It is undeniable that stopping is just as important as moving forward. And once trucks exceed around 8 500kg gross vehicle mass (GVM), braking systems move from vacuum/hydraulic to air/hydraulic and full air systems, utilising air-pressure energy to convert it into mechanical force. Vacuum just cannot create enough force fast and effective enough to deal with stopping power requirements over 8,5t GVM.

The modern air compressor is gear-driven and water-cooled and air compressors are a vital component in trucking technology - not only for stopping but also for all the other power assistance required such as gear-shifts. 

It is true to say that you can run out of brake fluid but you will never run out of air – even a minor leak somewhere in an air-brake system will always allow it to work. An air-brake system is thus very reliable and besides the fact that they are easier to maintain, road transport operators would prefer all trucks to be equipped with full air-brakes.

Air leaks have a costly knock-on effect, one of these being increased fuel consumption. The time taken to charge depleted air tanks on a 7-axle inter-link combination, combined with excessive idling to maintain pressures, adds up to excessive fuel usage. 

Air-dryers must match compressors

In analysing failures, national sales and marketing manager for WABCO Automotive SA, Enoch Silcock, comments on an interesting case study. 

“Paired with a WABCO compressor, an ‘alternative parts’ air-dryer did not provide sufficiently fast dumping speed on a truck tractor, causing the compressor power reduction valve (PR valve) to remain open for a fraction longer. This resulted in the PR valve bending and seizing.” 

Based on this example, Silcock states a simple rule: “The relationship of air-dryer and compressor is important and you need to replace any component with the same original parts specified and tested by the manufacturer during development of the vehicle components If they are not matched correctly, the efficiency of the air processing system is compromised and this may eventually lead to damage to compressor or air-dryer or both.”

He adds that the PR valve is a clever way of reducing compressor power consumption by ‘free-cycling’ during the unload cycle. With power reduction this eventually equates to fuel saving. 

He also points out that the oldfashioned unloader simply dumped excess air when the system pressure was reached while the compressor still pumped continuously. 

“This shortened the life of the compressor as it was continually working under added load. Today it is all about getting extended life out of components and replacing complete components when they do eventually fail.” 

Engine’s umbilical cord 

The days are long-gone when compressors used an independent lubrication sump and were routinely overlooked. Modern compressors share a diesel engine’s lubrication, cooling and air-cleaning system. This is what makes them ‘maintenancefree’. 

It’s quite clear that failure to adhere to OEM oil and anti-freeze specifications also damages a compressor. Dust ingress from poor engine air-cleaner maintenance will also wear out a compressor. Given all this, I have to ask if, when it comes to an engine overhaul, a compressor get equal treatment – I think not!

According to Silcock, new compressor prices vary from R7 000 to R15 000 depending on whether the compressor is single cylinder or  twin, a new vehicle with PR valves or older type of system etc. Repaired or overhauled compressors may cost between 60 - 70% of new depending on parts and labour costs – in many cases extended compressor life is not guaranteed. 

In a well-maintained situation compressor life of around 800 000km can be expected in Europe but not in South Africa.

And compressors take strain at altitude. Operating at 1 500m on the Highveld adds to the strain of maintaining pressure on a hot day when the air is thinner. Jimmy Brink, managing director of MICO, the Haldex distributors in SA, believes that truck original equipment manufacturers (OEM’s) should look at an ‘African spec’ for compressors. 

“A high duty-cycle of 80%, raised ambient temperatures, big air reservoir capacities and the demands of some routes shorten compressor life with the resultant impact on safety,” he says. In many cases, compressors do not reach 100 000km service life.

Oil carry-over

This is where oil gets into the air system through the air-dryer and into the air valves. It starts at the compressor and there are many causes. Each incident needs its own investigation. For example: 

• Damaged honing on the compressor barrel sleeve caused by dirt in the intake air falling into the compressor and leaving scratches on the barrel – damaged or clogged filters letting dirt into the compressor or hard carbon deposits. 
• Lubricant dilution through fuel leaking in the engine sump thins oil and decreases temperature so that the oil vaporises and then settles in the compressor. All compressors have a certain amount of oil carry over but a faulty or clogged air-dryer that no longer removes most of the oil from the air will let oil through that gets deposited in the valves as the air cools. 
• A cooling pipe that is too short lets hot air with oil vapour into the air dryer which then flows through into the valves. Air-dryers are not too effective at removing moisture above 60°C hence the cooling pipe is essential to lower the temperature of the air stream and allow the air dryer to function effectively. 

Carbon build-up 

Also indirectly a result of oil carryover, carbon build-up is the biggest cause of compressor damage. The oil gets hot and eventually burns turning into carbon. In a normal system, the carbon is pushed through the system and collects in the air-dryer or is blown out. However, any system will eventually build up with carbon and needs to be cleaned. And this, as Wabco’s Silcock points out, is where the problem starts. 

“The compressor is replaced or overhauled but the cooling pipe and air-dryer are overlooked. If the cooling pipe is clogged, the compressor is subjected to strain as it works harder, finally causing damage to the compressor valves in the head which in some cases can disintegrate due to the heat. If the air dryer is clogged, the same can occur or the airdryer is bypassed and the rest of the valves in the system are damaged due to dust, water or oil” 

Conclusion

Compressors are safety-critical components and ‘maintenance dependent’ and you should adhere to the following checks:

• Replace engine air-intake filters and air-dryer cartridges regularly. 
• Strictly observe engine oil change intervals. 
• Check for compressor cooling pipe carbon build-up.
• Match the air-dryer to OEM specified parts. 
• When overhauling/replacing the compressor, ensure the cooling pipe and air-dryer are checked as well. 

The final comment goes to Enoch Silcock:

"Teach drivers to drive conservatively and not to 
over-rev a cold engine in the morning to fill 
the trailer air tanks."

A tough call for an imperfect world.

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