The automotive exhaust market has grown in importance with the emphasis that is now placed on emissions control worldwide. What was once a relatively simple product has become increasingly complex and, writes Matthew Beecham, more expensive.
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The complete exhaust system is typically divided into the ‘hot end’ and ‘cold end’. The hot end includes the exhaust manifold, flex coupling, warm-up converter, air-gap down pipe, and underfloor converter. In other words, it is the end closest to the engine. The cold end includes the hangers and brackets, resonator, intermediate pipes, muffler, heat shields and exhaust tip.
The exhaust is one of the most highly stressed systems on a car. It must operate under extreme conditions—undergoing considerable changes in temperature. With severe vibration it is also subject to internal corrosion and damage from stones, mud, water and road salt.
The exhaust has three main functions—two environmental and one performance-related. The first function is to minimise carbon monoxide, unburnt hydrocarbons and oxides of nitrogen, and direct them out of the engine and away from the passenger compartment. The second is to minimise noise created by the car’s engine. The third is to help optimise engine performance by providing a continuous, smooth flow of gas out of the engine. The exhaust can also be ‘tuned’ to produce a distinctive and characteristic note, such as that of a Porsche or Alfa Romeo.
Technical developments in the hot end of the exhaust are highly influenced by the new emissions legislations. Consequently, we are seeing the emergence of new exhaust system technologies for both gasoline and diesel engines.
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By GlobalDataAs each new emissions regulation swings into force, more and more content is added to the exhaust in order to comply. This adds value to the overall price of the system. Manufacturers believe that with increasingly strict emissions regulations forecasted, the emissions control content per light vehicle will rise over the next few years as a result of the introduction of multiple catalytic converters per vehicle, heat exchangers and particulate filters. The emission standards are, however, technology-neutral. That means automakers are free to use whatever technology they choose in order to meet the current standard.
Caught in a trap
A particulate filter ‘traps’ (or collects) soot particulates flowing through the exhaust and burns them off periodically. Particulate filters consist of a ceramic block with a vast number of fine porous channels, which are alternately open or blocked on one or the other side. The exhaust gas carrying the soot passes through the ceramic wall into the adjacent channel, whilst the soot particles remain trapped on the ceramic surface.
Diesel particulate filters for OE applications remove particulate matter from diesel engine exhaust in a closed loop system. When gases carrying particulate matter flow through the filter’s substrate walls, particulate matter is trapped and collected in the filter. A regeneration process, triggered by the engine management system, then burns off the soot using either a fuel borne additive or a catalytic filter coating. Manufacturers recommend that the filters are completely cleaned or replaced at a service interval between 15,000 and 25,000 km.
Diesel particulate filters for aftermarket applications typically work in an open loop system wherein there is no communication between the filter and the engine management system to trigger the burning process. Instead, the design and the coating support continuous regeneration of the filter. The advantage of an open loop – or passive system — is it doesn’t require any calibration at the time of installation.
In the run-up to the introduction of Euro 5 in 2009, the current trend in Europe is toward coated filters. “Improving the thermal management of filters to improve regeneration characteristics, while minimizing the fuel usage for active regeneration, is a priority and will lead to greater use of close coupled filters,” said Dr.-Ing. Wolfgang Reuter, vice president, sales and engineering, Tenneco Emission Control International, Geschäftsführer Heinrich Gillet GmbH. “Additive-based systems remain a useful technology for low temperature regeneration and may gain popularity in the non-road segment as regulations take effect.”
There are moves being made to reduce the size of the filter itself through enhanced filter structures. “Basically, the size of a filter is generally defined by the amount of ash it has to store during its lifetime,” said Reuter. “This depends on the engine oil consumption as well as on the expected useful lifetime, e.g. 240,000km. Tenneco is working on improved filter structures that would reduce filter size significantly since ash storage would no longer be the deciding factor in filter sizing. Lower thermal mass and lower backpressure, both benefiting fuel economy, would be important ancillary benefits.”
In recent years, Tenneco has expanded its capabilities in diesel particulate filters and won new business in Europe. The supplier recently announced that its global production of particulate filters will expand to about 2.8 million units by 2012.
Catalytic conversations
Emitec, the German catalyst manufacturer founded in 1986 as a joint venture between Siemens and GKN, generally has a low profile, but has nevertheless become one of the biggest catalyst manufacturers in the world, employing over 800 people in Germany and the US. Its success has been built on its pioneering work building catalytic converters with metal substrates, and began with its launch of the electrically-heated Emicat device in 1994, which showed how heating a catalyst offered major improvements in emissions reduction. One of its recent inventions is a pre-turbo catalyst for diesel engines that makes use of the higher exhaust gas temperatures found before the turbocharger. The company is especially interested in the challenges of emissions reduction for diesel engines and sees major business potential there, both in Europe and eventually the US. Emitec is also working on emissions reduction for HCCI engines.
