Turbos with EGR are key to gas, diesel compliance, panel says

  • 26-Apr-2010 11:36 EDT
Ning Lei.jpg

Two-stage turbocharging with two-stage EGR delivers improved thermodynamics, driveability, and low-speed torque, said Ning Lei, Navistar's Vice President of Advanced Combustion Engines.

Just as the combination of direct fuel injection and turbocharging enabled the diesel-engine renaissance in the 1990s, so will the pairing of turbos with increasingly sophisticated exhaust-gas recirculation (EGR) strategies help both diesel and gasoline engines meet more stringent upcoming CO2 regulations, according to a panel of experts during the recent 2010 SAE World Congress in Detroit.

In the panel “Turbos and EGR are hot: What are the cooling and lubrication challenges that need to be addressed?” experts confirmed that turbochargers will play a major role in allowing ICEs to meet the proposed LEV3 emissions standards for light-duty vehicles. Additionally, the complementary technologies will be inseparable for cleaner off-highway engines in combination with aftertreatment systems.

Turbocharging with increasingly precise fuel and EGR control “will continue to help improve fuel efficiency and greenhouse-gas reduction for many years to come,” noted panel moderator Dean Tomazic, Vice President of FEV’s Engine Performance and Emissions group. The systems are “key to reducing NOx emissions,” he said.

Diesel-industry engineers are increasingly deploying EGR strategies for in-cylinder NOx control. Regarding commercial-vehicle diesels, the trend is shifting from a single turbocharger and EGR cooler per engine to two-stage turbos with two-stage cooling, noted Ning Lei, Director of Advanced Combustion Engines at Navistar.

“This technology delivers much improved thermodynamics, balancing driveability and low-speed torque,” she said, but two-stage systems add cost.

Use of boosting with EGR for in-cylinder NOx control enables mixing-controlled combustion and will help implement use of alternative fuels.

Lei asserted that integration of the various subcomponents and more powerful control algorithms—from the turbocharger and advanced-EGR system to the use of injection pressure, compression ratio, valve timing, and thermal management—is one of the most critical aspects of future diesel development. “It all has to come together without affecting driveability,” she noted.

Lei and the other panel experts expect a significant rise in diesel-injection pressures, “from today’s 2200-2500-bar, to beyond 2700-3000 bar in the near future,” she predicted, then asked the audience, “Will we use injection pressure for emissions control or fuel economy?”

Lei noted that Navistar achieved significant brake-specific fuel consumption (BSFC) gains on its latest 2010 MaxxForce 13-L heavy-truck diesel by taking a systems approach to thermal management.

“We’re using two-stage EGR with variable cooling; the low-temperature combustion means we don’t need very high rates of EGR,” she explained.

Innovation in turbocharger design also offers gas and diesel engine developers more opportunity to increase overall performance while lowering emissions. Karl-Heinz Bauer, Vice President and Chief Technology Officer of Honeywell Turbo Systems, noted that EGR puts significant mechanical stress (fouling, corrosion, particulate matter damage) on turbochargers, which can be solved by new compressor-wheel designs and material coatings.

Bauer also favors two-stage turbos—“high EGR rates at part load and good transient torque”—but said they’re expensive and require a larger package. He said variable-nozzle turbos improve EGR flow control while boosting engine braking capability, torque response, and average fuel efficiency.

For downsized boosted gasoline engines, cooled EGR rates of 15-20% help reduce pumping losses, lower exhaust temperatures, and help reduce spark knock and emissions. “Gasoline turbo engines are heading toward diesel-like designs” and combustion strategies, Bauer said.

Reducing thermal inertia is a major desire of gasoline-engine development teams. Turbocharger supplier BorgWarner is working on new materials technologies to reduce thermal inertia—when the turbocharger acts like a heat sink—to allow a greater volume of hot exhaust gas to enter the catalytic converter and thus enable quicker light-off, noted Heinz-Ulrich Froehm, Vice President of Sales for BorgWarner Turbo and Emission Systems in Germany.

Added Bauer of Honeywell, “Two-stage systems are not the way to improve cat light-off!” He said his company is developing thinner-wall housings combined with multilayer shielding for its future turbo units.

Aftertreatment strategies and technology, as well as combustion progress, also are affecting the turbo/EGR equation, noted Kevin Hoag, Associate Director of the University of Wisconsin’s Engine Research Center.

“[Gas] engine calibrations will call for reduced EGR as aftertreatment systems become more effective,” Hoag predicted. Selective-catalytic reduction (urea-based SCR) systems are quickly gaining efficiency, but backing off EGR in the engines adversely affects fuel efficiency, he said. Research at the University of Wisconsin continues in this area.

At the same time, Hoag said, advanced combustion systems “are progressing to the point of supporting much higher EGR levels.”

Ford’s Bob Fascetti had the advantage of actually having a state-of-the-art diesel—the new 2011 6.7-L V8 available in Ford Super Duty trucks—outside the AVL auditorium to refer to in his panel remarks. As Director, Large Gas and Diesel Engine Engineering, Fascetti sees clear benefits to using cooled EGR. But challenges remain, he said.

“We may need a mixing enhancement in the high-pressure loop. And the low-pressure EGR loop has greater transport delay, making it more difficult to control in-cylinder EGR rates during transient operation,” he said.

Another challenge to low-pressure EGR is it exposes the turbo compressor wheel and charge-air cooler to water condensation, which includes condensed acids, byproducts of combustion and aftertreatment. “Soot debris also can damage the compressor wheel—we may need a protective debris filter,” he said.

Looking at the future, Fascetti predicted boosted gasoline engines may require multiloop (high and low pressure) EGR, along with improved vehicle cooling systems.

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