21 Jun 1992

VOC running losses from canister equipped vehicles

Report no. 92/53: Six European vehicles fitted with carbon canisters have been tested under severe conditions to establish if evaporative losses of volatile organic compounds occur under European driving conditions - so called "running losses". The programme entailed the development of a point source measurement technique which has a number of advantages over other methods currently in use.Following the development and validation of the measurement technique, the six vehicles were tested at 28OC over a range of driving cycles on a gasoline with a Reid vapour pressure of 90 kPa.

None of the vehicles exhibited classical running losses, i.e. losses during higher-speed driving. This was due to the effectiveness of canister purging in these conditions.However, significant volatile organic compound (VOC) losses were observed for several vehicles during idle after a period of driving had heated the fuel.Substantial car-to-car variation was observed in the losses obtained. The losses were always more severe over longer idling periods, and more severe than hot soak over comparable periods. This may have important implications for urban pollution.Critical factors affecting running losses are fuel temperature and purging strategy. Higher fuel temperatures increase vapour generation and hence the canister charging rate.

Purging rates must be sufficient to overcome the charging rate. Larger carbon canisters (LCC) were found to be more effective than small carbon canisters (SCC) in reducing running lidling losses because of the extra adsorbent capacity available. Mitigation of re-fuelling losses is an added benefit. Systems that combine the canister with a pressurized fuel tank,in order to limit VOC charging of the canister, were shown to run the risk of VOC losses from sources other than the canister vent.

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