Closed loop systems - A deeper look

Fun fact: not all automotive air suspenFun fact: not all automotive air suspension systems use the same working principles! Therefore, it is important for a professional workshop to know which system they are working on: a conventional open system or an alternative “closed loop” air suspension system.

Properly diagnosing possible system errors or defects, ensuring the correct working procedures are followed, correctly installing new air suspension components, and checking the system and making it operational again after an air spring leak is different from the conventional open air suspension system. In this article, Arnott will provide more insight into the alternative closed loop system.

Inflation and deflation processes

Now that we know that specific technical knowledge and understanding is required, let’s take a look at the general differences between the two systems’ working principles before diving deeper into the subject.

Looking at the graphic above, it shows that the workings of the compressor are quite different in a closed loop system. Whereas in an open system the compressor compresses ambient air to fill the reservoir and/or air bellows, in a closed loop system the compressor uses the pressurized nitrogen that is already in the system. Additionally, in a traditional air suspension system (open system) excessive air is released into the open but in a closed loop system the nitrogen stays inside the system and excessive pressure is led back to the reservoir. It is important to keep this operating difference in mind when working on the closed loop system.

For inflation/additional pressure, the closed loop system uses two reversing valves that will open towards the valve block. Then the solenoid valve for the appropriate air spring opens and the additional pressure is transferred. When deflating, the excessive pressure is not discarded but cleverly routed back to the reservoir by opening the two reversing valves in the other direction. The excessive pressure is now transferred back into the reservoir. The pressure flow diagram below gives an indication of how the pressurized nitrogen is routed when an air spring is inflated or deflated.

The twist

Although it is called a ‘closed loop’ system and the diagram above shows how the compressed nitrogen is transferred from one area to another to inflate/deflate the system, don’t be fooled! A closed loop system does have the capability to make up for small amounts of nitrogen loss by drawing in and compressing atmospheric air. When the system has a leak, it will try to retain its required operating pressure by opening the (ambient) air intake. However, it can only compensate for a small loss of nitrogen and when the compressed nitrogen mass gets too low the system will stop functioning.

Indicators

Just like a conventional air suspension system, the system pressure will gradually drop when there is a big air leak present. Therefore, a symptom of an under-charged system is when it ceases to operate. iI a closed loop system, however, the ECU (Electronic Control Unit) continuously calculates the “nitrogen mass” as an indication of normal operation. If too low, error messages like “max payload exceeded” might appear on the dashboard (depending on the car manufacturer).

An indicator of an overcharged system is the audible discharging of excess pressure. This only occurs when a mechanic has not followed the correct procedure when recharging the system after a leak repair. An overcharged system can lead to damaged and defective components, along with serious safety consequences.

Tips & tricks

Now that Arnott has explained some of the basic working principles of a closed loop air suspension system, we would like to leave you with some key guidelines to follow when a vehicle needs repair or replacement on a closed loop air suspension system:

• The air springs/struts must be replaced if the vehicle has been driven/moved with one or more empty air bellows.
• The appropriate car manufacturer’s installation instructions must always be followed. If the instructions and specifications are not followed correctly, the system will continue to produce errors and malfunction.
• A vehicle raised with an empty air suspension system may not be lowered onto its wheels or be driven. When the air springs/struts are empty and the vehicle is lifted, a vacuum is created in the air bellows. If the vehicle is then lowered afterwards, creases might develop in the bellows resulting in premature failure of the part.
• Follow the correct inflation procedure. Failure to adhere to these procedures could lead to shearing of the electrical connection, or other additional damage to the part. Not following the correct inflation process can even cause the air spring to explode due to pressure in the wrong component or over pressurizing a component part.
• The nitrogen used to pressurize the air springs needs to be inside the system for the compressor to do its job. A closed loop compressor can only partially compensate for a pressure difference. If the system pressure gets below a certain value, the system needs to be recharged.
• In general, the output pressure in the system must be 17 bar after replacement of any spare part like air spring, air strut, compressor, and valve. Verify the manufacturer’s specification for the exact output pressure needed and always follow the recharging procedure as described by the manufacturer. Not following the correct inflation and recharging process can cause the air bellow to explode due to excessive pressure in the wrong component or over pressurizing a component part.