Does it really work? Isn't it just perpetual motion?

Posted: 21st November, 2018

Does it really work? Isn't it just perpetual motion?

When we talk to people about our system, we often get objections on the basis of “but it can't work, it's perpetual motion.”

In particular, they ask the following questions -

  • Where does the power come from to drive the compressor in the turbo-expander?
  • Surely there are losses in the system?
  • Isn’t it perpetual motion - you’re saying you’re getting something for nothing.

In this blog, we hope to dispel some of those concerns. We’ll start however with a quick reminder of how our system works.

The vehicle’s engine and its turbocharger are unchanged from a conventional installation.  The air from the turbocharger is cooled through an intercooler (charge air cooler) as normal and then is compressed by the compressor in our turbo-expander.

The air is then cooled in another intercooler before being expanded across the turbo-expander turbine which reduces the temperature of the exiting air to well below ambient levels if needed.  The now nicely cold air then enters the engine to mix with the fuel and provide the combustion to generate power.

Related: An Introduction to Active Charge Air Cooling [video]

And now, back to our original questions -

First off, how do we power the compressor? 

There’s no electric motor or geared drive from the engine.  Instead, the turbine provides the power to drive the compressor so it is self-powered.  The turbine is doing work as the air expands through it and this is where the power comes from and where the energy goes to once it has been taken out of the air.

Secondly, of course there are losses, just as there are in any system with fluid flow and machinery. 

The air arriving at the inlet manifold is at slightly lower pressure than it would have been if it had gone straight there from the car’s intercooler.  However, don’t forget that the air is a lot colder and hence more dense, so a power increase is already obtained.  What happens then is that the engine control unit (ECU) detects an inlet manifold pressure drop and calls for more boost from the turbocharger, thus ensuring that the inlet pressure is brought back to the desired figure.  And all this happens very quickly as the inertia of the turbo-expander is a lot less than a turbocharger.

The answer to the third question is therefore obvious.  It is not perpetual motion and we are not trying to contravene the Laws of Thermodynamics.

We compensate for the losses in the system by increasing the boost so we’re obeying the First Law.  And we use the work generated by our turbine to reduce the temperature of the inlet air and so we are satisfying the Second Law too.

If you have any questions on any of the points made above, do please get in touch and we would love to explain further.  We can also demonstrate our system so that you can see (and feel!) the actual temperature drop as it runs.  You can drive our test car and see a unit on our test rig.  There’s nothing like seeing to help believing!


Steve Hinton is Director and joint owner of Air Cycle Technology and responsible for guiding the growth of this exciting and innovative engineering and manufacturing business. With a First Class Honours degree in Mechanical Engineering, Steve has a wealth of experience from years in the rail industry and management consultancy and is a Fellow and past Chairman of the British American Project, a transatlantic leadership development and networking organisation.

Contact Steve direct on, call 01444 410564 or arrange a free consultation here.

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