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Stuttgart -- Mercedes engineers are using an innovative process to optimise the
climatic comfort of car occupants. With the aid of computer simulations, they
send new models on virtual test drives and calculate the air and temperature
distribution in the interior under different weather conditions. This computer
model is based on research carried out with a large number of male and female
drivers, who provided information about their personal comfort and the
temperatures at which they felt most comfortable. The SLK Class was the first
Mercedes passenger car to embark on such a computer-based climate test.
Controlling the climate in the interior is one of the most complex tasks in
automobile development. Since the vehicle speed, temperature, level of sunlight
and humidity change constantly when on the move, a car air conditioning system
must respond very rapidly and flexibly if the occupants are to experience a
uniform standard of comfort – neither feeling cold if the outside temperature
suddenly falls nor perspiring if it increases. People only feel really
comfortable if their climatic surroundings remain pleasantly constant.
In order to ensure this under all driving and weather conditions, climate
control engineers must commence their tests as soon as possible during the
development of a new car model. They are not able to wait for the first
prototypes or pre-series vehicles, as the technology and design have been almost
completely finalised by this stage.
This led to the development of "TIM" – the Thermo-physiological Interior
Model, which allows the climatic comfort of future Mercedes models to be
calculated and optimised in advance. At an early development stage it enables
the engineers to establish the ideal output for the heating and air conditioning
systems, how many ventilation vents are required and how large these should be
in order to ensure the constant climatic comfort which is typical for a
Mercedes.
"TIM": a virtual driver with almost every body function
"TIM" is the result of many years of work by DaimlerChrysler researchers on
human thermal comfort levels. For example, a large number of male and female
drivers provided the basic data for the so-called "equivalence temperature",
which corresponds to the temperature "felt" by car occupants and enables the
actual, perceived climatic comfort to be defined for each part of the body.
Specialists for example found that 80 percent of the individuals tested felt
most comfortable in summer when the equivalence temperature at the torso and
arms was between 19 and 28 degrees Celsius, while the comfort range for the
lower legs and feet is between 23 and 27 degrees Celsius.
The "TIM" computer model is based on these and other findings about the
sub-jective perception of comfort. It simulates most of the human body in a
total of 14 areas, also taking into account the blood circulation and relative
heat generation. The result is a virtual but certainly representative car
occupant, who is sent to all the climatic zones of the world by computer and
supplies Mercedes engineers with a mass of data. These are intended to answer
only one question: does he feel comfortable?
Airflow: comfort test at four million points of the interior
The SLK-Class is the first Mercedes model whose heating and automatic climate
control systems were developed both by computer and by practical testing. "TIM"
absolved test drives of many hours duration under the most varied driving and
weather conditions in this sporty two-seater. In addition, "TIM" was linked to
other computer programmes which for example divided the interior into up to four
million spatial units and measured the airflow, temperature and other comfort
parameters at each of these points.
On-screen readouts enabled the engineers to establish when the respective
feel-good temperatures were reached, and whether "TIM" indicated the right
comfort level. If required, a few key strokes at the computer were enough to
adjust the climate control system until the two virtual vehicle occupants began
to transmit satisfactory data.
Climate control: thousands of key data for all driving and weather
situations
In this way the engineers fed thousands of key data into the control unit of
the THERMOTRONIC automatic climate control system, subsequently verifying and
refining these in practical trials. Once the computer recognises a certain
situation on the basis of sensor data, it accesses the programmed values and
adjusts the air conditioning accordingly. This activates up to five electric
motors inside the air conditioning unit, which automatically open or close the
air vents to adjust the air distribution. At the same time the fan speed is
increased and the output of the compressor is adapted to the new conditions.
The results obtained from this computer simulation not only provide the basis
for the key data in the climate control system, but also help developers to
design the dashboard. The computer model gives them a precise indication of
where the air vents should be located, and how large they should be, to ensure
an effective i.e. draught-free air distribution. The larger the cross-section of
the vent, the lower the air speed and therefore the less draught to disturb the
occupants. The striking air vents in the dashboard of the SLK sports car follow
this principle, thereby combining form with function in an exemplary way.
Although they are not obvious, the six footwell vents on the underside of the
dashboard are no less important for climatic comfort. Their location is likewise
mainly based on the "TIM" simulation and other computer models. When the heating
is on, 80 percent of the air volume flows through these vents to warm the
thermo-physiologically most sensitive body areas of the occupants, namely the
feet.
Open-air experience: good heating performance even with the vario-roof
open
The intelligent climate control system of the SLK-Class also works when the
vario-roof is open. As soon as the occupants decide to enjoy open-air driving,
the automatic climate control system switches to special characteristic values
and adjusts both the air distribution and temperature control. The basic
settings for this are also supplied by computer models such as "TIM": the
computer, for example, simulates an open-air drive on a fine day at an ambient
temperature of ten degrees Celsius, calculating the airflow over and inside the
Roadster at 80 km/h.
At an early development stage, this enables Mercedes engineers to achieve the
right comfort (equivalence) temperatures at head level: for example by reducing
the air speed with the help of the standard draught-stop and large air vents
which direct warm air precisely at the car occupants. In conjunction with the
unique AIRSCARF neck-level heating system, this achieves a level of thermal
comfort unrivalled by any other convertible in this vehicle class.
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