TURBOCHARGER

  TECHNOLOGY 

The turbochargers manufactured and sold by Comp Turbo, embody the latest in small turbocharger design technology. The three main components that contribute to the turbocharger overall efficiency and it's performance on the engine are the compressor, bearing system and the turbine.

COMPRESSOR DESIGN

Referring now to the compressor component, a primary design  objective is to obtain the most mass flow through small diameter wheels,  thereby minimizing the inertia of the rotating assembly. The mass flow through  the compressor wheel is controlled by the net axial flow area at the inducer  inlet. Cutting back alternate inducer vanes opens up the flow area at the base  of the vanes and allows the hub diameter to be minimized. Obviously, a large  inducer vane outside diameter, along with the small hub, maximizes the net  inlet axial flow area. Usually, several inducer vane outside diameters are  employed to produce several different flow ranges from a single wheel casting.  The inducer vanes are made as sharp as possible along their entrance edges to  minimize entrance losses and this contributes to maximizing the net inlet flow  area and the flow range of the compressor.

It is usual to limit the inducer vane outside diameter to  about 75% of the wheel O.D. to limit the stress at the base of the vanes.  Exceeding the 75% limit can increase the vane base stress to values that can  exceed the material properties of cast wheels and force the wheel to be  machined from a billet. This is an unnecessary expense since 75% inducer wheels  made from economical casting material have adequate flow range for essentially  all commercial applications. There is no reason to use a full bladed inducer.  The evolution of small compressor performance took a giant step forward with  the development of wheels that employ alternately cut back inducer vanes.

It is desirable to select a relatively large number of  compressor wheel vanes to maximize the pressure ratio capability of a given  size wheel. The exit velocity of the compressed air can never reach the exit  velocity of the vanes, and this difference is termed wheel slip. To  illustrate this phenomenon, an approximation of wheel slip can be calculated by  using the Stodola equation from the literature:

Wheel slip = 1 – π/N where N is the number of vanes.
A 14-vane wheel would have a slip factor of 1 – π/14 = .776
An 11-vane wheel slip factor would be 1 – π/11 = .714

This comparison indicates that the 14-vane wheel will have a  significantly higher pressure ratio capability than an 11-vane wheel because of  its greater air exit velocity. The even vane number of the 14-vane wheel allows  the wheel to have all the advantages of alternate cut back vanes. A wheel with  11 full vanes could always have its flow range and pressure capability  increased by adding a vane and alternately cutting back the inducer vanes.

he airflow conditions at the wheel outside diameter are  very important for achieving high efficiency and broad range. A typical  velocity triangle representing wheel exit parameters is given below.

CU2 = U2 (1 – π/N)
U2 = tip velocity

Due to slip, the tangential component of the air exit  velocity is less than the wheel speed and the relative velocity, W2, dictates  the design of a backward curve in the vanes to match the relative exit velocity  so that the vane wake loss is minimized. Designing back sweep into the vanes as  they near the exit or O.D. improves both the efficiency and the flow range of  the compressor.
Consideration of all the foregoing design factors results in  the availability of broad range compressors with maximum efficiencies  approaching 80%, while still retaining relatively small size to minimize  rotational inertia.

Referring to the compressor casing, a re-circulation slot  can be designed into the casing located just inboard of the inducer inlet. This  feature can produce a lower surge line and broaden the flow range of the  compressor at high pressure ratio. The re-circulation slot is an outgrowth of  work done at NASA, where the flow range of axial flow compressors was enhanced  by several types of tip treatment. The re-circulation slot has been a useful  addition to small compressor design technology.

BEARING SYSTEM DESIGN

Referring now to the turbocharger bearing system, Comp Turbo  turbochargers utilize the latest in high-speed ball bearing technology. The  acceleration rate of a turbocharger is a function of the rotor inertia and the  friction losses in the bearing system. Conventional commercial turbochargers  use floating sleeve bearing systems that are a result of years of experimental  development. The floating sleeve bearings have an inner and outer oil film fed  by lube oil under pressure from the engine's lubricating oil system. They must  also employ a separate stationary thrust bearing that is fed lube oil under  pressure from the engine. The friction loss attributed to a stationary thrust bearing  is proportional to the fourth power of the radius and can amount to several  horsepower at the high speed at which turbochargers operate.

