Читать книгу Turbo: Real World High-Performance Turbocharger Systems - Jay K Miller - Страница 20

Variable geometry is the design of a turbine to vary its geometric size and therefore its capacity for swallowing exhaust gas flow. Variable geometry, or VG, turbos are a much more sophisticated form of turbocharging. Today’s high-speed diesels and gasoline automobiles demand higher efficiency. A wastegate does just what its name implies—it wastes energy that could otherwise be put to good use as energy to drive the compressor. There were working VG turbochargers as far back as the mid ’80s, but manufacturing costs and fuel mapping technology meant VG wasn’t quite ready for primetime.

In a VG turbine, a series of vanes is positioned around the turbine housing in such a way as to throttle the exhaust gas volume and pressure as it enters the turbine wheel. It works much like the garden hose spray nozzle end where you can vary the opening nozzle anywhere from nearly closed to wide open. This allows the same turbine to act as a small turbine at low engine speeds and as a larger turbine at higher engine speeds. Unlike a wastegate, with VG all of the energy is transmitted to the turbine wheel to build power.

Check out the inner workings of a variable geometry turbine with the turbine housing striped away. The series of vanes are positioned on a unison ring that rotates to open and close the vanes, thereby adjusting the amount of exhaust energy allowed to reach the turbine wheel. (Courtesy Honeywell Turbo Technologies)

Variable geometry turbines offer a slight efficiency gain over the same wheel and turbine housing design without VG. This is the result of the vanes being positioned at the most optimum angle of incidence relative to the turbine wheel. Tests have shown about a two-percent turbine efficiency gain with VG.

Commercial applications of VG turbos are now in the market. Detroit Diesel Corporation uses VG in their current Series-60 commercial diesel model engines. The latest GM Duramax, Dodge/cummins, and Ford Power Stroke diesels in pickups and midrange diesel trucks are also using VG turbines. While these applications are using VG for efficiency and emissions purposes, the fact that VG is now into the mainstream means it’s only a matter of time before its use changes the face of performance turbocharging in the automotive aftermarket.

To date there have been very few gasoline engine applications. The VG used on most production engines is presently applied to diesel. Diesel runs significantly lower exhaust temperatures than gasoline. The current VG designs are not very compatible with gasoline. But applying VG to gasoline isn’t as much about feasibility as it is affordability. Current VG components use parts with high nickel content. The use of cobalt would likely be required to be successful and durable for gasoline but would significantly drive up the manufacturing cost. Alternative designs for gasoline engines are being evaluated to find lower cost methods of durably applying VG. One such thought is a design where instead of a series of vanes, an axially sliding orifice progressively increases and decreases the port entry from the turbine housing to the turbine wheel blade tips. Holset brand turbochargers use this type of VG that’s now in production on the 2007 Cummins diesel in Dodge trucks.

This is the Cummins ISX engine in a Kenworth truck application. This late-model, emissions-certified commercial diesel engine uses a Holset variable geometry turbocharger to help create outputs from 385 hp and 1,550 ft-lbs peak torque to 600 hp with 1,850 ft-lbs, both at 1,200 rpm.