Mogas in Aviation

It is important to understand that gasoline is a complex mixture of various hydrocarbons and that mixture is not explicitly defined.
Each refinery produces a mixture that in its entirety act like what gasoline is expected to act like.
If it smells like gas(oline), burns like gas, ignites like gas, produces Fire, Heat, and Energy like gas, and vaporizes and evaporates like gas, then it can be sold as gasoline.
The vapor pressure of the gas-like product is a summation of all the constituent parts.
Most of the component hydrocarbons are liquid at normal room temperatures except for one.
The lightest component is butane, which in its undiluted form boils at very near 0°C, 32°F.
This makes it want to evaporate out of the gas at room temperature, much like the CO2 in a soda, that Boils at -109°F, wants to make the beverage go "Flat" if left in an open container.
As the butane evaporates the the Vapor pressure goes down significantly but the Octane rating is only slightly affected.
Butane is very volatile (vaporizes easily) but is an inexpensive additive to the fuel.  Winter gas is less expensive because of this.
It also helps NON-Fuel-Injected engines to start easier during cold weather. Snow-Blower, SnowMobile!
In a Carb, the fuel is mostly dribbled into the intake airstream and what doesn't readily evaporate coats the walls of the intake manifold, and once the engine warms up, it readily evaporates from the warm wall as it flows toward the intake valve.

In a Rotax Multiport fuel-injected engine, The fuel is sprayed at high pressure directly on the back side of the Hot Intake Valve.
It almost instantly flashes to vapor.
This is why fuel-injected engines are ready to go immediately after starting.
There is no need to warm up the intake manifold. The Head and the Valves heat up very quickly.
Any reduction to the Fuel Vapor Pressure has much less effect in a Fuel-Injected engine compared to a Carb'd engine.

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HERE is an interesting video demonstrating how to measure RVP and also how quickly over 10% of your fuel supply can be lost to the atmosphere.
If improperly stored, A 5-gallon can of gas can quickly become a 4-gallon can.

Two things I found very in the video that would be interesting to see comments on regarding RVP:

1. There appeared on the surface to be a relationship between octane & RVP which surprised me. He stated it was November so winter blend here in the test. As octane went up, RVP went down. He did mention when he saw the RVP of the 91 fuel (8.9) that it could have been a leftover summer blend but he had stated it was November....so what did he do, ie, bought two jugs of Nov winter fuel and then introduced either a stored jug he had from the summer (unlikely) or, the station he got the 3 fuel samples from in Nov still had some summer blend 91 left in their tanks, possibly because fewer people buy 91 octane? As I understand it however, stations are required to officially make the switch in September, so......? Yes, that's still possible so to a large degree he invalidated his own test which was very disappointing. The test left the impression that he was way less than very scientific from a methodology  standpoint. However, since the 89 fuel was at a midpoint RVP & the 87 was the highest with 91 at the minimum pressure, I'm still wondering if there is an octane relationship here? If we could be sure the 91 fuel was winter blend then it would indicated a conclusiveness that there actually is a relationship between RVP & Octane.

2. The part where he demonstrated that by exposing the winter 87 fuel (which was 13.8 psi or so) to atmosphere a couple of times, the RVP dropped quite quickly & SIGNIFICANTLY, well below even summer blend pressures by essentially "blowing off" what I assume was the butane & other vapors that is added to winter fuel. This really got my attention & got me to wondering if by leaving winter mogas exposed to a reasonably warm atmosphere (measuring the OAT during the process) for a period of time, say with an uncapped 5 gallon jug (not in your garage son... ?) that one could in essence end up with something approaching a summer blend RVP? Not very scientific I know (how much time to get the fuel to 7 or 8psi for example). Anyone willing to take that on?? I mean if a person left a 5 gallon jug of winter blend outside in the sun for 30 minutes and could end up with a summer blend RVP equivalent, that would certainly seem to solve alot of peoples concerns over vapor lock issues flying in winter on warm and/or high altitude days. I realize I'm being possibly too naive here, but at least "on the surface" so to speak, it seems there might be something here to consider?

Hi all

So glad most of you have the RVP issue in the target.  It is a real killer in the USA.  

The dirty secret is the fact that many states sell past the mandated dates for summer fuel buy getting exemptions.  Had a storm, some floods, oil from the middle east is slow...they have a ton of reasons.  The state gets an exemption to extend the change over date.  Add to that the stocking issue.  Butane is cheap, winter fuels are therefore less costs so as noted people stock up. 

On the iS you noted the bubbles in the return line and that makes sense.  On the pressure side from the fuel pumps on it is about 3 bar or 45 Psi.  As soon as it passed the pressure regulator it drops back to ambient and in doing so the gas can ....well it evaporates a lot.  The iS engines with the higher fuel pressures are not nearly as susceptible to vapor lock as a carb engine that only has .5 bar max after the pump.  

Food food thought if you have not seen this before....this was one of the crashes that caused the FAA to ask the ASTM F37 Light Sport standards group to change and add in fuel system testing to qualify fuels for any new Light Sport aircraft designs.  Vans has done extensive testing and it is extremely difficult to ignore RVP of the fuel. Sorry for the commencal but please watch the whole thing. 

https://www.youtube.com/watch?v=1xcCpZDSoiU