The oil films in conventional floating sleeve bearings have  significant viscosity that produces appreciable friction losses due to oil film  shear when the turbocharger rotor is accelerated and running at high speed. The  friction losses in the sleeve bearing systems and in the stationary thrust  bearings result in mechanical efficiencies in the middle 90% range in conventional  turbochargers.
The Comp Turbo turbochargers use a ball bearing system that  does not need a separate thrust bearing since the ball bearings carry both the  radial load and the axial thrust loads. There is little or no oil film shear in  ball bearings that operate with rolling friction only so that Comp Turbo  turbochargers accelerate much faster than conventional turbochargers that use  sleeve bearing systems. The Comp Turbo bearing system is a proprietary design  that is unique in the industry. It utilizes full compliment angular contact  ball bearings with ceramic balls. Compared with steel balls, ceramic balls in  ball bearing have a number of advantages.
According to a prominent ball bearing  manufacturer, bearing service life is two to five times longer than steel  balls, they run at lower operating temperatures and allow running speeds to be  as much as 50% higher. Also, since the surface finish of ceramic balls is  almost perfectly smooth, they have lower friction losses and lower vibration  levels. And, since there is less heat buildup during high-speed operation, they  exhibit reduced ball skidding and have a longer fatigue life.

All these  characteristics make ceramic ball bearings ideal for use in turbochargers where  they must operate at very high speeds and survive in a high temperature  environment. The full compliment bearings do not use a cage to position the  balls and this additional feature, combined with the ceramic material, provides  a combination that has minimal friction losses. The mechanical efficiency of  Comp Turbo turbochargers that use ceramic ball bearings can approach the high  90% range and this contributes to rotor acceleration rates that have been shown  to be faster than competition.
In the proprietary Comp Turbo ball bearing system, the  angular contact bearings are mounted in an elongated steel cylinder that is  free to rotate in the bearing housing. The outside diameter of the cylinder is  fed with lube oil and this outer oil film provides a cushion against shock and  vibration. Two angular contact bearings are mounted in tandem on the compressor  end of the cylinder in an arrangement that carries rotor thrust in both axial  directions. A single angular contact bearing is mounted under pre-load on the  turbine end of the cylinder and is free to move axially with shaft elongation  when heat is conducted down the shaft from the hot turbine wheel. The elongated  steel cylinder containing the angular contact bearings represents the complete  bearing system and can be inserted and/or removed as an assembly, making the  Comp Turbo turbocharger fully serviceable and rebuildable.

TURBINE DESIGN

The Comp Turbo turbine wheels are a unique design in that  they have vanes that are constant in outside diameter from inlet to exit. This  design feature maximizes the flow capacity of a given size wheel and allows the  use of reasonably small turbine wheels on large-size engines. One of the  largest losses in small turbine wheel design is the leaving gas velocity, which  is unrecoverable energy and is dissipated in the atmosphere when the exhaust  gas leaves the turbine casing. The Comp Turbo full-bladed turbine wheels have  minimal leaving velocity due to the large exit area, thus their leaving losses  are minimized, leading to higher turbine efficiency and greater flow range than  contoured turbine wheels. Conventional twin flow and undivided turbine casings  are available to match different engine exhaust manifold systems.

APPLICATION TECHNOLOGY

Racing applications require turbochargers that build boost  pressure as rapidly as possible, thus allowing the engine to develop high  torque at low engine speed and with boost capability that can produce very high  maximum power output. Comp Turbo turbochargers do exactly that. For example,  when mounted on one dragster, the Comp Turbo turbocharger produced 1.7 bar boost  in two tenths of a second and developed 650HP ready for takeoff. Now, that's  phenomenal response and very impressive.

In street applications, the acceleration rate of a vehicle  equipped with a Comp Turbo turbocharger is enhanced and moves the engine out of  inefficient operating regimes more rapidly. An improvement in number of gallons  of fuel used is the usual result when a vehicle is accelerated faster. Under  steady-state operation, the lower HP losses in the Comp Turbo turbocharger ball  bearing system means more power is available to the turbocharger compressor,  which results in higher intake manifold pressure. In most cases, higher boost  pressure can make an additional contribution to improving engine fuel  consumption.

Comp Turbo can supply turbochargers with various compressor  and turbine wheel trims to tailor their performance to exactly match specific  engine application requirements, whether they be racing, street, off-highway,  or stationary. Models available are described and listed in this catalog. Comp  Turbo also offers a rebuild service for conventional turbochargers as well as a  conversion service that can substitute a ball bearing system for conventional  sleeve bearings in many commercial turbocharger models. The sale of  turbocharger spare parts and accessories, such as waste gates, is also  available. Additional information about Comp Turbo, Inc. can be found on  website www.compturbo.com.

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