Often misunderstood, always awesome, the turbo is one of the greatest gifts to the automotive world.
At its core, an engine is an air pump, which creates the byproduct of horsepower and torque. A turbo simply increases the engine’s ability and efficiency in regards to moving air in and out of the engine. Rather than relying on the engine’s vacuum to suck air into the engine, a turbo forces the air into the engine.
A turbo has a few basic parts. Two wheels, which resemble fans, are attached to each other with a rod. The fans sit inside housings. The center section resides between the two housings and holds the bearings, as well as the oil journals and the cooling jackets.
As exhaust gas is produced, it passes through the exhaust housing, which causes the fan wheel inside the housing to spin. Because the exhaust wheel is attached to the compressor wheel, the compressor wheel also spins. The compressor wheel then sucks air into the compressor housing, and pushes it out and into the engine. The more exhaust gas is sent through the exhaust housing, the faster both wheels spin. The faster the compressor wheel spins. the more air is forced in and out of the compressor housing and then into the motor.
An important note, the nature of turbo-charging makes the air that is coming out of the turbo hot. On most applications, the air coming out of the turbo is cooled before going into the engine. To cool the air that has come out of the turbo, the air is sent through an intercooler. The intercooler is basically the same as a radiator, but instead of cooling water, it cools air.
This cycle of moving air is exponential and before you know it you are forcing air into the engine, and producing positive pressure inside the intake path of the engine. Positive pressure inside the intake manifold is referred to as boost, and is measured in Psi, Bar or KPa . When the proper boost pressure is achieved, the turbo must be regulated in order to not produce too much boost (although rare, you can have too much boost ).
To regulate the amount of boost a turbo makes, there is a wastegate. As the required boost pressure approaches, the wastegate begins to open. When the wastegate is open, exhaust gas goes through the wastegate instead of the exhaust housing of the turbo. By reducing the amount of exhaust gas passing through the exhaust housing, the exhaust wheel slows, the compressor wheel slows, and boost pressure is regulated.
A quick tutorial on electronic boost controllers. When you set boost, there are two settings, desired boost level, and gain. The desired boost level is the actual amount of boost you want to achieve. Desired boost pressure is generally represented as a percentage, which will basically be a percentage of the boost the turbo is capable of making. Gain, is how quickly the boost pressure will be achieved.
The more gain, the later the wastegate opens. Because the wastegate is opening later, or closer to when desired boost pressure is achieved, the more likely it is that the turbo will briefly make too much boost. When the turbo makes too much boost but then returns to the desired level is known as boost spike. You have to carefully balance gain to achieve quick spool, but not too much boost spike.
Also, each gear will make more boost than the last (the longer the gear, the more time the turbo has to build boost, and the more effectively it does just that. Same reason it is better to “roll into” the throttle rather than just dumping the gas pedal to the floor). Many boost controllers have per gear learning, in order to have the proper gain setting for each gear.
Lastly, there is boost creep. Boost creep is where the boost hit the desired boost level, but then slowly rises above the desired boost level. Boost creep is generally caused by a wastegate that is too small. I would say it can be caused by a turbo that is too big, but a turbo that is too big is just an oxymoron.
Turbo-charging. It’s just as cool as it gets.
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