Turbocharger Boost Pressure and the Waste Gate
The turbocharger has been developed to provide a greater volumetric efficiency for a given engine size, or in other words, to improve just how efficiently air can flow in and out of the combustion chamber.
Increasing the volume of air allows for an increase in the volume of fuel to be added, resulting in an increase in power output.
While we often associate the turbocharger with large high-performance engines, modern emissions regulations have seen an increase in the use of turbochargers linked to smaller efficient engines with reduced cylinder counts and capacities.
The use of the turbocharger allows for a reduction in engine weight, size, and emissions without sacrificing performance, which can only result in an increase in turbocharger applications.
The turbocharger is mounted in the exhaust manifold in such a way as to expose the turbine to the exhaust gasses leaving the combustion chamber on their route through the exhaust system. The turbine is linked to a compressor via the turbine compressor shaft, where any rotation of the turbine results in an identical rotation of the compressor.
The compressor is mounted in the air intake of the engine, increasing the intake air pressure and volume as the compressor’s rotational speed increases.
There is, however, an inherent characteristic to be aware of with the turbocharger operation. Turbo lag is the name used to describe the delay in response of the turbine and compressor shaft speed in relation to the volume of exhaust gasses passing over the turbine. In order to provide the greatest boost pressure in the minimum time, the engine is required to be subjected to wide open throttle (WOT) conditions, producing high volumes of exhaust gasses directed towards the turbine assembly.
While the boost pressure will provide an increase in the volumetric efficiency, accompanied by an increase in power output, over-boost conditions put the integrity of the engine components at risk and can result in severe engine damage.
Over-boost control is implemented by reducing the speed of the turbocharger compressor and turbine shaft speed. The given turbine speed is dependent on the flow and volume of the exhaust gasses passing over the vanes of the turbine. By redirecting the exhaust gasses away from the turbine, we can reduce the turbine/compressor speed and so reduce the turbo boost to a safe operating level.
By-passing the turbine is achieved by the use of a wastegate controlled directly by boost pressure applied to an actuator linked to the wastegate, or via the ECU applying pressure/electronic control signal to the wastegate actuator dependent of information received from the MAP sensor.
Do not switch off the engine immediately after revving as the turbocharger speed will remain high while the oil pressure supplied to the turbocharger will fall rapidly. This will lead to the undesirable condition of the turbocharger rotating at speeds in excess of 100,000 rpm with no oil supply/lubrication. After free revving a turbocharged engine, let the engine idle for 30 seconds before you switch it off.