There is a real difference in the chemical composition of summer grade and winter grade fuel. It's cold in the winter right? Combustion is not of liquid fuel but fuel in the vapor form. Makes sense to use smaller hydrocarbons in winter (high vapor pressure and cold temp) and larger hydrocarbons in summer (low vapor pressure and high temp) so you have relatively consistent drivability. Summer fuel will net better fuel economy.

 

SOUNDS valid, BUT- Remember: the working temperature WITHIN the engine will be relatively the SAME. This is part of the reason behind the added heat section of the intake throttle body(where hot coolant flows through it) to add heat to the incoming air(prior to the fuel being sprayed in)

As the fuel is sprayed in, it atomizes- becomes billions of droplets of roughly equal size: this creates a much greater surface area in the atomized fuel, absorbing the heat from the intake air, and vaporizing(turning into a vapor) more readily. This allows the flame front of ignited fuel to propagate much more rapidly across the piston.

The winter/summer blend is alleged to change the combustion byproducts- not the ability of the engine to more easily ignite the fuel or burn more readily...

Aircraft engines(reciprocating), for example, run (currently) on a 100 octane low-lead formulation, which is the SAME year around: This fuel is used in varying climates and temperatures from ground level to 10,000(OR MORE) in engines which are normally aspirated, or supercharged(allowing higher flight). There is no reason cars can't run the same formulation year round...

The "size of the hydrocarbons" argument is null- the fuel sprays through and exposes the same amount of surface area into heated air year round... With older carb models, the spray was NOT as fine, and there may be a little more validity to the concept- but- again normally aspirated aircraft are also quite usually carb equipped...

Scott

 

I mention the size of hydrocarbons (HC) because the longer the chain, the greater the intermolecular forces (London dispersion force to be specific) & the higher the boiling point (lower RVP). There's is likely a negligible effect on the spray properties of these different fuels, both in carb or EFI. They're all just non-polar solvents with low viscosity as far as spray properties are concerned. Just an example of HC length, methane is one carbon & obviously a gas at room temp, whereas something like hexane (6 carbon chain) is a liquid at room temp.

I was misinformed of the reasoning for the different fuels. I thought it was a start-up concern (summer fuel in (real)winter could be problematic?) not emissions based.

If I come across as a know it all, I don't mean to. I'm learning from you. Good discussion.

 

Another thing about modern gasoline is that it is made up mostly of aromatic compounds instead of straight chain hydrocarbons. Benzene, Toluene and Xylenes are the major constituents with many other substituted benzene compounds thrown in along with some straight chain hydrocarbons, alcohols and other organic molecules. I haven't studied the two blends in depth, we normally use a composite of several gasolines in the lab when we are analyzing soil samples for contamination, but I think it would be interesting to figure out just what the differences are between summer and winter blends.