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New Potential Exhaust Gas Aftertreatment Technologies for “Clean Car” Legislation
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Abstract
The California Air Resources Board recently adopted new passenger car regulations for future emissions levels. Some eastern US-states are considering similar legislation. Current exhaust aftertreatment technologies cannot meet the low emissions as defined in the new regulations.
New technologies will be neccessary if a further 90 % decrease of emission level are to be reached at the end of this decade. Since Volvo as the first car manufacturer introduced the closed-loop three way catalyst system 1977 (see reference-list 1), the catalyst - and engine optimization technology have matured to a 90 % reduction of the tail-pipe emissions. Even so, cold-start emissions represent the greatest contribution of the emissions from today's catalyst-equipped cars.
Good cold-start conditions of the engine together with an early lambda-control are favourable to low HC/CO-emissions in bag 1 in the FTP-test cycle. The use of an engine block heater to warm the coolant (10) before the engine is started, has been shown to be effective. Improvements can also be achieved by heating of the inlet air or the fuel-air mixture with a resistive heating system.
Such systems improve air/fuel mixture preparation and makes driveability and transient response better in the early phase of the cycle.
They only bring marginal improvement in catalyst light off time however, only by better driveability at retarded ignition settings and with secondary air injection.
There are several means to heat the catalyst, and solutions have been shown like the use of added fuel burners in the exhaust manifold, or with heating elements added to the main catalyst.
In many later presentations, technical solutions that activate the catalyst earlier during the cold-start and also before the engine-start, have been described (2, 3, 4, 5, 6, 7 and 8).
Several solutions how to achieve an “active” catalyst aftertreatment system have been described. Such systems have been based on a resistive electrical heating of a metallic substrate catalyst together with secondary air injection. In presentations so far, mainly preheating followed by an after cranking heating together with air-injection have been described. In this way, very low tail pipe emissions of unburned hydrocarbons (HC) and carbon monoxide (CO) have been achieved. NOx emissions have mostly remained at the same level or been significantly higher.
This paper will discuss the potential for an advanced electrically heated start-up catalyst integrated with the main under-floor catalyst. This prototype includes a non-preheated, (that is, key-on heated) catalyst system. The potential of the system will be viewed against the problems of the introduction of a high-power-consumer to the vehicles. This presentation will concentrate on the many theoretical and technological problems that have to be overcome to make an EHC-systems possible, rather than to present complete emission results and technical solutions. It is however shown that this technology may be one way to achieve the very low emission targets legislated in California for all pollutants in the exhaust gases, if the problems are successfully overcome.
VOLVO has for a long time carried out dedicated development of methanol (M15, M100 and later M85/FFV) vehicles, in order to find technical solutions to the particular problems associated with this alternative fuel.
The Californian legislative situation has made the development of FFV vehicles interesting, in regards to fuel economy ratings and in regard to the possibility of lower exhaust emissions. Of particular difficulty is the emissions of formaldehyde. The need for a very early light off of the catalyst system made the study of an active EHC catalyst system natural, and testing of EHC systems has been mainly linked to M85/FFV operation. The low exhaust temperatures for M85 combustion is a serious problem for such engines.
For reference, tests of the EHC technology applied also to gasoline engine operation have been carried out. This paper discusses the possibilities and potential of the studied EHC system in general terms for such an application.
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Gottberg, I., Rydquist, J., Backlund, O., Wallman, S. et al., "New Potential Exhaust Gas Aftertreatment Technologies for “Clean Car” Legislation," SAE Technical Paper 910840, 1991, https://doi.org/10.4271/910840.Also In
References
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