Vamac® and Viton®: leading the way in low-pressure EGR hose and sealing developments
Automotive exhaust gas recirculation (EGR) systems, designed to reduce oxides of nitrogen (NOx) from exhaust emissions, have been around since the 1970s. They work by recirculating already combusted exhaust back into the air intake to dilute the incoming air/fuel mixture, thus reducing combustion temperatures well below the 1300+°C peak at which NOx is formed.
To date, such systems have functioned at high pressure in a loop from the beginning of the exhaust line to the air intake manifold. However, high pressure EGR systems are fundamentally inefficient, and recirculate only 15% to 20% of the gases. Also, because of the pressures and temperatures involved, they require heavy metal pipework with limited flexibility.
While sufficient to meet current emissions legislation such as Euro 5, high pressure EGR loops will be less compliant with the more stringent Euro 6 regulations which come into force from 2014.
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Table 1 (source ACEA 2007) illustrates the progression of Euro emission limit standards from Euro 0 to Euro 6, requiring dramatic reductions in gasoline and diesel NOx emissions from 1000mg/km and 1600mg/km respectively, down to 60mg/km and 80mg/km over the period 1991 to 2014.
In response, OEMs are developing new and significantly more efficient low pressure EGR loops to help meet forthcoming emissions legislation around the world, led by Euro 6 in Europe, EPA 07 in the USA and NLT in Japan. The ultimate target of these exhaust emissions limitation programs is zero NOx.
The future of NOx reduction, particularly for diesel engines, is the low pressure EGR loop equipped with an EGR cooler. This new technology can recirculate over 50% more gases than traditional high pressure methods, by processing some 75% to 80% of total exhaust. Further EGR system advances should also eliminate the need for NOx post-treatment devices, especially for small turbo engines.
Schematic of a low pressure EGR system showing critical hose lines between exhaust manifold and exhaust outlet at extreme right, with intermediate EGR cooler. Vamac® ethylene acrylic elastomers and Viton® fluoroelastomers are at the forefront of developments in flexible hoses and hose-end seals, providing long-term temperature and acid resistance for these key applications. |
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Since the low pressure EGR loop operates at lower pressures and temperatures, rigid metal tubing can be replaced by rubber hoses and seals which are light and flexible, easy to install, do not transmit vibration and contribute to weight reduction. However, the components of exhaust gases can be highly damaging to many rubber materials.
The main components of exhaust gas from diesel and gasoline engines are NOx, acid, a small percentage of fuel, and particulates in the case of diesel. In addition, air + engine oil + acids + some fuel can be found in other air management lines such as positive crankcase ventilation hoses and other sensor hoses.The key questions related to future low pressure EGR loop technology are:
- Will standard rubber hose and seal components be capable of resisting these aggressive gases and fluids?
- Will the chemical environment change in the future for other air management applications like clean air ducts, hot and cold side turbo hoses, intercooler seals, quick connector seals, and air intake manifold gaskets?
Standard elastomeric materials such as halogenated elastomers (CR, CPE, CSM, ECO, ACM) can be adversely affected by the presence of acid (depending on concentration and chemical nature) and other exhaust components, while silicone does not provide an effective barrier to fuel permeation and engine oil.
The risk is that some widely used rubber products could be the weak link in otherwise highly sophisticated EGR systems because of low acids resistance, compromising long-term sealing ability and deNOx performance.
DPE offers two high performance elastomers that can meet demanding flexible hose, seal and connector specifications for low pressure EGR system applications: Vamac® ethylene acrylic elastomer (AEM) and Viton® fluoroelastomer (FKM). Standard recipes based on Vamac® and optimized Viton® recipes both demonstrate excellent resistance to exhaust acids and gases. Vamac® can provide a cost-effective EGR hose and sealing solution at peak temperatures up to 150°C, while Viton® can be considered the primary candidate for highly concentrated acid environments at peak temperatures up to 230°C.
It is important to select the appropriate Viton® grades and recipes because some acids can be very aggressive toward conventionally formulated FKM, causing blisters, swell and/or surface attack.
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Chart 1 shows the superior performance of grades of Vamac® and Viton® versus conventional halogenated elastomers, in terms of hardness change, tensile strength, elongation and volume swell, when exposed to EGR acid condensate for 168 hours at 90°C — typical exhaust gas condensate conditions that destroy CPE and ECO compounds.
The data indicate that while the lower cost option of alternative rubber materials may appeal, EGR systems OEMs should consider the greater cost of failure in operating conditions that exceed the capabilities of many elastomers.
Where life-time service, low permeation and high resistance to exhaust gases containing acids is a priority, Vamac® and Viton® can provide the right solution.
DPE elastomer technologists are working closely with OEMs, rubber compounders, rubber finished part manufacturers and automotive Tier 1 suppliers to develop new technology and new solutions to the challenges of new low pressure EGR loops. The company will shortly organize a webinar on elastomeric solutions to low pressure EGR loops, to provide a discussion forum on the state-of-the-art. More information on the webinar date will be available shortly.
For more information, please contact us.
