Externally Controlled Compressor
Externally Controlled Compressor
General information
The 2nd generation 911 Carrera is the first sports car to feature the "externally controlled compressor" system.
With an externally controlled system, the air conditioning operating panel has a direct effect on the compressor and thus on the cooling output. Given this, most changes here relate to the compressor or compressor control. The components, which you will be familiar with from the air-conditioning system used in the previous model, are not modified to any great extent by this system and are essentially identical from the point of view of design and operating principle.
A comparable externally controlled compressor system is also used in the Cayenne.
Advantages of an externally controlled compressor:
- Reduced weight due to removal of a compressor clutch - Total weight loss here: approx. 3 lbs. (1.4 kg)
- No need to push the clutch when engaging (high-comfort use)
- Refrigerant output is controlled more precisely and is demand-controlled, based directly on the evaporator temperature
System parameters
- Externally controlled compressor (electronic control valve)
- No magnetic clutch (compressor runs continuously)
- System is controlled via evaporator temperature (similar to the Cayenne)
- Control valve is activated from the air conditioner operating panel (new software)
- Some refrigerant lines are re-routed due to modified engine (DFI engine)
Operating principle of the externally controlled compressor
The externally controlled compressor (from Denso) is an air side-controlled swashplate (adjusting) compressor, similar in function and design to the compressors used in the previous models. The gaseous refrigerant has a pressure of 145 to 435 psi (10 - 30 bar) and a temperature of up to 212° F. (100° C.) when it exits the compressor at a maximum delivery rate. The compressor continues to run at the existing engine speed and is not just switched on via a clutch relay.
NOTE:
- The air-conditioning compressor switches off automatically at outside temperatures of below approx. 35° F. (2° C.) and cannot be switched on again - even manually - by pressing the "Auto" button.
- The pressure relief valve opens at a critical system pressure of 2030 psi (140 bar) to protect the lines from irreparable damage. The valve closes again when pressure compensation is complete. The valve is located directly on the compressor, on the high-pressure side.
An electronic control valve - integrated in the compressor - also allows the high pressure of the refrigerant circuit of approx. 145 to 435 psi (10 - 30 bar) into the crank chamber housing if necessary (a by-pass bore opens).
Like all similar compressors, the crank chamber is connected to the low-pressure side of the refrigerant circuit via another bore for the return (pressure compensation between the high and low-pressure side via the return bore). As a result, the angle of attack of the swash plate can be reduced (towards zero delivery) without any great force by a spring working on the plate.
The control valve is controlled by a variable PWM signal from the air conditioning control unit (operating panel). The control unit uses the signal from an evaporator temperature sensor as the basis for controlling the control valve (for more information, see "Evaporator temperature sensor"). The basic frequency for the valve is 400 Hz.
NOTE: At a coolant temperature of T > 244° F. (118° C.), a forced suppression of the air conditioning system is requested of the air conditioning control unit by the DME system via the CAN bus. For this purpose, the electronic control valve sets the compressor to zero delivery mode (both radiator fans running).
The piston stroke - as described above under pressure compensation in the crank chamber housing - can be varied continuously by changing the amperage rating of the PWM signal (opening a by-pass bore).
This type of control ensures a large stroke for a high (refrigerant) delivery rate and a small stroke for a lower delivery rate from the compressor. Since the compressor operates with a variable delivery rate, there is no need to control the cooling output of the air conditioning system by switching a compressor relay on and off. In other words: the compressor continues to run even when there is sufficient cooling output, but can operate with zero delivery (swash-plate deflection: approx. 3 degrees). In this case, the compressor is running without a load.
NOTE: A timed full-load shutdown (compressor function is disabled) occurs if the throttle position is greater than 92%. Speed is taken into consideration here when setting the driver setpoint torque.
Operation (air conditioning operating panel)
Air conditioning control is operated via an operating panel (air-conditioning regulator) with an integrated control unit in the center console. The operating panel looks similar to the operating panel used in the current sports car except for small changes on the operator interface. These changes are not relevant for the externally controlled compressor.
NOTE: Information is provided via CAN in order to dim the displays in the LCD display.
The most important changes on the operator interface are as follows:
- AC OFF (ECO function = compressor runs in zero delivery mode)
- Integration of the two buttons for air-conditioned seats (see also system description for "Air-conditioned seats")
- Position of some operating buttons changed
As on the previous model, all heating functions in the vehicle are controlled via one unit.
Actuators and sensors
Evaporator temperature sensor
The temperature of the evaporator is primarily used as the basis for controlling the electronic control valve in the externally controlled compressor. The sensor is not fitted directly in the evaporator vanes. It is located at a parallel position approx. 20 mm behind the evaporator. At this distance from the evaporator, the sensor can measure a homogenous air-flow temperature for all temperature levels.
The evaporator is also prevented from freezing, based on this measured temperature. If there is a danger of freezing, the air conditioning operating panel reduces the angle of attack of the swash plate (pressure compensation between high and low-pressure side) by triggering the electronic control valve and thus reduces the refrigerant delivery rate. There is now less refrigerant available for evaporation and this counteracts the possibility of freezing.
Position of the evaporator temperature sensor
On the sports car, the evaporator temperature sensor (arrow) is located on the back of the heater unit housing. The connecting cable for the sensor is routed close to the condensation outlet.
To remove this sensor, you must reach in between the bulkhead (passenger's side) and the rear wall of the housing. The sensor is released when it is turned a quarter-turn and can then be removed from the heater unit housing.
