VGT Turbo for Gasoline Engines
Despite additional costs 💰, VGT (Variable Geometry Turbine) TurboCharger offers lot of advantages including a significant increase of boost pressure ↗️ and a broader torque curve
There are several options💡in the technology design of VGT to make the flow to follow a certain path into the turbine ↩️
This video shows the most common design for pass car 🚘 in which a series of pivoting vanes surround the outer diameter of the turbine impeller.
Fig. 1- Cartridges’ movement to direct the flow in a VGT Turbo
An electronically controlled actuator changes the position of the vanes that guide the exhaust gas 💨 towards the turbine blades. For a given flow, the turbine speed increases as the vanes are turned towards their closed position ↩️ As turbine speed increases so does the resultant boost pressure
VGT (Variable Geometry Turbine), even if widely used in Diesel, is still not mass market in gasoline but is clearly a current major trend both for high performance 🏎 and everyday cars 🚙🚕🚗
VGT offers a lot of advantages for gasoline Engines:
For High Performance
Higher specific output + broader Engine torque plateau while keeping excellent drivability (improved transient at low speeds) + better combustion 🔥 (less back pressure = lower pumping losses, less knocking 💥)
Mass market CO2 reduction:
Enabler for Miller cycle because high boost compensates for low volume efficient while high turbine efficiency balances lower exhaust gas enthalpy
The main challenge for VGT in SI Engines is to cope with higher exhaust temperatures 🌡 (compared to Diesel). This means not only the usage of specific materials (strength on the contact surface + scaling resistance) but also complex Engineering for clearance to cope with thermal expansion 🔛 vs sealing issues
Another difficulty is the coupling with twin scroll manifold that ensures exhaust pressure pulses from adjacent cylinders do not adversely affect scavenging when the exhaust valve is open
Additionally, with direct injection, VGT needs on deal with soot accumulation that can block 🚫 the movement of the vanes and affect the Turbo durability
Some example of gasoline Engines using VGT :
Dodge Shelby CSX-VNT had a Garrett VNT (Variable Nozzle Turbine - another sub-option) turbocharger
Honda tried VAT (Variable Area Turbine) in Japan 🇯🇵 in the ACURA
Porsche in 2006 used 2 BorgWarner BV50 pivoting vane VTGs
The last VW EA211 TSI evo couples a Honeywell Turbocharger with a VGN (Variable Geometry Nozzle) turbine with a Miller cycle
The new Ford EcoBoost 500
