Читать книгу Turbo: Real World High-Performance Turbocharger Systems - Jay K Miller - Страница 21

The future for turbochargers is very dynamic. Today, carburetors are a distant memory on the street and fuel injection reigns supreme. Emissions, engine durability, and turbo compatibility are significantly enhanced through the use of electronic fuel injection. Gone are the large-bore torque monsters of the ’60s and ’70s. Today’s highly refined engines are smaller and making more horsepower per cubic inch than ever before.

Turbochargers are also so much more refined. A wide selection of turbo models and flow ranges are available. Fixed geometry, wastegated turbos, and even full variable geometry is now in production on many applications. Advanced materials and designs solve several problems of the past, and getting an engine to work with its turbo system is now so possible for virtually anyone because of the available system components and integrated electronic engine management and support systems.

It used to be that large turbos required for small engines to build high horsepower suffered from excessive system lag at vehicle launch. Today, with higher aerodynamic efficiencies, ball bearings, sophisticated fuel mapping, and two-step/anti-lag strategies, lag is simply another resolvable tuning issue.

Holset Brand turbos use a slightly different design in their Variable Geometry. Instead of the vanes themselves actuated by a unison ring, the vanes remain in a fixed and optimum aerodynamic position. A shroud plate moves over the fixed position vanes to progressively open and close the orifice leading from the turbine housing volute through the vanes to choke the turbine for low-end performance or open up the swallowing capacity for high RPM operation. (Courtesy Cummins Turbo Technologies)

Over the next few years the automotive manufacturers will use turbochargers more and more. The cost of a turbocharged V-6 is less than the cost to produce a naturally aspirated V-8 with the same horsepower, and the cost of a turbocharged 4-cylinder is less than that of a V-6. Diesels will continue to command more market share and every diesel will have a turbocharger because they have to in order to meet emissions standards.

While today’s high-performance aftermarket products are very advanced and allow for superior turbo tuning, wastegate control is still the dominate means of tuning a turbocharged engine to provide high amounts of low-end torque, while still allowing the engines to rev to their potential. I remember Steve Arnold, Honeywell Turbo Technologies’ director of innovation & new concepts, referring to the wastegate over 25 years ago as an “aptly named device.” Full variable geometry is now making its way into production vehicles. Most of the new commercial and consumer diesels coming out are equipped with full variable geometry turbochargers. While the present use of variable geometry is mostly applied to diesel applications, it’s only a matter of time before continued refinements in materials and design become mainstream on gasoline automobiles.

By the year 2010 new design concepts will begin production such as Garrett’s new Low Speed Turbo (LST). The LST is a creative concept that uses two compressors staged in series, or compound compressors, designed integral into one compressor cover driven by a common turbine. The LST is capable of comfortably achieving higher pressures without over-speed to the turbo and limiting its durability. In fact pressure ratios of 6:1 are well within this design concept, and the flow range of the compressor, surge to choke is nearly double that of its similarly sized single-stage wheel. The LST applied with a variable geometry turbine will make for some interesting aftermarket applications.

The Garrett LST (Low-Speed Turbo) uses two separate compressor wheels, driven by a single turbine. They are positioned in a first-stage, second-stage arrangement to achieve higher boost pressures from lower shaft speeds. This reduces compressor wheel stress for longer in-service life. The first-stage compressor wheel is aluminum while the second stage is made from titanium that will better withstand the high heat from the first-stage during second-stage compression. (Courtesy Honeywell Turbo Technologies)

Another new design that will likely see production in 2010 is the Garrett Single Sequential Turbo (SST). This design uses essentially two compressor wheels in one casting that share a common backwall. The two compressors flow in parallel. Their application is intended for high-BMEP (Brake Mean Effective Pressure) engines that have a wide speed range such as turbo diesels used in light-duty pickups, SUVs, and passenger cars. Due to the twin compressor wheels flowing in parallel, the overall rotor mass and resultant moment of inertia is reduced by around 40 percent. This directly effects turbo acceleration. The choke to surge is about a 3:1 ratio and the turbine efficiency gains from this compressor design are in the range of 10 percentage points, which is a huge gain.

The automotive aftermarket will always want its horsepower. The turbocharger of today is a high-tech engine component that possesses a great amount of sex appeal and gear-head attraction. The turbochargers of tomorrow will continue the trend of even higher sophistication and increased efficiency.

The new Garrett SST (Single-Sequential Turbo) uses two Siamesed compressor wheels formed into one wheel casting, driven by a common turbine shaft. Note how each compressor wheel discharges into a common volute in the compressor cover. (Courtesy Honeywell Turbo Technologies)