Engine manufacturers have been increasingly pressured by legislation and economics to reduce emissions and deliver improved fuel economy. A common strategy is to downsize and downspeed engines, and then compensate for the affected transient drivability through turbocharging, transmission and axle adjustments. This strategy can be highly effective, but there is a limit to how far it can be exploited before transient boost pressure, and hence transient torque, becomes too detrimental. Turbocharger manufacturers have mitigated this transient lack of exhaust enthalpy using technologies such as VTG (variable turbine geometry) and two-stage charging, especially with EGR-equipped air systems. Parallel to this issue but not separate is the scenario that sometimes the engine has more exhaust enthalpy than what is needed to power the turbocharger compressor. As a function of driver behaviour and the type of vehicle mission, the usage typically oscillates between these two conditions – too much energy or not enough.