Just how small can you make an engine? Two researchers from the University of Stuttgart and the Max Planck Institute for Intelligent Systems, Valentin Blickle and Clemens Bechinger, successfully shrank the Stirling heat engine down to a single, microscopic particle. The engine is so small, in fact, that the random fluctuations in position due to Brownian motion cause variations in its work output. This microscopic Stirling engine is controlled using a pair of highly focused lasers.

Stirling engines, named after the Scottish inventor who created them in 1816, offer the highest theoretical efficiency of any heat engine—the same as the Carnot efficiency. Due to pesky entropy and the second law of thermodynamics, you can’t get all the heat you put in back out as work. The efficiency of any heat engine, then, is just the ratio of output work to input heat. The Carnot efficiency, conceived by Nicolas Léonard Sadi Carnot (the father of thermodynamics), gives the maximum theoretical efficiency of the engine and depends only on the temperature range within which the engine operates.

The typical, macroscopic Stirling cycle consists of four steps: an isothermal (constant temperature) compression, isochoric (constant volume) heat addition, isothermal expansion, and isochoric heat removal. The heat addition and removal processes operate through the engine walls, making this an external combustion engine as opposed to internal combustion engines like gasoline and diesel, where the heat exchange occurs in the working fluid. The steam engine is another example of an external combustion engine.

In order to create a microscopic version of a Stirling engine, the researchers sandwiched a tiny, 2.94 μm melamine bead in a 4 μm water gap between glass slides. Heat was added and removed by near-instantaneously increasing and decreasing the surrounding water bath—it shot between room temperature (22°C) and 86°C in less than 10 milliseconds—using a laser.

They used another infrared laser to create an optical trap for the bead. Increasing and decreasing the stiffness of the trap (done by varying the beam intensity) functioned as the isothermal compression and expansion steps. The expansion, for example, occurs when the energized particle (from the heat addition) relaxes and is able to move freely, due to the stiffness of the trap decreasing.

Now, since we’re dealing with a single particle, rather than a continuum, the position—and extracted work—fluctuates randomly due to Brownian motion. In a macroscopic system, well described by the laws of thermodynamics, the large number of degrees of freedom negate the effect of microscopic fluctuations. Here, however, they are obvious in a plot of the extracted work.

The team observed a small difference between the work extracted during expansion and the work spent during compression in each cycle, and measured the average production of work over time. They suggest this was caused by the greater variation in position at the high temperature condition, where the particle moves around more due to higher energy.

Probably the most exciting result of the study is the high efficiency obtained: 14 percent thermal efficiency, which corresponds to 90 percent of the Carnot (and maximum possible) efficiency, 15.5 percent. This may seem low, but it depends only on the ratio of the high and low temperatures, 76°C and 22°C for this particular experiment.

The authors also found that some work is unavoidably dissipated to the surrounding environment, especially when the engine operates at higher frequencies, outputting more work. Conversely, the efficiency increases with increasing cycle time, only reaching the 14 percent value at low frequencies. This means that the optimal power is a competition between dissipation and efficiency, something the researchers plan to study further.

Blickle and Bechinger talk about future microscopic machines benefiting from this research, and it’s certainly an accomplishment to create a single-particle engine, but direct applications of this aren’t clear. One issue is that the engine is completely externally controlled by the two lasers, so a different approach would be needed for a self-contained device. However, the concept is fascinating, and provides new insights on thermodynamics at such small scales.

Nature Physics, 2011. DOI: 10.1038/nphys2163 (About DOIs)