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Exhaust Gas Energy Recovery Technologies


kscarbel2
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Turbo Compounding

A 25 to 30 percent energy loss occurs when exhaust gases exit the engine. While turbochargers successfully recycle some of that energy, there are additional recovery technologies available.

One that Scania and Mercedes-Benz have both taken advantage of is turbo compounding. Similar to a turbocharger, the turbo-compound approach recovers waste-exhaust energy. The second turbocharger, the turbo-compound turbine, spins at 55,000 rpm. This motion is passed through turbine gears and a hydraulic coupling, then through the timing gears to the crankshaft. Stepping down the revs produces a useful boost in torque, which adds momentum when it reaches the flywheel. This means extra driving force without increased fuel consumption.

* Scania in 1991 became the world’s first truckmaker to mass produce turbo-compound equipped truck engines.

** Daimler began production of turbo-compound equipped engines (OM472 / DD15) in 2008.

One future possibility is a turbo-compounding equipped engine paired with a supplemental electric turbocharger in lieu of a conventional turbocharger. The turbo compound system would provide constant power, while electric turbocharger actuates during low speed overtaking and hill climbs.

Exhaust-Driven Motor Generator

An alternative to the complex and costly turbine-to-crankshaft gearing associated with turbo compounding systems is the exhaust-driven motor-generator.

With an exhaust turbine, compressor wheel and generator all mounted on a common shaft, trucks could use the recovered electrical energy to eliminate the engine-driven alternators.

The generator can also operate as a motor to preemptively spin a conventional turbocharger to eliminate low rpm turbo lag.

In the case of a hybrid truck, the recovered electrical energy would be stored to power an electric drive motor located between the engine and transmission, providing supplemental power during low speed overtaking and hill climbs.

In both cases, these new forms of supplemental power during peak demands would allow the use of smaller engines resulting in increased fuel economy.

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The exhaust-driven motor generator (turbo generator) is a brilliant and viable concept that has been waiting for technology to catch up with the idea. Now it largely has. Taking advantage of free waste exhaust energy while at the same time eliminating the engine (belt) driven alternator for enhanced fuel economy is intriguing.

John Deere was experimenting with the concept nine years ago (Note the John Deere-powered Navistar 8600 on page 23).

https://www1.eere.energy.gov/vehiclesandfuels/pdfs/deer_2006/session6/2006_deer_vuk.pdf

Honeywell (formerly Garrett AiResearch) worked on the concept in the late 1990s, but I believe their approach (with an electric motor sandwiched between the two turbo housings) was flawed.

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