Cover Story ACOusTiCs - Rieter

Cover story ACOusTiCs
Temperature after 15 hours
2 °C
Engine block temperature after 15 hours.
Insulation thickness: 20 mm
0 20 30 40 50 60 70 80 90 100
Several of the engine-based measures
needed to improve fuel efficiency (such as
higher injection pressure, direct petrol
injection, downscaling to three or even
two cylinders, turbocharging) also have a
negative effect on the radiated noise. Tests
on a vehicle with a 2 l petrol engine [4]
show a potential of a maximum 1.5 dB
reduction in exterior noise by optimising
the existing body-mounted parts. In 2007,
a system of engine-mounted parts weighing
less than 2.5 kg was presented [5].
The system provided reductions in radiated
noise in the engine test bench of 3 to
4 dB. Equipping parts of the engine airintake
system with porous acoustic materials
integrated into a double-shell panel
provides significant improvements in the
exterior noise quality of a diesel vehicle
[6]. The most recent study conducted on
a Nissan Infiniti with a 3.7 l V6 petrol
engine shows that body-mounted engine
encapsulation can reduce powertrain
noise by 5 dB, 1. Overall, the studies
conclude that a combination of material
optimisation and body-mounted and
engine-mounted elements will be needed
to achieve the legal requirements for significant
noise reduction.
ConCePts For tHermo-aCoustiC
engine enCaPsuLation
Body-mounted encapsulation is made of
components originating from acoustic
treatment: the bonnet absorber, outer
bulkhead and under-engine shield. New
elements are the vertical elements along
the front beams and connected to the
% Coverage
Temperature after 15 hours
1 °C
above-mentioned elements in order to
form a tightly enclosed engine compartment.
It is also necessary to use electrically
actuated shutters or other appropriate
front closing elements to limit heat
and noise leakage through the radiator.
The system must ensure an efficient flow
of air through the radiator for optimum
engine cooling during driving. It should
also block the dissipation of warm air
when the vehicle is parked. Since vibrations
and operating temperatures are
lower for body-mounted components,
lightweight materials such as foams and
felts can be used. On the other hand, the
surface to be treated with thermo-acoustic
materials is greater in comparison to the
engine-mounted system and cost-effective
development of the sealing system against
the bonnet can be challenging. Several
pass-throughs for the steering column,
drive shafts, cables and other interfaces
need to be integrated, allowing easy
assembly.
Engine-mounted encapsulation is
smaller but is fixed to surfaces that are
usually hot and exposed to more vibration
than the body. This requires more
stable materials with higher temperature
resistance. The subdivision of the encapsulation
into its elements must allow
access to the powertrain for maintenance.
In practice, it is not possible to
cover 100 % of the powertrain surface.
Some ancillaries such as the alternator or
the fuel injectors need to be cooled by
air. Coverage of 85 % is, however, the
minimum needed to achieve effective
thermal encapsulation.
Engine block temperatures after 15 hours.
Comparison of several levels of engine coverage
0 10 20 30 40 50
60 % covered 70 % covered 80 % covered
2 Temperature of the engine block after 15 hours of cooling, as a function of the thickness of the encapsulation (left) and of the area coverage (right)
14
Insulation thickness [mm]
7 mm thick encapsulation covering
80 % of the area is equivalent to 50 mm
thick encapsulation covering only 60 %,
2. Therefore, the first priority in the
development of an encapsulation is the
highest possible degree of area coverage
rather than the choice of highly insulating
materials.
The potential of both approaches was
evaluated in several vehicle tests, 3. It
was shown that engine-mounted encapsulation
has a slightly lower heat storage
capacity, although one needs to consider
that this design was more difficult to realise
on the existing engine due to lack of
space. In new engine developments, the
possibility of increasing the area coverage
and the average thickness of the insulating
materials improves significantly.
In the case of a body-mounted concept,
it is usually easier to obtain sufficient area
coverage, but the sealing of the system is
more difficult. The weight of currently
used systems for the thermal encapsulation
of a four-cylinder engine is estimated
at a maximum of 8 kg. For a B-segment
car with a diesel engine, 6 °C higher powertrain
temperatures in the NEDC translate
into a CO 2 reduction of about 3 g/km,
4. Considering the additional weight of
the encapsulation, the actual CO 2 reduction
can be calculated as 2.5 g/km. To
achieve a comparable reduction by means
of vehicle weight reduction, the weight
would have to be reduced by 40 kg. The
financial assessment of this advantage
should also consider the new CO 2 limit
values, which foresee monetary fines for
fleets that exceed the limits.