Waste Heat Recovery for Trucks

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This submission shows a more effective method of recovering waste heat energy in commercial vehicles than the Rankine cycle systems and TEG's as used on the DOE SuperTrucks. The reason for utilizing waste heat is to reduce the vehicles fuel usage. There are four main sources of heat rejection; the exhaust, charge air cooler, EGR cooler, and the radiator. Each of these sources require very different Rankine cycle systems which would be very difficult to combine into a single system. Rankine systems are also very complex, expensive, bulky, potentially dangerous, and require sophisticated control systems. In addition their efficiency is rather poor.

By combining the compressor outlet to the uncooled EGR, these four sources of energy can efficiently be converted to electrical power utilizing three Stirling engines, Each of these engines would be sized for its application and utilize high pressure helium as the working gas. Their efficiency will be higher, the controls much simpler, and they would be fail safe.

In the comparison shown in the attachments, a smaller diesel engine could be utilized to generate similar vehicle performance to a larger baseline engine. Besides for the reduction in cost of the smaller engine, the conventional starter, alternator, and 12v battery pack could be eliminated. A high voltage battery would replace the current 12v battery,and could provide some regenerative braking, torque filling during transitions, and hotel loads. At road load the three Stirling engines (exhaust, intake, coolant) would provide a net 8%, 3%, and 6.5% reduction in fuel consumption respectively, and at full load these gains would be 9%, 4.5%, 6%. The coolant Stirling gain assumes the use of a higher temp non aqueous cooling system. Assuming a 2.5% gain from hybrid operation, the vehicle could see an overall 20% net improvement in fuel mileage. This would increase an already good 7.5 mpg to 9 mpg saving about 2700 gal of diesel per year with a cost savings of around $8500 per year.

Currently there are lower cost ways of achieving this level of fuel saving namely aero devices and lower rolling resistance tires, but these improvers are already becoming standards in the industry so future gains will become much harder to achieve. Potential raising fuel prices and or tightening effeciency regulations will eventually drive truck manufactures to more extreme measures. As estimated these Stirling engines would be 10"-12" in diameter, and 18"-24" in length.

A lower cost system could use just the exhaust Stirling and no buffer battery between it and the traction motor. This system would lose the downsizing benefit and hybrid effect, and would need to retain the conventional batteries, starter, and alternator. The fuel savings for this system would be about $3400 per year. This type of system may also be applicable to passenger cars and light trucks.


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  • Name:
    Robert Rowells
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    I'm an automotive engineer that specializes in engines and I have a passion for improving efficiency.
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