A large share of hydrocarbon emissions from vehicles comes from evaporation from vehicles fuel system. Modern vehicles with positive ignition engines have therefore for more than 15 years in Europe been equipped with systems for evaporative control. During the Swedish in service surveillance testing program evaporative emissions were measured on positive ignition vehicles already in operation on road. Results of this program imply a failure rate of more than 30 percent on evaporative emissions. These results differ from a parallel in service surveillance testing program conducted in Germany, where less than 10 percent of the tested vehicles exceeded the limit for evaporative emissions. Based on these results further investigations were started to analyze the evaporative emission of vehicles driven on Swedish roads.
One of the major differences between Sweden and Germany that could explain the results was the fuel quality. In Sweden the petrol contained 5 percent ethanol whereas in Germany the fuel did not contain any ethanol at that time. A number of references point out the effect the ethanol has on both the capacity of charcoal canister and plastic materials. This suspicion was also proved to be right by results from some of the vehicle manufactures analysis. These showed high concentrations of ethanol in the charcoal canister.
A special project is also conducted where two similar vehicles are exposed for different fuel qualities. One of the vehicles uses the Swedish market fuel with 5 percent ethanol and the other uses reference fuel without ethanol. The vehicles are driven with the fuels up to 10,000 km. Evaporative emissions are measured in the beginning and in the end and charcoal canister capacity every 500 km.
In the program charcoal canister capacity was also measured. In most cases a low capacity could be connected to high evaporative emissions in the test. In some cases the capacity could however not explain the results. Also taking into account different tank materials gave better explanation of the results. Some tank materials show clear problems with permeation losses.
Factors influencing the charcoal canister capacity are size and geometry of canister and the quality of the carbon. The permeation through the tank and pipes are influenced by the material. Also the purge strategy is very important. Some vehicle types do hardly any purge during city driving and cold starts. This means that the system for preventing evaporative emissions do not work during conditions where it is most important for human health.
There are large global differences in the legislation on evaporative emissions. The US regulation includes durability testing, in service conformity and OBD for evaporative control. The European legislation does not contain anything of this. The only thing in the European legislation is the type approval which is done on reference fuel not containing any ethanol. The long term effects seen in this program will not be prevented by going to a reference fuel containing ethanol during type approval. To prevent this either durability or in service conformity testing is needed.