When and why did turbochargers become so complicated? First, pressure controlled wastegates, then solenoids that control vacuum to a diaphragm. Now, electronic actuators and hydraulic solenoids control complex internal mechanisms within the turbine housing that vary the air ratio. The variable turbo concept has actually been around a long time and has been used in small automotive gas and diesel engines for years. The actual purpose and function of them really is quite ingenious and contrary to popular belief, they are not as troublesome as they are made out to be.
There are several different designs of variable turbine technology, but all are using the same concept. A standard turbo without so much as a wastegate can only provide boost to the engine in a direct correlation to the amount of exhaust the engine produces. The more exhaust pressure and heat, the faster the turbine wheel is driven and the more boost produced. This limits how much performance can be achieved from the turbo.
The smaller the turbo the faster it will spool up and provide boost. The downfall is that a small turbo reaches its choke point at a lower flow. This means that there is a point at which the pressure coming into the turbine housing exceeds the capability of the housing and wheel to exhaust. If the engine is still accelerating at the choke point, the turbo will become an exhaust restriction and rob the engine of power. By moving to a larger turbo, more air can be moved. The downfall to a large turbine size is the increase in the amount of energy needed to get the turbo up to speed to begin providing boost. This delay in boost is known as lag. Manufacturers have struggled for years walking that tight-rope between lag and restricting the engines volumetric efficiency.
The wastegate was one of the first forms of variable geometry. Its essential function was to allow a turbine housing to have a decreased size allowing for higher exhaust speed at lower RPM’s. Once the exhaust pressure neared the choke point, the wastegate would open and allow exhaust an additional path to atmosphere and lowering the exhaust pressure.
Essentially you have two different air ratios within one turbocharger. The wastegate is typically controlled by a solenoid, or by using the boost pressure created to operate a spring loaded diaphragm.
This was the first step at making mid-sized engines both economical and able to perform both low engine RPM’s for city driving and at highway speeds.
A variable geometry turbocharger, or variable nozzle depending on manufacturer, is exceptionable in that it basically acts as if it were several different sizes of turbocharger throughout the RPM range of the engine. By simply changing the velocity of the air across the turbine wheel, the turbo is capable of producing boost while the engine is at low RPM’s.
Opening the VGT mechanism reduces the backpressure and allows the turbine housing to flow sufficient air for the volumetric efficiency of the engine. The ability to control boost independent of engine speed and exhaust energy has revolutionised the light duty diesel pickup industry. Horsepower ratings are climbing faster and torque curves are widening.
Variable turbochargers do however have a challenge that will make them more susceptible to failures than previous styles. What’s that problem? Moving parts.
Imagine having anywhere from five to 25 moving parts in the worst possible environment imaginable. The temperatures ranging from ambient to 1300°F, condensation from heat cycles causing rust, leaky EGR coolers depositing sticky coolant and additives, and the never-ending enemy soot. The VGT mechanism is constantly battling contaminants that seem to want nothing but its demise. On a properly running engine, the chances are you can expect a long trouble free life from your turbo. Problems begin to arise, however, when you factor in things as simple as driving habits. The addition of EGR to the diesel engine is the equivalent of “diesel cholesterol” as it deposits unburnt fuel and soot into the turbo and back into the intake manifold.
Exhaust gases are recirculated back into the engine through the intake at light load conditions. As the gas passes through the EGR valve, EGR cooler, and intake, particulate matter from combustion is carried with the gas and deposited along the way.
These particulate deposits begin to cause performance problems. These deposits can create intake restrictions by clogging the intake, EGR valve malfunction by restricting movement, and deposit soot into your VGT components causing them to seize or react slowly.
These issues then actually cause more particulate creation and deposits and the problem worsens exponentially.
The key to long life of your VGT turbo basically boils down to ensuring proper maintenance of your engine and addressing any issues with engine performance as they arise. The majority of all VGT failures can be prevented and allow you to enjoy that boost performance from idle to wide open throttle, just the way it’s intended.
Source: Engine Professional magazine
