Hot air and confusion over compressed-air hybrids


When Lund University in Sweden unleashed a press release claiming that “normal air could halve fuel consumption,” techno sites across the Web responded by regurgitating it verbatim, with few, if any, questions asked about the underlying development. A quick Google search turned up more than 6,000 sites, even some claiming to serve physicists, rebroadcasting the lead sentence: “Every time a car brakes, energy is generated,” a statement any first-year engineering student should know is bogus. The press release, and the echo chamber on the Web, goes on to present an ambiguous explanation of an air or pneumatic hybrid, all based on a doctoral thesis.

The author, Sasa Trajkovic, claims city buses could cut fuel consumption by 60%. But Trajkovic makes no mention of the cost of changing the infrastructure, the safety of the scheme, or the cost of an air-hybrid bus.
A key feature of the Mr. Trajkovic’s hybrid is that it recoups 48% of the energy dissipated during braking. According to his thesis, which seems more about valves than hybrids, applying the brakes would force the braking wheels to turn the internal combustion engine, which is switched off during such times and no fuel flows. Turning the engine would slow the wheels while the pistons would compress air, which would be kept in a tank somewhere on the vehicle. As soon as the driver hit the gas pedal, the engine would restart. When the engine needed a boost -- as when passing, climbing a hill, or just pulling away from a stop light, the compressed air would either be fed back into the engine, much like the compressed air from a turbo or supercharger, or the compressed air would be used to run the engine.

It’s virtually impossible to explain Mr, Trajkovic’s concept based on the original press release, which is appallingly light on facts: no explanation of the technology, and more ambiguity than a presidential debate. One might hope the shortcomings can be chalked up to just a bad translation from an original written on Swedish. But the uncritical reaction of the thousands of so-called techno sites is disconcerting. Perhaps they just want to jump on the “hype the hybrid” bandwagon.

We contacted Mr. Trajkovic, now an engineer at Volvo. He readily agrees that energy is not generated during braking, merely transformed or converted. He attributes that error in the original press release to media people at his university uneducated in thermodynamics.

He also says that the key to improving mileage for a hybrid bus by up to 60% depends more on giving the engine a quick start/stop feature rather than regenerative braking. Quick start/stop lets the combustion engine shut down when stopped or idling for more than several seconds. “It is unfortunate that this was not mentioned in the press release,” says Trajkovic. “But it is all stated in my theses.”


Lund University, the original press release
Sas Trajkovic’s papers
Others have written intelligently about air hybrids. One concise explanation: Pneumatic Hybrids: Urban Powertrain Of The Future?

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.