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Compressor Wheel Design

14 April 2016

 

 

 

 

 

 

 

Some compressor wheels, like this example from an RB25, are made of plastic. Though they are light and aid spool they are extremely fragile and can't run high boost

 

 

 

 

TR30R Compressor Wheel (wearing a witches' hat)

This TR30R rally compressor wheel has a relatively small inducer and trim to best suit a restricted engine

 

 

 

 

Figure 1 - Comparison of an 82mm cast and an 82mm billet HTA compressor wheel

 

 

 

 

 

Bigger isn't always Better...

Compressor wheels design is one of the greatest factors determining a turbocharger’s performance. What follows is a brief guide of the factors which influence compressor wheel design, armed with this information it will be easier to make an informed decision about your next turbocharger’s specification. If you have any questions don’t hesitate to contact Owen Developments.

Aluminium is the most common material for compressor wheels to be made from, though magnesium, titanium and plastic wheels have been designed in the past. In large volume production cast aluminium compressor wheels are popular for their relatively low material production cost while being strong enough to withstand the loads encountered at high RPM in operation (over 150,000 RPM in some cases). However cast wheels have some flaws inherent to their manufacture, such as small voids and internal defects, which can be mitigated but not eliminated. In contrast, rolled or formed aluminium billet has a more consistent internal structure. Therefore compressor wheels made from billet are considerably stronger than their cast competitors thanks to the resulting improved mechanical properties.

The main effect of the disparity in strength is that cast wheels have to have thicker blades and a thicker hub than billet wheels to safely bear comparable stresses. This increases the weight of a cast wheel relative to a billet wheel; an 82mm cast wheel weighs 121g, the same diameter billet wheel weighs 93g, which compromises the transient response of wheel. The thicker hub and blades also hinder the aerodynamics of the wheel, as greater restriction exists. This limits maximum airflow through the wheel.

Though billet wheels can avoid the oversized hub problem to a large degree they have a different issue; the geometry of the wheel with a skinny hub can look perfect on paper but still be impossible to manufacture on a 5-axis mill (though billet wheels have much greater flexibility of design than cast wheels). Cast wheels have an analogous problem in that their design must allow them to be released from their mould, accommodating for these problems means that some potentially beneficial designs are unworkable.

Compressor wheel aerodynamics have changed dramatically over the last 20 years to accommodate changes in the market. Performance engines from the mid-1990s would have approximately 0.5 bar of boost pressure, whereas now some cars are running considerably over 1 bar from the factory.  Aftermarket turbos are expected to push close to 2 bar. This move to higher flow, higher boost and therefore higher power has catalysed the development of billet wheels from several companies, especially in the aftermarket and motorsport where the higher cost of manufacturing a billet wheel over a cast wheel can be easily justified by performance gains.

There are several factors which influence compressor aerodynamics design; maximum flow rate, optimum RPM, the operating pressure ratios it produces and the efficiency they cumulatively generate. Depending on the turbocharger’s intended application one of more of these attributes may be considered more desirable and given priority over the others, for example a compressor wheel for a static engine power generator would prioritise efficiency in a narrow band, instead of having a wide operating range like an automotive turbocharger. Almost all compressor wheel designs are compromises and are more suited to a particular application than others.

Comparing the individual aerodynamics of one compressor wheel against another is not as simple as ‘this one had 14 blades, therefore it’s better than one with 11’, or even ‘this one is larger so it must make more power’. Every configuration has benefits and disadvantages in terms of pressure ratio or flow rate, some configurations are favoured by OEMs simply because they’re quiet, which is above all other considerations for passenger road cars.

An interesting comparison is between an 82mm cast compressor wheel and an 82mm HTA billet compressor wheel. Even though they are ostensibly the same size the cast wheel is rated to 650hp, the billet to 710hp. Part of the reason for this can be seen in Figure 1; the hub of the cast wheel (highlighted blue, 20mm diameter) is considerably larger than the hub of the HTA wheel (highlighted red, 14.5mm diameter). This means even though the HTA has a smaller inducer diameter it has has gained a few back millimetres compared to the cast wheel, increasing its flow and therefore potential power while also losing weight which helps spool and transient response.

Also clearly visible is that the blades on the cast wheel are perpendicular to the shaft, whereas the blades on the HTA are angled. This geometry gives the HTAs an advantage as the effective length of the blade has been increased giving the wheel a larger swept surface area, without increasing the inducer size.

There are infinite comparisons to be drawn between compressor wheels in the same way the aerodynamics of race cars can be compared, what matters more than anything is that the compressor will suit your application. Contact Owen Developments to discuss your turbocharger needs with the technical sales team.

Owen Developments is one of Europe’s premier high performance forced induction specialist, with a proven track record of supplying turbochargers to a global customer base covering the motorsport, performance aftermarket and OEM sectors. Owen Developments is the sole turbo supplier to the British Touring Car Championship, the highly competitive one-make MINI Challenge and Indy Lights (Indycar’s feeder series).

 

 

 
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