Freezing problems are obvious and
they have required the development
of completely new hydraulic systems,
which has taken place.
What is widely underestimated
though, is the high-temperature behavior of DEF. Chemical decomposition
accelerates dramatically at temperatures above 150°F, independent of
system pressure. Constant heat over
185°F is already critical, and at temperatures above 270°F solid byproducts are formed that can ultimately
destroy the SCR system.
As such, operating temperatures are
in the range encountered by off-road
machinery — this must be accounted
for when the system is designed. It is
very difficult to offer standardized “how
to do it right” answers, as there are so
many different off-road machine appli-
cations. But physics reveals that some
factors have a universal character.
Good insulation helps. Heat energy
that is not introduced to the system
does not need to be cooled. So the
design of the interface between the
injector and the exhaust system, as
well as the proximity to other heat
sources in the surrounding area, has
to be considered.
Cooling has to be cool. Whatever
means are used to cool the injector,
the medium should be cooler than the
critical temperatures of the aqeuos
A closer look at an SCR injector in an
off-highway machine application. It is
interersting to note that while mechani-
cal shock loads would appear to be
the greatest challenge to SCR off-road,
actually thermal issues with diesel
exhaust fluid (DEF) potentially present
the most hurdles.
urea solution. DEF recirculation has
proved to be very efficient, as the
tank temperatures are generally able
to be kept low enough. Together with
an effective insulation, the continuous heat transfer rate is so small that
operation even with a tiny residual of
fluid is possible.
The use of engine coolant, on the
other hand, is rather limited. Without an
additional cooler it is simply too hot and
rather than cool the DEF, it adds heat.
Passive cooling is nearly impossible because the whole environment
of the injector needs to be temperature controlled, which is not feasible
in off-road vehicles.
The fear some have concerning SCR
systems is often exaggerated and gen-
erally groundless. But it’s also fair to
(Photo courtesy of Scania.)
say that implementing SCR systems is
not as simple as it might seem.
The bottom line is that off-road
applications require due diligence in
the layout and setup of the SCR system. With close cooperation between
the SCR system supplier and the
application engineers at the engine
or machine manufacturer — along
with a requisite amount of flexibility
on the part of both parties — even the
most ambitious projects with pressing
implementation dates can be accomplished successfully. dp
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