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The compressor cover, like the compressor wheel, is typically made from an aluminum alloy. The compressor cover has several design areas to be noted. The nomenclature of these areas is important to proper communication as you seek help in choosing the correct turbo or while bench racing. If you intend to turbocharge your own vehicle, then you definitely want to know how to “speak turbo.”

The compressor cover has four areas that can be easily seen from the outside: (1) the volute, (2) the discharge, (3) the eye or inducer, and (4) the inlet connection diameter.

The diffuser portion of the compressor is a critical portion of the overall compressor design. The diffuser is not an actual component, but rather the optimized path for the air as it leaves the compressor wheel on its way to the compressor cover volute. The function of the diffuser is to turn the rapidly compressed and high-speed air leaving the compressor wheel into static pressure as it fills the volute. Most diffusers used in automotive applications are the vaneless type formed by parallel walls between the compressor cover and the bearing housing face.

The diffuser has to have a minimum diameter in order to have an effect upon the highly turbulent air exiting the compressor wheel. This is the reason the compressor cover outside dimension is typically so much larger than the outside diameter of the compressor wheel—there has to be sufficient diffuser area. Most diffusers used in automotive applications are the vaneless type formed by the parallel walls between the compressor cover and the bearing housing face, or seal plate, depending upon the design of the turbo.

From an inside view the following areas can be seen: (1) the compressor cover contour, (2) the cover inducer diameter, (3) the bearing housing connection, (4) the bearing housing pilot diameter, and (5) the parallel wall diffuser face.

A compressor cover cut in half reveals a better view of its design features machined into the casting. Air flows into the compressor inlet (1) where a bell-mouth flow nozzle is frequently formed to aid in the smooth air transition into the wheel. The inducer diameter (2) forms the limiting flow of the compressor and it is this diameter that is frequently regulated by certain racing bodies to limit turbocharger size and hence the engine’s power output. The compressor contour (3) cut into the cover matches the contour cut in the compressor wheel, but provides a typical running clearance of anywhere between 0.009–0.012 inch, depending upon model and use, but in some models the contour can be as much as 0.020 inch. The diffuser face (4) forms one side of the parallel wall diffuser. The bearing housing flange, or seal plate, forms the other wall once the turbo is assembled. The volute section (5) gathers the air as it exits the compressor wheel. Note how the cross-sectional area becomes larger as the volute approaches the compressor discharge because it gathers more air and the increased size slows the air helping to convert it from high-speed flow to static pressure.

Vane-type diffusers are extremely efficient but have flow range restrictions and are therefore almost never seen on automotive applications. This goes back to optimizing efficiency within a specific flow range. While the vane diffuser does the better job in air diffusion at a specific match point, it becomes an impedance to flow rather quickly once the flow demand is greater than that compressor is optimized for. At higher rotational speeds, the slip angle of the air leaving the wheel tends to shift and can lose its optimum angle of incidence relative to each vane. The presence of the vanes will ultimately create a blockage of total mass-flow and therefore cause earlier compressor choke.

A close-up of an assembled turbocharger cutaway shows the parallel-wall vaneless diffuser that converts the highly turbulent air exiting the compressor wheel into static pressure. The diffuser face on the compressor cover forms one side of the diffuser while the other wall is formed by the seal plate or bearing housing flange. (Courtesy Diesel Injection Service Company, Inc.)

The vane type diffuser is superior in raising overall compressor efficiency, but it is restrictive and tends to inhibit compressor flow range. For this reason it is normally seen only on steady state or engines with a very narrow RPM operating range. (Courtesy Diesel Injection Service Company, Inc.)

Compressor covers have an A/R rating, or area over radius relationship. However, compressors are not very sensitive to A/R ratios and therefore manufacturers seldom allow you to choose. The compressor cover A/R used to be adjusted more frequently when straight radial wheels were commonly used to shift the narrower compressor flow range to better match up with different engine ratings. The BCI-type wheel design basically does away with that tuning variation, leaving the compressor cover’s A/R rating as a relatively unimportant variable.

The inducer bleed is a ring cut around the inlet of the compressor eye or inducer diameter that allows an inlet bleed of air from the compressor wheel. This inducer inlet bleed helps to stabilize the airflow when the compressor is operating close to the surge point. In many cases a compressor can become unstable when it is operated close to its minimum flow at a given pressure ratio. The airflow can be rather choppy and become superheated causing the engine to run poorly. The inducer bleed helps to broaden the flow near surge and stabilize the airflow for smoother engine operation.

