When compared with traditional gasoline cars, plug-in hybrids, like the 2012 Toyota Prius Plug-in Hybrid and the 2013 Chevrolet Volt are already capable of astonishing gas mileage figures. 

But how do you make plug-in hybrids even more efficient?

One answer is to add a larger traction battery pack, but the larger a battery pack, the heavier it is, meaning gas mileage can suffer. 

A team of researchers at the University of Stuttgart and the Ohio State University think they have another solution, however: waste heat recovery systems. 

Using waste exhaust heat to improve efficiency isn’t new. In fact, many automakers use simple heat recovery systems to divert waste heat from the exhaust back into the air intake to help improve engine warmup times, reducing engine wear and improving gas mileage. 

The simple Organic Rankine Cycle (ORC) waste heat recovery system proposed by the researchers from Stuttgart and Ohio however, is slightly different. 

As the team explained in their presentation of the paper at the ASME Internal Combustion Engine Division 2012 Fall Technical Conference, the system takes excess exhaust heat and turns it into electricity. 

The system passes the hot exhaust gasses over pipes containing Pentafluropropane--a hydroflurocarbon refrigerant known as R-245fa--transferring thermal energy from the exhaust into the refrigerant. 

Because it has a lower boiling point than water, the refrigerant expands, turning into a vapor that can power a turbine to generate electricity. 

2012 Toyota Prius Plug-In Hybrid, Catskill Mountains, NY, Oct 2012

2012 Toyota Prius Plug-In Hybrid, Catskill Mountains, NY, Oct 2012

After passing through the turbine, the refrigerant is then condensed back into its liquid form before recirculation.

With internal combustion engines wasting as much as 35 percent of the energy put into them as heat, ORC systems could have a dramatic effect on improving gas mileage for future cars. 

Already, the researchers say they have recovered up to 10 percent of engine waste heat under highway driving conditions using a prototype plug-in hybrid designed by Ohio State University. 

That translates to a potential 1.9 kilowatts of electrical power, or a 7 percent improvement in gas mileage. 

It’s worth noting however, that the system hasn’t yet been tested in any production plug-in hybrids, although both BMW and Nissan have worked on similar systems in the past.


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