top since it goes all the way to the outer edge of the tank, or nearly so.

Expansion region to handle freezing makes sense. I never really think about that where I live, as it only very rarely gets down to freezing, and even then, just for a few hours at night. Freezing this fluid would only be a problem if the tank was completely full, which it very rarely is. In fact, the stores in Los Angeles cannot sell 1X washer fluid with methanol in them (AQMD rule - they say that all that methanol in washer fluid was a significant source of VOC, contributing to smog). The bottles are marked something like "protects from freezing down to 32F", which is so stupid it hurts, but there it is. I put it right up there with "gluten free water" and the like.

 

I make up washer fluid a gallon at a time. Pour a little of 1 gallon of distilled water into the windshield fluid tank, add 1/4 cup of washer concentrate into the gallon water bottle, mix, add the rest to the car. Except it never all fits in the Prius because of the tank design. Usually I put the 1/8th or so of a gallon that is left in the wife's car. Not this time though, as her Accord just came back from a timing belt job and they had filled the windshield washer tank right to the top. (Also left a rag in the engine compartment, so good thing I had a reason to go look before it got picked up by a belt or something.)

So this time, it had to go into the Prius tank, into the (normally) unreachable air section at the top. A short piece of plastic tubing that was on the shelf was bent hard until it would briefly hold a U shape, then it was dunked into the tank and poked up into the air pocket. Blew air through the tube (by mouth, bad idea, tube tasted terrible) to be sure it was clear. Then poured the rest of the fluid in the top. This time it went in because the air could exit the trapped space through the tube and could not compress. There was still about 1/3 of an inch of air in the trapped space when done. A more rigid J shaped tube would have been better, but I didn't have one.

Anyway, problem solved, and unless there is a sudden and astronomically unlikely freeze in Southern California in the next few days the car should be fine.