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Originally Posted by heyjae
This cannot be true. Gas stations sell 89 octane by mixing 87 octane and 91 octane at the pump (gas stations only have 2 tanks: 1 for 91 and 1 for 87... 3 if they have diesel). They do mix.
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Well, yes... and no.
Mid-range octane is an alternative. It is the product of not mixing the base stocks of gasoline, but rather the dilutiion of the additives present in higher octane fuel. This is usually a 65.25:1 mix of the premium with regular.
It's more of a marketing alternative than anything. I'm not aware of any car which
specifies mid-grade fuel. These are usually designed for premium, but have ECUs which will 'allow' lower grades to be used. Most engine software today follows the principle of 'follow the knock sensor'. That is they run the most retarded spark they can until a knock sensor indicates a need to advance the spark. Since this is done many, many times per second, the driver rarely experiences knock.
I mentioned the need to agitate or mechanically mix the fuels. This is not done in the tank, but at the pump itself with blending valves and so mathematically, complies with the state law that the
minimum octane on the pumps sticker is being pumped. But, not any batches of 87 and 91 will do. Only batches from the same refinery and manufacturer, since different brands use different proprietary formulations and chemicals such as MBTE or Ethanol as octane boosters. Not all of these will mix and in some cases will cancel each other out.
The base stocks do in fact separate with time, but with the now dilluted additives attached. If it didn't, gasoline couldn't even be refined since the refining process relies on differing specific gravities to separate the components of crude components.
If you plotted the DMEs spark adavance, you'd see one setting for 65% of the tankful and another for 35% of the tankful. The driver would be oblivious to the different settings because no knock would occur through the tankful.
