The concept of turbocharging is simple enough: Employing a turbocharger allows more air and fuel to enter the engine’s cylinders to provide more power than a naturally aspirated engine, says Alex Miser, components communications manager at Cummins, a heavy duty diesel engine manufacturer.
This is accomplished using a compressor that draws in air, compresses it, and supplies it to the engine. The compressor is driven by a turbine which is powered by some of the engine’s exhaust gas.
“In turbocharged engines, the combustion air is already pre-compressed before being supplied to the engine,” says Thom Miles, senior manager of turbo sales and marketing at automotive supplier BorgWarner. “The engine aspirates the same volume of air, but due to the higher pressure, more air mass is supplied into the combustion chamber. Consequently, more fuel can be burned so that the engine’s power output increases related to the same speed and swept volume.”
Turbochargers have been used to improve the performance of internal combustion engines since the early 1900s, but it was not until the 1950s that they began to see use in heavy duty commercial vehicles. Since then, turbocharger design has evolved to allow greater power output and efficiency from diesel engines.
Having more moving parts tends to lead to more fail points, however. While turbochargers have come a long way in terms of durability and longevity, it is still imperative to perform preventive engine maintenance as outlined by the manufacturer in order to maximize the life of the turbo.
Ensuring turbo longevity
Turbochargers of the past had many common fail points, says Johan Agebrand, director of product marketing at heavy duty OEM Volvo Trucks North America.
“Since the fuel injection in today’s engines is controlled electronically, this also means that turbos can be controlled indirectly with electronics,” Agebrand says.
The advances in engine technology mean fewer common fail points on modern turbochargers.
Today, the turbocharger itself is also designed to last the life span of the engine, BorgWarner’s Miles adds. It does not require any special maintenance, and periodic inspections are sufficient.
Bearing inspection
The frequency and type of inspection required can depend on the type of bearing the turbocharger uses. Turbochargers today spin at very high RPMs and require special bearing systems, Volvo’s Agebrand says. They typically use one of two types of bearing systems: a full floating journal bearing system or a ball bearing system.
“The journal bearing type allows the shaft to spin on the inside of the journal bearing, yet the bearing is free in the bearing housing, therefore the bearing spins inside the bearing housing too,” Agebrand explains. “In general, the bearing spins at two-thirds of the turbo shaft speed. Since there are clearances on both sides of the journal bearings, a fair amount of shaft play can be felt when checking even a new turbocharger.
“Over time this clearance will grow as the bearings wear to a point where contact can start to occur of the turbine wheel or compressor wheel,” he continues. “An experienced technician can typically recognize when the play is too excessive and may elect to change the turbo. Also, it is important to maintain good oil quality as oil coking in the turbine side bearing will lead to early failures.”
The ball bearing system design, on the other hand, utilizes specially designed high-speed ball bearings, according to Agebrand. These ball bearings offer the advantage of lower friction for faster spool up and mechanical efficiency. Additionally, since a ball bearing design does not allow the outer bearing to rotate in the bearing housing, it has less shaft play and therefore tighter turbine and compressor clearances can be maintained to improve overall efficiency.
Agebrand notes the ball bearing design does not wear as much over time and no periodic clearance check is recommended.
Engine maintenance
Regardless of bearing type, the speeds at which turbos operate can generate an immense amount of heat which – without proper engine maintenance – can lead to bearing failure. Len Copeland, Detroit product marketing manager at heavy duty OEM Daimler Trucks North America, says this is one of the most common fail points of turbochargers today.
“Today’s diesel turbos run at such high speeds, they must run at near zero friction levels to keep temperatures down,” Copeland says. “So bearing cooling is critical as well as bearing lubrication.”
Most diesel turbochargers today are cooled and lubricated by oil from the engine. Therefore, if the engine’s oil is contaminated or run past its change interval, the turbo will also suffer from this lower quality oil, reducing lubrication and putting the bearing at risk.
“To ensure that the turbocharger’s lifetime corresponds to that of the engine, the vehicle owner must adhere to the engine manufacturer’s service instructions,” Miles says. “One of the most common causes of turbocharger damage is the lack of adequate lubrication of the turbo’s rotor assembly and … bearings. Therefore, the vehicle owner must follow the engine manufacturer’s recommended oil and filter change intervals.”
Miles adds that inspection and maintenance of the engine’s air filter system is also critical to turbocharger longevity.
Poor air cleaner maintenance can lead not only to reduced fuel economy and engine performance, but a plugged air filter can also cause compressor stage oil pullover in the turbo, leading to lost lubrication of the bearing, Copeland says.
Additionally, foreign object debris can be catastrophic to a turbocharger. Agebrand notes this can be a common issue when servicing other parts of the truck such as air filters.
“Foreign object debris are things that enter the air inlet such as a dropped bolt from a technician,” he says. “We have also seen rags getting sucked into the air inlet. Many technicians use a rag to protect the air inlet from bolts and other items they might accidentally drop into the inlet pipe, but those rags are sometimes easily forgotten when they connect everything back. Once the engine is started, the rag gets sucked right into the engine.”
Agebrand suggests a better practice for technicians to consider would be using shower caps, or something, similar that sit visibly on the outside of the inlet pipe, not jammed inside like a rag. This makes it much harder for the technician to forget to remove when reassembling the system.
Oil leaks
Another example of a turbocharger fail point is oil leakage, which can lead to reduced performance, increased oil consumption, and emission non-compliance, says Cummins’ Miser. Cummins’ latest oil sealing technology is designed to reduce these risks through the development of a more robust sealing system.
According to Miser, the new sealing technology for Holset turbochargers allows turbo downspeeding, downsizing, oil leakage prevention on two-stage systems, and enables CO2 and NOx reductions for other technologies. He says the technology has also improved thermal management and reliability of the turbocharger.
Advances in turbochargers over the last century have significantly reduced common fail points and improved efficiency and power output of engines. However, regular inspection and engine maintenance – especially oil changes and air intake maintenance – remains critical to turbo longevity.
