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How Turbines Are Designed

The exhaust turbine's job is to convert the energy in the moving and expanding exhaust gasses into rotating kinetic energy of the shaft and turbines. The compressor turbine's job is to convert that rotating energy into the movement of the air that enters the engine. This air is compressed and (unfortunately) heated. The turbines in a turbo charger are measured by the sizes of each stage of the turbine. A turbine has two stages: the inducer stage and the exducer stage. The size and shape of each stage determines the shape of the turbine's fins and ultimately the characteristics of that turbine.

For the compressor turbine (or "compressor wheel"), the inducer part of of turbine is at the end of the shaft and can be seen by looking into the intake of the turbo compressor housing (looks like a fan). The blades that you see there extend into a larger diameter at the other end of the turbine. This is the exducer stage. The inducer on the compressor turbine is responsible for generating the vacuum at the compressor housing inlet that pulls air into the compressor. The air then "rides" the fins towards the exducer stage, which is a larger diameter, and gets sling-shot towards the outside of the compressor housing. The housing collects this moving air an expels it through the housing's outlet. The way that the air leaves the fins causes it to swirl as it leaves the housing. Since the intake manifolds on the early Turbo I engines were so close to the turbo, an anti-swirl fin had to be installed in the turbo outlet duct to stop this air motion from effecting the flow characteristics of the intake manifold.

The exhaust turbine also has an inducer and exducer, but because the exhaust turbine has the opposite function of the compressor turbine, the two are switched. The exhaust gasses are directed towards the outside of turbine through a nozzle. This is the inducer stage because it is the part of the turbine that collects the gasses. As the energy from the gasses is transferred into the turbine, the gasses slow down and exit the turbine through the exducer stage.

There are several factors that determine the performance of a turbo charger. The three most important ones are the type of exhaust turbine, the A/R ratio of the exhaust housing, and the size of the compressor turbine. Usually, it seems that the exhaust turbine is just referred to as the "turbine" and the compressor turbine is referred to as the "compressor wheel".

The Exhaust Turbine
The exhaust turbine design is a balance between absorbing as much energy from the exhaust gasses as possible and allowing the gasses to flow as easily as possible. This is closely related to the size of the exhaust housing. A larger turbine can absorb more energy from the gasses and spin the shaft with more torque and speed, but too large a turbine will restrict the flow of exhaust such that engine performance is greatly reduced. Typically, the inducer is only slightly larger than the exducer on the exhaust turbine. Generally, you would want to stick with the stock turbine because it's size is not nearly as important as the compressor turbine's size. If you want to reduce restriction through a smaller housing, you can have the turbine "clipped", which reduces the size of the fins and allows more air to flow around the turbine.

The Turbine Housings
The exhaust and compressor housings on turbo chargers use a "scroll" design. For example, the exhaust housing's scroll is where the exhaust gasses enter the housing and are directed at the turbine. It's basically a smooth, tubular chamber that surrounds the turbine with a slot all the way around that acts as a nozzle to direct the exhaust gasses at the turbine. It's called a scroll because it slowly gets smaller in diameter as a goes around the turbine. This pressurizes the gasses, forcing them out of the slot/nozzle at a fast rate. In turbo-terms, the scroll is measured by the cross-sectional area of the scroll's "tube" "A" and the distance from the center of the "tube" to the turbine shaft "R". The values by themselves are not meaningful to the user and for the most part, R does not change much for different housings, but by dividing R into A, you get the A/R ratio. So, the A/R ratio of the exhaust housing refers to the size and shape of the scroll that is cast into the housing. It basically determines how restrictive the housing will be, versus how quickly the turbine will spin up. A lower A/R ratio (smaller scroll area, A) results in a more restrictive housing. This restriction speeds up the exhaust gasses and increases the amount that the gasses will expand. It's the speed and expansion of the gasses that causes the turbine to spin. So with a low A/R ratio, the turbine will spin up quicker, but as engine output and rpms increase, the restriction of the housing begins to build up too much back pressure on the engine, which reduces performance. A good rule of thumb for when there is too much back pressure is when the pressure in the exhaust manifold is more the half of the pressure in the cylinder. So basically, a larger A/R ratio will improve your engine's top end, while losing some mid range power and increasing turbo lag. A smaller A/R ratio will help the bottom and mid-range, but may effect the top end.

On the compressor side, the housing also features a scroll design, but it has the opposite function. The air leaving the compressor turbine has a lot of speed, but not much pressure. The scroll on the compressor housing starts small and gets larger as it approaches the compressor outlet. This collects the air and builds up air pressure. So, the compressor housing is designed to convert the speed-energy of the air coming off of the compressor turbine into pressure-energy, which is much more useful to an engine.

The Compressor Turbine
The size of the compressor turbine determines the maximum amount of boost that the turbo charger can produce. It also effects the spool-up time of the turbo. The type of compressor wheel is usually designated as its "trim", which is a value that describes the inducer and exducer sizes. Typically, the exducer is significantly larger than the inducer on a compressor turbine.

So in conclusion, a turbo charger's design becomes a balancing act between these three factors.