Thermal to Kinetic Energy
All, a fun thread, but all of you are missing an important piece of information, the direct relationship between thermal energy and mechanical kinetic energy.
I'll skip the reason for a compressor, and go straight to why higher compression yields higher efficiency. The Brayton and Otto cycles are similar, if you look at the volume, pressure and heat curves. The important question is HOW does heat energy get converted into mechanical energy inside either a piston or a turbine engine? Remember that thermal energy in a gas is directly related to the kinetic energy of the individual molecules within that gas. The temperature of a gas is the average temperature of all the molecules within the volume being measured, as some will be hotter and some cooler. The temperature of an individual molecule is the amount of kinetic motion that molecule is experiencing. The hotter it is, the faster that molecule is vibrating or moving.
The bottom of a piston and the end of the exhaust nozzle of a turbine engine are both exposed to the ambient outside pressure. When an air/fuel mixture is compressed and ignited, the resulting heat energy is directly transferred to mechanical motion, when individual molecules transfer their heat (kinetic energy) to the metal molecules of the piston or turbine blade (think Newton's cradle). In other words, the transfer of many gas molecule's kinetic energy to the metal of the piston or turbine blade, causes the metal parts to move. This happens because the heat and kinetic energy of the gas molecules are used to create pressure within the engine by design, which causes movement of the piston or turbine blade. When that movement occurs, the gas molecules give up some of their kinetic heat energy to the moving parts, and exchange it for mechanical motion or work, which is just another form of kinetic energy (again think Newton's cradle). This lowers the temperature of the gas, since it has now given up some of its heat energy in exchange for mechanical motion.
When higher compression (piston) is used, the pressure difference after ignition between the top of the piston and the ambient outside pressure at the bottom of the piston is greater, and therefore more kinetic energy is transferred to the piston, exchanging more heat from the gas to the piston's movement. If you look at the Otto cycle curves, most of the work and most of the energy transfer takes place in the top half of the power stroke, and this is why ignition timing is so important for efficiency. Bad ignition timing means heat is released by the fuel at the wrong time, and doesn't transfer as much heat to mechanical work, thus the EGT is higher when the timing is off.
For the turbine, a greater pressure ratio means a higher pressure is created in the combustion chamber, thus the pressure differential between the combustion chamber and the exhaust nozzle (across the turbines) is greater, thus more exchange of the heat (kinetic) energy of the gas into mechanical motion. In a turbofan, this is really helpful as turning the fan produces much more thrust then the residual pressure out of the exhaust nozzle (which you still use).
To summarize, the whole point of greater compression or pressure ratios, is to create greater pressure differentials across the moving parts, so more heat within the gas can be exchanged for mechanical work. This is what creates greater efficiencies.
Last edited by Flight Safety; 28th Nov 2011 at 23:29.