Empa researchers optimize diesel cat
(October 1, 2015) The
scandal surrounding VW has thrust nitric oxide (NOx) emissions from diesel
vehicles into the limelight. Owing to the different engine technologies, these
have always been higher than in gasoline-powered cars. On the other hand,
diesel consumes less fuel. If a way can be found to “denitrify” diesel
emissions efficiently, we would have an economical, clean engine. Empa
researchers are thus working hard on optimizing the catalytic converter
technology for diesel.
Laser-optic measuring techniques at Empa’s engine lab enable
modern NOx reduction systems to be optimized for diesel engines.
Diesel engines burn their fuel more efficiently compared to
their gasoline counterparts – and thus emit considerably lower quantities of
the greenhouse gas carbon dioxide (CO2) – mainly because diesel engines work
with excess air in so-called lean-burn mode. Due to the “excess” oxygen in the
exhaust gas, however, the three-way catalytic converter we know from gasoline
engines, which breaks down more than 98 percent of the toxic NOx in the exhaust
gas, cannot be used in diesel engines.
In order to clean diesel exhausts of NOx, a method is used
that was originally developed for the denitrification of power plant exhaust
gases. Around ten years ago, the first trucks with this new technology hit the
roads. The method uses an aqueous urea solution bearing the tradename “AdBlue”
to convert the NOx into harmless nitrogen via various chemical reactions in a
special SCR (selective reduction) catalyst that has been optimized for NOx
reduction. AdBlue is carried in a separate tank in the car and needs to be
topped up every now and again, usually while the vehicle is being serviced.
However, SCR systems are considerably more complex than a
conventional three-way catalytic converter in gasoline engines. For instance,
the AdBlue dosage needs to be set precisely to the amount of NOx emitted by the
engine. Too low a dosage does not yield the NOx reduction prescribed by law and
too high a dosage results in undesirable ammonia emissions. And at temperatures
below 200°C, AdBlue tends to form residues that eventually clog up the SCR
catalyst. Consequently, SCR systems have to be adjusted specifically to the
various engine types and expected load change – i.e. drivability – and
optimized, which is a complex and thus expensive process.
EURO-6: same NOx
ceiling for gasoline and diesel
SCR systems have only just started to be used in diesel
cars. The limits currently prescribed in both Europe and the US require SCR
catalysts to reduce the NOx level in diesel exhaust gas by more than 95
percent. Moreover, thanks to the EURO-6 norm introduced in September 2014, the
same NOx limits now apply for gasoline and diesel vehicles for the first time.
Beforehand, the latter had always been allowed slightly higher NOx emissions in
Europe.