Team claims to have made world's smallest steam engine

13 December 2011

The team, made up of physicists from Stuttgart University and the Max Planck Institute for Intelligent Systems, claims that its micro-sized engine could be used towards making highly efficient, small heat engines.

In a regular Stirling engine, a gas-filled cylinder is periodically heated and cooled so that the gas expands and contracts. This makes a piston execute a motion with which it can, for example, drive a wheel.

’We successfully decreased the size of the essential parts of a heat engine, such as the working gas and piston, to only a few micrometres and then assembled them to a machine,’ said Stuttgart University’s Valentin Blickle.

The working gas in the Stuttgart-based experiment is represented by an observable plastic bead, measuring three micrometres, which floats in water.

The physicists replaced the piston, which moves periodically up and down in a cylinder, with a focused laser beam whose intensity is periodically varied. The optical forces of the laser limit the motion of the plastic particle to a greater and a lesser degree, like the compression and expansion of the gas in the cylinder of a large heat engine.

Prof Clemens Bechinger, head of the physics institute at Stuttgart University, told The Engineer: ’This is quite similar to a weight which loses energy when you let it drop in a gravitational field. In our case, the energy of the particle at a given position inside the laser trap is proportional to the laser intensity. So, when you reduce the laser intensity, the particle loses energy.’

Similarly, compression and expansion must take place at different temperatures. Bechinger said this is because the two processes are completely symmetrical. He explained: ’When you compress and expand at the same temperature, the net energy is zero. Therefore, as already realised by Carnot and Stirling, both processes must be performed at different temperatures.’

The researchers replaced the coal fire of an old-fashioned steam engine with another laser beam that heats water suddenly, but also lets it cool down as soon as it is switched off.

‘We’ve developed the world’s smallest steam engine and found that the machine really does perform work,’ Bechinger said in a statement.

‘This was not necessarily to be expected, because the machine is so small that its motion is hindered by microscopic processes which are of no consequence in the macroworld,’ he explained.

Despite some sputtering caused by water molecules colliding with the particle, the engine is said to have run well.

‘Our experiments provide us with an initial insight into the energy balance of a heat engine operating in microscopic dimensions,’ said Bechinger. ‘Although our machine does not provide any useful work as yet, there are no thermodynamic obstacles, in principle, which prohibit this in small dimensions.’