Turbochargers for gasoline vehicles
While previously mainly having been applied to diesel engines, the market for turbocharged gasoline engines started to take off around 10 years ago. As emission regulations became stricter, the application of turbochargers provided a perfect solution for downsizing by reducing the fuel consumption without penalty in engine performance and drivability.
The concept of turbocharging is fundamentally the same for gasoline and diesel engines. However there are some differences due to the different combustion processes of the fuels.
In a gasoline engine, the air to fuel ratio needed for ignition is very sensitive and needs to be controlled precisely. In a diesel engine the air to fuel ratio is always “lean” due to the excess of air. The exhaust gas temperatures are therefore generally higher from a gasoline engine than from a diesel engine (~950-1050C°). Thus, the turbocharger for a gasoline engine requires advanced materials to withstand the high thermal loading conditions. Special alloys, including expensive materials such as nickel, are commonly used for these turbochargers.
The heat transfer from the hot turbine area to the bearing housing is protected from these high temperatures by the application of a full water jacket and an optimized internal heat shield. This avoids any heat damages on the bearing housing as well as on other components.
As the European emission legislations are pushing new boundaries, many future gasoline applications will rely on variable geometry (VG) solutions.
A VG system makes it possible to adjust the turbine operating point continuously during all engine operating points, which improves the fuel efficiency significantly. Furthermore, MHI has developed technologies for gasoline vehicles such as twin scroll and dual entry turbines, which provide a significant reduction in turbo lag.