Advanced turbocharges key to emissions-era diesels: managing airflow critical; Honeywell chooses VNT and AVNT as technology path for commercial truck diesels.
Obviously, turbocharged diesel engines are not a new concept, but as the emissions box continues to shrink globally, new and more precise engine-boosting systems, in a variety of designs and concepts, are coming into the marketplace. And VGT--variable geometry turbocharging of all different types--are right at the heart of those developments. There are certainly more turbochargers in the future for all sizes of diesel engines.
Honeywell's turbocharging unit, Torrance, Calif, one of the diesel industry's major suppliers of turbocharging systems, has selected VNT--variable nozzle turbine and AVNT--advanced variable nozzle turbine for its GT35-40 turbochargers and the double axle GT42 and GT45 models, as its primary technology paths for commercial truck sized diesel engines, in the 200 to 600 hp range.
"For engine manufacturers using EGR for N[O.sub.x] reduction and particulate traps for PM reduction, VNT has an important role in driving EGR, in accommodating the airflow, managing the airflow, while driving the EGR," said Steve Arnold, Garrett's director, innovation and new concepts at Honeywell's turbocharging unit.
"When you introduce EGR to the engine, the engine still needs the same amount of fresh airflow, but you are now moving a lot more gas flow through the engine and you have to find a way to do that. With VNT technology you can maintain the same amount of airflow and also drive the EGR."
For larger engines in the commercial truck range, Honeywell has continued to develop its existing double axle variable nozzle turbine turbos that range from the GT42 through the GT45 models.
The non-VNT GT47 to GT55 product family, now incorporates an upgraded bearing system, as well as aerodynamic improvements and are custom matched for commercial applications to 700 hp. Recent developments in aerodynamic design allow for an inertia reduction of up to 25 percent, according to Michael Cirone, director, commercial diesel product line at Honeywell.
The top end of the range, the Garrett GT60-70 turbo family, now features two compressor designs and three wheel sizes to cover a wide range of pressure ratio and flow requirements.
Smaller output engines are being served by VNT turbos with wastegate options. The Garrett GT15 compact turbocharger is specifically designed for small displacement diesel and gasoline engines in passenger cars, light-duty trucks and marine applications. The GT15's application range is for 1.3 to 1.6 L (60 to 80 bhp) diesel engines and 1.0 to 1.4 L (85 to 120 bhp) gasoline engines.
But it is in the middle, Garrett's GT30 to GT45 range, especially in commercial vehicles, tracks and buses, where much of the excitement is today. Honeywell is currently in production with AVNT versions of ira Garrett GT35, 37 and 40 turbochargers on International's 6 L V8 and 8.7 L, in-line six-cylinder medium-duty diesel engines with a fourth installation due to be announced shortly. The turbochargers are manufactured at the Honeywell turbocharging unit's new facility in Mexicali, Mexico.
Arnold said the design of the three AVNT turbos is very similar, using many of the same components sized to the specific requirements of the turbocharger model.
Honeywell started down the AVNT path in the mid 1990s as the next step in providing optimum airflow for diesel engines. "The idea behind the development program was to create a new value proposition for the customer with variable geometry turbocharging," Arnold said. "For commercial diesels, we had a very good VNT, however the value proposition wasn't sufficient to drive it forward based on fuel economy, drivability and performance."
Arnold said Honeywell approached the AVNT project as a complete system design with an eye on developing a simple, high performance variable nozzle turbine with integrated electronically controlled actuation and position feedback.
The result was an AVNT design that includes a compact turbine housing, integrated actuation, no external actuators, no brackets and 59 percent fewer parts in the turbine housing assembly compared to earlier designs. The turbochargers utilize molded-in-metalmetal injection molding and powdered metal technology to eliminate machining on smaller parts.
One of the keys in AVNT design is the variable geometry mechanism itself, a hotly debated technical subject. For its AVNT, engineers at Honeywell decided on a pivoting synchronized cascade of vanes. The vanes rotate by way on a shaft that is pressed into the turbine housing.
A ring, which moves the vanes in unison, was designed to interface directly with the vanes through a tab-and-slot arrangement. This, Arnold said, eliminates the need for a shaft to penetrate through a cavity and for an arm or gear to be welded or attached to the vane assembly.
The vane and tab were designed with geometry that enables area contact between the tab and unison ring over much off the travel. The ring and wines rotate at the same rate with 20[degrees] of rotation in each part.
The axial floating of the unison ring provides the vane side clearance control of the AVNT. A pressure differential is created across the two faces of the unison ring by the acceleration of the flow through the vanes. The pressure differential moves the unison ring into contact with the vanes and eliminates vane side clearance, Arnold said. Further, the vane movement has been moved into the flow stream, eliminating the need for a separate cavity and welded arm.
A key design decision in the Garrett AVNT project was actuation. Typically, Arnold said, actuation had been an afterthought in many turbo designs. But it was here the system design concept also came into play with the actuation, variable geometry mechanism and center housing designed as a unit.
Arnold said the control and actuation of a variable geometry turbocharger are critical and must also be robust in controllability and durability. "The actuation system has to enable the engine manufacturer to control the turbocharger precisely to obtain the maximum benefits in meeting emissions, as well as performance and transient response targets."
In looking at actuation options, Arnold said the designers realized that the turbocharger already had a hydraulic source connected to it in the bearing system oil supply. Further, the pressure was low and matched the actuation force requirements off being proportional to the mass flow through the turbine.
"We thought, why not use the oil that's coming in for lubrication as the power source for the actuation?" Arnold said. "That's what we did and we didn't have to bring in an additional power source. From there we used a four-way hydraulic spool valve that is proportionally controlled by the engine ECU."
The electrohydraulic actuation system achieves true vane position through the basic architecture of the hydraulic circuit eliminating the need for a microprocessor and position feedback sensor. The four-way electrohydraulic spool valve is utilized with force feedback. The proportional solenoid force is balanced against the force generated by a spring compressed by a cam follower. The cam follower tracks the vane position riding a cam mounted on the vane control shaft.
The four-way valve is connected to a double-acting piston, the movement of which is transmitted through a rack and pinion gear into rotation and control of the vane control shaft.
A pulse width modulated proportional solenoid provides infinitely variable control. Further, the design is completely self-contained with only a single electrical plug-in to the engine's ECU. As a result, all variable forces are removed from the control loop; aerodynamic loading, friction and oil pressure.
The politics of emissions also came into play during the development process. "Part way through the development process, we wound up with the consent decree, so there is now additional motivation to put in a variable nozzle turbine because of the synergy with driving EGR and making accommodations in airflow for EGR," Arnold said. "Also, you can put more air into the engine at low operating speeds and increase the torque. So a lot of manufacturers are looking to increase the peak torque value when they apply VNT."
Arnold also said engine braking is another performance benefit. "You can enhance engine braking by closing the vanes and if you use a decompression brake you can supercharge the braking cycle and enhance the braking power."
"The benefit of VNT is that it gives you a controllable air source and that provides good transient response and drivability," Arnold added. "You can spool the turbo up much more quickly by closing the vanes down and the driver perceives a significant improve ment in drivability." *
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|Title Annotation:||engine technology|
|Publication:||Diesel Progress North American Edition|
|Date:||Oct 1, 2003|
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