Particulate matter (PM) has become arguably the most demonised exhaust pollutant, especially among those who stubbornly refuse to acknowledge the true benefits of diesel power, notably its unrivalled fuel efficiency with correspondingly low CO2 emissions. Anti-diesel prejudice is most virulent in North America, based on visible evidence from the past of smoky exhaust stack ‘chimneys’ on heavy trucks. In consequence, gasoline engines still reign supreme in the US and Canada, at least in passenger cars and SUVs. Even in pick-up trucks, the acceptance of diesel power is only now gaining ground, driven largely by rising fuel prices.
In Europe, where diesels are taken much more for granted, there nevertheless remain concerns about PM emissions and their effect on human health. Fine (so-called PM10) and extra-fine (PM2.5) particles, which are able to penetrate the lungs and lodge there, are known to aggravate respiratory diseases. Larger particles are effectively filtered out by the lungs, while those smaller than 2.5 microns across are handled by the respiratory system as effectively a benign gas rather than as solids. As such they are exhaled more or less harmlessly.
For that reason EU legislators and their health expert advisers came to realise, several years ago, that the current exhaust PM limits, specified in terms of total mass, included all particles, regardless of the health risk they pose. The limits are expressed for cars and vans in grams per kilometre (g/km) and for over-3.5 tonne gvw vehicles in grams per kilowatt-hour (g/kWh).
The PM mass limit of 10mg/kWh is, in other words, quite incompatible with the PN limits that the EC wants to impose.
A more meaningful indicator of PM health risk is now deemed to be the number of particles in the dangerous size range. A particle number (PN) limit of 5×1011 (that is 5 times 10 to the power 11) per kilometre has now been determined by the EU for Euro 5 and 6 certification of diesel cars and vans.
For vehicles above 3.5 tonnes gvw, whose existing emission limits, including PM mass, are expressed in g/kWh, the European Commission would like also to include a PN limit in Euro 6 truck and bus emissions legislation, due to come into force in 2013. But the Commission’s Euro 6 limit proposals, though supposedly now set in stone as far as PM mass (and NOx, CO and HC) is concerned, show blank spaces under the PN column headings. A footnote says ‘yet to be decided’.
There are several reasons for that procrastination in Brussels. One is tied up with some parallel dithering over CO2 limits, where controversy continues to rage over the matter of how the CO2 issue - directly related to fuel consumption - can be addressed. Legislative challenges arise most problematically in the case of the same engine, identically rated (in power and torque) installed in say a 30 tonnes gvw vehicle and also in a 40-tonner.
Obviously in the heavier truck, the engine will emit more ‘real life’ (per kilometre) CO2. It is something which also impinges on the particle number issue. How hard a diesel engine has to work for its living inevitably affects exhaust particle size distribution. The controversy over future truck and bus CO2 limits is now being matched by heated arguments between commercial vehicle manufacturers and the legislators on the latter’s PN proposals.
the accompanying PN requirements now on the table in Brussels would, if they became law, make the fitment of a bulky, heavy and costly DPF unavoidable
Scania’s head of powertrain Jonas Hofstedt, talking last week to AutomotiveWorld.com, voiced the industry’s concerns. He pointed out that the Euro 6 PM mass limit proposal of 10mg/kWh corresponded to a PN figure of about 3×1013 /kWh. But, said Hofstedt, the EC wanted to set much lower particle numbers of just 8×1011/kWh on the world harmonised steady-state test cycle (WHSC) and only 6×1011/kWh on the accompanying transient cycle (WHTC). (The two world-harmonised cycles are due to be applied for the first time for Euro 6 certification.)
Hofstedt points out that reducing particle numbers - by whatever means - necessarily also reduces particle mass. Those Euro 6 particle number limits now being proposed by the EC accordingly correspond to a PM mass limit of only about 3mg/kWh - say one third of the already agreed Euro 6 PM limit. The PM mass limit of 10mg/kWh is, in other words, quite incompatible with the PN limits that the EC wants to impose.
Some manufacturers, including Scania, claim to be able to meet the Euro 6 PM mass limit through advanced engine technology, ie without needing a diesel particulate filter (DPF). But, crucially, the accompanying PN requirements now on the table in Brussels would, if they became law, make the fitment of a bulky, heavy and costly DPF unavoidable. And the filter would have be of the latest ‘fine pore’ ceramic substrate type rather than the open steel mesh variety and, as such, very costly and heavy. Furthermore, the additional exhaust back pressure imposed by such a filter would bring an inevitable fuel consumption penalty.
Such a requirement would, of course, mean a surge in demand for ceramic DPFs. No wonder some truck and bus manufacturers suspect heavy lobbying of the EC and its technical advisers by catalytic filter makers such as Johnson Matthey and BASF-Engelhard.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.