The actual interior temperature value and the setpoint interior temperature value are compared by left and right master controllers. The differential signal resulting from the comparison influences the controlled variable (Y-value) for control.
The operating range of the master controller is configured more widely than the operating range of the downstream controllers. The different IHKA functions, such as the automatic flap and blower facility, are dependent 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 using a separate regulating circuit and is not a disturbance variable for 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 control is a proportional regulator (linear amplifier).
The interior temperature can be set individually in 4 zones, additionally separated at the front for up and down. Each desired temperature is set by mean of a control wheel which only has blue and red markings. The temperature range that can be set by the occupants 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 of both temperature selectors on one side are set to maximum heat. Interior control is thereby cancelled. The heat exchanger temperature is adjusted up to the maximum of 90°C.
Exception: The MAX HEATING function is cancelled so that the water valves remain closed during independent ventilation operation.
MAX COOLING:At setpoint setting to maximum cold, MAX COOLING is activated individually for the left and/or right sides. Interior control is thereby cancelled. The heat-exchanger temperature is lowered to the minimum of 5°C and the heater valves are closed.
DEFROST:During this function, the interior regulation is disabled. When the DEFROST function is terminated, the system is matched to normal operation to prevent subsequent strong cooling.
Correction of setpoint values
Outdoor temperature input: the setpoint 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 specified 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: The control panel contains a temperature sensor with inside sensor fan for recording the interior temperature.
Heat-exchanger sensor: Heat exchanger sensors are installed on the left and right sides in the air flow of the heater for recording the diskharge 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: Coolant temperatures of up to 120°C occur due to the use of map cooling. In order to avoid damage to the air conditioner, the heat exchanger temperature is restricted to 90°C. If faulty water valves cause the heat exchanger temperature to exceed 98°C, map cooling is deactivated in the DME via the K-CAN bus.
Auxiliary water pump: An electric auxiliary water pump is installed in order to ensure adequate water flow through the heat exchanger 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 grill 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 inevitably influences not only the temperature but also the air volume in the entire system with each opening change of a flap, a complicated calculation depending on the influencing variables and disturbance variables is necessary. The actuators for the hot and cold air flaps are bus-controlled step motors.