How the turbo copes with different engine demands

Wastegate turbo

SO FAR, I have described a turbo whose speed, and therefore the amount of air charge pressure, is dictated by the speed of exhaust gas flowing through its turbine. In practice, driving in different gears, up hills, towing, accelerating and other criteria demand that the turbo supplies air at an ideal pressure for the occasion, so the air pressure needs to be modulated or controlled. On earlier turbos this is achieved by a wastegate – a flap valve that allows exhaust gas to bypass the turbine, thus lowering the turbine speed and reducing the air charge pressure (known as boost pressure) as the flap valve closes, and boost pressure increases. The wastegate is controlled automatically according to the charge pressure transmitted via a rubber pipe from the compressor outlet to the actuator. Later models, such as the Td5 Discovery had a solenoid-controlled wastegate, influenced by signals from the engine ECM.

Variable geometry turbo

This mechanical system has always worked well, but was superseded in the early Noughties by mechanically-operated and electronically-controlled variable vane turbochargers (VVT), also known as variable geometry (VGT) turbos. These use adjustable vanes inside the turbine housing to control the flow of exhaust gas passing through the turbine, thus controlling the turbine speed and modulating the compressor output and the charge air pressure into the engine. This allows boost pressure to be far more accurately matched and modulated to the engine’s running conditions, producing an even more efficient engine and more low-speed torque. Variable vane/geometry turbos appeared on Discovery 3, Freelander 2 and Defender TDCI models, and on Range Rovers and Sports. The variable vanes are operated by a electronically controlled actuator according to engine running parameters detected in the engine management system.

Turbo lag

The earliest turbos were a compromise, achieving a boost pressure via the wastegate, but without the flexibility to benefit the engine over its complete rev/ load range. The problem was that the driver pressed the accelerator, then the engine picked up speed, then the resulting increase in exhaust gas flow acted on the turbine, and the turbocharger finally accelerated to produce full boost. That time delay between pressing the accelerator and developing boost was known as turbo lag (a lack of throttle response). A small turbo with lighter components could respond more quickly, but couldn’t generate boost over a large rev/load range. Conversely, a large turbo could produce boost over a large range, but the weight of its components meant it would take more time to accelerate up to speed (to spool up), which means more turbo lag.

So later developments have all been about increasing turbochargers’ response and flexibility to achieve a more efficient engine system. The flexibility of variable vane turbos, and the use of twin turbos on later models address these concerns.