The JEC research collaboration completed a major test programme specifically designed to investigate the influence of gasoline vapour pressure and ethanol content on evaporative emissions from modern passenger cars as determined using the current European regulatory test procedure. The first report was issued in 2007 and the programme is still on-going.
As a consequence of European regulatory framework aimed at promoting the use of ethanol and other biofuels, the question has arisen as to whether the vapour pressure limit for ethanol/gasoline blend should be relaxed with respect to fuel specifications laid down in the Directive 98/70/EC (subsequently amended by Directive 09/30/EC). The test programme was conceived to assess specifically the effect of ethanol/gasoline blends on evaporative emissions with the objective of providing a technical basis for discussion on this issue.
Seven gasoline passenger cars representative of EURO 3-4 technology were tested for evaporative emissions with ten different test fuels.The test fuel matrix comprised 60 and 70 kPa hydrocarbon base fuels with 5 and 10% ethanol splash blends and 5 and 10% ethanol matched volatility blends. The evaporative emission tests were carried out according to a test protocol agreed by the partners of the programme and based on the European homologation test procedure.
Key messagesThe test procedure showed considerable influence on the results in particular repeating the test procedure, without additional conditioning between tests above regulatory prescriptions, resulted in an increase of the carbon canister weight with successive tests. In other words, the test protocol was not able to return the vehicle to a consistent condition at the start of each test. The increase of the canister weight may not represent real-world operating conditions as the canister load depends on the typical driving patterns. The canister weight problem made it difficult both to obtain representative emission measurements from the various vehicles and to determine fuel effects. Test results confirmed that vapour pressure (DVPE) is a key fuel variable for evaporative emissions. Increasing fuel vapour pressure above reference increases evaporative emissions. However the effect of vapour pressure is strongly non-linear. Ethanol blends with final DVPE around 75 kPa gave considerably higher evaporative emissions than lower volatility fuels in most of the vehicles. Differences between fuels with DVPE in the range 60-70 kPa were small.
Due to the combination of DVPE variations, the presence or absence of ethanol, and significant changes of canister weight, it is difficult to draw any reliable conclusions on the influence of individual parameters.
- Results obtained in tests where extra purging of the canister was carried out suggest that differences in evaporative emission measurements on fuels in this volatility range could be reduced if a more extensive canister conditioning procedure was adopted.
- The engineering margin built into the system may also explain the reduced fuel effect.
Ethanol might influence evaporative emissions also via other mechanisms.
- Ethanol is known to be more difficult to purge from carbon canisters (as are heavy hydrocarbons), so could reduce their working capacity.
- Unfortunately the poor repeatability of the main data set has not allowed quantifying the relative size of these effects.
- Multiple additional tests on one vehicle showed that ethanol containing fuels with matched volatility gave higher emissions than the hydrocarbon fuels.
Ethanol increases the fuel permeation rate.
- Emission tests resulted in fuel permeation through plastics and rubbers as a potentially significant contributor to evaporative emissions.
Data show clearly that volumetric fuel consumption (litres/100 km) increases with increasing ethanol content, roughly proportionally to the oxygen content of the fuel. However there was no effect of ethanol on energy consumption.
The test programme was designed to explore only the effects of ethanol and fuel vapour pressure on evaporative emissions from a range of latest generation canister-equipped gasoline cars using the EU Evaporative Emissions test procedure. Other parameters like test temperature profile, presence of ethers in the fuel, fuel permeation and the long term effect of ethanol and water on carbon canister working capacity have not been addressed by this programme.