Performance Under Pressure

Turbos transform exhaust to power

Published in the June 2008 Issue June 2008 Exhaust, Turbos

Air is a remarkable thing. Although it is basically unnoticed, it is a source that sustains life and influences everything it comes in contact with. And as the saying goes: If some is good, more has to be better.

That's definitely the case when it comes to diesel engines.

So the trick is to get more air packed into a cylinder so it can do its magic with a little diesel fuel. Now that's putting some pressure on performance.

Basically, the process of a combustion engine is simple-air and fuel are compressed in a combustion cylinder where the generation of heat from the compression creates an explosion which sends the pistons down, spinning a crank shaft which simultaneously pushes another piston up, compressing air and fuel and starting the process all over. The more air and fuel you can pack into a cylinder, the bigger the bang, the faster the rotation and the more power generated.

During this process there are other things going on inside the combustion engine. One such event is the removal of the spent gas and air out through the exhaust.

Over the years, engineers have discovered ways of utilizing the process of removing exhaust into a way to create more power and performance to an engine. The most popular concept was to use the flow of exhaust air to spin a turbine which powers a compressor that packs more air back into the original process of air/fuel mix. Thus, the creation of a turbocharger.

Your basic turbo consists of an inlet feeding exhaust gas into a turbine housing which spins the turbine wheel while allowing the exhaust gas to exit out your exhaust pipe. The turbine wheel is attached to a main shaft which is attached to a compressor wheel inside a compressor housing. It has an ambient air intake that draws in fresh air, compresses it and channels it to an air cooler. (Compressed air will heat up by nature. By cooling it down before introducing it to your combustion engine, you reduce engine temperatures-which is a very good thing.)

Since you have more air being packed into the engine, you can increase the flow of your fuel, thus creating more bang.

To understand the process and benefits of putting a little air under pressure, we contacted Craig Gibbs, manager of Technical Applications at Honeywell's Garrett Independent Aftermarket, to get his insight on the process of maximizing air pressure, flow and efficiency to improve the performance of a diesel engine via turbochargers.

Gibbs says the process of designing the most effective turbocharger is very complicated and requires a lot of testing. There is no magic formula for size, weight and number of blades on the turbine. "We wish there were," Gibbs says. "It would make our job as engineers a lot easier."

He explains that the aerodynamic efficiency is constantly being juggled against mechanical properties of the wheel design including blade strength-especially when the product is applied in severe applications such as high duty cycles, high cyclic operating conditions (high altitude, stop and go hard acceleration duty cycles such as a transit bus) or extreme speeds.

"There is a constant battle to provide the most efficient aerodynamics and good responsiveness, yet with something strong enough to survive the demands of the application," Gibbs says. "A heavier wheel in theory will be less responsive due to the wheel's inertia. The weight can be affected by the overall size of the wheel, the size of the core hub, the number of blades, blade thickness, the type of material (applies to both compressor and turbine wheels)."

Gibbs says lightweight materials for wheels have so far not been successful (such as ceramic) due to durability issues. They are not robust to foreign objects going through the exhaust, unlike a steel wheel.

Even the size of the turbine plays a role in the performance of a turbocharger. A smaller, lighter turbine represents less lag and a larger, heavier turbine represents better high-end boost. So what is the optimum balance in turbo size?

"One way Honeywell engineers have addressed this issue is with the development of the patented variable nozzle turbine (VNT)," Gibbs explains. "By manufacturing a turbocharger with adjustable vanes, you get the best of both worlds. Turbochargers with adjustable vanes are able to provide a wide range of flow with high-end boost (as needed) along with minimal lag."

VNT turbochargers are currently utilized in light-duty applications such as the GM 6.6L Duramax engines.

The boost pressure and air flow rate are determined by the energy of the spinning turbine and its relationship with the compressor. "Each wheel size has its own power requirements and it depends upon what boost level you're operating at," Gibbs explains. "There is a wide range of power required."

It comes down to the basic design of the diesel engine.

"Most engine designs have cylinder pressure limits and the vehicle's system will have a limited cooling capacity," he explains. "An engine's core design and its system capability dictate boost levels."

Gibbs says the concept of matching a compressor wheel to a turbine wheel is generally a matter of finding the optimal balance of air flow capacity and wheel speed. "Housings are designed to be optimized for a particular wheel family and there is no relationship between the size of the two end housings," he says. "They are separate elements that are designed specifically to the wheel they're mated with."

