You will find the figures mentioned in this article in the German issue of MTZ 9/2005 beginning on page 670.
HC-Adsorber zur Minimierung von Kohlenwasserstoffemissionen bei nicht betriebenen Fahrzeugen
HC Adsorber to Minimise Hydrocarbon Emissions from Vehicles at Rest The subject of this article is the reduction of evaporation emissions in the air intake system of gasoline engines. Using as an example the HC adsorber “Pleatsorb” by Mann+Hummel, the important function of activated carbon as a hydrocarbon absorbent is described. The “evaporation emissions” research project from the FVV (Internal Combustion Engine Research Association), in which Mann+Hummel is actively involved, is also outlined.
1 Introduction In recent years, there has been continuous further development of technology in the after-treatment of exhaust gases in order to meet legal standards limiting emissions from motor vehicles. In 1961, a law was passed in California which demanded that crankcase gases were recirculated into the combustion system. In 1977, Volvo introduced the three-way catalyst as part of a secondary exhaust gas aftertreatment system, which was a milestone towards improving air quality in the USA. Finally, as part of the “Federal Clean Air Act” in the 1980’s, attention was focussed on the formation of ozone as a critical element of smog. Hydrocarbons (HC) significantly contribute, as so-called precursors, to the formation of ozone in the atmosphere through photochemical reactions.
Authors: Christian Hoppe, Frank Pfeiffer and Jens Sohnemann
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In order to reduce HC emissions, it is necessary not only to reduce HC exhaust gases, but also to improve the emissions characteristics of vehicles at rest. Even when the engine is switched off, hydrocarbons from the tyres and from plastics are released by evaporation into the atmosphere. Since the beginning of the 1970’s, attention has therefore also been focussed in the USA on the continuous improvement of emissions characteristics of vehicles at rest. The front-runner with regard to legislation is the CARB (California Air Resources Board) in California. The laws are in force in four other States in the USA – New York, Massachusetts, Vermont and Maine – as well as in California itself. New vehicles are classified according to the evaporation emissions measured from the entire vehicle. The well-known SHED test is employed. In this process, total HC emissions, such as for example from the tyres, coatings or tank systems, are measured in a specified testing cycle. The aim of the CARB is to substantially reduce emissions from motor vehicles and achieve as far as possible “zero emissions” in a so-called ZEV (zero emission vehicle). In order to meet these requirements in the future, it will be necessary to introduce new design features to reduce the emission of hydrocarbons from the whole vehicle, and from parts of the fuel system in particular.
2 Legislation Within the legislation from the US Federal Authority EPA (Environmental Protection Agency) and CARB, a distinction is made between exhaust gas emissions and evaporation emissions. The latter are recorded in EPA Tier 2 [1] and CARB LEV II [2]. The CARB requirements to be implemented in stages by 2007 are defined in Figure 1. In a special process, accounts are set up for each manufacturer regarding the emissions characteristics of the vehicles sold. The OEM’s also have the opportunity to earn so-called “credits” for the registration of zero emissions vehicles (ZEV). A lifetime guarantee of reliable function over 150,000 miles or 15 years, as well as permanent, non-detachable installation of a hydrocarbon adsorber in the air intake system is required by law.
3 The Air Intake System as a Source of Emissions In the air intake system, Mann+Hummel has introduced new developments to reduce HC emissions from vehicles at rest. Along the entire air intake, there are several potential sources of evaporation emissions. With conventional engine designs, intake valves remain open, depending on the position of the camshaft. Any unburned hydrocarbons in the combustion chamber will
diffuse, due to the difference in concentration compared with the outside environment. These hydrocarbons can then eventually escape into the atmosphere through the intake manifold and the air filter. Leaking injection valves and/or wet walls in engines with channel injection make by far the largest contribution to HC emissions from the air intake system. Over a longer period, fuel evaporates here, due to the specific vapour pressure of the fuel mix. All ducts connected to the air intake are also potential sources of HC emissions, including the crankcase ventilation, vacuum tubes and the backflush tube from the tank canister. A number of measures must be taken to successfully reduce HC emissions from the air intake system. These include the correct selection of low permeation materials, for example flange seals between the intake manifold and the cylinder head as well as air intake components. With regard to polymers, fluoro-rubber (FPM) or polyamide (PA) based thermoplastics have proved suitable. In addition, it is also possible to select injection valves with particularly low leakage rates. Through optimised design of interfaces, and reduction in the number of interfaces between the air intake components, the amount of micro-leakage contributing to total evaporation emissions can be further reduced. Those HC emissions caused by diffusion can only be reliably prevented by a “barrier” in the air intake, the HC adsorber. Figure 2 shows some emission sources in the engine, as well as HC adsorption elements in tube design, or as flat adsorbers on the clean air side of the filter. The function of the HC adsorber is to pick up the majority of the HC emissions created when the engine is switched off, and then return them to the intake air when the engine is re-started. For years, Mann+Hummel have been working on developments which are integrated into the air filter system. As a result of this work, a number of patents have been submitted, and the first series project has been undertaken. The company will start series production of several other developments in the near future, as a consequence of stricter legislation.
