Editor's Note: This article was orginally published Jan 15, 2013. Some of the information may no longer be relevant, so please use it at your discretion.
The diesel engine would be a shadow of its current self if it weren’t for the turbocharger. Having said that, when was the last time you saw a new diesel that didn’t have a turbocharger? Aside from small tractor engines and the like, the naturally-aspirated (non turbo) diesel has pretty much gone the way of the dodo.
This makes sense, because the turbocharger has made the diesel engine better in virtually every respect. Diesels are now more efficient, produce more torque and horsepower, and have much cleaner emissions than ever before. While a host of technologies have been utilized to make this happen, a good deal of the credit has to go to the turbocharger.
Diesel engines have gotten better over time, and this has been enabled by an evolution in turbocharger technology. Early turbos were limited in their capabilities, and these shortcomings had a proportional negative effect on diesel engine performance. The diesel turbocharger of today is capable of performing across a much broader engine speed and load range, yielding tremendous efficiency and emissions benefits. And we can expect even more from our diesel powertrains as new generations of turbocharger technology are introduced.
Turbochargers are relatively simple machines and are very reliable overall. But like everything else, things can and do go wrong, and automotive service professionals are called on to make them right again. Even if you only work on gasoline engines, turbochargers are becoming more common and you will need to become familiar with them. Let’s start by looking at basic turbocharger operation.
Laws of physics
A turbocharger is constructed by attaching a turbine to the end of a shaft with a compressor wheel at the opposite end. The turbine is located in the engine exhaust stream, so it sees very high temperatures.This requires the use of materials such as ductile iron for the housing and nickel or titanium alloys for the turbine wheel.The compressor, on the other hand, operates much cooler because it handles filtered air at ambient temperatures. Thus, the compressor end of the turbocharger typically uses aluminum for both the housing and the compressor wheel. The turbocharger may also incorporate a wastegate, which would allow exhaust gases to bypass the turbine wheel if boost pressure rises above a certain threshold.
The basic operation of the turbocharger is both simple and elegant. Start by using waste exhaust gases to spin the turbine (at speeds that can exceed 250,000 rpm). His recovered energy is utilized to drive the compressor wheel, which forces fresh air through a charge air cooler (CAC) and into the diesel engine’s intake manifold. More air in the combustion chamber means more fuel can be burned, thus producing increased power and torque. Turbocharging benefits diesel engines by:
- Increasing engine power density
- Increasing engine torque at low and medium RPMs
- Increasing fuel economy
- Reducing emissions
- Enhancing DPF regeneration
- Maintaining power at high altitudes
Of course, it can’t be quite that simple. If a turbocharger is sized large enough to work best at peak engine output, it won’t work nearly as well at lighter loads where lower exhaust flows are produced. In contrast, a single small turbocharger would respond well at low loads, but would lack capacity for high engine output. This dynamic doesn’t affect engines such as generator sets that run at a steady load and speed. However, automotive diesels operate over a broad range of engine RPMs and loads, so a single fixed geometry turbocharger cannot meet these requirements.
Another critical issue regarding turbocharger performance is a phenomenon known as “turbocharger lag.” When the driver pushes down on the throttle, more fuel is injected into the engine’s combustion chamber. However, turbocharger boost will not increase until the extra fuel creates greater exhaust gas flow. This results in a flat spot (hesitation) as the turbocharger takes time to “spool up,” and can also cause an increase in black smoke as an overrich mixture occurs. Turbocharger lag gets worse with larger turbo units, which require that much greater exhaust flow to build boost. Clearly, what is needed is a turbocharger package that will provide adequate boost under all engine operating conditions.
There are a number of possible approaches that can be taken to maximize turbocharger performance. However, the real challenge is to do it at the lowest possible cost without increasing vehicle weight, or the space required. This is a tall order, but automotive engineers have come up with some creative ideas to meet these objectives.
