The quantum afterburner
Feb 4, 2002
Quantum mechanics could be used to improve the efficiency of heat engines, according to Marlan Scully of Texas A&M University in the US. Scully has calculated that exhaust energy from an engine could be used to power a laser. This would improve engine performance beyond that of the ideal Otto cycle without violating the laws of thermodynamics (M Scully 2002 Phys. Rev. Lett. 88 050602).
Scully considered a simplified version of the Otto cycle, a sequence of operations similar to that carried out by a standard car engine. In this four-step cycle, a hot gas expands in a cylinder doing useful work before it loses heat to its surroundings; then it is compressed and heated again.
But Scully added two extra steps. After the heat has dissipated the gas is passed through a laser-maser cavity at constant temperature, where it deposits useful energy. This cavity is then reheated once the system as a whole has been reheated, again at a constant temperature.
The crux of Scully’s system is that the heating of the cavity does not raise the kinetic energy of the atoms in the gas, as the normal heating stage does. Instead it increases the internal energy levels of the atoms, making them emit photons. Scully calculated that this liberated quantum energy was greater than the extra work that would have been produced if the additional heat had instead been ploughed into a normal four-stage cycle.
Scully points out that this result does not violate any laws of thermodynamics. He also analysed an equivalent system in a Carnot cycle – which, unlike the Otto cycle, runs at the highest efficiency permitted by thermodynamics – and found that quantum mechanics could not be used to improve the engine’s performance.
He hints that he has already worked out how to test his idea in the lab and that he has conceived several novel laser systems. His idea has been dubbed a ‘quantum afterburner’ because of its parallel with the devices that extract useful energy from the exhaust of a jet engine.
About the author
Edwin Cartlidge is News Editor of Physics World