4 Requirements and Dimensions of the HC Adsorption Element HC adsorbers are fitted on the clean air side after the air filter element, and are thus protected against contamination and moisture. They can either be placed directly in the air
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filter housing or in the clean air duct. Figure 3 shows the main installation layout in the air filter housing. With regard to the design of the HC adsorber shown in Figure 3, the following factors must be taken into account.
ties of the activated carbon such as grain size, the size and distribution of pores determine the adsorption and desorption characteristics with fuels. In addition, the activated carbon must as far as possible be resistant to adsorption of water or moisture. The dimensions of Pleatsorb meet the customer’s demands and are based on the package available and the required adsorption capacity of the HC adsorber. The major advantage of Pleatsorb over flat HC adsorbers is its higher adsorption capacity and lower pressure loss within the same package, Figure 4.
4.1 Air Flow Restriction The HC adsorber can be fitted in the clean side of the air intake system to achieve the necessary adsorption performance and at the same time minimise pressure loss. With regard to flow, it is suitable to install the HC adsorber in the air filter housing. It can thus be accommodated easily, and is tamperproof. Loss of flow through the HC adsorber is primarily determined by the area, the structure of the activated carbon layer and the installation point. It is for example possible to install the HC adsorber in by-pass, as a partial full-flow (e.g. 50 % cover) or fullflow filter. Optimised flow is achieved, both in the computer design of the total air filter system and in part tests, in order to supply the required quantity of air to the engine.
4.2 Design of the HC Adsorption Element The design of the HC adsorber is primarily determined by the required butane working capacity (BWC). When establishing the minimum working capacity, factors to be taken into account include the number of injection valves and their leakage rate, the amount of air required by the engine and the length and/or volume of the clean air side of the air intake. As it is a lifetime part, the relevant ageing effects must also be taken into account. It is well known that hydrocarbons with a higher pressure within the ambient air or crankcase ventilation will bond strongly with the activated carbon. However, to ensure adequate working capacity throughout the lifetime of the adsorber, an appropriate reserve of BWC must be available.
5 Pleatsorb – an Innovative HC Adsorber from Mann+Hummel The HC adsorber Pleatsorb from Mann+Hummel is pleated a storage medium consisting of layers of non-woven material and activated carbon. The pleats are fixed by means of side bands. With the HC adsorber, particular attention is focussed on sealing of cut edges. Reinforcement parts can also be attached to the end pleats. The effectiveness of Pleatsorb depends on the correct choice of adsorbent media and the activated carbon it contains. The proper-
6 Fastening and Installation Position of the HC Adsorber HC molecules are heavier than air and therefore tend to move by gravity to the lowest point of the air intake system. A by-pass arrangement effectively utilises this effect, Figure 3, so the Pleatsorb is located beneath the clean air outlet in the air filter housing. Diffusing hydrocarbons in the air filter thus “fall” directly onto the Pleatsorb and are retained. As the Pleatsorb is outside the air flow and is thus exposed to lower forces, such as for example engine pulsations, the HC adsorber Pleatsorb can be attached tamper-proof to the air filter by an L-shaped side band. The medium can be adhered or welded on the housing through the lateral edge of the side band. When fitted in by-pass in this way, a compromise is achieved between reduced/no loss of flow and adequate adsorption of hydrocarbons. With a full-flow arrangement, which covers the entire free cross-section of the air filter, higher forces must be taken into account. It is therefore preferable for it to be integrated by means of a holding frame. Depending on the installation, the frame may have a supporting grid on one or both sides. It is securely and permanently mounted through adhesive, vibration- or ultrasonic welding. The technical specification of the air intake system specifies stringent requirements regarding pressure loss, dynamic load, as well as resistance to temperature, ageing and mechanical load. The technical spec. of the HC adsorber also lists specific requirements.
7 Computer-simulated Calculation of HC Emissions In addition to carrying out its own research and development, Mann+Hummel is also working with several OEM’s and suppliers MTZ 09/2005 Jahrgang 66
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on the FVV (Internal Combustion Engine Research Association) “Evaporation Emissions” project. The aim of the project is to develop a testing process which can establish the concentration of hydrocarbons along the air intake system according to time and location. The results of the research should help to quantify more effectively the emission rates from the various sources of hydrocarbons, such as injection valves or fuel residues. Furthermore, the results should make it possible to create simulation models on the distribution of HC emissions from the air intake system. Through this, optimum design and positioning of the HC adsorber may be established early in the development phase. It would also substantially reduce costs for the vehicle manufacturer, avoiding the significant costs incurred with conventional measurement of evaporation emissions in the vehicle SHED chambers.
