Due to the downsizing of an engine, the importance of a turbocharger, in the internal-combustion engine system, has been increased. Since as the bigger engine creates the more power, the downsized engine must need a turbocharger to increase its output.
[Turbocharger, used for HINO Ranger]
As well known, one turbocharger is used with one engine for a passenger car. However, for a truck with a big capacity, it is required to use twin turbochargers or triple turbochargers to increase the engine’s output.
Of course, the same as the engine, the bigger turbocharger is used, the more power the engine can create. However, there is a big disadvantage to a big turbocharger, and it is called as the turbo lag.
The turbo lag simply means the delayed time that the turbocharger starts to run after you hit the accelerator. Normally, the turbocharger runs when the exhausted gas returns to the turbocharger and turns the turbine wheel. Therefore, the bigger turbocharger is used, the more time is taken to accelerate. This gives you the dilemma of giving up the reaction for output or abandoning the output to increase the reaction.
[Air and exhaust flow chart]
The twin turbo is used to solve this dilemma. The twin turbo, consisting of a small turbo and a large turbo, is designed to activate in different patterns of low speed and high speed to minimize turbo lack and to be used without any inconvenience in large vehicles such as trucks and buses, by increasing responsiveness and power.
[Cycle of twin turbo]
In the low-speed section, the exhaust gas flows to both the small and large turbochargers so that the reaction force is increased during acceleration. In the high-speed section, exhaust gas flows only to the large turbo, which is designed to prevent the engine from the over-output and preserve the output constantly.
Of course, since two turbos are used, there is a disadvantage that the repair cost doubles once a failure occurs. However, there are many trucks that use the twin turbo system because it gives the more than doubled efficiency, comparing from just using only one turbocharger.
[Triple turbo]
Not only for large vehicles, the twin turbocharger system is also used in sports cars and luxury vehicles because of the advantage that the output is also increased as more turbos are installed.
[BMW M550d xDrive]
For example, BMW's M550d xDrive, which was launched in 2012, has two small turbos and one large turbo as shown above. At the low-speed section, the large turbo starts to move, and then the two small turbos sequentially run when accelerating. It is designed to produce stable output in both low-speed and high-speed sections.
Today, the technology has developed, and the vehicles equipped with quadra turbo are available. However, when you consider the limited place of a car’s engine room, installing more than four turbos is not imaginable because the four turbos’ size and weight are as big and heavy as one engine. Therefore, rather than the number of turbos, the performance of the turbo or a replacement of the turbo may become the main issue in the future.