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.
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.
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.
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.