Electric auxiliary heater

The electric auxiliary heater using the PTC principle (resistance with positive temperature coefficient) heats the air flow for controlled-temperature air in the passenger compartment. The electric auxiliary heater accelerates the heating of the passenger compartment, especially at low ambient temperatures and during the cold-start phase. In principle, the electric auxiliary heater works like an electric radiator.

The heating elements in the electric auxiliary heater are PTC resistors. The heating elements are made up of individual ceramic semiconductor resistors. As of a certain temperature, the resistance of the heating element has a positive temperature coefficient. That means: The warmer the heating elements become, the higher their resistance. When the electric auxiliary heater is switched on, a high level of current flows. With increasing heating, this level of current falls. This limits the maximum power consumption. These electrical characteristics of the resistors enable a temperature of approx. 120 °C in the area surrounding the heating elements that presents no problem for the heater/air conditioning system. This "physical" overheat protection is ensured, even in the event of a blower failure.

The following electric auxiliary heaters are installed:

• Electric auxiliary heater in the heating and air-conditioning unit (only vehicles with diesel engines)

  The electric auxiliary heater is a separate component and is installed in the heating and air-conditioning unit. The electric auxiliary heater is activated by the control unit of the integrated automatic heating and air-conditioning system (IHKA control unit). Depending on different signals (e.g. temperature signal from the rear footwell temperature sensor, signals from energy management), a percentage power request for the electric auxiliary heater is generated in the IHKA control unit and sent on the LIN bus The maximum electrical power output of the electric auxiliary heater depends on the resources of the vehicle electrical system. As a general principle, only the excess power of the alternator is available to the electric auxiliary heater.

  Fig 1: Identifying Electric Auxiliary Heater

  Courtesy of BMW OF NORTH AMERICA, INC.

• Electric auxiliary heater in rear left and rear right (only IHKA High with FKA)

  The electric auxiliary heaters are part of the rear automatic rear air-conditioning system (FKA) with left-right separation. An electric auxiliary heater is installed In every footwell ventilation duct. Depending on different signals (e.g. temperature signal from the rear-cabin footwell-temperature sensor, signals from energy management), a percentage power request for the electric auxiliary heater is generated in the FKA control unit. The output request in % is transmitted by means of a pulse-width-modulated signal.

  Fig 2: Identifying Electric Auxiliary Heater (Only IHKA High With FKA)

  Courtesy of BMW OF NORTH AMERICA, INC.