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I was reading an old article about the use of TEGs (thermoelectric generators) in place of alternators in cars to improve efficiency. The main problems that engineers face with this technology is the horrid efficiency of peltier elements (5-9%), their low operating temperature (not suited for direct exposure to the heat of the exhaust), and cost. So I was thinking... why not use a Stirling engine? It can withstand a lot higher amount of heat and has an efficiency up to 50%. Considering that a car running on a freeway consumes about 10kwh per 100km to move and 35kwh goes out of the exhaust, the potential of recapturing some of this energy seems very appealing to me. Is it because of the weight? Is it the cost? Any ideas?

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The answer is virtually the same for "Why don't we use Stirling cycle [instead of Otto] engines? "

Cost (especially adding an entire auxillary system), weight (or power/weight ratio), complexity...

The answer is virtually the same for "Why don't we use steam turbines to power our cars?"

Ultimately Internal Combustion engines are here to stay. Consumerism is driven mostly by fuel cost, less by pure "efficiency" or environmental concerns. And now with high Air Fuel Ratio "lean-burn" technologies like Ecoboost, SkyActive, etc, more hyper efficient diesels like the VW/Audi TDI, use of once "exotic" materials involving composites, laminates, nanoparticles... we are seeing dinosaur burning IC monsters that are more efficient than hybrids, while still maintaining emissions readings lower at the tailpipe than the intake air.

This response is certainly just my opinion, and doesn't address the technical aspect -- but @ihniwid's comment certainly does.

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    "The answer is virtually the same for "Why don't we use Stirling cycle [instead of Otto] engines? " Not exactly. Stirling engines would be a bad choise as a prime power source. They respond slowly to rpm change. "Cost (especially adding an entire auxillary system)" hybrid cars have more complex auxiliary systems and have known some popularity. Im not saying that a stirling would replace an otto engine. I am just wondering if a constant mechanical load could be removed by use of waste heat. – PaulB Jun 3 '16 at 14:45

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