Two-stage turbocharging
In terms of pure performance, the best solution is to use multiple (two or three) turbochargers. One such approach that is being used in light-duty diesel engines is two-stage regulated turbocharging, also known as series sequential turbocharging. “Series” means that the output of one turbocharger feeds directly into another; “sequential” is the shifting of work from one turbo to the other depending on the mode of engine operation. This system is designed to rely on a small turbocharger to provide rapid boost response at lower engine speeds, and then press the large turbo into service as the engine approaches peak output. A familiar example of two-stage regulated turbocharging is the Ford 6.4 liter PowerStroke diesel. This design is also used extensively in late model BMW passenger car diesels.
In two-stage regulated turbocharging, a large turbocharger (known as the low pressure or LP turbo) is used as the first stage of compression. The LP turbo feeds boost air into a smaller turbocharger (known as the high pressure or HP turbo) and then on to the engine intake manifold. Ideally, there would also be CACs located after each turbo to increase air density and maximize boost performance. However, it is more common for a single CAC to be used after the HP turbo.
Exhaust gas flows in the opposite direction from the boost air, flowing first through the smaller HP turbine, and then continuing through the LP turbine. A valve is used to progressively bypass the HP turbine and send exhaust gases directly to the larger LP turbine as engine output increases. A compressor bypass valve may also be used to “short circuit” the HP compressor on the air intake side, effectively eliminating the smaller HP turbo at high engine outputs.
The performance advantages of two-stage regulated turbocharging are clear. However, the downsides are also significant, as this approach adds more weight and uses more space under the hood. What about designing a single turbocharger that can adapt to the various engine operating conditions?
Variable geometry turbochargers
Since it isn’t always practical to use two turbochargers, another turbocharger design has become mainstream technology. The Variable Geometry Turbocharger (VGT), also known as Variable Turbine Geometry (VTG), has been in use in light-duty diesels for more than 20 years. The basic idea is to modify the turbine section of a large turbocharger so its flow characteristics can be adjusted during engine operation. With this approach, a large turbocharger can operate like a small turbo when a diesel engine is at lower engine speeds and loads.
The most common VGT design uses variable pitch vanes that guide exhaust gases on to the turbine wheel. The vanes, which have a similar profile to an aircraft wing, are located around the circumference of the turbine. The vanes are all connected by a unison ring, which rotates to adjust vane pitch (angle). The PCM is responsible for the operation of the unison ring, and can control it using either a hydraulic or electromechanical actuator.
At light engine loads, there is limited exhaust gas energy to generate boost. When more boost is required, the vanes are rotated to a closed position. This creates a nozzle effect, where high velocity exhaust gas is directed on to the turbine wheel and increases its speed. This generates boost very rapidly, reducing turbo lag.
As engine load rises, exhaust gas flow also increases. If the turbocharger compressor is already running at high speed, this could cause an overboost condition if left unchecked. The turbine vanes are opened progressively, allowing exhaust gases to flow more freely through the turbine section. This reduces the nozzle effect at the vanes, which helps limit turbocharger speed and peak boost pressures. Most VGT turbochargers do not use a wastegate and instead use the variable vanes alone for controlling boost.
VGT turbochargers play a major role in diesel emission control. This plays out in a number of ways, including simple efficiency gains that result in lower fuel consumption. A bigger factor, however, is the enabling of diesel exhaust gas recirculation (EGR) systems. With boost pressure in the engine intake, EGR gases don’t flow very well because of a reduced pressure difference between the intake and exhaust manifolds. To overcome this, diesel engine management systems will close the vanes in the VGT turbocharger, which increases exhaust backpressure. Combining this action with closing of an air intake throttle helps increase the pressure difference between the manifolds and EGR gas flow increases.
Another benefit of using a VGT with a diesel engine is the ability to close the vanes to create exhaust backpressure when starting in cold ambient temperatures. This helps warm up the engine more quickly, and retains heat better during engine idling.
Best of both worlds
VGT turbochargers cost less and have a smaller footprint than multiple turbocharger systems. However, they do not perform as well either, so (like everything) a compromise is made. This is the job of the engineer, who has to decide what is most important and strike a balance between these competing factors.
