Thermal management plucks CAFE's low-hanging fruit

  • 07-Dec-2017 04:30 EST
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Chart shows thermal upgrades, their estimated effect on fuel economy and cost per 1% improvement. (ITB Group)

Thermal management systems may offer the most 'bang for the buck' in the quest to improve vehicle efficiency and thus meet stringent U.S. Corporate Average Fuel Economy standards. Although some of the 'low-hanging fruit' in this area has been picked, opportunities still exist to further improve thermal management before engineers must shift to more costly investments.

This was the widely-expressed belief by speakers at the 2017 SAE Thermal Management Systems Symposium, led by a keynote from Sean Osborne, director of the ITB Group, a consulting firm specializing in the subject.

The need for improvement becomes urgent in 2018. That's because when the 2019 model year begins, the NHTSA raises the penalty for missing the CAFE target, from the present $5.50 per 0.1 mpg shortfall. Initially the new penalty was set at $14 per 0.1 mpg, but the industry was granted a review, based on claims of negative economic impact and objection to the way an inflationary adjustment to CAFE was applied. Environmental groups are fighting the review delay.

Market shift to trucks, SUVs an issue

To date, automakers have met the CAFE standards, but the shifting of the market from passenger cars to light trucks and SUVs has tightened the margins, and forced high use of retained and/or purchased CAFE credits.  Fines paid by OEMs during the 2010-14 period include $46.2 million by Jaguar Land Rover; $28.2 million by Daimler; $17.4 million, Volvo; $4.8 million, Porsche, and $3.6 million Fiat Chrysler (FCA).

FCA, with its 2015 sales concentrated in Jeep and Ram trucks, also used 33.4 million credits and was projected to use 62.0 million for 2016. The amounts it spent to accumulate the needed credits is confidential.

Cost of installing new technology has to be balanced against buying credits. FCA was the first U.S. maker to go across the board with R-1234yf, a low-global-warming refrigerant that when used carries significant credits (13.8 g CO2 for cars, 17.2 g for trucks).

To stimulate sales, the Chrysler Pacifica plug-in hybrid (PHEV) is cheaper in transaction prices than the gasoline-only model for the most popular configurations. FCA has yet to install direct fuel injection on its high-volume 3.6-L V6, but it has a system fully engineered and ready to deploy when its use is more cost-effective.

Flow valves, heat recovery

For conventional powertrains the lowest-hanging “fruit” to date, according to ITB's Osborne, consists of such features as high compression ratio, liquid-cooled EGR, idle stop-start, low-friction lubricants and active grill shutters. Of the energy produced by the engine, 26% is diverted to the cooling system and 32% to the exhaust, according to Jaguar Land Rover estimates based on the NEDC—New European Drive Cycle. Other opportunities in the thermal management area are  focuses of attention.

BMW has been using a duty-cycle-controlled electric water pump for more energy-efficient cooling on its gasoline engines for several years. But coolant flow valves may be a less-expensive alternative for engines with complex coolant flow patterns based on temperatures in the cylinder block vs. head(s) and ambient temperatures.

Further, recent research by Bosch, in its TMSS presentation, indicated a flow valve system also could contribute to range extension in battery electric vehicles (BEVs). Using the GT Suite from Gamma Technologies, a Bosch team projected that recovering heat from electronics, along with a heat pump, could increase BEV range by perhaps 25 km (16 mi) by avoiding re-use of a battery heater to enhance battery capacity. This approach also was seen as less expensive than adding battery kW·h capacity in cold ambient.

Although exhaust heat recovery seems likely to be among the higher-cost systems available, two already are in use on U.S. market Toyota Prius and Hyundai Ioniq/Kia Niro hybrids. They’re basic systems, with a heat exchanger in the exhaust transferring heat to engine coolant for faster warmup.

In the Niro/Ioniq split system, early heating of the block reduces engine friction, increasing use of the all-electric mode. So fuel economy in the EPA city cycle was increased up to 7%, claims Kia for the Niro. This is much more than 1% estimated for such a system by a Tenneco/Computational Science Experts Group (CSEG) research team led by Tenneco’s Dr. Dipanka Sahoo. It indicates selection of the right application is key.

The Tenneco/CSEG estimate is based on 85% effectiveness in the first 250 s of warmup and an average of 19% heat capture over the entire cycle.

Other systems the Tenneco team also evaluated converting exhaust heat into mechanical work or electricity, which can reduce alternator load and/or serve as hybrid assist devices.

One is the “TEG” (thermo-electric generator), which uses exhaust heat to provide the high temperature of a temperature differential that produces electricity from a solid-state device that is the reverse of the Peltier effect used for cooled-heated seats. The thermo-electric materials are rare-earth type, not inexpensive (particularly those with higher efficiency). However, work in nanotechnology could improve efficiency and minimize need for rare-earth substances, both in TEG and Peltier applications, the Tenneco team believes. Fuel economy improvement was projected at 0.7%.

Thermo-acoustic potential evaluated

The thermo-acoustic traveling wave system also was evaluated.  It uses a pair of heat exchangers (one hot from absorbing exhaust heat, one cold).  A porous medium called the regenerator, in between, establishes a temperature gradient between the two, using helium as a working fluid. This results in an oscillating gas flow, generating sound (acoustic) waves that are amplified and flow through a tuned duct into a mechanical device (a generator/ power transducer) to produce useful mechanical work or convert to electricity.

The device can be compact and requires no rare earth or other exotic materials. In itself it has no moving parts. And although its fuel economy improvement was calculated at just 0.45% on an EPA city cycle, the TA device develops high power on the highway cycle, and  including that the improvement was 2.7%.

Organic Rankine cycle was cited as another choice, but because it results in larger-size systems the Tenneco group said it had more likely application in trucks. It’s somewhat akin to an A/C system,  using an evaporator to capture heat and build pressure on a working fluid (a refrigerant).  Then it releases the pressure through an expansion device that performs mechanical work.  Residual heat is rejected through a condenser. See related http://articles.sae.org/15755/.

Available data led the Tenneco researchers to develop fuel economy numbers modeling a Class 8 truck, in which three steady-state operating conditions were investigated. Heat recoveries were 56%, 59% and 64%, accumulations of 67.7kW, 84.9kW and 121.9 kW of energy, with conversion efficiencies of 11-13%, and fuel economy improvements of 3.6-4.1%.

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