Читать книгу Turbo: Real World High-Performance Turbocharger Systems - Jay K Miller - Страница 19

The turbocharger can be applied in a variety of ways. The intended use of the vehicle tends to dictate what type of turbo system is best applied to the engine. The most common system is the single turbo matched to a specific RPM and horsepower point.

Gale Banks Engineering makes a marine twin-turbo kit for the 454-ci big-block Chevy that produces nearly 900 hp. Note the water-cooled exhaust manifolds, and pull-through carburetor arrangement.

The twin turbo set-up is a highly specialized system where two identically sized turbos are matched to approximately one half of an engine’s need for airflow. A typical application is the V-8 where 4-cylinders feed each turbo. This approach is typically used for racing where rapid acceleration is needed and the smaller wheels inside each turbo spool up faster than one larger unit would. It is also used in the street rod market for the design beauty that comes from the more sophisticated, symmetrical look. This is commonly found in open-air engine bays where builders are as much showing the engine as they are the rest of the car.

Manny Cruz of the Bronx, New York, uses a pair of Garrett GT42 turbos on his 3.0-liter V-6 Mercury Cougar, which races in the Pro Rear Wheel Drive Sport Compact class. The compressor outlet from each turbo is routed forward where they enter the aluminum cased water-to-air aftercooler (1), and from there, the aftercooler combines the air from both turbos into a single duct boost tube (2) that enters the engine’s intake air plenum (3).

Note: There are important considerations to the twin turbo concept given today’s technology as will be discussed more in Chapter 6, Designing a Turbo System.

Multi-stage (or compound) turbo systems are a very exotic method of turbocharging and are usually reserved for extreme applications such as tractor pulling or similar high-performance applications. However, Caterpillar manufactures a 550-hp truck engine that uses two turbos staged together to reach higher boost levels and air density than could be durably achieved using only one turbo. Using a first-stage, second-stage compressor allows for higher air density from the higher boost pressure without the overspeed condition that would be required to achieve the same total pressure ratio from a single turbo. This type of system also contains what the application engineers refer to as “all kinds of map width,” a concept that refers to a compressor’s mapped flow range potential discussed in Chapter 3. It has essentially the turbo surge margin of the small turbo and the choke flow output of the larger turbo.

The robust design of the diesel engine can withstand this level of power output and commercial diesel engines are sold and priced by horsepower. Independent truckers like higher horsepower because they never know what their next load will be, or whether or not they’ll be hauling up through the mountains. In a typical staged turbo setup, the exhaust leaves the first turbo and enters the second turbo. Similarly, the compressor outlet of the first stage is fed into the intake of the second stage turbo, much like a two-stage air compressor in a typical workshop. Tractor pullers have used these types of two, three, and even four stage turbo systems to build incredibly high pressures and achieve power levels that the engines could not withstand in a work environment. In fact, many times they don’t even last through one full-pull! The pressures seen in these extreme applications frequently reach over 100 psi, while still others have achieved boost levels as high as 200 psi!

This C15 Caterpillar engine is popular among many line-haul fleets and owner-operators requiring high horsepower. This 15-liter engine uses a compound turbo setup, which is commonly, but incorrectly, called a twin-turbo system. Twins are two identical turbos where each turbo is matched to flow one-half of the engine’s air-mass demand and each turbo feeds the engine directly. In a compound turbo system, one turbo feeds the next for a compound effect to create a second stage of compression. The engine comes in ratings ranging from 435 hp with 1,550 ft-lbs of torque to 625 hp with 2,050 ft-lbs of torque. While these ratings may not sound astonishingly grand to high-horsepower enthusiasts, the engineering feat is that these commercial engines can produce this output all day long, and not require engine rebuilding for 500,000 to 1,000,000 miles!

Another variation to the single turbo system uses a wastegate. The wastegate is located either inside the turbine housing or as an add-on device that is installed in the exhaust manifolding upstream of the turbine. The wastegate is applied to an application such as an automobile where turbo boost is desired at lower engine speeds, so a small turbine is used. Yet the engine’s natural operating RPM is over a wide range and will frequently achieve an exhaust flow volume that would over-speed the turbine and choke the engine. The wastegate allows a simple bypass whereby a valve opens at a preset point and allows the excess exhaust energy to bleed off. This allows more exhaust flow past the turbine instead of choking the engine, while also allowing boost to be controlled, as the power driving the compressor is regulated.

The Garrett Model GTP38 as shown was used on the 7.3-liter Ford Powerstroke diesel engine in heavy-duty pickups. This turbocharger used a pedestal mount and employed a wastegate assembly that allowed improved low-end response. (Courtesy Diesel Injection Service Company, Inc.)

Wastegates are a vital component in any gasoline engine due to the wide RPM range of operation. Recent history has seen wastegates also used on several commercial and consumer diesels. The wastegate is the simplest form of variable geometry. They are widely used and have good acceptability and durability in most applications, but there is a superior concept.