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Tác giả :
Automotive engineers may have thought that after a century or more, the problems of handling heat in all parts of the powertrain had been solved. But another is emerging that particularly affects commercial vehicles (CVs) and off-highway equipment: keeping exhaust heat only where it is wanted.




The new challenge affects exhaust systems and involves a combination of the auto industry’s decades-old conflicting demands: efficiency, legislation, packaging and cost.


Terry Graham, managing director of thermal management specialist Zircotec Group, warns that expected upcoming global emissions standards will require improvements to the engine, catalyst and DPF (diesel particulate filter), each of which is likely to increase exhaust gas temperature.


But there is an added challenge, said Graham: “It’s a desire to keep heat in the exhaust to maintain turbocharger efficiency and response. Zircotec believes that internal temperatures in some cases will rise from 500°C to levels in excess of 700°C. In some instances, obtaining higher performance from catalysts and filters may require an increase in size, putting pressure on overall packaging as hot exhaust system components encroach on other systems that could potentially be vulnerable to heat damage.”


Reducing heat transmission


So Zircotec is now looking at ways of reducing heat transmission throughout the entire structure of the exhaust, including the exhaust manifold, thus reducing heat transfer from the hot exhaust gas to the outer containment to help maintain exhaust gas temperatures. This would also potentially have a significant impact on cold-start and the time taken for systems to warm up. It should also cut the operating temperature of the containment structure, reducing the thermal demands on the material and allowing more economic choices.


“Even the metal clamps holding the exhaust system can be a major source of heat loss. To overcome this, Zircotec is working with a supplier on ceramic coatings options to reduce this effect,” revealed Graham.


Safety legislation already limits external temperatures. Apart from flammability, the exhaust on a CV is more exposed to pedestrians. The maximum external temperature, currently as high as 480°C (896°F) on some components, could be mandated down to just 70°C (158°F), believes Graham.


“The traditional solution would be to add insulation around the affected areas, but conventional materials would typically need to be up to 70 mm thick to achieve the thermal barrier performance required,” he said. “There just isn’t the room for such a thickness in many applications without re-routing the exhaust line and introducing knock-on effects that would compromise other systems and overall packaging, to say nothing of the cost involved.”


On modern vehicles, heat management is often carefully controlled to reduce warm-up times and ensure sufficient heat is delivered to key systems.


But Graham estimates that the point is approaching where there is only just enough high-grade heat in the exhaust gas to drive these systems, so a different approach is required: “For reasons of efficiency, one of our clients is aiming to maintain the exit gas temperature at the tailpipe to no less than 87% of the temperature at which it leaves the engine. To achieve this we have to analyze the heat transmission through each individual component of the exhaust system.”


Aftertreatment considerations


A further key focus for Zircotec is on thermal management of the aftertreatment systems to significantly improve clean-up efficiency. Graham said Zircotec’s technology can be used inside and around the DPF and catalyst to increase the internal gas temperature and ensure heat is delivered and focused where it is needed, yet simultaneously reduce external heat transmission through improved thermal barrier performance. Together with one of its Tier 1 customers, the company is working on the insulation of a modular catalyst unit that would meet future requirements.


To reduce the packaging space required, Zircotec is investigating the use of its ceramic ThermoHold-based heat shield material, both with and without an integral conventional metal heat shield. By incorporating small integral air gaps, solutions have been devised that can operate in different environments and orientations to improve thermal barrier performance.


“The heat shields can deliver significant heat protection within a confined space, providing a highly cost-effective way to meet packaging requirements,” stated Graham.


He added that one of Zircotec’s significant long-term projects is to improve the thermal management of the exhaust manifold. To achieve this, the company has a collaborative program in place with a major vehicle manufacturer, and with exhaust component suppliers, via two “very different” approaches to examine the incorporation of ThermoHold ceramic coating within the manifold.


Prototypes have been produced, with the Zircotec ceramic-based material used and applied by a manifold supplier. Graham says early trials, and associated test results, are “very promising.”

Source: sae.org
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