ORIGINAL: highhorse
Barry explain how you can get a increase in Thrust using diesel fuel.. Am I missing something here? Doesn't the RPM of the turbine determine thrust? If your ECU settings stay the same for the MAX RPM, then where does the increase in trust come from when using diesel fuel over JET A?
Dennis
Great Q Dennis, and if the above explanation doesn't do it for you then please let me try, in simpler terms that help me, anyway: For recips it's intake, compression, power, exhaust. Turbines= Suck, squeeze, bang, blow.
The thrust isn't ''made'', if u will, by the rpm.
Rpm does the suck and squeeze part (nit pickers hold ur piece, we are NOT talking about high bypass engines),
but the amount of thrust can be enhanced by a ''higher calorie'' fuel, which gives more bang and blow at the same level of suck and squeeze (RPM). Diesel fuel has more calories than Kero and JetA, and more energy is released when it's burned. That extra energy makes for more bang, which makes more blow(aka Thrust).
Hope that helped?
Don.
Sorry, no, not true. We are talking of gas turbines that work on the Brayton cycle here, not piston engines.
If the fuel have more specific heath, then at same mass of fuel burned will produce more heath. This heath will increase the temperature of the air on the intake of the turbine wheel, expanding it. This expanded gas will make the turbine wheel to spin faster, and at this point the ecu will "see" that there is more rpm than maximum and will reduce the mass of fuel to restore the correct amount of heat so that the turbine wheel turn at the correct speed.
Anyway, I can think of 2 possibilities of that a different fuel make different thrust, but hardly applicable our engines. One is that the thrust of a engine is the mass flow in the exhaust (grams of air trough the exhaust of engine each second) times the exhaust speed. The massflow at the exhaust is the addition of the weight of the intake air added to the weight of the fuel burned. So using a heavier fuel with less heath capability will increase the thrust at the expense of a higher fuel consumption by weight. But to put thing on perspective, a engine of 90N of thrust use 210gram of air and 4,4g of fuel each second, so doubling the weight of fuel used it would increase only a 2% the thrust. The second possibility is that there is some unburned fuel past of the turbine wheel that burn in the exhaust, making a sort of reheat, out of the control of the ecu, but this would seem to me a remote possibility.
About the pump power between the diesel and kerosene, my particular experience is that the pump worked harder on my test. Diesel, at least here, is thicker than kero, so the pump should work harder to have it trough the engine injectors. Could be that a engine with bigger diameter injectors be less sensitive to thickness of fuel, and this, coupled to the less mililiters (not grams) of fuel needed for same heat, be unnoticeable by reading the pump power.
Gaspar