Wide World of Turbo
In 1905, Switzerland’s Dr. Alfred Buchi patented an exhaust-driven, intercooled supercharger. At the time, Buchi was the head engineer of the Sulzer Brothers Research and Development firm (and maybe as a reward, they gave him a gold pocket watch and some Cognac).
In 1915, Buchi developed a prototype turbocharger designed for diesel engines. The idea didn’t set the world on fire back in the day, but it did years later. If Buchi’s life would have been a Hollywood movie, he would have obsessed over the “failure” of his invention, drank gut-rotting whiskey during his waking hours, sold bad pencil sketches to tourists, and concluded his descent into madness by mailing a piece of his ear to a sweetheart.
But the industrial world eventually woke up and learned that turbos go with diesels like steak and eggs. Chances are that the diesel (or special ed.) bus you rode to school was turbocharged. Earlier applications of turbos found their way onto aircraft seeking higher altitude performance. The commercial and industrial diesel applications are where turbochargers gained most of their acceptance before and after World War II.
The compressor side of the turbocharger is the champagne-cork-flying, cocktail-dress-wearing, triple-bean-dip-parting side of the turbocharger. The compressor’s impeller sucks cool ambient air through its center inlet, pressurizes it, compresses it (heating the cooled air) and then send it towards the intake tract for more glorious power.
Compressor Housing- The compressor housing is cast in aluminum for superior heat dissipation (and trick racing turbos are sometimes cast in magnesium.) Cooler intake charges yield greater power. Compressing the ambient air invariably heats it, and also gives us the excuse to mount large air-to-air intercoolers aft of the compressor outlet. The size of the interior cavern of the housing is tuned to the level of pressure generated by the compressor wheel. The larger the compressor housing, the more volume of air is allowed to flow.
Compressor Inlet Wheel- The ambient air inlet (located in the center of the housing) is where the compressor wheel captures air in the curvatures of its fins and pressurizes the housing and intake tract. The impeller turns much faster than the engine’s rpm (higher than 130,000rpm) in many cases. Usually the compressor wheel is bolted to the opposite end of the shaft that the turbine wheel is friction-welded to. Compressor wheels are normally manufactured of very light materials, such as magnesium or aluminum.
Wastegate- Wastegates are valves that bleed exhaust energy from the turbo housing when pressure reaches a preset point. The diaphragm, usually referenced from an intake manifold pressure signal, compresses the spring tension that hold the valve shut then dumps exhaust energy when the preset pressure is reached. When the diaphragm or oil-filled cylinder opens the valve, they direct exhaust energy away from the turbine housing, reducing boost pressure once the desired boost level has been achieved.
The
rear end (or boiler room) of the turbo assembly is the turbine. The turbine
wheel is driven by exhaust gas energy, which spins the compressor wheel that’s
attached by a common shaft. Exhaust gases exit through the center outlet and
out the back of the car. The design characteristics of the turbine and
compressor are what affect turbo performance. Each engine and desired power
output must be matched to the appropriate turbocharger. This entails selecting
a compressor with the correct wheel size, housing size, and wheel speed for the
desired performance envelope.
Turbine Housing- The turbine housing is manufactured of ductile iron or, in some cases, a high nickel iron that can tolerate tremendous heat. Race turbo housings are often die-cast stainless steel. The turbine housing acts as a tunnel, directing flow of exhaust gas over and through the fins of the turbine wheel. Those blades drive the shaft that connects to the compressor wheel. Slowing down the exhaust gas flow results in backpressure that increases the turbine’s internal pressure and temperature. Turbine housing size can be designed and sized to reduce backpressure and alter the turbine wheel’s performance and response. Small turbines will spin faster than a larger turbine with the same exhaust energy, and will improve the compressor response at the expense of restricting exhaust flow at higher rpm. Larger turbos will continue to produce power in these high rpm.
Turbine Wheel- The speed of this rotating assembly is influenced by the design of the turbine wheel and the size and internal porting of the turbine housing. Materials popular for turbine wheel design typically reduce inertia of the assembly while improving durability under extreme heat and pressure. Mar-M-235, ceramic, and titanium-aluminide wheels have all been produced in the quest to improve turbine response to exhaust pulses, the compressor’s spool up time, and response. Shape, angle, and arrangement of the fins on the turbine and compressor wheels affect boost characteristics, and are subject to timeless research in turbo fabrication and timing.
Turbine Exhaust Inlet and Outlet- Superheated exhaust gas flows from the exhaust manifold (not the housing) to the edges of the turbine wheel causing it to spin. Exhaust gases exit from the center of the turbine impeller. The turbine is a radial flow device because exhaust gas flows through the housing, over the outside of the turbine wheels fins, and through the center exit.
Turbine A/R Ratio
Jack K. Yamaguchi has this to say about the series 4 turbocharger: “The twin scroll turbocharger concept was developed jointly by Mazda’s Department Five and by Hitachi, and is manufactured by Hitachi having been designated HT18S-2S. It’s turbine-side bearing area is now water-cooled, and the main body is further protected by high temperatures by a dual-skinheat shield. The 1 blade turbine, of impact design, is 64 mm (2.52 inch) in diameter. It’s scroll area, the nry path for exhaust gas, is divided into two passages separated by an integrally cast wall; one, called the secondary scroll, has a trap-door-like gate operated by intake vacuum. The turbine thus has two cross sectional area (a) factors, A being the scroll’s smallest area as used in calculating turbocharger performance characteristics: 6.00 sq. cm (.93 sq. in) for the primary path and 9,48 sq. cm (1.47 sq. in) for the secondary. The other factor is R, the distance between the turbine-shaft center and the center of area A: 59.4 mm (2.34in) for the primary path, 60.4 mm (2.38in) for the secondary. Thus the twin-scroll turbocharger has two A/R ratios: 0.4 for the primary and 1.0 for the primary and secondary combined.”
As for the compressor side, “its 12-blade compressor is 63 mm (2.48 in) in diameter.
So there you have it…the turbine for all purposes has a 1.0 A/R…Note, just because a turbine housing is larger doesn’t mean it’s going to have a higher A/R (ie: T04)