Restricting Air Flow

Since the turbine can restrict exhaust flow, there is a little disruption in the engine's ability to exhaust its spent gases. However, the power gain from this process far exceeds any negative effect it may have to air flow.

"Engine exhaust flowing through the turbocharger's turbine generates horsepower to spin the turbocharger's compressor," Gibbs explains. "The compressed air entering the engine yields more engine horsepower. In effect, the overall power of the engine is increasing."

To improve the efficiency of this process, Garrett GT turbochargers utilize dual ball-bearing cartridges, which have been shown to produce a dramatic improvement in spool-up response with significant reduction in friction loss, increased thrust bearing capacity and higher durability (see Garrett Turbochargers 2008 Catalog for detailed information).

Gibbs also points to new compressor and turbine wheel blade designs which have improved the overall efficiency of both sides of the turbocharger, resulting in engines that spool-up quicker and do not have to work as hard for the same boost levels. Plus, separate back plates allow the user to upgrade the compressor without changing the center housing.

The process of converting exhaust into turbo-driving performance tends to generate more exhaust than what is required to spin the turbine. One of the functions of the wastegate is to balance what is needed and what is excessive as the exhaust passes through.

"The wastegate allows the turbo to be sized to produce more low engine speed boost without over-boosting the engine at high speed,"_ Gibbs says. "It essentially is a means of controlling boost pressure."

To decrease the temperature of the compressed air, intercoolers are utilized.

"The intercooler drops the temperature of the boosted air and effectively increases the density of the air going into the engine, yielding more potential horsepower," Gibbs explains.

Honeywell engineers have examined every aspect of the turbocharger in their efforts to create more efficient power, including the design of the main shaft. Rather than staying with the standard "journal bearing"_ technology that relies on oil lubricating and facilitating the spinning of the shaft, Garrett has designed ball bearing cartridges in a single sleeve system that contains a set of angular contact ball bearings on either end. Not only does this improve the responsiveness of the system which converts into instant power, but it reduces the amount of oil required to provide adequate lubrication to the system, thus reducing the chance of seal leakage.

The maximum tolerances of the bearing system are tested for rotor-dynamic stability beyond the maximum turbocharger operating speeds to ensure its durability throughout the life of the turbo.

Heating Things Up

Another area where Honeywell engineers have spent a fair amount of time researching is in the negative effects of heat soak-this is, where a hot object tends to continue to emit heat after the engine (and cooling process) has been shut off.

"Years ago Honeywell engineers addressed heat soak back coking from high temperature applications by creating and applying water-cooled housings," Gibbs explains. "For example, this kind of product is typically used in passenger car and light truck applications where you can have very high under-hood temperatures. The housings allow water to flow around the bearings which keeps the operating temperature under control. Even after the engine is shut down, the water will continue to thermally siphon through the bearing housing and absorb the heat soak back from the turbine until it cools down."

Different Applications

Turbo technology has created significant advancements in the performance of combustion engines. It has actually led to a down trend in engine size while improving engine performance.

"There are some differences in requirements between these classes of engines and therefore in turbo design requirements," Gibbs explains. "One example is the flow range requirement of a high speed range light truck engine incorporating exhaust gas recirculation resulting in a significant challenge for turbocharger flow range design compared to a low speed range heavy-duty engine needing high boost for maximum power. The turbocharger solutions are distinctly different design."

According to Gibbs, Honeywell has a team of more than 500 engineers working together worldwide, in efforts to continue to develop advancements in turbo technology.

Gibbs says the United States, however, is the current leader in diesel turbocharger technology due to our strict emission standards and the technologies that must be developed in response to customer needs.

Due to the large range of emission requirements there is quite a variety in world markets. "For example, third-world countries are allowed to operate with a much higher percentage of older designs whereas U.S. requirements are among the most stringent, demanding constant changes in technology," Gibbs explains. "The next big milestone in emission regulations will occur in 2010 and Honeywell engineers have been hard at work developing new diesel turbocharger technologies in response."

Gibbs says the ever-increasing standards are continuing to force innovations in technology. "Emission standards around the world are without a doubt the number one driver in technology. And this is not solely a western phenomenon. For example, China recently announced the introduction of emission standards for heavy duty trucks."

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