21pzr
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Joined: 2/11/2011
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Been following this for a couple of days. Funny how people think that higher octane equates to higher energy. Octane is a measure of ignition delay. The higher the octane, the longer the fuel takes to ignite. Gasoline, whether produced in the 40's or now, has a fairly small range of "specific energy", or the amount of energy released per gallon (liter, cubic meter, or pound). As others have stated, you only benefit from higher octane if you have a high enough compression ratio. I can remember back to my younger days of tinkering with muscle cars in the 60's and 70's, and having a tricked out Dodge hemi that would knock on any fuel other than Sunoco Ultra, which was a 95+ octane. This is why many people pay too much for gas in their cars, buying premium, with its higher octane, when their engine doesn't need it. Premium does not give you any more horsepower per gallon than regular.
The other track of this thread, that of leaning out fuel mixtures, or retarding spark timing is really reminiscent for me as well. Changing timing on airplane engines as JWE and Pax mention is great for optimizing your engine to whatever fuel you bought that day. The reason most planes don't have this anymore is the much more uniformity of avgas than was the case in WWII. I mean, come on, floating drums of avgas ashore (water contamination and salt), straining through chamois! Again, anyone who worked on the old muscle cars knows as Pax does about leaning out the mixture. Much of the gasoline used by a carbureted engine is not used to provide power, it is used to absorb heat while evaporating in the cylinder, to keep the fuel/air mixture cooler, helping the tetra-ethyl lead octane booster to keep the fuel from igniting too soon, preventing knock and loss of power. Not to say that the fuel didn't burn, but that "excess" fuel plus timing meant that much of it was burning while the piston is already going down, so little energy is transferred to the propeller. This is what makes water injection so effective. The old Indy cars with the Offenhouser engines used a massive turbocharger, ran a very lean fuel mixture, and used water injection. It was really funny to see these cars with a fairly small engine running at 200 mph until the turbo blew, and then they could only do about 40mph! Methanol is used today as both an oxygenator (less pollution) and an octane booster, since no one really wanted all the lead dioxide fumes in the air that we had before unleaded gas. As Pax says, it cools the mixture, alcohol burns slower than gasoline, and the burning adds its own energy.
As a 35 year merchant ship chief engineer (all diesel time, sorry, steamboateng), and a tankerman, I will add my $.02 to Steamboat's comments about crude and bunker. "Sweet" and "Sour" crude refer to the sulfur content (hydrogen sulfide having a rotten egg smell, hence sour). Metallic sulfur does contribute to engine wear, but the most damaging aspect of sulfur in fuel is that when it is burned, the sulfur dioxide produced will combine with the water vapor also produced, and at low exhaust temperatures will condense as sulfuric acid, which will really accelerate your engine wear. One point that Steamboat didn't mention about Bunker oil is that while the density is different between Bunker-C and Navy Special, the real difference is that Navy Special has a lower viscosity (thinner) than Bunker-C, which allows for the lower heating before burning in the boiler. The navy was interested in reducing auxilliary equipment (less heating equipment), and using less steam to heat fuel and more steam to power the propeller.
Bill
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RE: Octane rating - 
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