punkalouver,
Automotive and aviation fuels, as we know them, are those in the Hydro-Carbon Methane series, rangeing from gaseous Methane, CH4, at the bottom, through to Hexadecane at the 'top end' of heavy fuels used for aviation kerosene. Automotive fuels usually comprise Septane and Octane, and immediately after, the range from Nonane to Hexadecane is used for aviation turbine fuel.
Volatility is high, and flash point is low, at the lower end of the series, and, as we progress through the series, volatility decreases, and flash point increases. (If you want to smoke, and give me the choice of being in a room full of Methane or Octane vapour, against a room awash with Hexadecane, I'll go for the Hexadecane room every time).
If we examine the range of fuel used for aviation turbine aircraft, we look at the range beginning with Nonane, Decane.....through to Hexadecane. (Nonane is very close to Octane, the highest cut used for automotive fuels).
It is absolutely true that "less dense fuel while still refined within the specifications of Jet-A, is more burnable than more dense kerosenes". The problem is that "more burneable" (lighter) fuels have less calorific value than denser fuels, and calorific value is what you need to produce thrust. Burneability is NOT directly related to calorific value. If you were to use a lighter grade of aviation kerosene, you would need a commensurately larger volume to achieve the same task as would a heavier fuel. All fuel tanks are eventually volume limited, and a full tank of light fuel will carry you a lesser distance than a heavier fuel.
Consider this, if I need 7500 Kg (16535 Lb) of fuel for a particular journey, with a 2000 Kg (4409 Lb) reserve, I will need to load 9500 Kg (20944 Lb) of fuel. If I load fuel of SG 0.75, I must carry 12667 Litres (3346 US Gal) of fuel, and have 2667 Litres (705 US Gal) remaining after the flight. If I load fuel of SG 0.85 for the same flight, I must carry 11177 Litres (2953 US Gal) of fuel, and have 2353 Litres (622 US Gal) remaining after the flight.
It does not surprise me then, that, if you load a lighter fuel, you will have a greater VOLUME at the end of the flight, the weight will, of course, be the same as if you'd taken the heavier fuel.
Even this is something of an over-simplification. It considers only that lighter fuel have less calorific value per unit of volume than do heavier fuels. Heavier fuels also have greater calorific value per unit of mass than do lighter fuels. This is not usually considered, as in the normal ranges of the mixes used for turbine fuel, calorific value per unit of mass does not vary too greatly. If, however, we did allow for the difference in calorific value per unit of mass, you would need to load less volume and less mass of a heavier fuel, and have less mass and less volume at the end of the flight compared to the lighter fuels.
Regards,
Old Smokey
Last edited by Old Smokey; 18th September 2005 at 23:49.