Sealing Shielding

Seals and Vehicle Evaporative Emissions

best seal material to avoid vehicle emissionsReducing the environmental impact of operating a passenger car has long been a request from the environmental community, and that demand continues to spread. But how can something as small as a seal influence vehicle emissions?

The most obvious way to reduce the environmental impact of a vehicle is to burn less fuel while doing the same job, and to burn it more completely. In a previous blog, I mentioned several ways in which state of the art seal materials help improve overall vehicle efficiency. These include VG286-80 and VG109-90 used in gasoline direct injection fuel systems and VG292-75, VG310-75, and FF400-80 used in turbocharger coolant applications. But these are not the only ways that seals impact vehicle emissions.

fuel vapor sealsAnother critical source of emissions – and one that is directly impacted by seal material – is fuel vapor escaping from the vehicle’s fuel system. This happens whether the car is running or not, and it’s much worse in hot weather. Gasoline evaporates quickly, and that fuel vapor can be hard to control. Not only are vapor leaks much harder to prevent than liquid leaks, but fuel vapor can permeate through most rubber and plastic materials and escape into the environment  When it does, those unburned hydrocarbons contribute to smog and ground-level ozone pollution.

Fortunately, Parker has decades of experience in solving these types of sealing challenges.

Fuel vapor permeation rates can be measured directly through fuel vapor permeation tests such as the Thwing-Albert method, but this is a very slow process and it takes weeks to reach equilibrium. As a result, it’s not a good screening test for quick comparison. It’s much easier and faster to measure the volume swell with a liquid fuel immersion. While it’s not a perfect, it does provide a quick, “down and dirty” evaluation of different types of seal materials.

Table 1 shows the volume swell of various fuel-resistant materials after being fully immersed in two laboratory reference fuels. Using reference fuels eliminates the variability inherent in using actual pump gasoline from different manufacturers, refined at different times of the year, and from different batches. Fuel C is a little more aggressive to seal materials than regular American gasoline. CE-10 is a blend of 10% ethanol and 90% Fuel C, which makes it similar to most gasoline sold in the US. As you can see, there is a significant difference among different types of seal materials. Clearly, high fluorine fluorocarbon materials offer the best resistance to both fuels.

Table 1: Volume swell of various seal materials in fuel, 70 hours at 23 C per ASTM D471.

 

High ACN nitrile

Fluorosilicone

Traditional fluorocarbon

Low temp fluorocarbon

High fluorine fluorocarbon

Fuel C

16%

15%

3%

5%

2%

CE-10

21%

18%

6%

11%

4%

 

Because reducing the weight of a vehicle also helps with fuel economy, many fuel system components have changed from metal to plastic over the years. Not only does this increase the opportunities for fuel to permeate out of the fuel system, but it also increases the sealing challenges. Sealing plastic components brings three challenges not seen when working with metal components: parting lines, rigidity, and creep. Plastic components are almost always molded, meaning they have mold parting lines. Those parting lines frequently pass through the seal groove, and the seal must be able to conform around that imperfection to form a vapor-tight seal. Second, plastic components aren’t rigid like metal components; they move when a seal is pushed against them, so the amount of seal load force that can be applied is limited. Third, plastic components move over time in response to these load forces in a process called creep, which means the amount of squeeze applied to a seal reduces over time.

To get the best performance when sealing against plastic components, softer materials are key. Compounds in the normal 70 to 75 Shore A range generate so much load force that they deform the plastic and dramatically increase the creep rate of the plastic. In addition, harder compounds don’t seal well around plastic parting lines, so softer compounds are preferred over harder ones.

However, there’s a limit to how soft a compound can be. Hardness is controlled by the use of reinforcing fillers, and these fillers also help to reduce the fuel vapor permeation through the elastomer. Softer compounds usually have higher permeation rates. Also, extremely soft rubber compounds with no fillers tend to have poor resistance to compression set and compressive stress relaxation, which reduces the effective service life. The ideal balance tends to be in the 60 to 65 Shore A range.

Based on these two design criteria, the ideal compound for minimizing fuel vapor loss is a 60 to 65 Shore A, high fluorine fluorocarbon elastomer. For nearly 20 years, that exact solution, Parker’s VW252-65 compound, has sealed the fuel tank of almost every passenger car assembled in North America.

Now, Parker has taken another evolutionary step forward by improving the compression set resistance, low temperature performance, and fuel vapor permeation resistance with VW313-65, our next generation fuel system seal material. A comparison is shown in Table 2. Still a high fluorine fluorocarbon compound in the ideal 60 to 65 Shore A range, VW313-65 was also designed to be manufactured in all corners of the globe. This means seals can be made locally to best suit our customers’ needs to localize manufacturing and optimize logistics.

Table 2: Comparison of VW313-65 and VW252-65.

 

Hardness, Shore A

Compression Set, 336 hours at 200°C

Compressive Stress Relaxation, 1500 hours at 150 C

CE-10 volume swell

CE-10 fuel vapor permeation

TR-10

VW252-65

66

60%

42%

6%

24

-5°C

VW313-65

69

50%

72%

4%

18

-8°C

 

VW252-65 has long been the preferred material for minimizing fuel vapor loss from a vehicle fuel system, and VW313-65 offers further performance enhancements to provide for the next generation of vehicles.

 

For more information, please contact our Applications Engineers via our live chat service on Parker O-Ring & Engineered Seals Division's website.

 

Dan Ewing, Senior Chemical Engineer

 

 

This article contributed by Dan Ewing, senior chemical engineer, 

Parker Hannifin O-Ring & Engineered Seals Division.

 

 

 

3 Global Trends Driving Seal Innovation in the Automotive Industry

Seals and the Quest for MPG

Categories
Recent Posts by Author

Five Ways to Maximize Performance of Electric Vehicle Batteries

Electric vehicles are developing fast in line with growing demand. However, only by selecting proven, reliable, high-quality products for the effective thermal management and EMI shielding...

Intelligent Product Design of Polymer Components Enables Integration of Functions

The utilization of suitable materials based on specialized engineering and manufacturing know-how makes it possible to integrate several parts and/or functions in a single assembly component. In many...

Compound Cure System: How Much Does It Matter?

Frequently, our team in Applications Engineering receives a question along these lines: “Is your compound E0740 peroxide cured?” Usually, these questions are asked because an end customer specifies...
Comments

Comments for Seals and Vehicle Evaporative Emissions


Please note that, in an effort to combat spam, comments with hyperlinks will not be published.

Leave a comment





Captcha