Turbocharger System Description: Notes

The exhaust gases from the engine cause the turbocharger turbine wheel to spin. The turbine wheel is directly connected to a turbocharger wheel via a shaft. This means that the turbocharger wheel rotates at the same speed as the turbine wheel. The turbocharger wheel increases the pressure in the intake system which increases air into the engine. Thus it is possible to burn more fuel and the result is a higher torque and power. The engine control module (ECM) regulates this by releasing surplus pressure on the exhaust side when necessary.

The turbocharger's turbine housing is a dual-scroll type. There are two separate spirals in the turbine housing. The turbine wheel, however, is of a conventional design. Spiral A is supplied exhaust gases from the front bank of the engine while spiral B is supplied from the rear bank of the engine. Since the firing order is 1-2-3-4-5-6 with the front bank cylinders numbered 2-4-6 and the rear bank cylinders numbered 1-3-5, the exhaust pulses will alternate between spiral A and B. Alternating between the two banks of the engine has the following benefits:

• Quicker turbocharger response
• Considerably fewer disruptions between cylinders during the flushing phase.

By using the energy of the exhaust pulses as a propelling force rather than the more or less constant flow of exhaust gases, the turbocharger reacts quicker. There is also a lower average pressure in the exhaust gases, which is good for engine ventilation. Because there is a difference of 240 crankshaft degrees between the exhaust strokes in the respective exhaust manifold, the exhaust pulse will have subsided when the next cylinder opens its exhaust valves. There is little flow of exhaust gases from the exhaust manifold into the cylinder, resulting in efficient cleansing with low residual gas content.

The charge air pressure is mainly due to engine speed and load. At low engine loads, the exhaust gas volume driving the turbine is relatively small and all the exhaust gas needs to pass the turbine in order to drive the turbine wheel.

When the engine load is somewhat higher, the exhaust gas volume will also be larger, which the energy driving the turbo is greater and therefore forces more air into the engine.

If the engine load rises further, the exhaust gas volume produced by the engine will be greater than that needed to drive the turbine in order to provide the correct air mass per combustion. At high loads, the volume of gases reaching the turbine must therefore be limited so that the turbocharger produces the correct airflow. This is achieved with a valve, called wastegate, opening a bypass passage parallel with the turbine. The excess gas not required to drive the turbine passes through this passage.