If you’re going to do something like building a Porsche 911 that fits on the head of a pin, or make a microscopic medical pump, you need a microscopic engine. A team of researchers from the University of Twente in the Netherlands, the Russian Academy of Sciences, and Germany’s University of Freiburg have developed a micro-engine that burns oxygen and hydrogen, but there’s a small problem; they’re not sure how the thing works.

Making micro machines means making micro power plants, and with nanotechnology advancing by the day, it's a wonder that they aren't more common. Micro-engines are cheaper to produce than conventional ones, they can do things that larger engines cannot, and they could help their larger cousins operate more efficiently. The problem is that making an engine that is small, powerful, and fast is more elusive than first thought.

The main hurdle to be overcome is building a motor or actuator that can change energy into motion. Getting energy to a micro-machine in the form of electricity is simple enough, but the tricky bit is getting that electricity to do any useful work.

This is due to the problem of scale. Electric motors as they scale down generate less and less force. Combustion engines have an even worse time, because on a microscopic scale the small space of a combustion chamber with its comparatively large surface area carries heat away too fast, so combustion can’t be supported. Though there are alternatives, such as electroactive polymers and electrochemical actuators, these are limited in function and too slow to be practical.

The new 100 x 100 x 5 μm3 micro-engine is made of layers of polymer membrane 530 nanometers thick. It uses electrodes generating an alternating current to break down water into hydrogen and oxygen, in a chamber formed in the membrane. As the gases mix, they spontaneously combust. As they do so, the electric pulses are switched off momentarily, producing more power in a sort of piston-stroke effect.

That’s all well and good, but it’s also a bit embarrassing because the researchers aren’t at all sure how the engine works – more specifically, they aren't sure how the combustion is able to occur. It’s a bit like the famous bumblebee quandary; the maths says the bee shouldn't be able to fly, but it does.

Though the mystery has yet to be solved, the scientists think that it might be due to transitional nanobubbles less than 200 nm in diameter that form for fractions of a second in the chamber, that prevent heat from dissipating so combustion can occur. The researchers say that this may be counterintuitive, but these bubbles have already demonstrated such counterintuitive properties.

The scientists believe that if the details of how the engine operates can be worked out, it opens up the great potential of micromachines for future development.