The interior temperature actual value and the set interior temperature target value are compared by a left-hand and right-hand master controller. The differential signal resulting from the comparison is included in the control as an adjusting value (Y value).
The operating range of the master controller is configured more widely than the operating range of the downstream controllers. Various IHKA functions, e.g. automatic fan and flap control, depend on this control range.
Auxiliary control loops for the heat exchanger on the left and right correct disturbance variables. Disturbance variables can be caused by temperature changes as a result of air flow and water flow fluctuations.
The evaporator temperature is set with a separate control loop and has the effect of an interference variable on the system.
Two similarly independent control loops regulate the ventilation temperature on the left and right sides. This helps to achieve a stratification between the footwell vents and the ventilation vents.
The interior controller is designed as a proportional controller (linear amplifier).
The interior temperature can be set individually in 4 zones, additionally separated at the front for top and bottom. Each desired temperature is set using a control wheel that only has blue-red markings. The temperature range that the occupant can set is converted internally to a range of 16 °C ... 32 °C (approx. 60 °F ... 90 °F). The increment is 0.5 °C or 1 °F.
MAX HEATING: MAX HEATING is activated when both temperature selectors on one side at the front are set to 'maximum hot'. Interior control is thereby cancelled. The heat-exchanger temperature is regulated (raised) to the maximum of 90 °C.
Exception: so that the water valves remain closed during independent ventilation operation, the MAX HEATING function is overridden.
MAX COOLING: with the target value setting to 'maximum cold', MAX-COOLING is activated individually for left and/or right. Interior control is thereby cancelled. The heat-exchanger temperature is regulated (lowered) to the minimum of 5 °C and the water valves are closed.
DEFROST: During this function, the interior regulation is disabled. On terminating the DEFROST function, a match to normal operation takes pace to avoid subsequent severe cooling.
Correction of the target values
Outdoor temperature input: the target value is corrected as a function of the outside temperature. The effects of radiation from outside and surrounding surfaces of the passengers are thus compensated. The outside temperature is recorded in the bumper areas by an external temperature sensor and transmitted via the K-CAN bus to the IHKA. The outside temperature input and thus target value increase can be between +12 °C and -2 °C. The outdoor temperature input also serves as so-called pre-activation for a possible disturbance variable.
Interior-temperature sensor: a temperature sensor with inside-sensor fan is fitted in the operating unit to pick up the interior temperature.
Heat-exchanger sensors: Heat exchanger sensors are installed on the left and right sides in the air flow of the heater for recording the discharge temperature at the heat exchanger. The opening time of the water valves is derived from the values thus determined. The water valves are activated with pulse-width-modulated signals.
Filling station effect: When the valves are at zero current, the heat exchanger can fill up with water (filling station effect). To avoid this, the water valves continue to be supplied with current for three minutes after the drop in terminal 15.
Engine characteristics cooling: the application of characteristic map cooling means that coolant temperatures of up to 120 °C occur. To prevent damage to the air-conditioning system, the heat-exchanger temperature is limited to 90 °C. If the heat-exchanger temperature is > 98 °C due to defective water valves, the characteristic map cooling is disabled in the DME via the K-CAN bus.
Auxiliary water pump: An electrical auxiliary water pump is installed to ensure the necessary water flow even at low engine speeds. Activation takes place
A virtual flap setting (i.e. does not exist in reality) is calculated in order to achieve the temperature and air volume of the total air flow. The following influencing variables are taken into account in the calculation:
The settings of the cold and hot air flaps are corrected with the virtual flap setting calculated from the points listed. The correction is carried out in such a way that the desired air volume and temperature is reached at the air inlet grille through the mixture of hot and cold air.
The settings of the hot and cold air flaps determine the hot and cold air mass flows individually. They thus determine the total mass flow in addition to the ventilation temperature.
As this system necessarily influences not only the temperature but also the air flow rate in the entire system every time there is an opening change to a flap, a complicated calculation depending on the influencing variables and interference variables is necessary. Bus-controlled step motors serve as actuators for the hot and cold air flaps.