Some forecourt pumps use ambient temperature (or forecourt bulk tank temp) as a reference temperature, the temperature reading then effects forecourt pump pressure.. Or the fourecourt pump may just add a few bars of pressure to forecourt tank pressure. Assuming the vehicle tank is at the same temperature as the forecourt tank this makes for consistent fill speed because the pressure difference between pump and vehicle is around the same irrespective of weather etc. Some forecourt pumps have a pressure gauge, where one is fitted you'll usually see the gauge reads higher pressure in warmer weather and vice/versa. The fastest tank fills happen when the difference between pump pressure and tank pressure is greatest... Sometimes you'll find a forecourt pump that pushes 13 bar in cold winter conditions when tank pressure might only be 4 or 5 bar, this will make for a very quick fill because the pressure difference between pump and tank is (in this example) 8 or 9 bar when usually the difference might only be 3 or 4 bar.
Still you will be able to put more gas in the tank in cold conditions than warm conditions because (like you've said) the liquid gas expands with rising temperature (density of liquid decreases). On a lot of LPG pumps there are words to the effect 'calibrated for gas at 15degC'.... We buy gas by weight really, not volume. If we buy a litre's worth of gas in cold conditions we may not get quite a full litre by volume, if we buy a litre of gas in warm conditions we may get a bit more than a litre in volume but we'd be buying the same weight of gas and paying for the correct amount of gas on both occasions. Which means that for example if we buy 70 litres of cold gas it may not be enough volume to make the vehicle tank float rise enough to shut off incoming gas but on a warm day 68 litres may be all we can get in the vehicle tank, you may get a bit more gas in the tank in cold conditions than in warm conditions.
However, there is an aspect that can prevent LPG tanks filling to even float shut-off level in cold conditions. Air pressure in the tank (air trapped in the tank before the tank was sealed) can be the limiting factor that prevents the pump pushing any more gas into the tank. If a tank is sealed then we fill it to 80% with a liquid the air is going to be compressed into 1/5 of it's original volume so now the tank has 5 bar pressure (4 bar on top of atmospheric) even if the liquid is only water. For a 90% fill the air is going to be compressed into 1/10th of it's original volume so now we have 9 bar air pressure relative to atmospheric pressure - an LPG pump that only pushes 8 bar in cold conditions could never pump gas into a tank that has higher than 8 bar pressure (and even as pressure approaches 8 bar the speed of fill will drop to a slow trickle). In fact whenever air pressure in the tank means tank pressure is higher than (would be) gas pressure (if the tank didn't contain any air) it will slow the speed of the tank fill. You don't really notice the slower fill if the tank only fills to 80% because gas pressure in the tank is seldom below 5 bar because the weather usually isn't cold enough but if the tank valve has been modded to allow the tank to fill to more than 80% and/or weather is cool you do notice the difference in fill speed (and capacity) if there is any air in the tank. Where possible when I do a new install I flush air out of the tank, just put a couple of litres of gas in the tank then let it out (let it out in vapour form not liquid form), air comes out with the gas vapour. I also always modify the tank float to allow the tank to fill to 90% (or on my own cars to 100%).
On one of my own vehicles I have a 93L rated (water capacity) tank fitted, I have intentionally disabled the tank float shut off mechanism on this car, the other day in cold conditions I got 95 litres in the 93L tank lol. If I didn't use any of that gas out of the tank and just left it full to the brim until spring when the weather warmed up the tank would vent some of that gas, but as soon as I'd filled up I payed the cashier and drove away thus immediately started using some of that gas. The pump I filled at didn't have a pressure gauge but didn't fill the tank particularly quickly, so I imagine it's pressure would only have been (say) 8 bar. If I hadn't removed the air from this tank before I fitted it rising air pressure in the tank (as liquid gas level rose as I filled the tank) would have meant I'd only have been able to fill the tank to around 92% full (around 85 litres) even though I'd disabled the tank fill limiter with intention of allowing it to fill to 100%. With air in the tank I might have been able to fill the tank to a greater percentage of full in summer (when gas and pump pressure are highest but air pressure remains almost constant) than in winter, removing air means I can fill to the same percentage of full regardless of weather but I can still get more gas in my tank in winter because that is when liquid gas is most dense.
Over time the amount of air in a tank can change anyway. Usually when we draw gas from the tank we only draw liquid but when the tank is nearly empty of liquid some systems can run on vapour at low engine loads (particularly mixer systems and particularly on small engines), if the engine is fed on vapour from the tank some of that vapour will be air mixed with gas, so some air can be removed from the tank in these conditions. On the other hand most sequential systems have reducer output pressure referenced to manifold pressure and the manifold can be under vacuum, so on a small engine with a sequential system and reducer output pressure referenced to manifold pressure if we set too low a pressure for the system to switch back to petrol we could have a situation where tank pressure is even slightly below atmospheric pressure while we're running on vapour, which then might cause one way fill valves on the tank to open to allow air into the tank. E.g. if map is 0.26 bar while an engine is idling and a sequential system is set to switch back to petrol at 0.4 bar pressure tank pressure could fall to 0.66 bar before the system switches back to petrol, which is 0.34 bar of vacuum in the tank. This could only happen if the engine were left idling but still for this reason I don't set changeover pressure below 0.7 bar. Even though mixer systems are more likely to be able to run on vapour than a sequential system you won't have a situation with extremely low tank pressure on a mixer system because the mixer system doesn't have output pressure referenced to manifold pressure (except on a turbo), their reducers are referenced to atmospheric pressure. A tiny bit of air will get into the tank when the connection is made between forecourt pump and vehicle filler.
Last edited by LPGC
on Wed Dec 09, 2020 10:44 am, edited 5 times in total.