Die Sonderausgabe der MTZ hier herunterladen (6 MB - MS Motor ...

CAPABILITIES AND COMPONENTS
FOR COMMERCIAL VEHICLE ENGINES
One of the primary development tasks on the commercial diesel engine is to minimize NO x and PM emissions
without compromising the engine efficiency. Because of the complex interaction of combustion engine, turbocharger
(T / C) and exhaust gas recirculation (EGR), the layout of air and EGR management systems calls for a highly
integrated approach. To allow for this, Pierburg merged all its activities and components for the commercial vehicle
and off-road sector in its Business Unit Commercial Diesel Systems (CDS) in 2009.
COMPONENTS AND SYSTEMS
The future development of the commercial
diesel engine will be dominated by the
need to comply with the challenging emission
targets US2010 and Euro 6 for on-road
applications, and Tier 4 Final in the offroad
sector. The air and EGR management
system plays a key role here [1]. Thanks to
its broad experience in the field of intake
and EGR systems Pierburg is able to support
its customers in meeting these challenging
targets. Pierburg offers tried-andtested
charge management products such
as intake manifolds and throttle bodies,
products for emission control such as EGR
valves, EGR coolers, reed valves and secondary
air systems as well as solenoid
valves and electric actuators as positioning
and control elements. The new Pierburg
turbocharger rounds off the products associated
with the gas exchange system.
All the components are available and
put to use individually or in integrated
modules. The modules gain the advantage
that the customer does not have to configure
the individual components from those
available on the market, but is receiving a
system with tested and tuned components.
For the success of such development
tasks it is crucial to have an understanding
of the component interactions
and the effects on the engine process.
Accordingly, the first part of this article
recapitulates the main thermodynamic interactions
between the combustion engine,
EGR system and turbocharger. From
these interactions, component targets are
then derived in the second part.
100 Years of Kolbenschmidt Pierburg
NO X FORMATION AND REDUCTION
During combustion NO x is mainly formed
at high temperatures [2]. This chain reaction
is described by the Zeldovich mechanism
which, under certain assumptions,
can be restated and simplified to Eq. 1.
Eq. 1 d(nNO )
_____
dt ~ √ ___
_______
n n o2 N2
√ __
V [ 1 ___
√ __
T exp ____ – E
( R T m ) ]
Due to the finite reaction rate, the majority
of the NO molecules are borne not in the
hotter flame front, but in the burned gas.
Consequently, Eq. 1 mostly refers to the
amount of O and N in the exhaust gas.
2 2
According to Eq. 1, the following ways for
reducing NO formation are available:
: reduced quantity of O (in the exhaust
2
gas). This presupposes combustion
with a low AFR (a)
: low quantity of N (in the exhaust gas)
2
(b)
: high cylinder volume V during combustion
(c)
: low (exhaust gas) temperature T (d). This
highly effective measure in view of the
exponential dependency can be achieved
by intensive cooling of the fresh cylinder
charge in order to lower the temperature
at the start of combustion (d1), a high
cylinder load reducing the combustionrelated
temperature gradient (d2) and late
combustion timing (d3)
Options (c) and (d3) have to be ruled out
for efficiency reasons. (d2) must not counteract
item (a). Hence, the cylinder must
be charged preferably with the combustion
COMMERCIAL VEHICLE TECHNIQUE
products CO 2 and H 2 O. In addition, as the
ratio of the specific heats is lower for the
larger exhaust molecules, introducing EGR
slightly reduces the temperature gradients
during compression and combustion.
These conclusions are underpinned by the
behavior of the throttled, homogeneous
gasoline engine. For an identical, quantitative
charge dilution, EGR (�=1) has a
much greater effect on NO x reduction than
lean combustion (� ≥ 1) [2].
In practice, goals (a), (d1) and (d2)
have to be achieved by charging the cylinder
with cooled EGR gas with a low, but
still sufficient amount of O 2 [3]. Since the
concentrations of N 2 in the air and the exhaust
gas are almost identical, any charging
(including that by EGR) increases the
quantity of N 2 in the cylinder. However,
this in terms of (b) counterproductive effect
is more than offset by (d2).
AUTHORS
DR.-ING. MICHAEL BREUER
is Senior Manager Components
and Thermodynamics at Advanced
Engineering of Kolbenschmidt
Pierburg AG in Neuss (Germany).
DR.-ING. MARTIN HOPP
is Senior Manager in the
Business Unit CDS at Pierburg
GmbH in Neuss (Germany).
DR.-ING. KARL WÜBBEKE
is Director of the Business
Unit CDS at Pierburg GmbH
in Neuss (Germany).
79