The increasing number of catalytic converters inevitably leads to an increasing demand for noble metals. Yet limited resources and correspondingly high prices are prompting the development of converters with higher efficiency and lower catalyst ageing. “That tendency continues and can be read from the past where closed couple converter, thinner walls and high cell densities have significantly increased efficiency and thus lowered even cost,” said Wolfgang Maus, CEO of Emitec GmbH. “Now the risk of still rising noble metal prices will be a threat for NOx-Adsorbers and this supports the investments into SCR [selective catalytic reduction] technologies.”
Reuter added: “Noble metal costs, especially platinum and rhodium have increased substantially in the last years. Improvements in the washcoat technologies have allowed a reduction of noble metal content per catalyst.”
Manufacturers are working toward positioning the catalytic converter as close to the manifold as possible thereby making the catalyst smaller. Catalytic converters are also becoming more and more integrated into the manifold. “Close coupled is state-of-the-art today,” said Maus. “Stating this, it is also true that the position behind the manifold or even the turbocharger will not be close enough for diesel and gasoline engines with innovative combustion processes and therefore even colder exhaust gas temperatures. New challenges with regard to cold start efficiency will come up for all lean burn concepts. Pre-turbo-charger catalysts will be one innovation to satisfy that thermodynamic must. More development efforts are needed to solve that cold start issue with regard to particulate reduction.”
In fact, Emitec has managed to push back the technical boundaries on a number of fronts over the years, recently introducing its so-called Metalit catalytic converter. Maus added: “Environmental consciousness creates tougher environmental limits and those create higher expectations for a better and healthier environment. Engine technology has been forced to provide lower engine out emissions which has to be included into the future demand. Last but not least, we love cars and acknowledge that mobility has been and will be a main driver for prosperity. In that way we take the risk and develop components our customers might not yet want. Turbulent flow substrates fall under this category. After about three years of development and seven years of introduction they are now in real mass production with Volkswagen’s terrific new common rail engines.”
Hitting the right note
Exhaust sound used to be simply a by-product of engine and vehicle development, but for today’s sports and luxury cars, it is a carefully-manipulated ‘product’ that must fit in with the car’s character and the brand’s image.
Exhaust noise is caused by expulsions of gas from the cylinders. Indeed, the ambient conditions prevailing in an exhaust system are extreme: excessive heat, moisture, a high sound pressure level and severe vibrations. To reduce this noise, sound energy must be reduced. This is achieved in two ways: absorption and reflection. By the absorption method, sound waves enter a porous material where energy is converted by friction into heat. The absorbent material consists of mineral wool or fibreglass. In the reflection process, sound waves are reflected by baffles; the advancing and retreating sound waves cancel themselves out. Today, most exhaust systems use a combination of the two silencing methods.
Legislative and customer pressure is being placed on automakers to reduce the noise of their vehicles, both inside, for greater comfort, and outside for reduced environmental noise pollution. As with all aspects of car design, noise control is a compromise. A simple change of tyre material can reduce road noise, but at the cost of wear and safety. More sound insulation can make a car quieter inside, but at the cost of space, weight and fuel efficiency. Luigi Lubrano, R&D manager at Magneti Marelli Exhaust Systems, told us: “We think that, beside the efforts to develop and produce exhaust systems for the most advanced emission legislation, there will be an effort to prepare the exhaust system for the future 71 dB acoustic limit. When this limit becomes effective, it will have an impact on the exhaust weight and cost.”
In the recording studio, manufacturers have focused on acoustical engineering of exhausts to help OEMs differentiate individual brands. “In addition to technical and economical aspects, the emotional component of modern cars gains more and more importance,” said Eberspächer’s Dr.-Ing. Jan Krüger. “The sound of the exhaust system is one of these emotional components. Due to the legislative limitation of the pass-by-noise level, it is not possible to emphasise the sportiness of a vehicle by simply increasing the emitted sound pressure level. The main focus of the Eberspächer acoustic development is the fulfillment of legislative requirements as well as the maximum interior as well as exterior sound pressure targets set by our customers. However, besides that, also a deliberate sound design which emphasises the typical brand identity of the OEM is requested. At Eberspächer, this is taken into account by the use of psychoacoustic tools as well as careful jury evaluation including double blind tests carried out in our sound studio together with in-house as well as OEM experts. By this, a systematic comparison and time-efficient optimisation of the exhaust sound takes place.”
Exhausting business
On balance, vehicle emission control products and systems play a critical role in safely conveying noxious exhaust gases away from the passenger compartment and reducing the level of pollutants and engine exhaust noise to an acceptable level. Precise engineering of the exhaust system – from the manifold that connects an engine’s exhaust parts to an exhaust pipe, through the catalytic converter that eliminates pollutants from the exhaust, to the muffler – leads to a tuned engine sound, reduced pollutants and optimised engine performance. “OEMs are requesting more development support from system suppliers like Tenneco as the emissions control systems become more complex,” concluded Reuter. “Traditional emissions control suppliers have gone from being component suppliers to full system suppliers and now full technology suppliers.”
Matthew Beecham
See also: Global market review of exhaust systems – forecasts to 2014