It is fun to watch what is happening with the latest turbocharger system designs, because every trick in the book (and some new ones) are being used to push VGT technology closer to multiple turbo performance levels. One area with room for improvement is the mass of the turbocharger rotating element. With a conventional VGT, a large turbocharger with relatively heavy components is built to perform like a small turbocharger when needed. If the mass of the rotating components could be reduced, the VGT would be able to build boost even faster and therefore respond more quickly to changes in engine load. The key is to make it lighter, but still be able to maintain the airflow potential of a large turbocharger.
Honeywell has taken this path in the design of its VNT DualBoost turbocharger, found in the pickup version of the Ford 6.7 liter Powerstroke diesel. VNT stands for Variable Nozzle Turbine, which is Honeywell’s version of the VGT. The VNT DualBoost’s primary claim to fame is low inertia, meaning that the turbocharger’s rotating components have a reduced mass and can accelerate and decelerate much more quickly than a standard VGT. This was accomplished through the use of a unique compressor wheel with two small impellers mounted back to back. Each impeller has its own separate air inlet, and discharges into a common outlet. The VNT DualBoost is described as a two-stage sequential variable geometry turbocharger, despite the fact that both compressor wheels are driven by a common shaft and operate effectively in parallel.
“This (design) gives the Dualboost its inherent advantage in allowing a lower inertia rotor group while delivering the same air flow capacity of a larger turbocharger,” says Geoff Duff, Honeywell Regional Applications Engineering Leader – Americas.
The VNT DualBoost also uses ball bearings instead of conventional sleeve-type bearings to support the rotating element. This reduces mechanical losses and further improves turbocharger response.
The turbine section of the VNT DualBoost looks more like a conventional VGT, but includes a wastegate. The unison ring is hydraulically-actuated, and the vehicle PCM operates a spool valve that controls engine oil pressure on both sides of the actuating piston. When the engine is operating at high boost and RPMs are rising rapidly, opening the variable vanes may not be enough to control boost pressure. In these situations, the vacuum-operated wastegate is opened to prevent overboost and possible engine damage. Overall, the wastegate allows for more precise control of turbocharger boost and a high power point.
During engine operation, the outboard impeller receives air through the main air inlet in the compressor housing, much like a conventional turbocharger. The inboard impeller is fed by an alternate passage known as the high speed air inlet. While there are select conditions when airflow will stall on one side of the wheel or the other, the impellers operate in parallel with little difference in their performance. Using two impeller wheels instead of one changes the work/speed relationship of the compressor, which increases the performance of the turbine. This improves on conventional VGT performance and comes close to matching that of two-stage regulated turbocharging.
What goes wrong
While turbochargers are designed to last the life of the vehicle, things can and do go wrong. Like the engine itself, a turbocharger can only live a long and prosperous life if it has clean air and clean oil. Make sure to inspect the air inlet system regularly, and repair any problems you encounter right away. Changing oil at the correct intervals and using the best oil and filter you can afford is also good policy.
An illuminated Check Engine (MIL) light will often be your first clue that something is wrong with the turbocharger and/or associated systems. Turbochargers are designed as an integral part of any diesel engine, and are monitored closely by the vehicle onboard diagnostic system. OBD monitors related to the turbocharger include the boost pressure monitor, which is responsible for determining if turbo output is correct for any given engine operating condition. The vehicle’s PCM runs regular tests on all emissions-related systems, and will generate diagnostic trouble codes (DTCs) when tests fail. DTCs will often be the starting point for your diagnosis, which may also involve searching TSBs and service information databases such as Identifix or iATN.
In regards to scan tools, it definitely pays to have the most capable unit you can afford. In light-duty diesel repair, the litmus test is what sort of bi-directional testing the scan tool is capable of. As you can imagine, more is better, and generally the best option is the factory scan tool. Better scan tools are also capable of performing air management tests that do a comprehensive check on the turbo, EGR system, etc.
Turbocharger technology continues to evolve, so watch for new designs and know that diesel powertrains will only get better as turbocharger performance improves.
