Fuel Permeation

Introduction
To meet current partial zero emissions vehicle (PZEV) requirements, the automotive industry uses polymeric materials in fuel system components with very low rates of permeation. As a result, component testing can be very time consuming making development difficult. A study has recently been completed, supported by the UK DTI and an industry consortium, which aimed to produce more efficient testing techniques so that fuel system components (tanks, filler hoses, fuel lines etc.) with reduced rates of evaporative emission could be developed and validated more rapidly.

One part of the project was to develop an accelerated test methodology for determining permeation rates more rapidly. This has been achieved by using absorption tests (total immersion) that are quicker and easier to perform than the permeation tests that are currently used.

• The project demonstrated that the proposed technique of employing absorption to give rapid and accurate estimations of permeation characteristics is justified
  
• A methodology has been developed to determine the diffusion and permeation coefficients from sigmoidal-shaped absorption characteristics– a common occurrence for many polymeric materials in practice
  
• The use of elevated temperature and reduced test piece thickness to accelerate absorption testing by shortening the time taken to reach equilibrium has been validated statistically and against control data
  

In another part of the project, a micro-SHED test facility has been designed and constructed at MERL (Figure 2) to measure emissions from individual components of a fuel systems in a way that is comparable with regulated measurements by Sealed Housing for Evaporative Determination (SHED) but on a smaller scale.

Background
Fuel system components in the automotive industry are often made from various types of polymeric materials. As no polymeric material forms a complete barrier to the passage of fluid molecules, these components are a source of small but continuous evaporative emissions. Recent legislation, sets “near-zero” limits for evaporative emission globally (EURO II, III/2000 and CARB legislation). This legislation represents a major challenge for the use of polymers in automotive fuel systems. Hence, where polymers are used, reduction of fuel permeation is a major requirement in bringing about reductions in overall emissions to acceptable and legislative levels.

Accelerated Testing Methodology to Predict Permeation Rates
A test methodology has been developed to determine fuel permeation rates more rapidly by conducting absorption tests and applying a theoretical estimation. In addition techniques for accelerating the absorption tests have been developed and quantified.

For liquid permeation, the transmission cup mass loss test provides a direct and simple measurement of its rate. However, the disadvantages of this technique are that problems in achieving an effective seal may occur, and long test periods may be required before equilibrium is reached; moreover, only one test is possible per cup.

Micro-SHED Facility
To meet the increasingly stringent legal requirements for hydrocarbon emissions from the whole car imposed by authorities in the USA and worldwide, the need has arisen to identify which components give rise to hydrocarbon emissions. Large emissions can normally be detected with the aid of small hand held instruments or by laboratory methods based on measurement of volume or mass reduction. Once major leakages have been eliminated, the problem is to detect the materials and components that leak very small amounts, not due to material faults or poor sealing, but through the fabric of the material itself. This is called permeation. Being able to make these low level measurements reliably will enable the comparison of different materials under controlled conditions.

The general micro-SHED set up is shown in 4. Air is pumped in a circuit between a sealed test chamber and an FID for measuring total hydrocarbon content. The test object, filled with fuel, is placed in the test chamber. The FID measures the hydrocarbon concentration in the circulated air, either continuously for high permeation rates, or intermittently for low permeation rates.

The MERL test vessels were made in the form of tubes with detachable end plates . To obtain a good mass transfer and adequate mixing, a circulation fan is used inside the vessel.

Further work is required to develop the accelerated test methodology to account for the multi-layered construction of many components and to assess the effects of multi-component fuels. A new industry consortium is currently being formed to support this phase of the work.

For further information please contact Dr John Harris.