Gasoline fuel is not only highly combustible, it also evaporates easily. The fumes – which contain benzene – pose a health hazard when inhaled and in combination with sunlight and exhaust gases they can cause photochemical smog which is a concern in large cities. This is why modern gasoline cars have a system on-board to minimise hydrocarbon emissions.
A canister filled with activated carbon is used to collect all fumes from the tank. Of course the canister with the cumulated hydrocarbon (HC) needs to be flushed or purged from time to time. The air content, rich in HC, is fed forward to the intake manifold and flows into the combustion chambers where it gets burnt. This system has greatly reduced evaporative HC emissions. However, two challenges begin to threaten this success story: Firstly, the legislation on evaporative HC emission is set to become much stricter around the world over the next four to five years.
Secondly, purging the HC canister is currently done by using the vacuum that is generated in the intake manifold of a gasoline engine during part-load operation by so-called throttling. In highly efficient gasoline cars there is less or no vacuum available, though, because throttling is inefficient and is avoided wherever possible. In a gasoline mild hybrid car the engine may not be running at up to 20 per cent of the driving time, so no vacuum either.
“Continental has developed an electrified solution to further reduce HC evaporative emissions”, said Klaus Hau, head of the Business Unit Sensors & Actuators in the Continental Division Powertrain. “For the first time the active purge pump makes canister purging available independent from engine operation point and vacuum availability. With the active purge pump vehicle manufacturers will be in a much better position to meet oncoming HC emission legislation.” First SOP is planned for end of 2019.
Put very simply, the efficiency of modern gasoline cars is partly based on running the engine in the most efficient part of its map whenever possible – or not to run it all, for instance, in a hybrid car. Throttling in particular is avoided as the engine’s thermodynamic efficiency is reduced by throttle losses. This is increasingly unacceptable because environmental concerns and economic considerations unanimously advocate lower fuel consumption. “The need for higher engine efficiency is beginning to make vacuum a rarity in gasoline cars. As a result it is becoming more difficult to flush the HC canister because often the boundary conditions will not allow this”, said Stefan Grüneis, Product Mfor Continental Exhaust and Emissions Actuators.