Inducer bleed is a generic term that is also identified by the terms “map-width enhancement” and “ported shroud” compressor covers. Do not mistake this feature as one that will usually make a given compressor a good match if it wasn’t before. Inducer bleed is a development that mainly provides a slight amount of surge line movement, if any. Its real value is in stabilizing the airflow being supplied as you approach the surge point. Compressors all have slightly different characteristics. For example, a surge line for a compressor that was at 25 lbs mass at a 2:1 pressure ratio may have unstable airflow from 30 lbs down to the 25-lb surge point on the map. Engineering compressor maps, which are never shown to the public, will have this data on them (labeled “choppy air noise”). This assists the application engineer in avoiding an application where a given engine could enter this choppy air condition. Inducer bleed basically smoothes out this choppy area, and therefore in some cases does provide a bit more map width.

Inducer bleed shows how a small amount of the airflow is recirculated from the inducer back to the main inlet, thereby stabilizing the entire compressor stage when operated near surge. This design can also extend the max-flow range near choke-flow.

Inducer bleed is a loss mechanism however. The air that is bled out through the ring cut into the compressor cover from positive pressure is routed back into the compressor inlet and recirculates back to the wheel. This process induces additional heat because the air is slightly compressed more than once. This heat added lowers the compressor’s adiabatic efficiency. The volume of air flowing out of the inducer bleed ports is variable and is at its maximum as the compressor approaches surge. As the compressor extends further out onto its flow range the amount of air flowing out of the bleed port becomes less and will reach a static flow near the maximum efficiency area of operation for that compressor. As the compressor flow range extends further to the right from maximum efficiency, the bleed port can go from static to negative where there is actually additional air intake that will provide some extended flow.

In theory, if the compressor is well matched through dynamometer testing and flow measurement verification, and if that compressor was very stable near surge, inducer bleed would not be a recommended design feature due to the unnecessary reduction of efficiency and the added cost. But in reality there are several OEM cases when it does make sense when an engine’s air demand is just beyond where a given compressor will flow stable air. Additionally, aftermarket turbos applied without the aid of dynamometer testing and verification can be slightly mismatched. The double benefit of more surge margin and stability coupled with a slightly broader choke flow range makes sense.

Inducer bleed can cause a high-frequency noise as the blades of the wheel pass the interrupted openings that form the bleed ports. In some vehicles, where drivers spend their day listening to this high-frequency whine, it can become very annoying. Using Computational Fluid Dynamics, aerodynamists can go back and quickly design a full-bladed wheel that has the proper flow characteristics for that production engine, and the full blade wheel will then virtually double the number of blades passing the bleed ports, thereby doubling the frequency of the whine above an audible level. Problem solved!

So if you’re wondering whether a current production full blade wheel design you happen to see is something trick for your new gasoline project, think again. You’re still better off with a splitter blade wheel on a high-speed gasoline application. Besides, what high-performance enthusiast ever worried about turbo noise?

The contour cut into the compressor housing is matched to the compressor wheel. There is approximately a 0.009–0.012 inch running clearance between the compressor wheel and the compressor cover. Excess clearance causes a loss in efficiency while insufficient clearance can cause compressor wheel to compressor cover contact during operation, which will result in catastrophic failure. It should be noted however that a commonly misunderstood fact about today’s high efficiency compressors is that they can be rotated by hand and a slight contact between the inducer of the wheel and the eye of the compressor cover can be felt. This should not be call for concern. The oil pressure inside the bearing system will center the rotating turbine shaft and contact will not occur as long as the dimensions are correct.

The benefit of a ported shroud compressor cover allowing more surge protection can be seen in this compressor map where two maps are superimposed showing the same compressor with and without inducer bleed. (Courtesy Honeywell Turbo Technologies)

New turbos are often built “dry,” without the benefit of engine oil. The purpose is to accurately measure end thrust and rotor tilt. The rotor is the term used for the assembled turbine wheel and shaft assembly with the compressor wheel installed. Placing a few drops of oil into the oil inlet of the bearing housing and rotating the rotor by hand will quickly allow the tolerances to reveal themselves as correct and wheel-to-housing contact will disappear.

Close-up of compressor wheel in its operating position relative to the mated contour cut into the compressor cover. Clearances allow for slight gyrations in shaft motion during engine operation without the wheel coming into contact with the cover at high speeds, which would quickly fail the turbocharger. (Courtesy Diesel Injection Service Company, Inc.)