Mercedes-Benz SLK-Class: The latest computer models for climate control: Virtual driver tests climatic comfort in wind and weather
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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
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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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