4 ms·

I'm a bit suspicious of the qualification 'thermal' in 'thermal efficiency' which sounds like it's hiding something. From the wiki article on thermal efficiency

I'm a bit suspicious of the qualification 'thermal' in 'thermal efficiency' which sounds like it's hiding something. From the wiki article on thermal efficiency https://en.wikipedia.org/wiki/Thermal_efficiency https://en.wikipedia.org/wiki/Thermal_efficiency, and I'm no wiser for reading it: "For a heat engine, thermal efficiency is the fraction of the energy added by heat (primary energy) that is converted to net work output (secondary energy)." Does thermal efficiency = actual efficiency in the case of jet engines? I find that difficult to believe.

Neil44 6y ago
Yes thermal efficiency is a common rating for engines that work by burning fuel to make heat then turn that into useful power. You take the theoretical energy in a litre of fuel and work out how much of that the engine can convert into useful work.
deleted 6y ago
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agurk 6y ago
A heat engine is a device that converts heat into kinetic energy. The amount of heat that is converted into movement as a proportion of the total heat is the thermal efficiency. With a jet engine it's the amount of energy in the burnt fuel that's converted to both rotation in the shaft and the speed of the expanding burnt gasses exiting the back. Propulsive efficiency is how good the engine is a converting this energy into thrust. The Concorde's engines were a type where all the propulsion was from the fast gasses exiting at the back, i.e. a turbojet. Efficiency is less than 100% as energy is taken from the gasses to power the engine and not all the gasses exiting the rear usefully push the aircraft forwards. I don't have the exact number for the Concorde to hand but looking at [1] from my previous post, we can see it was typically around 60% in 1970, but these numbers are likely for (low bypass) turbo fans. These work in a slightly different way where they have more energy in the rotation of the shaft and less in the gasses going out the back. The energy in the shaft drives the big fan at the front which pushes a large volume of air more slowly out of the back. This is to address the problem of the gasses which exit the back but don't usefully push the aircraft forward. To get the total efficiency of the jet you have to multiply the thermal efficiency by the propulsive efficiency. edit: changed division to multiplication
- gsnedders 6y ago
  > To get the total efficiency of the jet you have to divide the thermal efficiency by the propulsive efficiency. Also need to consider combustion efficiency (i.e., the percentage of fuel turned into heat as an input to the heat engine), though for gas turbines this is typically ~99.9%.
- shawabawa3 6y ago
  > divide the thermal efficiency by the propulsive efficiency Multiply surely? Otherwise reducing the propulsive efficiency would increase overall efficiency
  - agurk 6y ago
    Yes, you're absolutely right. Edited my comment to correct it.

Neil44 6y ago

Yes thermal efficiency is a common rating for engines that work by burning fuel to make heat then turn that into useful power. You take the theoretical energy in a litre of fuel and work out how much of that the engine can convert into useful work.

deleted 6y ago

[deleted]

agurk 6y ago

A heat engine is a device that converts heat into kinetic energy. The amount of heat that is converted into movement as a proportion of the total heat is the thermal efficiency. With a jet engine it's the amount of energy in the burnt fuel that's converted to both rotation in the shaft and the speed of the expanding burnt gasses exiting the back. Propulsive efficiency is how good the engine is a converting this energy into thrust. The Concorde's engines were a type where all the propulsion was from the fast gasses exiting at the back, i.e. a turbojet. Efficiency is less than 100% as energy is taken from the gasses to power the engine and not all the gasses exiting the rear usefully push the aircraft forwards. I don't have the exact number for the Concorde to hand but looking at [1] from my previous post, we can see it was typically around 60% in 1970, but these numbers are likely for (low bypass) turbo fans. These work in a slightly different way where they have more energy in the rotation of the shaft and less in the gasses going out the back. The energy in the shaft drives the big fan at the front which pushes a large volume of air more slowly out of the back. This is to address the problem of the gasses which exit the back but don't usefully push the aircraft forward. To get the total efficiency of the jet you have to multiply the thermal efficiency by the propulsive efficiency. edit: changed division to multiplication

gsnedders 6y ago
> To get the total efficiency of the jet you have to divide the thermal efficiency by the propulsive efficiency. Also need to consider combustion efficiency (i.e., the percentage of fuel turned into heat as an input to the heat engine), though for gas turbines this is typically ~99.9%.
shawabawa3 6y ago
> divide the thermal efficiency by the propulsive efficiency Multiply surely? Otherwise reducing the propulsive efficiency would increase overall efficiency
- agurk 6y ago
  Yes, you're absolutely right. Edited my comment to correct it.