Following the BWC to ASTM, the working capacity of flat media was determined at Mann+Hummel. In order to create more realistic conditions, this test was carried out with 500,000 ppm butane in the mixture with nitrogen rather than pure butane. In addition, rupture testing with adsorbent material was carried out to obtain information on the dynamic behaviour during adsorption and desorption. In the model for emissions testing, butane is used, rather than gasoline. Butane displays a Type l isothermal pattern on activated carbon, according to IUPAC (International Union of Pure and Applied Chemistry) [4]. Depending on the application, the butane concentration must be adjusted in the measurements. Figure 6 shows isotherms at different temperatures. Isotherms are diagrams to visualise the correlation between balanced load and concentration. The concentrations for different applications are entered in the text fields along the axes. Depending on the application in the car, different optimised types of activated carbon can be used. As well as adsorption of butane, the adsorption of water on activated carbon must also be taken into account, as there are differences in the relative humidity of ambient air in the air intake. There will be competing adsorption of water vapour and hydrocarbons. Typically, water vapour shows an IUPAC Type V isothermal pattern on activated carbon. It is however possible to arrange the activated carbon in such a way as to shift the sharp increase in isotherms as far as possible towards high relative humidity. This is shown in Figure 7. The processes taking place when the engine is switched off are marked in an isothermal field in Figure 8.
7.1 Investigation of Adsorption Behaviour – Integral When examining adsorption performance and functionality of HC adsorption elements, a variety of test methods have been discussed. They range from tests on the adsorbent itself - mainly activated carbon - to vehicle SHED measurements. In addition, tests have been defined on media and on adsorbers ready for installation. As already established from components of a fuel supply system, useful information on hydrocarbon emissions from the air intake can also be obtained from component SHED measurements. Figure 5, for example, shows an air filter in a component SHED.
7.2 Investigation of Adsorption Behaviour – Activated Carbon The activated carbon itself is characterised according to grain size, bulk density, hardness, moisture content, pressure loss and other tests. The corresponding test methods are available from the American Society for Testing and Materials (ASTM), from the Deutsches Institut für Normung e.V. (DIN) and the European Council of Chemical Manufacturers’ Federations (CEFIC). Specifically for automotive applications, the butane working capacity (BWC according to ASTM D 5228) measures the performance of the activated carbon [3]. The BWC refers to the mass or bulk volume factor, and indicates how much butane can be reversibly adsorbed into the carbon. An adsorption and desorption cycle is carried out and the changes in mass are recorded in each case. 20
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8 HC Adsorber Testing To meet the requirements defined above, a technical specification is drawn up for series release at Mann+Hummel. Various tests are carried out, measuring the resistance against temperature over longer periods, at alternating temperatures, against vibration and/or pulsation. As well as evaluation of mechanical resistance, the BWC is measured before and after these tests. In this way, the changes in adsorption characteristics can be assessed. Tests on media resistance covered oil, petrol, ice, water and crankcase ventilation. In addition, it was possible to measure the effect of different HC adsorption elements on the pressure loss of the entire air filter module.
9 Series Application of the HC Adsorber Figure 9 gives an example of series application of the HC adsorber for the new 3.5 V6 engine PZEV (M272) from DaimlerChrysler AG. Mann+Hummel supplies the air filter module. This engine will be available in the new M-class and in the current SLK series. Through optimum packaging and geometry of the HC adsorber, it was possible to achieve a very air flow restriction and consistent adsorption/desorption throughout the lifetime of the vehicle.
10 Summary An increasing number of measures are being implemented by vehicle manufacturers to meet the stricter legislation on HC emissions in the North American market. Particular attention is focussed on components in the fuel supply system. For this, it is necessary above all to analyse and chart emissions behaviour in the air intake system. The measures taken will however depend on the seal of the injection valves, particularly with large volume engines containing 6 or more cylinders. To ensure approval of the vehicle by CARB, Mann+Hummel offers an effective and economical solution with the HC adsorber Pleatsorb. Through its pleat geometry, this HC adsorber provides very good adsorption characteristics and at the same time air flow restriction. The use of computer-generated calculations will provide better opportunities in future to establish suitable positioning in the air filter with adequate operating capacity at an early stage of development. All the tests currently required to validate the design of this development are based on individual specifications of each vehicle manufacturer. This leaves potential for a future unification of these specifications.
References [1] Environmental Protection Agency (EPA). Code of Federal Regulations (CFR) 40, Part 86 [2] California Air Resources Board (CARB). California Code of Regulations (CCR), Title 13, Motor Vehicles [3] American Society for Testing and Materials: ASTM D 5228-92 (reapproved 2000). Determination of the butane working capacity of activated carbon, 2000 [4] Sohnemann, J.; Kümmel, R.: Aktivkohle zur Abscheidung von Kohlenwasserstoffen in Pkw-Tanksystemen. In: Steinmetz, E. (Herausgeber): Emissionen aus Kraftstoffsystemen von Pkw. Haus der Technik Fachbuch, Band 15. Renningen: Expert Verlag, 2002
