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Estimating Real Time Diurnal Permeation from Constant Temperature Measurements
Technical Paper
2001-01-0730
ISSN: 0148-7191, e-ISSN: 2688-3627
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Event:
SAE 2001 World Congress
Language:
English
Abstract
Using the results of Constant Temperature (CT) Permeation Measurements to estimate Real Time Diurnal (RTD) permeation emissions has a number of practical advantages. In particular, Constant Temperature measurements are easier to set up and control in a laboratory environment, and Constant Temperature measurements provide for data checks using simple self-consistency tests that are not possible with Real Time Diurnal measurements. Furthermore, there is no need to repeat permeation measurements for each separate real-time temperature profile of interest. The same two Constant Temperature measurements can be used to estimate permeation performance for many different temperature cycles - for example, the temperature cycles prescribed by CARB, EPA, and EEC, or the different temperature profiles experienced by separate fuel system components during a vehicle SHED test. This paper reviews the scientific basis for the procedure, discusses both its strengths and its limitations, and explains the applicability of Constant Temperature measurements to situations involving multi-component fuels and to fuel subsystems made of several different polymer materials.
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Citation
Lockhart, M., Nulman, M., and Rossi, G., "Estimating Real Time Diurnal Permeation from Constant Temperature Measurements," SAE Technical Paper 2001-01-0730, 2001, https://doi.org/10.4271/2001-01-0730.Also In
References
- United States Code of Federal Regulations, 40 CFR §86.101 to §86.157.
- California Code of Regulations, Title 13, Division 3, Section §1976, “Standards and Test Procedures for Motor Vehicle Fuel Evaporative Emissions. See in particular: Evaporative Emission Standards and Test Procedures for 1978 and Subsequent Model Motor Vehicles.
- For the EPA cycle TMIN=72°F and TMAX=96°F. For the European Stage III cycle TMIN=20°C and TMAX=35°C.
- Speciation of Evaporative Emissions from Plastic Fuel Tanks. Fead E., Vengadam R., Rossi G., Olejnik A. and Thorn J.; SAE Technical Paper 981376.
- This second requirement is important in view of the 15 years/150,000 miles Useful Life standards. For certain multilayer constructions exposed to certain fuels, several years may be required in order to attain steady state; an example is the co-extruded plastic fuel tank exposed to ethanol containing fuels such as CE10 or IE10.
- This, of course, assumes that there is no leakage at the connecting points between different components.
- Assuming the Arrhenius dependence of permeation on temperature given in equation (A-1) and assuming that the relevant α is known (see Appendix A, Eq. (A-3)), it is possible to estimate from the results for one of the cycles (say the EPA cycle) the results that would be obtained in the other cycles (CARB, EU Stage-III). However, a single RTD measurement does not provide any information on α. In other words, a separate measurement is required to obtain α. It should be noted in this context that, since the CARB cycle is the most severe of the three, results obtained using the CARB cycle do provide an upper limit to the results expected from the EPA or the EU stage III cycles for a specific fuel.
- A number of new laboratory procedures for measuring evaporative emission are currently being prepared within SAE. These include: (a) a proposed “Method for Speciation of Fuel Losses from Polymeric Fuel System Materials”; (b) a proposed “Test Procedure to Measure Fuel Permeation of Elastomeric Hose or Tube by Weight Loss”; (c) a proposed “Test Procedure to Measure the Fuel Permeability of Materials by the Cup Weight Loss Method”; (d) a proposed “Test Procedure to Measure Fuel Permeation by the Mini-SHED Method”. In addition to these procedures SAE J1737, “Test Procedure to Determine the Hydrocarbon Losses from Fuel Tubes, Hoses, Fittings and Fuel Line Assemblies by Recirculation”, has been used by a number of laboratories in the last few years.
- SAE J1681, “Gasoline and Diesel Surrogates for Materials Testing”, revised September 1999.
- “Estimating Real-Time Diurnal (RTD) Results Using Constant Temperature Permeation Tests”, a technical information report currently in preparation for ballot by the SAE Fuel System Supply Forum.
- See for example: “Barrier Polymers” by De Lassus P. in Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, Wiley, 1992; Vol. 3, p. 946-948.
- We use the notation P* to distinguish the solvent vapor transmission rate from the permeability P; the latter is commonly defined as the amount of solvent that in a unit of time permeates through a unit area of a polymer wall of unit thickness at steady state, provided that the difference, Δp, between the solvent partial pressures on the two sides of the wall is unity. Contrary to what is often assumed the permeability P is not a constant, rather it is a function, P(p), of the solvent partial pressure, p, to which either side of the slab is exposed. By contrast, the vapor transmission rate is defined for situations where on one side of the wall there is either liquid solvent or saturated solvent vapor and on the other side there is no solvent. If p′s is the solvent saturated vapor pressure the relation between P* and P is:
- Note that P*(T) is a property only of the material and of the solvent involved; it does not depend on the geometry of the specimen used to measure it. Suppose, for example, that a monolayer plastic fuel tube of overall area A and wall thickness L contains liquid toluene and that at steady state at temperature T it permeates an amount fss (e.g., fss grams) of toluene per unit time; then, P*(T) for toluene in the plastic material making up the tube is / Here, L is assumed to be small compared to the diameter of the tube (i.e., a thin-wall tube) so that the effect of the tube cylindrical geometry can be neglected.
- One may view the diffusion process of a solvent molecule through a plastic material as a succession of steps or jumps from one microscopic location to the next. For example, the solvent molecule may have to go through a bottleneck or a narrow passage formed by the polymer chains of the plastic material. As these chains move with respect to each other because of microscopic thermal motions, the bottleneck or passage may widen so that the solvent molecule has a chance to go through. In this interpretation α is a measure of the energy (activation energy) required to overcome the barrier; it depends on both the properties of the solvent and of the polymer. The microscopic factors that influence the value of the activation energies (e.g., of α) are the microscopic interactions of the solvent molecules with the polymer chains (note that, to a first approximation, all hydrocarbons will behave similarly in this regard), the molecular size of the solvent molecules, and the degree of internal flexibility of the solvent molecule.
- If the percentile change in P*(T) per degree centigrade at the temperature T can easily be shown to be equal to: Here, both T and α are supposed to be measured in degrees Kelvin.
- The speciated vapor transmission rates Pi*(T) are defined as the amounts of the i-th fuel component that in a unit time permeates through a unit area of a polymer wall of unit thickness, provided that the wall is exposed to liquid fuel (or its saturated vapor) on one side and to the atmosphere on the other.
- Data confirming eq. (A 8) in the 40 to 60°C temperature range exist for: (i) toluene and isooctane from fuel C in HDPE, nylon12, polyacetal, polyketone (CARILON), GFLT elastomers and FKM elastomers; (ii) toluene, isooctane and ethanol from fuel CE10 in HDPE, nylon12, ETFE, polyacetal, polyketone (CARILON), GFLT elastomers and FKM elastomers.
- Note that if both P*A and P*B are underestimated (or overestimated) the two errors in eq. (A-13) tend to cancel each other so that the error (Δα/α) is smaller.
- For example, plotting the logarithm of P*(T) versus 1/T it should be possible to find a curve β(1/T) that interpolates between the experimental data. Then the expression can be used to describe the temperature dependence of P*(T).